Aeroplanes & Conveyor Belts

[d2tw4all]

I hope there are some physicists here, an argument has been going on for many years between myself and someone else, anybody who has any insight, please answer the following question and you will help to resolve this debate!

A 747 jetliner weighing 163844 kg (the empty weight of a 747) lands on a treadmill that is 500 meters long running in the opposite direction of the plane and doesn't apply any brakes whatsoever, relying only on whatever friction is in the bearings of the wheels to slow it down. The speed the aircraft lands is 200kph, and the speed that the treadmill is moving in the opposite direction of the plane is 200kph. Assume the tires and landing gear withstand the impact, remember, little to no brakes are applied at any time. Also, it's autonomous so there is no margin for pilot/human error, assume it's a "perfect" landing onto the treadmill. WHAT WILL HAPPEN.

Thanks!
Tom Lavoie

[Artman]

The wheels rotational speed would accelerate to twice the speed of the treadmill and the plane would continue to move toward the far end of the treadmill till it goes off if it. Sort of like taking a friction car and moving it against the ground which is staionary gives one speed to the wheels, but to get an even higher speed to the wheels push them against a piece of wood that you are dragging in the oposite direction.

That is my opinion of what would happen.

[d2tw4all]

Ok, just so I am clear, what's your physics background, for the record.
Thanks a lot for replying, anyone else have an opinion?

Tom Lavoie

[enigma]

Originally posted by d2tw4all
please answer the following question and you will help to resolve this debate!


If you don't mind me asking, what are the points the two of you are arguing?


A 747 jetliner weighing 163844 kg (the empty weight of a 747) lands on a treadmill that is 500 meters long running in the opposite direction of the plane and doesn't apply any brakes whatsoever, relying only on whatever friction is in the bearings of the wheels to slow it down. The speed the aircraft lands is 200kph, and the speed that the treadmill is moving in the opposite direction of the plane is 200kph. Assume the tires and landing gear withstand the impact, remember, little to no brakes are applied at any time. Also, it's autonomous so there is no margin for pilot/human error, assume it's a "perfect" landing onto the treadmill. WHAT WILL HAPPEN.



Well, you've got a big qualifier in there when you say that the tires and landing gear withstand the impact. If that is the case, then, just as Artman described,the plane will slow down a little bit by the time it gets to the end of the runway... maybe a little bit more than if the plane landed on a normal runway without applying brakes.

The problem is that the wheels and landing gear are not designed to sustain a 400kph landing. My SWAG factor tells me that the bearings would overheat, locking the wheels, and causing them to overheat and explode. Big mess.

For the record (since you asked): senior year aerospace engineering major.

[Artman]

For the record, my physics background is mostly practical and not academic. I've been doing Mechanical Engineering for about 20 years.

I drew the situation out to arrive at my guess.

[d2tw4all]

I kind of want unbiased opinions to the question and that's why I didn't identify the two opinions, I just need the most reputable people I can find to answer the question as best they can, assuming the landing gear can withstand the landing and have bearings that don't sieze up. Thanks a lot for your answer, I hope others will or if anyone knows a physics professor who could comment, that would be even better.
Thanks!
Tom

[enigma]

Originally posted by d2tw4all
Thanks a lot for your answer, I hope others will or if anyone knows a physics professor who could comment, that would be even better.


While others are more than welcome to comment, if you assume that everything with the landing goes smoothly and safely, there really isn't much to the problem. Definately not enough that would need a PhD to answer it (not to mention that you'd probably get a better answer from an engineer for any 'real-life' type question).

Assuming the landing gear works at the higher speed it's feeling, the only thing which I can think of which would differ between an airplane on a normal runway would be the potential for a higher coefficient of friction in the bearings due to the higher velocity. That would translate into a small decrease in the plane's speed relative to a normal plane's speed at the end of the runway.

My $.02

[LURCH]

Agree with the answers already given. Although I am not a physics professor, I have been a pilot. One fo the first things they start drilling into your head in flight school is that the plane "feels" airspeed, NOT ground speed. The exceptions heve already been stated; the gear would be subjected to more friction, I would add that the inertia of the gear would be a greater factor. The mass of the landing gear would have to go from stationary to whatever rpm's they spin at 400 kph, rather than zero to 200. This would make the instant of touchdown quite a jolt, and the plane would lose slightly more speed than it would on a stationary runway. Not enough to make much difference in the rollout distance, though.

Now if, rather than a treadmill, you used a wind machine, that would be a different story. Generate a 200kph wind above a stationary runway, and land your 747 with its nose into that wind, and you can actually get it to hover, and then make a vertical landing!

[zoobyshoe]

I'm confused about the brake
stipulation. Are you saying that
the planes "autopilot" or whatever
it operating it, refrains from
applying the brakes after touch-
down?

[d2tw4all]

Correct, no brakes are applied. The general consensus seems to be that the plane will undoubtedly roll off the runway, and that the speed of the wheels will be the sum of the forward velocity of the plane plus the speed of the treadmill in the opposite direction. To me this seems logical but the other party involved does not see this. Instead, he believes that the aircraft landing on the treadmill presents an "infinite plane" for the aircraft to roll on, thus significantly reducing the landing distance required. Obviously if you used brakes the treadmill might provide an advantage in that regard, but resulting in doubling the stresses on the landing gear and it would have the same net effect of doubling the braking capacity on the plane. You have all thus far confirmed my belief, and I thank you!
Tom

[Artman]

By reversing the direction of the treadmill to match that of the plane and varying the speed that it moves as the plane slows to a landing, you could land the plane without the wheels turning much at all and save stress on the landing gear. [:D]

[russ_watters]

Originally posted by Artman
By reversing the direction of the treadmill to match that of the plane and varying the speed that it moves as the plane slows to a landing, you could land the plane without the wheels turning much at all and save stress on the landing gear. [:D] Yeah. You could land, set the parking break, and use the brakes on the treadmill to slow and stop the plane. It would stop faster and wouldn't wear down or heat up the brakes on the plane.

With the treadmill going in the opposite direction, all you do is overheat the tires and wheel bearings. The plane won't stop any faster than if it were on a regular runway with no brakes (which is to say it won't stop). The primary force slowing the plane down before the brakes are applied is aerodynamic drag.

[d2tw4all]

I definitely see how running the treadmill in the same direction as the plane would allow you to essentually "set it down" on the treadmill and then bring it to a stop. You would still have to decellerate it over the same relative distance as you would landing it on a runway, to maintain the same stresses on the plane and it's contents though, so I see no advantage in that from the distance to land aspect...
Tom

[zoobyshoe]

Whenever I ride in a jet I hear
a fairly sudden change in the
sound of the engines that happens
after touchdown but before coming
to a complete stop. I've always had the impression they were
reversing the direction of the
turines to help bring the plane
to a stop.

What is this noise, in fact?

[d2tw4all]

It's called "reverse thrust" and it's pretty much as you said, they are making the engines blow forwards instead of reverse. They can't run the engines in reverse, however, so for jets they usually deploy thrust reversers which are basically pieces of metal that redirect the air coming out of the engine forward instead of backwards.
Tom

[zoobyshoe]

Thanks Tom,

Very cool info. Every time I stop-
ped to think about my assumption
they were reversing the direction
of the turbine something told me
it was insane to think they could
do that without the engines coming
to a full stop first, but then I'd
leave the airport and forget the
whole question.

So if they did not engage reverse
thrust, or flaps, or brakes,
how far off the treadmill runway
would the plane go and how bad
would the crash be?

[d2tw4all]

I would say the crash would be pretty bad, but that wasn't the basis of my argument... No problem on the info... On a side note, propeller driven planes also employ reverse thrust, however usually they actually alter the pitch of the blades to achieve negative thrust, just so you know...
Tom

[sm0ke]

We are having a heck of a discussion on a board I am a member of. Maybe you guys can provide some insite?

---------------

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

[G01]

No it wouldn't. Because the plane will move with a velocity equal and opposite to the conveyor belt. It will stand in place with respect to the air. Because of this, the plane will not be able to move throught the air, and the pressure under the wing is equal to the pressure above the wing. Since a plane takes off because the pressure under the wing is greater than above, (because it is moving through the air -look into the Venturi Effect), the plane will stay on the ground.

[rivermud]

This is a quote from NASA. The Venturi effect is based on Bernelli's theory.

When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

Hope that helps. There must be flow/velocity in order for the plane to lift off.

[sm0ke]

I understand the whole lift aspect.

The main part i'm concerned with is if the plane would have the ability to move in the first place. Here is a quote from someone on the board...

"There are four forces that govern an airplane at any given time: lift, weight, thrust and drag. Lift and weight oppose each other but can be disregarded in this question because all we are talking about it whether or not the airplane can accelerate in reference to the surrounding air. Once we get acceleration and airflow we can get the lift and weight but we don't need to talk about that now.

The throttle of the airplane is advanced and propeller/jet of the airplane produces thrust. We now have a forward vector, say 500 pounds of force, for a small cessna. In order for the plane to remain stationary an equal and opposite force needs to be introduced. Everybody follow? This opposite force would be drag.

We need to account for 500 pounds of drag. Right now the only drag is the force of friction in the wheel bearing. Has anyone here ever tried to push a cessna? It's not very hard...maybe 50 pounds of force at the most. So now we have 450 pounds of force acting in the forward direction. The conveyor belt itself does not impart any friction or drag to the airplane. It will accelerate, gain airspeed, and take off

Let's think of it a different way. Let's say the plane is landing. It approaches the runway at 100 knots and is 1 foot above the ground. As posed in the original question the conveyor belt is moving in an equal and opposite direction. So it's moving 100 knots backwards. As soon as the plane touches down, what happens? nothing...it continues its rollout as if the runway were stationary. It doesn't just automatically come to a stop. The airspeed indicator of the plane would read 100 but the wheel speed would be 200. (apologies to Youens at flightinfo.com)

People cannot separate a car/bike/walking on a treadmill with an airplane. Same thing would happen if you put the airplane on ice and hit the throttle, it will still accelerate at the exact same rate as on a runway (not a safe operation however). Try that with a car/bike/your foot and you'll get drastically different results."

[brewnog]

No it wouldn't. Because the plane will move with a velocity equal and opposite to the conveyor belt. It will stand in place with respect to the air. Because of this, the plane will not be able to move throught the air, and the pressure under the wing is equal to the pressure above the wing. Since a plane takes off because the pressure under the wing is greater than above, (because it is moving through the air -look into the Venturi Effect), the plane will stay on the ground.


Sorry, this is not right. The aeroplane's wheels are not powered, - the forward motion of an aeroplane on takeoff is produced by thrust from the engines. For these purposes, it makes no odds what speed the runway is moving relative to the plane, since the undercarriage wheels are free to spin at whatever speed they're being driven at.

[C12_805]

I understand the whole lift aspect.
The main part i'm concerned with is if the plane would have the ability to move in the first place.

To answer your orignal question, No - there wouldn't be forward movenment. If the conveyer offsets any increase in trust... So no matter if there's 500# or 1K# of trust, it won't move if the conveyer can keep up.
In a common situation is the differential relationship between air-speed and ground speed. If an aircraft has a set speed of let's say 75knts and it [hypothetically] hits a head wind of 75knts, it's ground-speed would be 0. Even though the lift + trust is present, the plane isn't moving forward.

[James R]

To get lift, air must flow over the wings. In the given situation, it doesn't, since the wings remain stationary relative to the air. So, the plane won't take off.

[thowonasweaterandgo]

an airplane isn't a car, the wheels aren't driven. the propeller/jet will pull/push the plane regardless of what the wheels are doing, given they're not locked or something. brewnog got it right, think about it.

[erok81]

We are having a heck of a discussion on a board I am a member of. Maybe you guys can provide some insite?


:rofl:

I figured it would only be a matter of time before this thread made it over here. I think it is up to something like 57 pages on the other board.

I still don't see how it wouldn't take off. Once the planes engines over come the friction from the landing gear it will move forward no matter what the conveyor belt is doing. Now, if planes were propelled by the wheels, then the conveyor belt would keep it from taking off. Since they aren't, it will take off.

[sm0ke]

:rofl:
I figured it would only be a matter of time before this thread made it over here. I think it is up to something like 57 pages on the other board.
I still don't see how it wouldn't take off. Once the planes engines over come the friction from the landing gear it will move forward no matter what the conveyor belt is doing. Now, if planes were propelled by the wheels, then the conveyor belt would keep it from taking off. Since they aren't, it will take off.


i really want to know. i've gone back and forth so many times my head is spinning.:surprised

[moose]

It would definitely take off.

This question is awesome because it is easy, yet people start talking about lift and all.... It tries to confuse them I guess.


If the belt was able to move at incredible speeds, then the friction would be even with the jets. However, this is not the case here!

[FredGarvin]

To answer your orignal question, No - there wouldn't be forward movenment. If the conveyer offsets any increase in trust... So no matter if there's 500# or 1K# of trust, it won't move if the conveyer can keep up.
In a common situation is the differential relationship between air-speed and ground speed. If an aircraft has a set speed of let's say 75knts and it [hypothetically] hits a head wind of 75knts, it's ground-speed would be 0. Even though the lift + trust is present, the plane isn't moving forward.This wrong on two accounts. Forst, the only force the conveyor can exert on the aircraft is friction due to the aircraft's weight. It will never be able to overcome the entire amount of thrust created by the aircraft. Also, the situation here, you could care less about ground speed. Indicated airspeed is what is your concern for lift. In your exapmle, the ground speed would be zero, but what is the relative airspeed for the wind going over the wings? It is not zero.

Think about it this way; every runway in the world is moving at the speed of the Earth's rotation and aircraft manage to take off.

The aircraft would take off. The only thing that would be holding the aircraft back is friction. Once the thrust overcame that friction, the airplane would move. Granted, it wouldn't be good for the wheels because the wheels would be rotating at the combined relative speed, but the aircraft would still move. Since the only thing in contact with the ground is the wheels, the propulsive force would have to be through the wheels for the conveyor to be effective (like Brewnog pointed out). Since the force is in no contact with the conveyor, the conveyor doesn't come into play. The only way for the aircraft to not take off would be to restrain the aircraft. In that case you wouldn't need the engines, the wheels would just freely rotate.

[russ_watters]

The question is slightly unclear: if the conveyor "tracks the plane's speed" and keeps it exactly zero, then obviously, no, the plane would not take off as it would be stationary with respect to the air. I don't see what could possibly be difficult to understand about that.

The way the question is worded though, it could mean to say that the plane moves forward at 150kts, while the conveyor belt moves backwards at 150kts, for a rotational speed of 300kts for the wheels. In that case, as long as the wheels don't burn off, the plane would take off.

Perhaps the wording is all that is causing the problem there...

For example, your quote in post 3 doesn't obey case 1, it just questions whether case 1 is physically possible. IMO, case 1 likely is physically possible - a plane with it's throttle just slightly above idle would be able to stay stationary on a treadmill as the case requires.

Post 6 rases the same objection, but there, you're starting to get into the realm where case 1 breaks-down: a conveyor would have to be moving pretty fast to generate enough friction to overcome the thrust of the engines - and that wouldn't really be possible. But again, that's not what case 1 is saying, so it isn't really relevant here.

So again, the clarity of the question is the only problem here. If the question is specifying that the plane is stationary then the plane is stationary and doesn't take off. Simple. If the question is specifying that the plane is moving, then it most certainly can take off. Either way, the question is probably meant to specify the speed of the plane, so get the question clarified and the answer becomes easy.

[DaveC426913]

The question is slightly unclear: if the conveyor "tracks the plane's speed" and keeps it exactly zero, then obviously, no, the plane would not take off as it would be stationary with respect to the air.

I think there is some confusion at this point.

Exactly *how* does the conveyor belt keep the plane's speed in check? (It can't.) The wheels are free to move - the conveyor belt can speed up as much as it wants, but the plane still has forward thrust from the prop.


(This is tricky! It looks deceptively simple at first - initially, I was positive the plane would not take off...)

[Ki Man]

okay... this is the way i see it

the conveyor belt means nothing. as the plane goes forward the conveyor belt goes backwards with the same amount of force, but since the forward moving power comes from the engines mounted somewhere on the wings/fuselage and not the wheels, and the wheels merely rotate freely on an aircraft, the plane will go forewards while the wheels just spin twice as fast as they usually would.

its good to be back. what did i miss?

[James R]

I think I've changed my mind on this.

The plane's engines push air backwards. The resulting reaction pushes the plane forwards. At the same time, there is friction between the wheels and the conveyor, trying to pull the plane backwards.

If the brakes were on, the wheels would start to slip on the conveyor until the maximum possible friction force was being applied. This would be less than the thrust provided by the engines, so the plane would move forwards.

Even with the brakes off, there is a maximum force the conveyor surface can apply to rotate the wheels. After that is exceeded, the wheels will slip. The plane will still start moving forwards. Once that happens, there is air flow over the wings. When the plane gets up enough forward speed to generate sufficient lift, it will take off.

The only way to prevent this would be to physically tie the aircraft down.

[alaska cajun]

To start with,I know nothing at all about physics.

A question was asked on a sportbike forum of which I'm a member and I thought I would ask it here to see if there realy is a answer to it.

Here it is

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."

The question is: Will the plane take off or not?


This is a link to the thread on the sportbike forum.

http://www.sportbikes.net/forums/showthread.php?t=297891

thanks
Michael.......aka/alaska cajun

[Gokul43201]

I believe another member of that board brought the same question here a couple of days ago. The discussion is going on here : http://www.physicsforums.com/showthread.php?t=101259

[russ_watters]

So perhaps we could see the initial question...? The wording of the initial question makes all the difference here.

[Wilbur Wright]

So perhaps we could see the initial question...? The wording of the initial question makes all the difference here.
A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

The answer is that the plane will take off with little difference from the manner it would on solid ground.

We know the vessel must move, or there can be no lift.

There are only two forces at work here, the force of the engines pushing the craft, and the force of friction on the wheels.

Since the conveyor is only moving at the same speed as the vessel, rather than with equal and opposite force, the force of friction on the wheels will never come even remotely close to equal or overcome the force of the engines.

The conveyor could even be moving considerably faster than the fuselage, and flight will still occur because the force of friction is so much lesser than the force from the engines. :smile:

One needs to remember that unlike a car, the wheels on an airplane do nothing but spin freely. It's no different that pushing a matchbox car up a treadmill, where the wheels on the matchbox are the landing gear, the car itself is the aircraft, and your hand is the engine of the plane. The speed of the treadmill makes next to no difference in the force exerted by you to move the car against the rotation of the belt.

[G01]

Sorry, this is not right. The aeroplane's wheels are not powered, - the forward motion of an aeroplane on takeoff is produced by thrust from the engines. For these purposes, it makes no odds what speed the runway is moving relative to the plane, since the undercarriage wheels are free to spin at whatever speed they're being driven at.

Yes, I see my mistake, you're right, my bad.

[DaveC426913]

BTW, as far as the conveyor goes, consider this:

If the conveyor is able to supply ANY non-trivial amount of drag upon the plane's forward movement then - clearly, the wheels are NOT doing their job! They're not supposed to cause drag! If they are, it means they're ALSO going to cause drag when the plane is going down a regular runway to take off!

[Jeff Reid]

Edit: what's being ignored on my part is that the conveyor belt just speeds up how fast the tires spin. Otherwise the plane just takes off as normal. If the plane was tied down so it couldn't move horizontally, then then propwash would have to draw enough airflow to create lift.

[MythBusted]

http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B
This may help you get a visual on the problem

[Moe88]

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

[Jeff Reid]

There is already another thread here on this.

Edit: The following would only apply if the plane was tied down so it couldn't move, not the same situation.

The answer is only if the planes engines can produce enough airflow to cause the plane to fly even though it's not moving relative to the ground.

No full scale plane could do this, but there are some high powered radio control models that can do this. The most extreme examples are "27 cell" (old ni-cad terminology), F5B limited motor run gliders. The motors produce over 2 1/2 horsepower, geared to drive a 16 or 17 inch prop (folding) on a model weighing about 5 pounds.

[Cyrus]

lol mythbusted, I heard what you said in the end of that video....ahem........

[Jeff Reid]

Wow, brain fade on my part, wasn't paying attention, assuming that the friction from the tires rotating isn't excessive, the plane takes off in normal fashion. The free wheeling tires just spin faster while the plane takes off.

[Jeff Reid]

Wow, brain fade on my part, wasn't paying attention, assuming that the friction from the tires rotating isn't excessive, the plane takes off in normal fashion. The free wheeling tires just spin faster while the plane takes off.

All the conveyor belt does is cause the tires to spin faster. Other than some energy consumed by rotational kinetic energy and friction from the tires rotating faster, the plane is going to take off pretty much as it normally would.

If the plane was tied down down via a tow hook so it couldn't move forwards, then my extreme power situation where the propwash draws enough air across the wing to lift the plane would apply.

[russ_watters]

It's all about the wording of the question, the phrase "The plane moves in one direction" implies that the plane is moving with respect to the air, so the plane does take off. See the other thread...

[pervect]

If I'm understanding the operation of the conveyer property, it would not even stop a car. A car whose spedometer read 60 mph would be going 30 mph in one direction (say east), while the road went 30 mph in the other direction (say west)

[pervect]

If I'm understanding the operation of the conveyer properly, as described it would not even stop a car. A car whose spedometer read 60 mph would be going 30 mph in one direction (say east), while the road went 30 mph in the other direction (say west).

It's unclear if this was the intent of the question.

If the conveyer was intended to stop a car, the conveyer will reach its maximum possible speed in order to attempt to stop the plane. This could be achieved by deriving the feedback signal to the conveyer not from a measurement of the speed of the plane relative to the ground, but by deriving a feedback signal from the rate at which the wheels of the plane were turning. (The original question didn't really clarify exactly where the feedback signal was coming from, hence the ambiguity of the question).

It would probably be good to include a physical upper limit to the speed at which the conveyer can operate in the problem statement if the problem is modified so that the conveyer actually does attempt to stop a car.

[Noodler]

My friends and I have been having a physics based argument for some time and we have reached a point where we need a resolution. I believe for someone trained in physics it is a very easy question and I was wondering someone could shed a little light on this.

Thanks a lot!

The situation:

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will the plane move forward or stay stationary?

[brewnog]

http://www.physicsforums.com/showthread.php?t=101259

This has been done to death.

[KingNothing]

Good question, one that makes you think twice. Not because of the conveyor, but because of how planes work. Yes, the conveyor could stop a car, because a car uses the ground to move.

Think about it like running on a treadmill. If you just run and the treadmill keeps up with you, you won't go anywhere. Now think about if you had legs that could move incredibly fast (like the wheels on the plane), and simply pushed off the wall. You would still move.

[webtim-1]

See what you guys think
Taken from another forum, this question was put forward and argued with incredible conviction from 5 angles or more, to reach either of two possible answers.
It's a pretty simple question but had some brilliant rows about all sorts of things going on to justify the answers.




Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

[moose]

http://www.physicsforums.com/showthread.php?t=101259

Yes it will take off

[pallidin]

A plane does not require wheels to provide for forward motion.
Perhaps a way to look at this is similar to your description:
Imagine a plane, without wheels, yet it's belly is on a sheet of slick ice. Will the plane move forward? Yes.
Perhaps some of the confusion is with respect that a car will NOT go forward in the same scenario you described. In a car's case, the wheels ARE providing the forward motion by virtue of being DIRECTLY COUPLED to the engine output. A plane DOES NOT have that set of circumstance.

[russ_watters]

Duplicate thread....locking.

[AJHank,Jr.]

The plane would rise if the air on its lifting surfaces was moving across them faster than the plane's stall speed. Aircraft carriers head into the wind so that the speed over ground is lessened, relative to the speed of the (moving) ship. If the plane must go 140 kts relative to the air it is flying through, and there is no wind, it will only need to go 120 kts relative to the deck of a carrier going 20 kts in the same direction as the takeoff. Into a headwind of 20 kts, in that scenario, the plane will have sufficient life to take off at what would appear to be 100kts to someone standing on that same carrier. Speed through the air is what counts. A plane could beon a conveyor going 1000 kts and keeping that pace, but if the air over the control and lift surfaces (wings) does not move, the plane will not go up.

[curvedlogic]

If you go to pprune/forums/jet-blast you will see what pilots and engineers think of this problem. you may never want to fly again.

[Dan Shenk]

Aside from the question of whether or not the conveyer belt can actually prevent forward movement of the plane, and all other esoteric mumbo jumbo, planes fly because of airspeed, not becaues of ground speed. If air does not flow across the wing, the plane will not fly.

Care to rephrase your question, Smoke?

[mewmew]

So on another forum we are in a huge argument and so I come to you guys for help with the following:

Suppose you have an airplane on a runway that is a huge conveyor belt. The conveyor belt moves with the opposite velocity of the plane at all times, what happens when it tries to take off?

Now, I think we have two cases at hand, one where the runway moves with the opposite speed of the plane itself, and one in which the runway moves with the opposite speed tangental to the outside of the tire.

Can anyone shed any light onto how this would actually work, assuming just newtonian mechanics and what not.

I think it will take off in both cases. but am looking for a solid argument for each. Thanks

[simon009988]

i not sure what u mean but i think the plane wouldn't take off because a plane only files because there is a difference in air pressure on the wings and if only the runway is moving and not the air molecules the plane would stay still (if only the tires were moving exactly in opposite to the runway)

[mathman]

The ability of a plane to take off depends on its speed relative to the air, so what is happening on the ground is relevant only as it affects air speed. An example of this principle is taking off from an aircraft carrier. The carrier is pointed against the wind and the plane takes off against the wind. Similarly at airports the preferred direction for takoff is against the wind. An analogous logic is used for landing, also against the wind, to get minimum ground speed for given airspeed.

[mewmew]

I understand that, I know how a plane flys(well, the basics atleast) and know that this problem comes down to if the plane can move or not. I, as well as others I have talked to, say that the conveyor belt will only make the wheels spin faster but have little effect on the plane. The wheels aren't locked like on a car, so the plane can generate forward movement relative to the earth independent of how fast the wheels move. Say the plane is going 100 m/s relative to the earth, the coveyerbelt is going -100 m/s relative to the earth, so the wheels, or a point on the wheel, should spin at 200 m/s, if this was a car it would be easier, but because its a plane the situation is harder to imagine.

[berkeman]

Who cares what the wheels are doing. If the plane is not moving relative to the earth and the associated air, the wings cannot generate lift. This seems like a pretty straightforward puzzle. Why are you guys debating it somewhere? Care to make a little wager... :-)

[mewmew]

It is not as easy as you make it sound, just because the belt moves backwards and the plane is moving forward(with equal and opposite speed) doesn't mean the plane is going to feel the backwards motion, because it has wheels independent of its engine that can spin however fast they want. It isn't really a straight forward question.

[berkeman]

Wanna bet? You got a PayPal account? :-)

[mewmew]

No where in the question does it say the plane is stationary in respect to the earth.

Here is what I am thinking:

The moving belt should only cause a small change in the planes speed, but it will cause the wheel of the plane to spin. If you have a treadmill and something on wheels you can try it, as it is easier to imagine or even actually do. If you turn on the treadmill you can put something on it that has wheels like a tonka truck and then with little force move it in the opposite direction the treadmill is moving, can you not? If this is the case than certainly the plane could do it because its engine is just like your hand and then it could of course take off.

I am more interested in what the forces all are that cause this to happen and what is going on with those forces, as I don't really understand all of that part of the question.

[Danger]

How bloody many threads do we have on this thing, anyhow?
The plane will take off unless the belt is moving so fast that the wheel bearings seize up.

[berkeman]

No where in the question does it say the plane is stationary in respect to the earth.

Well, I guess it was where you said this:

Suppose you have an airplane on a runway that is a huge conveyor belt. The conveyor belt moves with the opposite velocity of the plane at all times, what happens when it tries to take off?

"Opposite velocity at all time" is the part where I guess you threw me. You need to define your reference frame better (the earth?), and define what the velocity of the treadmill is with respect to it, and what the velocity of the plane is with respect to the reference frame.

[RandallB]

So on another forum we are in a huge argument I hope you’re talking about some other forum web site, not double threading on this one.

Now, I think we have ...
one in which the runway moves with the opposite speed tangental to the outside of the tire.
No this would be the same a stationary runway, the tangential speed of the tire in contact with the runway is always ZERO. The top of the tire would be moving forward at double the speed of the axle. Tires don’t leave skid marks on takeoff.

This is a logic problem that requires assumptions: assuming zero wind and the airplane speed unlike a car is measured against the wind and (because no wind) the stationary markers on the sides of the treadmill runway.
And since the runway only moves as a complement to the movement of the airplane it’s only important the runway move backwards at takeoff speed. As defined in the problem it only gets there if the plane also does so moving forward & therefor takes-off.
Note also that the bottom of the tire is still moving at Zero with relation to the treadmill which is moving backwards. The axle moves with the plane so the top of the tire is moving at 4 times takeoff speed.

Pretty simple, what’s to argue about?
I assume since you can afford one expensive runway you can afford wheels that don’t seize up when over spun.

[mewmew]

How bloody many threads do we have on this thing, anyhow?
The plane will take off unless the belt is moving so fast that the wheel bearings seize up.I searched and found nothing, searching for conveyor, airplane, and plane. Also, if you read through I made it clear that I pretty much know what the answer is, as I posted an example and what not. I am interested in the forces involved that make it so.

I hope you’re talking about some other forum web site, not double threading on this one. Yes, I am.

the tangential speed of the tire in contact with the runway is always ZERO. Why does the tangental speed have to be 0 for a tire rolling on a regular runway?

My main question, as stated above in an earlier post, was what where the forces acting on the system. I have pretty much figured that out so it doesn't much matter, its nice though to see some people who know what they're talking about explain things sometimes though. If this question is a sore topic or anyone has a link to the other thread please tell me and Ill close this one.

[Integral]

See this (http://physicsforums.com/showthread.php?t=101259) thread.

[Chris Rox]

Move at exactly the opposite speed and direction to that of the plane


will the plane be able to take off?


People have been arguing with me for days no saying that it wont.

this is my theory and i would like a few of you on here to let me know if i am right or wrong.


The treadmill is set to move in the opposite direction to the plane but at the same speed

so if the body of the plane is travelling at 20 mph to the left then the belt of the treadmill is travelling at 20 mph to the right

people have been constantly arguing with me that this means the plane will be standing still then.

This is my theory.

the plane is powered by jets which use the exhaust gases to provide the thrust.

the wheels on the plane arent powered and do not provide any drive or forward motion

as the plane apllies the thrust will it be able to move forward and eventually take off?

i am the only one out of about 40 people that says yes, they are all saying no it will sit still because the treadmill is going in the opposite direction to the plane at an equal speed.


the wheels of the plane and the treadmill are in no way related to the jets of the plane


so if the plane is travelling at 200 mph to the left then in theory the treadmill will be travelling at 200 mph to the right

so

will the plane be sitting still?? Or will the wheels be turning at 400 mph in the direction of the treadmill and the plane will be travelling at 200 mph and be able to take off??



PLEASE HELP, IT FEELS LIKE ME AGAINST THE WORLD IN THIS ARGUMENT!!

[ZapperZ]

Er... what is with this question that seems to be popping up every couple of weeks? Is some website running a contest or something?

Please read these two threads that have appeared already. Continue your discussion in the last thread.

http://www.physicsforums.com/showthread.php?t=103024
http://physicsforums.com/showthread.php?t=101259

Zz.

[Chris Rox]

thanks to everyone who agreed the plane would take off


i knew the plane would take off,

people were too busy worryong about lift and air pressure, the plane has to move first to acheive both of these

the point being argued here is wil the plane move in the first place to be able to achieve these factors??

the answer

yes

so will the plane be able to take off??


yes

[w_benjamin]

actually, it won't fly.

[Danger]

How do you figure that? This thing has been put to bed.

[w_benjamin]

The amount of energy the conveyor exerts on the plane is directly proportional to the amount of energy the plane exerts on the conveyor effectivly cancelling each other out.

[russ_watters]

The amount of energy the conveyor exerts on the plane is directly proportional to the amount of energy the plane exerts on the conveyor effectivly cancelling each other out. While that's true, that amount of energy (force, really) is relatively small because the wheels of the plane spin freely. Because of that, the conveyor cannot, in reality, exert a strong enough force to stop the plane (though it can spin the wheels so fast they burn up).

Again, you have to be careful with the wording of the problem. The consensus here is that the wording of the problem says the plane moves forward with respect to the ground and the conveyor moves backwards with respect to the ground, making the wheels spin at twice the plane's forward speed.

Let me reiterate that all of the difficulty with this problem is all in the wording and the problem's adherence to reality. Had it been better stated (both here and in whatever forum it was first posted in), it would be very, very simple. Ie, modified to match reality, the question would simply be:

Can a conveyor belt prevent a plane from taking off?

The answer from a theoretical standpoint (assuming negligible friction in the wheels) is a simple no. From a more practical standpoint, it's probably still no, but there is always the chance that the wheels could burn up before the plane reached takeoff speed.

[w_benjamin]

This is where we might agree to disagree. To me, wheel speed is defined as how much distance a wheel COULD travel at a given rpm. This is how race teams and testing facilities for cars define it. When they're testing a vehicle on a dyno, the vehicle speed is considered 0 while the wheel speed is whatever the have going at that particular time. It is also how they plot data for tire grip when a tire is spun up. At the start the vehicle speed is less than the wheel speed. The car eventually builds vehicle speed until the car moves forward at the same rate as the wheel.

[w_benjamin]

Remember, if you put a car on a set of rollers, and run it for 150,000 miles, the car didn't go anywhere, but does the drivetrain stil have 150,000 miles on it?

[w_benjamin]

The original wording of the question is thus:

"Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off? "

[Danger]

In the case of a car on a chassis dyno, look at it this way. If you have that sucker spooled up as fast as it can go, sitting still, and push it from behind with another vehicle, it's going to move forward. Think of the propellor or jet engine as that other vehicle.

[w_benjamin]

An imperfect scenario as the second car is not turning the wheels of the first. In the question, it states the conveyor will not turn any faster than the wheels of the plane will. Therefore, the conveyor does not contribute to the turning of the wheels. If the conveyor does not turn the wheels of the plane, what does? (I'm going somewhere with this, trust me.)

[Wishbone]

im new to this debate, however, from the question at hand, can we assume that the plane is stationary? Are we dealing with a treadmill situation where the force you push on the ground, there is a seperate force of the belt pushing you backwards? If this is true, then the plane obviously wont take off...

[Danger]

An imperfect scenario as the second car is not turning the wheels of the first.
And the engine of an aeroplane has no connection to the wheels.

[w_benjamin]

Ah, Newton's Cradle! The force acting upon an object does not need to be connected to the object. The thrust of the plane begins to try to move forward. This includes the wheel. Since the wheel is in contact on the ground it can either a)skid or b) roll, depending on which is easier. The thrust is indeed acting to turn the wheel.

[Danger]

Okay... let's try it this way then. Suppose you have one foot on solid ground, and the other in a roller skate on an opposing treadmill. If you push forward with your anchor leg, what will happen?

[w_benjamin]

If the treadmill works as stated in the question, it won't move.

[w_benjamin]

And you're not pushing against a solid object in the question. You're pushing against a fluid(believe it or not), so the scenario again an imperfect one.

[Danger]

Man... I'm running out of examples here. Let's try it again.
Based upon the fact that we're already considering the wheels to be indestructible with perfect bearings:
Eliminate the wheels completely and treat both the plane and the treadmill as frictionless surfaces. Now fire up the jets.

[w_benjamin]

"Based upon the fact that we're already considering the wheels to be indestructible with perfect bearings:"

Why would I do that? The wheels aren't going to be going fast enough to need that.

On a frictionless surface, then yes the plane would take off. The question does not give you that parameter, however, so you shouldn't use it in your argument.

[w_benjamin]

You also dont need to eliminate the wheels if you have a frictionless surface as the wheels aren't going to turn, hence the conveyor won't either.

[Danger]

The original post actually states that the conveyor matches the plane's speed, not the rotational speed of the wheels. If the plane is not moving forward, therefore, the belt will not move backward. Now, just for the heck of it, substitute a Harrier hovering over the belt in place of the frictionless body. Again, same result.

[russ_watters]

If the treadmill works as stated in the question, it won't move. No, the way the question reads, the treadmill doesn't move until the plane starts moving with respect to the ground. Once the plane starts moving forward with respect to the ground, the conveyor belt starts moving backwards with respect to the ground. But since the plane is moving with respect to the ground, it is also moving with respect to the air - so it takes off.

If the question meant what you are saying, not only would the plane not move, but the treadmill wouldn't move either. Nothing at all would be happening.

As yourself this: if the plane isn't moving with respect to the ground, how does the treadmill know what speed to move at?

[ukmicky]

Won’t fly. No increased air flow over the wings means no lift.

The engine is producing thrust in order to push the plane forward through the air, which increases the forward momentum of the plane which increases the speed of the airflow over the wing which then produces lift.

However the plane is attached to the ground through the wheels and the ground is moving in an opposite direction and counteracting the forwards velocity of plane so that it always remains at 0

which means no increased airflow over the wings. so no lift.

But if the engine was given full power a prop could theoretically produce enough movement of the air over the wings to produce some lift but it wouldn’t be enough as the flow would be moving inefficiently with a corkscrew motion.

A quick question if I may though. If the plane had jet engines on the wings and the engines were placed on full power would the air movement and vortexes created from the engines sucking in air and pushing it out again create enough circular motion over the wings to create lift?

[russ_watters]

However the plane is attached to the ground through the wheels and the ground is producing an equal and opposite force to the engines, keeping the plane forward velocity at 0. That isn't what the problem says. The problem mentions the speed of the treadmill, not the force. Not the same thing.

[FredGarvin]

However the plane is attached to the ground through the wheels and the ground is moving in an opposite direction and counteracting the forwards velocity of plane so that it always remains at 0
which means no increased airflow over the wings. so no lift.
No. No and no. The plane is not "attached" to the ground in any fashion other than the force of friction.

If the plane had jet engines on the wings and the engines were placed on full power would the air movement and vortexes created from the engines sucking in air and pushing it out again create enough circular motion over the wings to create lift?What vorticies are going to be created by the engines? We work very hard with airframers to make sure that flow is as undisturbed as possible on both the inlet and exhaust. The wings and fuselage are the most probable sources of vorticies and those are usually restricted to the wing tips and roots.

[Danger]

There are far too many groundhogs on this thread. :grumpy:
Will everyone please stop and consider that those of us who fly the bloody things have a pretty good idea of how they work.

[w_benjamin]

y'know for a physics forum, this place doesn't use the laws of physics very much to solve 'em. And the ORIGINAL original question that started this whole thing goes thus:

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?

[w_benjamin]

back to my original statement for the original question..., not the garbled one that sm0ke produced: no, it won't fly.

[pervect]

No, the way the question reads, the treadmill doesn't move until the plane starts moving with respect to the ground. Once the plane starts moving forward with respect to the ground, the conveyor belt starts moving backwards with respect to the ground. But since the plane is moving with respect to the ground, it is also moving with respect to the air - so it takes off.
If the question meant what you are saying, not only would the plane not move, but the treadmill wouldn't move either. Nothing at all would be happening.
As yourself this: if the plane isn't moving with respect to the ground, how does the treadmill know what speed to move at?

This is the most literal interpreation of the original question. However, such a system would not keep a car from moving, much less a plane, as I remarked earlier. For instance, if the car moved west relative to the Earth at 30 mph, the conveyer would move east relative to the Earth at 30 mph, making the speed of the car relative to the conveyer 60 mph.

It seems likely that the intent of the conveyer was to keep the car from moving, but without more clarification of the question by its originator, we can't be positive of this.


Of course both versions of the question have been answered already.

[w_benjamin]

With russ's version, that's abosultely right. The version I was given was as stated above, and that is a VERY different question.

[hypermorphism]

The conveyor and wheels are mostly irrelevant. The air is not attached to the ground, and the airspeed is the only indicator that matters. The airplane is certainly not attached to the ground. The question is similar to asking whether a plane will take off on ice, or whether a groundspeed of 0 means a plane is falling out of the sky.
This question made AVweb (http://www.avweb.com/news/columns/191034-1.html) as analogous to the "fatal" downwind turn.

[Danger]

That's a great link, Hyper! Thanks.

[FredGarvin]

y'know for a physics forum, this place doesn't use the laws of physics very much to solve 'em.Care to revise that statement?

We'll believe AvWeb, but we won't believe the physics board. What has the world come to? At least they had the means to actually perform representative tests.

[russ_watters]

This is the most literal interpreation of the original question. However, such a system would not keep a car from moving, much less a plane, as I remarked earlier. For instance, if the car moved west relative to the Earth at 30 mph, the conveyer would move east relative to the Earth at 30 mph, making the speed of the car relative to the conveyer 60 mph. That's true, but a treadmill could also be programmed to keep a car stationary (or going whatever speed it wants), while it could not be programmed to keep a plane stationary.

For the plane, the only way anything even happens is for the treadmill to respond to the forward motion of the plane. ---because (as has been pointed out a good dozen times now) the car drivetrain is directly connected to the treadmill, whereas the plane's engines are not.

So the scenario you and w_benjamin are describing would simply be pointless - nothing at all would happen. That's why I don't think thats likely to be the correct meaning of the (as I've said - not well written) scenario.

[Danger]

May I humbly request that this thread be locked before I'm forced to wriggle my fingers through the ADSL line and choke someone? :biggrin:

[ukmicky]

.
What vorticies are going to be created by the engines? We work very hard with airframers to make sure that flow is as undisturbed as possible on both the inlet and exhaust. The wings and fuselage are the most probable sources of vorticies and those are usually restricted to the wing tips and roots.


However we are dealing with a plane which is not flying but standing still with its engine on full power, pulling in and pushing out large quntaties of the surrounding air. so to say there will be no vorticies created around the wing is madness

[russ_watters]

I feel the need to add some more. Those who are saying that the plane would not take off are basing that on reading the problem as defining the plane to be stationary. Since the question is poorly worded and barring a clarification from the writer, it is possible to interpret the question that way. However, the problem with doing that is two-fold: First, if the problem simply reduces to: "If a plane is stationary, will it take off?", why even bother asking the question? It's so basic that it's pointless. Second, just defining that the plane is stationary does not address the question of whether such a thing is physically possible. And while we engineers are arguing about what is physically possible, those who are saying the plane is stationary are simply assuming it without basis in physical reality. I think if you analyze the problem - think through the steps of how it would work - you'd find that it would be useless to ask the question you are describing for the two reasons above.

However, if you guys do think that your position is physically possible, please follow the steps of how it would happen and explain it. If you do that, I think you will find the scenario falls apart. Let me start it:

Step 1: A plane is sitting stationary on a stationary treadmill.

Step 2: The plane fires up it's engine and begins to accelerate.

Step 3: The treadmill senses the motion/acceleration.
-------Step 3 is a toughie: how can the treadmill sense the acceleration? Unlike a car, which exerts a direct force on the treadmill in order to accelerate, the plane does not. About all you could do is sense the motion with sensors along the surface of the treadmill.

Step 4: The treadmill responds to the motion of the plane and begins to move.....how fast?
-----Step 4 is where the scenario completely falls apart. Since the treadmill is not capable of exerting a direct force on the plane in the way that it can on a car, it is not possible for the treadmill to instantly react to it's motion and keep it stationary like it can a car. It could keep a car stationary regardless of what the car does - accelerate, decelerate, whatever. With the plane, the only way the treadmill can react is by speeding up - and while the treadmill accelerates, the plane is still accelerating with respect to the ground. The treadmill can't stop the plane and keep it stationary in a stable situation like it can with the car.

The only physically possible way to stop the plane is to accelerate the wheels until the wheels actually fail - heat up, burn up, disintegrate, ripping the landing gear off the plane. If the problem was meant to allow that, fine - you could always make higher speed wheels and then the problem becomes a battle between the engineering of the wheels and the acceleration capability of the treadmill. But again, that's not a very useful discussion. Plus, the scenario is uncontrolled - the treadmill is not under any sort of active control - it's not reacting to anything the plane is doing, you just turn on the treadmill when the plane fires up it's engine and see who wins the battle.

[jackie_nkm]

hi,
On ground, the force of the propeller makes the plane go forward irrespective of the wheels being present. wheels mere act as ion reducers( not as in car where the are the driving force generators)

this riddle can be solved by this:

Normally,
if the plane were moving with 150knts on runway w.r.t. ground , at that moment ground is also moving at 150knts w.r.t. to plane. the plane is lifted up bcos of the wind also moving at nearly same speed opposite to it and due to all that Bernoulli stuff.

Here,
when the plane is moving with 150knts, and the belt is moving at 150 knts the plane cannot move forward bcos it is already at 150knts speed w.r.t. belt(just as above case) since the plane is on belt. and belt is at 150knts w.r.t. plane (just as above case). so plane wouldnt move w.r.t. stationary ground on which the belt is kept.
Same is case with the air, which is staionary w.r.t. plane since air is not in frame of belt but in frame of ground. and so plane cannot take off.NO opposing force on the wings same from bernoulli eqns.

[jackie_nkm]

sorry spelling mistake
hi,
On ground, the force of the propeller makes the plane go forward irrespective of the wheels being present. wheels mere act as FRICTION reducers( not as in car where the are the driving force generators)

[krab]

Russ is right. The thing that everyone forgets is that the jet is pulling on the AIR, not the runway. With the brakes off, the wheels and the runway are not going to apply any force to the plane and so whatever they are doing is entirely irrelevant to whether the plane takes off.

[russ_watters]

when the plane is moving with 150knts, and the belt is moving at 150 knts the plane cannot move forward bcos it is already at 150knts speed w.r.t. belt(just as above case) since the plane is on belt. and belt is at 150knts w.r.t. plane (just as above case). so plane wouldnt move w.r.t. stationary ground on which the belt is kept. How is such a situation physically possible? Ie, answer the question implied under step 4 of my previous post: how can the conveyor impose a force on the plane equal to and opposite of its acceleration force?

[w_benjamin]

As I posted earlier, there are two different versions of this; one thats tracks plane speed, and one that tracks wheel speed. The first one is easily answerable, yes it will fly. The second one is the one I am intrigued with.

[FredGarvin]

However we are dealing with a plane which is not flying but standing still with its engine on full power, pulling in and pushing out large quntaties of the surrounding air. so to say there will be no vorticies created around the wing is madness
LOL! OK. I'll stick with my madness.

[FredGarvin]

May I humbly request that this thread be locked before I'm forced to wriggle my fingers through the ADSL line and choke someone? :biggrin:
You just got my vote if there is ever an election for PF Prime Minister.

[w_benjamin]

I emailed the author of that link with the version of the question I have and got this response:

That's a very good question and you are correct, different than the one
I
looked at.

I guess I'm troubled by the "match the speed of the wheels" language
because the feedback loop gets kind of strange. The propeller will
pull
the airplane forward, causing the wheels to begin to turn. Does that
then
mean the conveyor will begin to turn in the opposite direction at the
same
speed the wheels are turning? If so, then it causes the wheels to turn
faster. It sounds like a feedback loop that would very rapidly
accelerate
the wheel to exceed the tire speed of the airplane, blowing the tires
and
thus possibly preventing the takeoff. Sort of a feedback loop of "the
faster it goes, the faster it goes".

Having taken off upwind, crosswind and downwind and in seaplanes on
moving
water, I suspect I'll never make a takeoff on a conveyor belt.

All the best,
Rick

[Danger]

One problem with that: there's a tremendous amount of hydrodynamic (right term?) drag on the floats of a seaplane until it gains lift. That would be equivalent to having partial brakes applied to one on the conveyor, and is thus irrelevant to this discussion.
PS: Thanks for the vote, Fred... and for using the right title. :biggrin:

[russ_watters]

As I posted earlier, there are two different versions of this; one thats tracks plane speed, and one that tracks wheel speed. The first one is easily answerable, yes it will fly. The second one is the one I am intrigued with. Yes, I know - and you haven't answered the question about how the second one is possible. I emailed the author of that link with the version of the question I have and got this response:

That's a very good question and you are correct, different than the one
I looked at. So the author says that your interpretation of the question isn't what was meant. Great! So the answer become a simple "yes" again!

[KingNothing]

This thread's dragged on for far too long.

[ukmicky]

Some of us are being too clever for our own good and assuming that the treadmill is more than your basic treadmill which moves by a external force being applied to it, but rather a more advanced system which is able to accurately sense the forward motion of the plane and is able to compensate and adjust its own speed of movement in the opposite direction. but then we're still not taking into account that the wheels on the plane can move independently to the motion of the plane.:blushing: :smile:


However an advanced system with motion detectors could attempt to prevent the plane from taking off if in response to the forward motion of the plane the front of the treadmill could lift up allowing the plane itself to halt its forward motion and fall backwards due to it own weight, but even then if the engines were powerful enough it would still be able to take off vertically.

[Danger]

While that's an unnecessary complication to the picture, ukmicky, it does serve to illustrate another aspect of it. Given enough thrust (or wind) the plane could have zero ground speed and still take off.
I remember once seeing a German Stork aeroplane, which has just about the lowest take-off and landing speeds that you can find, flying backward over the runway. The wind was about 5 knots faster than its airspeed.

[w_benjamin]

My response was to show that the article linked to had no bearing on the version of the question I asked, and that the author indicated the in my version he probably wouldn't be able to take off, given his experience in taking off on moving terrain(ie:a river) "So the author says that your interpretation of the question isn't what was meant. Great! So the answer become a simple "yes" again!" No, that was an observation, not a statement. As I said, there are two versions of this, one which was easy to answer and one which is not. If the conveyor is tracking wheel speed, then I don't think the plane will take off. The conveyor is reacting to the force being applied to the wheels from the thrust of the plane. As for "Given enough thrust (or wind) the plane could have zero ground speed and still take off.", I reject that statement. The thrust is used for forward momentum which the conveyor is cancelling out, similar(but not exactly) to when brakes are being applied to the wheels.

[ukmicky]

Place a shopping trolley on a conveyer, stand behind it and hold on to the handle so it can't move backwards or forwards .
Now get someone to turn on the conveyor so it moves backward at 5 mph.
WHAT WILL HAPPEN
The wheels will turn on the trolley and you will feel a slight strain on your arms as they fight against the friction created in the wheel bearings.

Now turn the conveyer speed up to 200 mph
WHAT WILL HAPPEN
Not much ,you will still be able to hold on to the trolley and prevent it from moving.

Now increase the speed of the conveyer to 1000 mph
WHAT WILL HAPPEN
once again not much ,the force required to hold the trolley still hasn't increased by any significant amount and if you could prevent the bearings from getting hot causing extra friction the amount of strength required to hold to trolley at 5 mph would basically still be the same as it is at 1000 mph.

Now apply that to the plane, once the engines have overcome the friction in the wheel bearings, you would still have more than 90 percent of your engine power to move the plane forward to take off speed. All the conveyer does is cause the wheels to rotate faster and nethier the wheels or conveyer have any bearing on how fast or slow the plane moves once the friction in the wheel bearings is overcome.

Its not a car the wheels are not connected to a drivetrain

[w_benjamin]

Ah, the classic trap. In your trolley example, the conveyor is the actor, and the wheels are the reactor. In the question, it is the other way around, and it does make a difference. Use this analogy instead: Get a little cart with wheels on it, and mount an electric fan onto it thats big enough to move the cart maybe 2-3 mph. Start up the treadmill and set it for maybe 10 mph?(arbitrary number) Start up your fan cart, and place it on the treadmill, holding it so the wheels get up to speed with the treadmill, then let go. If the 'it will fly' crowd is right, the cart will move forward at the 2-3mph it did on solid ground. If the 'it won't fly' people are right, the treadmill will move the cart backwards until it falls off the back of the treadmill. This treadmill doesn't have the precision of the one in the question, but the principle elements are there.

[Danger]

"Given enough thrust (or wind) the plane could have zero ground speed and still take off.", I reject that statement. The thrust is used for forward momentum which the conveyor is cancelling out, similar(but not exactly) to when brakes are being applied to the wheels.
You can reject it until you're blue in the face, but it's a fact. If a 152 with a take-off speed of approximately 80 knots is facing into a 90 knot wind, it will take off without even starting the engine. Assume that there's a cable from it to the ground to prevent rearward motion, and it will continue to fly like a kite. If you replace the 152 with a Harrier, it will take off vertically with zero ground speed because it has enough thrust to overcome gravity.

Get a little cart with wheels on it, and mount an electric fan onto it thats big enough to move the cart maybe 2-3 mph. Start up the treadmill and set it for maybe 10 mph?(arbitrary number) Start up your fan cart, and place it on the treadmill, holding it so the wheels get up to speed with the treadmill, then let go. If the 'it will fly' crowd is right, the cart will move forward at the 2-3mph it did on solid ground.
Which it will, assuming no friction in the wheel/axle system. That's what we've been trying to tell you all along.

[w_benjamin]

"You can reject it until you're blue in the face, but it's a fact. If a 152 with a take-off speed of approximately 80 knots is facing into a 90 knot wind, it will take off without even starting the engine. " You are absolutely right. However, it has no bearing on the question as there is no wind. As such all you have left is thrust. At zero ground speed, thrust will not lift the plane. If that were true, you could put the brakes on full on a plane, load up with thrust, and just take off with no wind to aid you. Unless you have a Harrier (which you did mention), I don't think you're going to be doing that anytime soon. As for the second part, well, this is where we disagree. You believe the cart will move forwards, and I believe it will move backwards.

[w_benjamin]

Using the same analogy against ground speed, try this: Take a cart with wheels and set it on the ground. 0 mph, right? No movement? Take the cart and once again put in on the treadmill going 10 mph, holding it until the wheels get up to speed. If you let it go, using your logic, it should stay at 0 mph. In other words, it won't move on the treadmill, the wheels will just spin underneath and it will sit in the same spot indefinately. That's an experiment you can try very easily.

[f91jsw]

This thread has definitly dragged on for way too long for a physics forum. The simple answer is:

The plane will take ALWAYS take off NO MATTER WHAT SPEED the conveyor belt moves with.

For those who can't see it intuitively it's simple Newtonian physics: The engines produce thrust, which means a large force on the aircraft. Barring another force that counteracts this large force there's nothing stopping the aircraft accelerating according to F=ma. In the given problem there is no such counteracting force except for the very small friction force from the wheels. As the aircraft accelerates it will gain speed in the reference frame defined by the airmass and the surrounding landscape, and it will take off. This will happen in all the discussed cases, no matter what feedback loop exists between the aircraft/wheel speed etc. and the conveyor belt.

It's a fun problem by the way, I will remember it and try it on my first year physics students in the future.

Johannes

[Danger]

Thanks for weighing in, Johannes. I'm seriously at my wits' end trying to figure out how to make him understand. (And I suspect that you soon will be as well.)

[w_benjamin]

Johannes, if you're a physics teacher, then I weep for the future of our country. You're dismissing the wheels as a small frictional force, and your agreement with him Danger, shows that it is YOU who lack the understanding. You're saying that just because the wheels only need a small amount of energy under normal circumstances, that they will need no more in this one? Different circumstances do indeed require different amount of energy to the wheels. (and if you think that the wheels have no energy being transferred to them, you're sadder than I thought-- see "Newtons Cradle"). What if the wheels weren't exactly round? More energy would be needed. What if the wheels are not on a smooth surface, such as sand? More energy would be needed. And what if the wheels have a counteracting force being applied to them? More energy would be needed. The more counteracting force, the more energy is needed. If the conveyor provides such a force (and yes, it can be done through the wheels) then it is theoretically possible to hold the plane in place using that force. The plane only moves forward after all objects that hold it back have been overcome. The energy for that ability to overcome is the thrust. If you use all the energy from the thrust to overcome the energy holding back the plane, there's none left to move the plane forward. What if you put a two foot high lump in front of the tires? The wheels roll just as freely as before, will they still need the same amount of energy to turn as on flat ground?

[f91jsw]

I'm not a physics teacher, I just teach physics between doing physics research (in the UK).

The problem formulation did not state that the plane was starting from a sand trap or that the wheels were locked. You can't invent new circumstances to save your theory.

BTW, hint: look up a physics textbook for the distinction between "energy" and "force."

Johannes

[w_benjamin]

You're right, those were just other examples of how more "force" might be needed to move the plane forward. And you're right in that I used the term energy in place of force. I will give you the question again, and let you read it carefully:

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?

Note that this version says "wheel speed". That means the wheel speed cannot exceed the conveyors speed no matter how much "force" you convert the thrust into as the conveyor always corrects itself to match the speed of the wheels. Also note that the conveyor is acting as a reactor to the "force" applied to the wheels.

[ukmicky]

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
Note that this version says "wheel speed". That means the wheel speed cannot exceed the conveyors speed no matter how much "force" you convert the thrust into as the conveyor always corrects itself to match the speed of the wheels. Also note that the conveyor is acting as a reactor to the "force" applied to the wheels.

How can the conveyer match the speed of the wheels ,when any change to the speed of the conveyer will affect the speed that the wheels are moving.

Also the wheels can still move without affecting the motion of the plane as the wheels on the plane are not connected to any drive train .


And by moving the conveyer in the opposite direction of rotation of the wheels .( im trying to visualise which way the wheels turn on a forward moving plane) wouldn't the conveyer be moving in the same direction as the plane is moving, pushing it through the air slightly faster than it would have gone with just the engines as it would help combat the frictional forces of the wheel bearings

[russ_watters]

You're dismissing the wheels as a small frictional force, and your agreement with him Danger, shows that it is YOU who lack the understanding.... [bla, bla, bla...] benjamin, how much friction do you think there is in an airplane wheel? I'll help you: the engines exert hundreds, even thousands of times the force of the friction on the wheels. So yeah, that qualifies it as "a small frictional force".

Also, you are aware that friction is not dependent on speed, right? Increasing the speed of the wheels does not increase the force of friction. If the friction force in the wheels is 1/1000th the force of the engine at 10mph, it is still 1/1000th the force of the engine at 100mph. So again, the only way for the conveyor to halt the acceleration of the plane (the plane does accelerate with respect to the ground, no matter how you slice the question) is for the conveyor to quickly speed up to the point where the wheels melt and come apart before the plane becomes airborne.

But again - that's a pretty useless way to view this issue. The more counteracting force, the more energy is needed. If the conveyor provides such a force (and yes, it can be done through the wheels) then it is theoretically possible to hold the plane in place using that force. That's nonsense. The only way the conveyor is capable of exerting a force is through the friction in the wheels and that force is constant. Nothing the conveyor does can increase that force. [caveat: there is viscous friction due to the lubricant, but just as the regular dynamic friction is much less than the thrust, the viscous friction is less still.]

So is that all the problem is here: you think friction is dependent on speed? That's a common mistake due to the heating of the wheels. People think that since faster=hotter, faster also equals more force. Nope. I think you may need to review your Newtonian physics.

[krab]

Man, I've seen some pretty lame threads. This one is right up there with questioning 0.999...=1.

[Doc Al]

krab's right. I think we've had enough.

[ice87]

Someone sent this to me, and apparently it caused quite a ruckus on another forum. I couldn't make any sense out of their responses, so I decided to put it to the great minds of NeoWin. I didn't find it anywhere, so sorry if this is already on here. The Question:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Please post a reason.

Can this plane take off given no winds?

Theres a quite a bit of discussion on this over at neowin. http://www.neowin.net/forum/index.php?showtopic=413477

[Pengwuino]

Screw this, I can't figure this out yet, I need to think about it.

I have a feeling it will take off as long as its engines are capable of producing enough thrust to reach take-off speed while having to deal with the frictional force created by the tires going twice as fast.

[AJerman]

Figured I'd just copy and paste over to here the explanation I gave over at Neowin. I actually voted no at first, then realized quickly my mistake. Then I brought it down to my father who is an aircraft mechanic, and my grandfather who loves planes and asked them. I actually had to spend about 30 minutes explaining to them how it would still take off before they understood what I was saying. It's a fun one to throw out there though.

Wow, I have to say that this is a very good one, more of a good one in the way that it causes great conversation. I initially voted no, but I want that vote back, I was wrong. It will and it won't take off. I didn't read every post before this, so I'll just post this to explain what would happen rather than reply to anyone.

So first off, it will take off because an airplane get's it's forward motion, it's thrust, from it's jet engines pushing air. No matter what the wheels on the ground do, a jet engine at full throttle CAN NOT sit still without being strapped to the ground or having some sort of friction greater than the power it is providing (which is quite a bit and wheels cannot cause anywhere near enough friction to do this). It's going to push the air and move forward, and if the ground is moving the same speed as the plane then the wheels will end up spinning at double the speed, but the plane will continue to move forward with the same thrust as it would no matter what the runway is doing. That's simple enough if you know a little physics to figure out. If the wheels are providing no power to be turned into forward motion, then the runway moving cannot change the amount of thrust you are going to have.

The problem is that we, in general, think of thrust as something that comes from what is making contact with the ground. Our feet pushing against the ground make us go forward. The wheels on a car spinning against a stationary ground make a car go forward. However, on a plane, the wheels that are on the ground don't make the plane go forward, the jet engines sucking in air, compressing it, mixing it with fuel, igniting it, and shooting it out of the back, produce the thrust and forward motion of a plane. Therefore the wheels have nothing to do with moving forward, and what the ground is doing has nothing to do with it either.

Also, the plane will take off in roughly the same distance as it would on a stationary runway. With the wheels spinning twice as fast, there will be a small increase in friction, however this wouldn't cause much difference in the long run.

But now seems like a good time to say why it likely WON'T take off. We're going to use 180 mph as the take off speed for our jet (which is similar to that of a 777). This means that the wheels will be going 360 mph at take off. At this speed the wheels, bearings, and tires are unlikely to hold up and will end up grounding the plane. But hey, our question is theoretical, so lets ignore these issues, or assume our plane in question has been reinforced to withstand any of the above factors. Besides, where are you going to get a conveyer belt this big anyway?

[chroot]

The question has already been proposed here, and the conclusion is that YES, the plane will take off, despite the very imprecise wording of the question. It's already been beaten to death on this forum.

http://www.physicsforums.com/showthread.php?t=101259

- Warren

[vincentm]

This question has caused quite a ruckus on the other forum i post on, so i decided to post it here since alot more members here are more competent in there physics and math.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

[Doc Al]

This has been discussed to death here as well: http://www.physicsforums.com/showthread.php?t=105314

[HallsofIvy]

Airplane lift is due to the motion of the wings relative to the air (that's why airplanes prefer to take off and land into the wind). If the wings are unmoving relative to the air, the airplane will not "take off" no matter what the speed relative to the ground is.

[maverick6664]

As this thread (http://www.physicsforums.com/showthread.php?t=105342) is locked, I post here again.

As I have a pilot's license, this is interesting.

The answer is queer; because it's impossible to make such a conveyer. So the answer is "cannot be determined." Airplane's engines are made so the airplance accelerates "relative to the air", not to the ground. See the propellers or jet engines blow winds backwards. So such a conveyer cannot "stop" the airplane assuming we ignore the friction of wheels of the airplane. That airplane will accelerate as usual relative to the wind (so called "airspeed") regardless of the conveyer, and will finally take-off.

But this will be against the intention of the conveyer, because it's supposed to "stop" the airplane. The answer is "this condition is impossible, so doesn't have an answer."

Yes, an airplane's lift is produced by the speed to relative wind to the airplane. So if the advancement of the airplance is stopped in other ways than that conveyer, it cannot take-off, but in this experiment, the airplane will advance relative to wind regardless of the conveyer.

[Danger]

There's a very good reason why that thread, and others on the same subject, are locked. We're all pretty much sick of the topic by now. The plane will take off, and there isn't too much point in beating it around any more.

[russ_watters]

Yes, the only thing that can prevent the plane from taking off is bad wording. So what's left to discuss...?

[raid517]

Someone sent this to me, and apparently it caused quite a ruckus on another forum. I couldn't make any sense out of their responses, so I decided to put it to the great minds on this forum. I didn't find it anywhere, so sorry if this is already on here. The Question:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Please post a reason.

I am really looking for a conclusive answer. My instinct is to say no, but I would like this confirmed by some hard physics and maths if possible.

GJ

[fargoth]

ok, so you got an airplane that doesnt move... whats your point?
i didnt get what reason you want...
EDIT: (im wrong here :blushing: )

[Homer Simpson]

There was some thread on this I saw a couple weeks back. Basically, it the wheels will turn however fast they want. As soon as the jets fire up the plane will move relative to the stationary ground. Any treadmill beneath it will just make the wheels spin faster. This would only be a problem if the wheels were motorized to make the plane move, but thats not the case.

On the other hand, If the treadmill was running at the same speed in the same direction as the jet as it fires up, the jet would take off with no movement of the wheels.

You dont need math or physics for this type of stuff, just think about it.

[Danger]

I seriously think that if this question shows up one more time, I'll scream. :surprised

[raid517]

Well this is true and this is what I thought. But I thought that this was a good place to shoot it in the head for once and for all.

Everything I know of says that it's a stupid question and of course the aeroplane wouldn't take off - but when you have 50 different guys challenging your assumptions, you can begin to doubt yourself.

GJ

[Danger]

Take a look at the other threads on the same subject, which mercifully have been locked. The plane will take off.

[fargoth]

oh yeah, i forgot about the fact that the wheels arent motorized, if the jets are what causing the movement the airplane would move... though i think that if the wheels spin too fast there would be lots of heat generation and the plane would move slower. (but if the wheels are ideal, it just means they would move twice as fast)

[raid517]

Point me to these other threads please?

GJ

[Danger]

Look at 'Classical Physics' in the post entitled 'Airplane question: revisited'. There's a link in the OP to another.

[deadmoon]

hi
if you had a plane on a treadmill would it take off?
the treadmill is moving in the oppsite way of the plane at the same speed

thanks

[Danger]

Welcome, deadmoon. Unfortunately, you introduced yourself with a question that has absolutely been done to death here. If you check back a few pages, you'll see a very thorough examination of the subject. Bottom line: yes.

[brewnog]

KILL IT! LOCK IT! QUICK! KILL IT! KILL IT TIL IT DIES FROM IT!

Argh!

[russ_watters]

The answer is yes, unless the question is so poorly worded as to be meaningless. See this thread: http://www.physicsforums.com/showthread.php?t=101259&highlight=treadmill

[Doc Al]

And this: http://www.physicsforums.com/showthread.php?t=108477

[R1ch]

Hi all ive just signed up to the forum.
This question seems to have got over the net a bit so sorry if its a re-post

A plane is sat on a runway with some sort of conveyer belt, when the plane moves the conveyer moves in the opposite direction matching the planes speed exactly. Can the plane take off?

[russ_watters]

Sorry, we've discussed this ad nauseum and won't go over it again. Do a search of the forum for the topic.

In short, though, the problem is so poorly worded as to not have a definite answer. Regardless, an airplane takes off when the airflow over the wings is fast enough to create enough lift to make the plane fly. That's it. It doesn't matter what the wheels are doing at the time (the plane can even be completely stationary if it has a strong enough headwind).

[Kaldarin]

If an airplane is placed on top of a conveyer belt which turns in the oposite direction of the plane with the exact speed as the plane, will it ever take off?

[Pengwuino]

This has been discussed over and over and over and over and over and over and the answer is basically yes with the qualification that the question is worded too poorly to be worth anything. Please search the general physics area.

[ZapperZ]

I'm almost going to insist a 1-year moratorium on this question.

http://www.physicsforums.com/showthread.php?t=101259

Zz.

[kmarinas86]

Sorry, this question is too fun too resist.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

Well, to understand how the system works, let's change the settings first:

Instead going 100% of the speed in the opposite direction, have the conveyor belt go 700 knots in the opposite direction. The implications of this are obvious now since the air would be flowing backwards on the wings not forward, therefore the plane will be affected by this movement.

Another thought experiment. Put a Honda civic on a steel treadmill. If it has a spoiler, is the spoiler being used? It isn't. The only wind being generated is from the exhaust and the contact of the air with the moving surfaces of the vehichle and the steel treadmill. The body as a whole is not generating a lift or downforce as if it were travelling fast on the road.

[Gokul43201]

This exact question has been discussed in an earlier thread in Physics.

Read the discussion there :

http://physicsforums.com/showthread.php?t=101259

[russ_watters]

...and will not be discussed again.

The question isn't fun, it's dumb. The only reason there is any doubt as to the answer is because the question is poorly worded and unrealistic (how can the conveyor exert a force on the plane?).

[MrsScruffles]

I'll hope you'll all excuse the intrusion, but I've fled here from another forum in the hope that you will be able to settle a debate that has been raging for far too long. The original question that has causes such furore is this:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not?


I did try searching your forums, and saw a similar question with a car, but not a plane. I'm sat in the "no" branch, because I believe that there are no aerodynamic forces at work because essentially the airplane is stationary, and only the wheels are in motion, so you wouldn't get lift. However, the oppposing argument seems to be that the force from the jet engines would cause lift off regardless. I don't think any of us have a good enough grasp of physics to ever formally end this debate. Our formally peaceful forum has turned into something out of Gullivers travels, but instead of everyone divided on which way to eat a boiled egg we're going crazy over a hypothetical plane.

Twenty two pages later and the argument is still going.

So can someone kill this question dead for me?

[Pengwuino]

What forum is this?!?!?! We've recieved this question like 20 times!

[Mk]

Yeah. What Forum Is It!! Do We Have To Wipe Their A** For Them?? I think we need to do a DNS attack on them. And SPAM, yes Spam... :devil: :devil:

[MrsScruffles]

Sorry, as I said, I did do a search and couldn't see it. The question I found was about cars. And as far as I know, I think it's quite viral, and been spread from place to place. Sorry for wasting your time.

[fasterthanjoao]

the plane isn't actually moving wrt anything other than the belt so theres no lift from air? surely.. hmm..

[FredGarvin]

Board searches are pretty fickle. If you do a search on "conveyor" you will get this thread (which links back to some of the original threads)

http://www.physicsforums.com/showthread.php?t=112124&highlight=conveyor

Please, someone go out into the ether and beg that this topic die a horrible death that it so rightly deserves and smote the boards that started it...amen.

BTW...what board is it that has had this raging debate going on for so long?

[ZapperZ]

This question is fast becoming the Most Annoying Question (MAQ).

Zz.

[P.G]

Hey people. I have a problem that I dont know the answer to, but you guys could probably help. Its quite a hot topic on other forums. Anyways, here it is.

A plane is standing on a runway that can move (some sort of conveyer belt). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Can the plane take off?

I say no personally but ive been told it can :uhh: help!

[chroot]

This same question has been answered many, many times already, largely by people like you, P.G, who have come here from another forum. This question is no longer welcome here. Use the search function to find previous discussion on it.

- Warren

[userjayson]

What would happen if a plane was approaching a runway at 200 mph and the runway (conveyor belt) was moving at 200 mph in the opposite direction. I think if your wheels could take the sudden jolt, they would start turning at 400 mph. Since the plane is being thrusted buy a jet not attached to the wheels, it would continue to go its original 200 mph untill the jet was shut off. Once that happens, at what rate do you slow the conveyor belt down so the that the plane is not forced of the back end of the belt. I cant get my mind around this one.. I heard a radio talk show host talking about this one day and was wondering if were possible.

[Integral]

Please use the search function. Airplane or conveyer ought to get what you are looking for.

[Kastey]

i have a some trouble in solution of this problem:
Airplane is standing on conveyer. Rate of movement of this conveyer automatically (without delay) concide with rate of movement wheels of undercarriage. Conveyer move to side reverse of course fly up airplane. This system is switch on. Question: airplane fly up or not?

[Astronuc]

Need more information. Each aircraft has a take-off speed based on the lift and weight of the aircraft.

Student is expected to show work (equations and computation) when asking for assistance.

[Kastey]

I think, what in order to this problem weight of this aircraft absolutely unimportant. This aircraft be able to fly up from usual landing strip. question:
can it fly up from conveyer?

[Gokul43201]

Read the guidelines.

We can not help you with schoolwork unless you show what effort you have made.

[dav2008]

This isn't homework. This is the airplane on a treadmill question that has been discussed ad nauseam.

Do a search for "treadmill" on the forums. You should find at least 10 other threads on this subject.

It looks like this is the longest thread on the subject: http://physicsforums.com/showthread.php?t=101259

[DaveC426913]

The very short answer:

An airplane's ability to take off has nothing to do with the movement of the ground under its wheels and everything to do with the movement of air over its wings.

[Astronuc]

This isn't homework. This is the airplane on a treadmill question that has been discussed ad nauseam.

Do a search for "treadmill" on the forums. You should find at least 10 other threads on this subject.

It looks like this is the longest thread on the subject: http://physicsforums.com/showthread.php?t=101259 I wondered about that as soon as I saw "conveyor".

And as DaveC426913 has indicated, the solution has to do with the airspeed (airflow) over the wings. The air provides lift via the wings, which must be greater than the weight of the aircraft in order for the aircraft to 'takeoff'!

However, if the thrust of the engines are not increased to offset the force of the conveyor (once the aircraft lifs), then the aircraft will simply reduce speed due to drag and fall downward to the ground/conveyor. It's simple physics.

The question is ill posed and describes a physically unstable system.

[Gokul43201]

This isn't homework. This is the airplane on a treadmill question that has been discussed ad nauseam.I'm more than aware of the letter, but certainly not as sure as you about the former.

If some teacher decided to pose this question to a class (heaven forbid), then we're violating the Homework Help guidelines by responding to a query that includes no original work.

It's not our job to speculate about whether or not this is coursework. The location of this thread determines its fate. If the OP has posted in error, s/he can correct that with a PM to a mentor.

[Kastey]

This questions are discussed on many many forums. On some of it this discussion last 3-4 years. Number of posts about this problem amount to 10 000. I simple want to know, what you think about it. Most of people incline to "The question is ill posed and describes a phyically unstable system."

[Astronuc]

This questions are discussed on many many forums. On some of it this discussion last 3-4 years. Number of posts about this problem amount to 10 000. I simple want to know, what you think about it. Most of people incline to "The question is ill posed and describes a phyically unstable system." The problem as stated in the OP is ill-posed, i.e. it is too non-specific.

Replace conveyor or treadmill with catapult, and one has the situation on a naval aircraft carrier. Navies around the world have demonstrated that one can lauch an aircraft, but if the aircraft leaves the carrier deck without sufficient thrust, it crashes shortly after takeoff.

As soon the the force applied by the conveyor, treadmill or catapult is removed, the plane begins to decelerate because of drag and the lift (which is dependent on air speed over the wings) decreases as the forward velocity decreases and the plane descends.

This should not be hard to understand - and it certainly does not warrant 1000's of posts on countless websites. :rolleyes:

[moose]

Astronuc, I can't seem to understand what you are saying. Are you saying that yes it will fly because the thrust has nothing to do with the ground, or are you saying no for some weird reason?

[Astronuc]

Astronuc, I can't seem to understand what you are saying. Are you saying that yes it will fly because the thrust has nothing to do with the ground, or are you saying no for some weird reason? Under certain conditions, that being forward thrust >= drag, and lift >= weight, the aircraft will takeoff.

However, as soon as the aircraft 'takes off', i.e. loses the force from the conveyor, treadmill, or catapult, unless thrust (force) is produced by the aircraft to match drag, drag will cause a rapid deceleration, and resulting lift will decrease, and plane will come back down.

When flying (cruising) in air, there is a force balance. Lift = weight, and drag = forward thrust. If one cuts off (stopped) the engines in flight, the plane would immediately decelerate (forward) and that would cause a rapid decrease in lift, which would cause the plane to plumment. That is not flying as much as it is falling!

[Pengwuino]

Is there some sort of virus spreading around the internet that forces people to ask this same freaken question over and over and over? Does anyone know the source of this? Search the forum, it's been discussed beyond all reason.

[Astronuc]

Lock the thread! Please!

[chroot]

Please don't feed the trolls, Astronuc.

Kastey, this topic has indeed been discussed ad nauseum, mainly because people like you come here from other forums, don't bother to search, and post the same question again. It's not even an interesting physics problem -- the answer is definite and very clear, though the question is poorly worded.

Do not discuss this topic further here.

- Warren

[Brad117]

I wasn't really sure where this would go, and it seemed most sensable to place this question here since it wasn't a homework problem, just a random critical thinking problem. And with the anwser, could you also give a good explanation and proof.

A plane is sitting stationary on a runway. The runway is a giant conveyor belt that will instantly and inversely match the speed of the plane's wheels (wheels move at 10mph forward, runway moves 10mph in the opposite direction). As the plane accelerates the runway matches the planes speed.

Will the plane move? Will the plane take off?

[Jeff Reid]

This has been posted before. The plane isn't wheel driven, it's air driven by the propeller (or jet). The wheels just spin faster, might slow the rate of acceleration a tad, but the plane would still take off (assuming the belt doesn't move so fast to cause the tires to explode from fast rotation). So if the plane takes off at 100mph, the tires just spin at 200mph, as long as they don't explode, there's no problem.

[Hootenanny]

This has been posted before. The plane isn't wheel driven, it's air driven by the propeller (or jet). The wheels just spin faster, might slow the rate of acceleration a tad, but the plane would still take off (assuming the belt doesn't move so fast to cause the tires to explode from fast rotation).

I can't see how the plane would infact take off. If there is no air flow over the wings (because the plane is stationary relative to the air) then the wings generate no lift. :confused:

[Brad117]

This has been posted before. The plane isn't wheel driven, it's air driven by the propeller (or jet). The wheels just spin faster, might slow the rate of acceleration a tad, but the plane would still take off (assuming the belt doesn't move so fast to cause the tires to explode from fast rotation). So if the plane takes off at 100mph, the tires just spin at 200mph, as long as they don't explode, there's no problem.

yeah I don't quite get what you mean. I sort of thought of it as like a man running on a treadmill, and assuming that the man runs at a constant rate of 8mph and the treadmill is at 8mph also, then the man will stay stationary. But since this is an airplane, and theres no foward momentum...then your saying the airplane would vertically lift up in the air?

[Jeff Reid]

Once again, the plane starts off stationary, but doesn't remain that way once the propeller starts producing significant thrust. Again note that the plane isn't wheel driven, it gets its propulsion by accelerating the air backwards. As it does this, the plane accelerates forwards, the tires will rotate at twice the speed that the plane is moving forwards at, because of the rule you made about the conveyor belt. If take off speed is 100mph, then the tires will be rotating at 200mph when the plane takes off.

Now if it was a wheel driven vehicle, like a car, then the car wouldn't move, but your example is a plane which isn't wheel driven.

If it was a rocket powered motorcyle, like the one in this video, then the tires would rotate twice as fast at the motorcycle, but the bike will still accelerate forwards at a great rate. All of it's acceleration is due to acceleration of air backwards, not force applied to the wheels. Unless the tires exploded from the rotational speed, the bike would still accelerate very fast. Note this is a very short video, the bike is very quick:

rktbike.avi (http://jeffareid.net/real/rktbike.avi)

[Hootenanny]

Ahh, makes sense now Jeff thank you. I've just read through the other thread loacted here (http://physicsforums.com/showthread.php?t=101259)

[Doc Al]

This has same question has been asked--and answered--many times already. Please look at the link that Hootenanny provided if you need more.

[cowmoo32]

Could it take off? I'm assuming yes because the wheels aren't what propel it forward. Once there was enough thrust to counteract the initial movement backward due to the friction of the wheels on the ground, it would be able to move forward. Am I correct in thinking this?

[Integral]

Here is one thread (http://physicsforums.com/showthread.php?t=101259&highlight=treadmill) resulting from a search on "treadmill" There are more.

[david90]

Can an airplane take off if it is traveling on a conveyor belt that travels in the opposite direction of the airplane and the conveyor belt speed = to the speed of the airplane?

[Doc Al]

Sorry, but this one's been done to death. Search the archives and you'll find several threads.

[mtaylor]

This has been posted on other forums and has cause massive arguments

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

[Matt1]

This has been posted on other forums and has cause massive arguments

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

The control system is going to ensure that the plane's speed is always zero. If the plane is not moving, do you think it will take off?

errr....wait.....no. the plane will take off. the moving tradmill will simple ensure the wheels of the plane move twice as fast.

[lufbrajames]

This has been posted on other forums and has cause massive arguments

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

I would say no, because to take of the plane needs to generate lift. And the lift is created by the airflow over the wings, if the plane is not moving, it means that there is no airflow, therefore it wont fly.

-James Hart
-Loughborough Uni, UK

[neutrino]

This has been posted on other forums and has cause massive arguments


Why do you think this forum is any different? :rofl::wink:

There are a many threads on this question, here, open and locked. Use the search feature.

[Danger]

Welcome to PF, Mtaylor. This subject has been beaten absolutely to death. Do a quick forums search and you'll find more than you'll ever want to read about it. The simple, straight answer is that it will take off. There's no point in debating it again.

[joshd]

It can take off just fine. The plane moves forward as usual (the wheel bearings are the magicaly frictionless ones, OK?) and takes off, since the plane's engines are pushing against the air, not the groud, the moving ground has no effect apart from to make the wheels spin twice as fast as it takes off.

[Matt1]

It can take off just fine. The plane moves forward as usual (the wheel bearings are the magicaly frictionless ones, OK?) and takes off, since the plane's engines are pushing against the air, not the groud, the moving ground has no effect apart from to make the wheels spin twice as fast as it takes off.

good call. I wasn't thinking properly when I posted that.

[Jeff Reid]

Unless the tires explode from rotating at twice the speed, as posted the jet takes off because the engines exert a force against the air not the ground , taking only a tiny bit longer because more work is used up adding kinetic energy to the wheels and tires.

[Doc Al]

As Danger said, this topic has been done to death. Check the archives!

[Xori]

I read this puzzle recently,

There is a plane on an endless treadmill. The thing about this treadmill is that however fast the plane is moving forward, the treadmill goes the same speed backwards.

Would a plane be able to take off of this treadmill?


The only answer I can come up with is that the wheels would eventually blow up and the airplane crashes.

But this is probably not the right answer.

Any thoughts?

[D H]

Not again! I predict this thread will soon be locked or deleted. Look at this thread (http://physicsforums.com/showthread.php?t=101259&highlight=treadmill) instead.

[Doc Al]

This one's been beaten to death several times over in various threads.

[russ_watters]

I know Doc already locked this, but I'm going to throw in my $.02: This is a stupid question. What matters for an airplane is how fast the air is flowing over the wings. Since the question badly worded, there is no way to know how fast the air is flowing over the wings without arguing over the wording of the problem and various assumptions and complicating factors. Thus: stupid question.

[solidon]

A plane is on a conveyor belt or treadmill, a very very very long one. It is slowly accelerated to a speed of 700MPH backwards. The treadmill is designed with a moving structure placed behind the air plane stopping drag/air-resistance from affecting the airplane. That is the plane is moving back without its wheels spinning, just as if it were at rest.

The plane begins accelerating foward, and reaches a top speed of 600 MPH.(the moving structure designed to cancel drag air-resistance follows behind it.). Does the plane move foward with relation to the earth or backwards?

The person claims it moves backwards, but some people claim it moves foward with respect to the earth. Who's right?

[chroot]

This has been answered so many times on this forum that it's no longer welcome (or interesting).

- Warren

[Cyrus]

http://video.google.com/videoplay?docid=-7083501107445407444&q=plane+on+treadmill&total=24&start=10&num=10&so=0&type=search&plindex=0

Wow, what a shocker. Now, if you search this on the internet, PF is one of the hits on the first page of google, so I KNOW they had to have seen the answer posted here.

[D H]

Unfortunately, I doubt that that video will stop the psychoceramics. I predict we will be plagued with (and locking) plane-on-a-conveyor belt threads for years to come: "The MythBusters didn't set things up right." "That wasn't a real conveyor belt." "That wasn't a real airplane." "That video doesn't prove anything."

[mgb_phys]

Did they have birds flying off perches inside it ?

psychoceramics
Brilliant - I hadn't come accross that term before.

[jimmysnyder]

I'm new to this myth. The plane was going 40 mph forward and the belt 40 mph backward and so the plane should have been stationary relative to the ground. But it wasn't, you could see it moving past the cones. Therefore, I conclude that the plane's speedometer measures speed through the air, not relative to the ground. If the plane is going at 40 mph through the air, then I suppose that is sufficient to rise. The motion of the belt seems irrelevant.

Now I see the point. The airplane moves by pushing the air back with the propeller, not by pushing forward with the wheels. That's a pretty elaborate experiment to confirm this simple fact.

[mgb_phys]

The motion of the belt seems irrelevant
It is, but the only way to get that fact into some people heads appears to be with a hammer and chisel!

[Matt1]

I'm new to this myth. The plane was going 40 mph forward and the belt 40 mph backward and so the plane should have been stationary relative to the ground. But it wasn't, you could see it moving past the cones. Therefore, I conclude that the plane's speedometer measures speed through the air, not relative to the ground. If the plane is going at 40 mph through the air, then I suppose that is sufficient to rise. The motion of the belt seems irrelevant.

Now I see the point. The airplane moves by pushing the air back with the propeller, not by pushing forward with the wheels. That's a pretty elaborate experiment to confirm this simple fact.

yeah, that's a lot of work to prove this obvious fact.

[tribdog]

wow, so a plane can take off no matter how fast it's wheels are turning. who woulda thunk it.

[D H]

I'm new to this myth.
The silly question has hit this forum many times since your join date. Those discussions must have either flown right over your head, or since the mods got sick of the silliness quickly, the threads must have been locked/deleted before they took off.

I predict that this thread, for instance, will soon be locked because some pyschoceramic will chime in with their bits of wisdom. At the forums at the MythBusters website the post-show discussion is now over 40 pages long (:bugeye:), mostly by people who claim that they didn't bust the myth. The plane took off right before their eyes, the MythBusters even did one of their "Warning: Science Content" bits to explain why the plane does take off, and they still claim the plane won't take off.

[jimmysnyder]

The silly question has hit this forum many times since your join date. Those discussions must have either flown right over your head, or since the mods got sick of the silliness quickly, the threads must have been locked/deleted before they took off.
I don't pay attention to all the threads. I probably only enter about 30% after reading the subject line. Perhaps someone could do a psychological profile on me based on what threads I enter. It may be that two threads started some time apart have the same subject line and yet I entered one and not the other. I not only didn't hear of this myth from the forum, but not from any other source either. That's not surprising, most of the myths I hear of, I hear of from this forum.

[Mk]

The plane took off right before their eyes, the MythBusters even did one of their "Warning: Science Content" bits to explain why the plane does take off, and they still claim the plane won't take off.Wait a second, I thought I understood this myth.

I understood it to be, a plane uses some sort of force, like a gas jet, to attempt to propel itself forward. However, the conveyor belt keeps the plane's x-coordinate constant. Is that right?

[D H]

However, the conveyor belt keeps the plane's x-coordinate constant. Is that right?No real conveyor belt, acting on the plane solely through its freely-rotating wheels, can do that. The plane has wheels for the very reason that wheels reduce friction. If you insist on endowing the conveyor belt with some magical force, I in turn insist on endowing the plane with magical, frictionless wheels. With this, the silly "plane-on-a-conveyor" question reverts to the equally silly but age-old problem of an irresistible force versus an unmovable object.

The basic problem with the myth is that it has a flawed premise. Such a conveyor belt contraption can indeed keep a car stationary because a car has driven wheels. The flaw in the premise is that a plane does not have driven wheels. End of story, end of myth.

One of the things the MythBusters did was to put a model car on a treadmill. While the car was in gear, the treadmill was easily able to keep the car stationary. Then they took the car out of gear. Because wheels only reduce rather than eliminate friction, the can began to move backwards. However, it only took a very small force (exerted by Adam's hand) to keep the car stationary.

The same applies to the plane on a conveyor belt. The conveyor belt exerts a tiny force backwards on the plane. A plane moves forward by forcing air to move backwards relative to the plane. The forward-directed force resulting from the plane's propellers forcing air to move rearward easily overcomes the tiny rearward-directed force resulting from the plane's wheels moving forward with respect to the conveyor.

The only way to keep the plane stationary is to strap it down to the conveyor. A plane securely strapped down to the conveyor (or to the tarmac) will not take off. However, that is not how the question was posed.

[FredGarvin]

A pox on the person that put this question out to the world. Fie! Fie!

[RonL]

The only other time i made the "mistake of watching" myth busters, they were building a steam powered cannon, boy what i could have done with the money they used to make that program:rolleyes:

[Lord Ping]

Just remember not to switch the treadmill on before switching the plane on... otherwise it really will go backwards.

[chemisttree]

The plane didn't really fly... the earth just moved away beneath it. Simple optical illusion.

[russ_watters]

Wait a second, I thought I understood this myth.

I understood it to be, a plane uses some sort of force, like a gas jet, to attempt to propel itself forward. However, the conveyor belt keeps the plane's x-coordinate constant. Is that right? Now you see where the argument comes from. The question is ill-formed, so the entire argument is over that point and the minutae of how it could come to be true. It's just dumb.

[Dark Prism]

Before I get the, "This has already been talked about in another thread," let me just state my case.

The question here is not about taking off, but of landing.

I was discussing the Mythbuster's experiment of the airplane on a conveyor belt with some friends of mine, who all agreed that the experiment was flawed in it's wording and/or approach. It seemed obvious to us that the plane would take off, being that the propeller was pulling the plane forward through the air, and the wheels had little to do with it. I assume this is the general consensus on a physics forum as well. The point of this, however, is that my friend posed this question-
If a plane were to land on a conveyor belt moving in the opposite direction to it, wouldn't it be able to land with almost no distance?

What followed was a very lengthy discussion, the outcome of which was that everyone thought I was nuts because I argued that while it might be possible to stop the plane in a very short distance, it would cause countless injuries because it would stop so abruptly. what they argued was that the plane would slow down like normal because of the conveyor belt.

I would very much appreciate some intellectual input on this. Please let me know if I haven't made the problem clear.

[mgb_phys]

You need the plane to stop in the same time as before, to keep the same acceleration and so the same force on the passengers.
But the wheel speed on landing will be higher because you have the extra conveyor speed so you would have to apply more wheel braking - assuming you are using wheel brakes to slow you down. If you used purely engine braking (thrust reverses) it would make no difference.

[Dark Prism]

For arguments sake we'll say that braking is done the way a 747 usually brakes, that being reverse thrust and wheel brakes.

Your answer seems like it doesn't follow the question, although that is probably my fault. Let me restate.

A plane is landing. Instead of a long runway, you have a short conveyor belt. The plane is travailing 100mph north. The conveyor is not going anywhere, but it's speed is 100mph, south. The plane lands on the conveyor belt and the conveyor is slowed from 100mph to a stop. If the plane lands on the conveyor belt, and its forward momentum is transfered into the conveyor belt, will it stop without the need for a long runway with no adverse effects on the plan or passengers? Will the conveyor belt negate it's forward inertia?


My argument is that when the plane touches down, if it does stop, it will probably destroy the plane and/or kill the passengers because it's inertia is cut abruptly, being that I don't believe that the conveyor belt will "catch" the plane's inertia. Alternatively, is it even possible to stop the plane on the conveyor belt, as the plane's 100mph is air speed, and the conveyor belt's 100mph is ground speed?

[Danger]

I agree with Mgb. Under normal circumstances, with a light passenger aeroplane, you don't use the toe brakes until it's time to turn from active to a taxiway. Runways are long enough that the bird slows down just fine by itself; differential braking is used for steering. If you did apply the brakes on a conveyor belt, something would come unglued.

[mgb_phys]

The plane lands on the conveyor belt and the conveyor is slowed from 100mph to a stop.
If it takes the same time for the conveyor to stop the plane - there is no difference, if it takes less time there will be more force on the plane and passengers.
If the plane lands on the conveyor doing 100mph in the opposite direction then you just have to (wheel) brake more gently.
It's exactly equivalent to landing with a 100mph tailwind - you would have the same indicated air speed but would probably blow all the tires !

[FredGarvin]

The thing that people fail to realize is that, no matter what the wheels are doing, the forward flight of the aircraft is the wing relative to the surrounding air. When you're coming in at, say, 100 MPH IAS, that means the wing has a relative velocity to the wind of 10 MPH. The ONLY way this would work is if you had a wind generating device that matched the required wind speed at the second the wheels hit the ground.

[D H]

OMG. Kill one plane-on-a-treadmill myth and another just as ludicrous in from its ashes.

The MythBusters already addressed this new incarnation as well. They drove a car up a ramp trailing behind a moving truck. The car's wheels changed their rotation rate nearly instantly as the car transitioned from rolling at speed with respect to the road to nearly at rest with respect to the ramp. The bulk of the car's momentum is in the car, not the wheels. The same applies to the plane landing on a moving treadmill. The bulk of the plane's momentum is in the plane itself, not the wheels.

In this case, problems would arise should the pilot stupidly lock the brakes the instant the plane hit the conveyor belt. So what? Problems would arise should the pilot stupidly lock the brakes the instant a plane hits a run-of-the-mill tarmac.

[Dark Prism]

I guess what I can't conceive here is that the plane is going forward, and when it hits the conveyor belt, it isn't supposed to go forward any more. The idea being that it really is still going forward, but on the conveyor belt now. This just doesn't seem possible to me.

[D H]

The plane has free-rolling wheels. The plane landing on a moving conveyor belt is no different physically than a plane landing an a normal runway while flying with a tailwind or against a headwind.

[Danger]

The plane has free-rolling wheels.

Exactly. It's the application (if any) of the brakes that makes the difference between this and the take-off scenario. Suddenly, they're no longer free-rolling. You're quite correct that the effect would be the same as locking them up upon first touching a runway. Most tricycle gear planes wouldn't mind too much, but it could be devestating to a tail-dragger.

[Dark Prism]

See, I would think that the plane would continue off the end of the conveyor, as it still has it's main forward inertia through space.

Maybe this is what your saying...

[DaleSpam]

They drove a car up a ramp trailing behind a moving truck. The car's wheels changed their rotation rate nearly instantly as the car transitioned from rolling at speed with respect to the road to nearly at rest with respect to the ramp.Hehe, just finished watching the new Knight Rider!

[Danger]

Hehe, just finished watching the new Knight Rider!

Loved it! A lot less hokey than the original, and I really liked the way that they integrated Hasselhoff into it. I knew that he was going to be a guest star, but I expected some crappy 'formula' type of approach. This was handled very nicely. It was great that it was a stand-alone movie (ie: no cliff-hanger), but still left me wanting more. I hope that they pick it up as a series.

[Cyrus]

Do you think an aircraft carrier is stationary when those airplanes land on it?

QED

How many times do we have to hear this crapy question?

Thread locked, poster banned.

[Danger]

Do you think an aircraft carrier is stationary when those airplanes land on it?

Do you think that Earth is stationary when an aeroplane lands on it?
The question still comes down to the matter of having or not having brakes applied.

[Dark Prism]

Do you think an aircraft carrier is stationary when those airplanes land on it?

The difference being that the aircraft carrier is moving the same direction the plane is landing. Not only that, but it still doesn't answer the question of stopping in a small space. When those jet fighters stop abruptly on an aircraft carrier, they use giant tow wires and stop quickly enough to jerk the pilots around.


Also, I humbly apologize for wanting to know more about the world around me.

The question still comes down to the matter of having or not having brakes applied.

So what your saying is that if there are no brakes applied, it will continue off the conveyor, but if brakes are applied it will stop quickly?

[mgb_phys]

So what your saying is that if there are no brakes applied, it will continue off the conveyor,
Yes - imagine a plane landing on ice with skis if you prefer - it doesn;t matter what the ground is doing.

but if brakes are applied it will stop quickly?
Yes - the brakes will have to be applied a bit more gently because you don't want them to go backwards as soon as the touch.

[MonstersFromTheId]

Actually the idea of "landing on a conveyor belt" has already been tested, though not by Mythbusters.

At an airshow, a small plane was landed on what was billed as "the world's shortest runway" as a stunt.

A large platform was mounted to the top of a motorhome. The motorhome matched the landing speed of the small plane, speeding up and slowing down as necessary to stay under it as it landed.

As you might expect, the wheels of the plane barely turned as they settled on the platform.

Once the plane had settled its weight on the platform the motorhome simply slowed to a stop.

Here's a link to where you can see it on YouTube:

http://www.youtube.com/watch?v=ihvJuMlRFVs

[russ_watters]

I guess what I can't conceive here is that the plane is going forward, and when it hits the conveyor belt, it isn't supposed to go forward any more. The idea being that it really is still going forward, but on the conveyor belt now. This just doesn't seem possible to me. It is still going forward wrt the ground and air. The conveyor belt has no way to stop the airplane besides making the wheels spin faster and increasing the braking force.

Btw, the first answer you got (and pretty much everything that followed followed it) is correct.

Sorry, this thread isn't any better than all the other ones on the subject that we've banned. It's a simple question that people make just complicated enough to confuse themselves. Drop the complications and listen to the simple answers. There was nothing wrong with the Mythbuster's test - you're adding irrelevant complications to it and convincing yourself they matter. They don't.

Locked.

[knoxjeff]

I think some people may have a misconception of friction, including me about ten minutes ago.

As I set out to demonstrate a stationary plane due to frictional forces the wheel bearings place on the plane I found a problem in my logic. I had imagined a different universe in which friction INCREASES with velocity, which, I found out, didn't happen to be true in my current universe.

If the friction increased proportionally in the wheel bearings with the increase of the rotation of the wheels, then it seems reasonable that we could arbitrarily set the backwards force due to friction on the plane to exactly equal the forward thrust simply by increasing the conveyor speed.

As I understand it, friction only depends on the Coefficient used and the Force between two objects. And, both the Coefficient and the Force (gravity and mass of the plane) remain constant in the case of the plane-on-a-conveyor experiment. So, what force am I missing that is canceling out the forward thrust?

But what if we changed the experiment to allow ridiculously high friction wheels? Assuming a fully loaded Cessna 182 weighing in at 1160 kg and a thrust of 1,000 N the normal forward force (with coeff of friction 0.001 for the wheel bearings) would be 1,000N - 1160kg * 9.8m/(s*s) * 0.001 or about 990N. But if the wheels had a coefficient of friction of, say, 0.088 (the coefficient of sliding friction of copper on copper) then the total forward force would be about 0N which means the conveyor would create enough force via friction to keep the plane stationary. But I don't know of any wheels that use copper slabs in place of bearings.
Do I have an error in my calculations of this last example (besides blatantly ignoring significant digits)? I've been out of physics for quite a while now. Maybe I should be using rolling friction for the wheel bearings, and it doesn't have anything to with the weight of the plane?

[TVP45]

I Googled Cesna and found a picture of the instrument panel. The "speedometer" is unmistakably an air speed indicator.

[mgb_phys]

The ATR 42/72 (a french twin turboprop commuter) is almost unique in having powered wheels so it can be manouvered easily in small airports without the props running. So it might also be the only aircraft with a speedometer!

edit. Of course this has nothing to do with conveyor belts

[ray b]

have you seen the chopper on a turntable version?

[knoxjeff]

have you seen the chopper on a turntable version?

lol, That made me laugh. A whole new ball of wax has just opened up before my eyes.

[geoff wrights]

Hi everbody, I,m new on here but had to join looks really interesting, however the real reason for searching out a forum like this is to try and have a question answered that was put to me and is really niggling me.
Please take a look at the question I have pasted . Can you help with the correct answe and reason why?
geoff
--------------------------------------------------------------------------------
An aircraft is standing on a runway that can move. The aircraft can move in one direction, while the conveyor belt moves in the opposite direction. This conveyor has a control system that tracks the aircrafts wheel speed and tunes the speed of the conveyor to exactly the same as the wheels, but in opposite direction. There is no wind,
The pilot begins to add thrust to the engines.

Will the plane take off

[fedaykin]

Damn it, this was even tested on Mythbusters.
Aside from some increased friction in the wheels, there will be no effect. It will take off normally (assuming you don't have a massive backward starting velocity that would screw the plane up.)
I don't think an airplane that provided its thrust through wheels would be very effective.

[berkeman]

Hi everbody, I,m new on here but had to join looks really interesting, however the real reason for searching out a forum like this is to try and have a question answered that was put to me and is really niggling me.
Please take a look at the question I have pasted . Can you help with the correct answe and reason why?
geoff
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An aircraft is standing on a runway that can move. The aircraft can move in one direction, while the conveyor belt moves in the opposite direction. This conveyor has a control system that tracks the aircrafts wheel speed and tunes the speed of the conveyor to exactly the same as the wheels, but in opposite direction. There is no wind,
The pilot begins to add thrust to the engines.

Will the plane take off

This has been discussed at length. A search on the PF for "conveyor" finds the threads pretty quickly.

http://www.physicsforums.com/showthread.php?t=212151&highlight=conveyor

Thread locked.

[Jwink3101]

NOTE: Never mind the question! I anwered it myself.
Anyway, if you want to read it:
************************************************** *******************

I search quickly, but if this has been posted, please direct me and delete this threads...Anyway:

There was a mention in the XKCD this week about the airplane/treadmill connundrum. I am including the wording I found below:
"plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

So, everyone who claims to be a physicist says it will take off. If it was just a free standing belt that could spin, I would agree, but I think the way it is asked the airplane will not take off.

Assume bearings that are frictionless bearings but not frictionless wheels. So, once the plane starts moving forward, the treadmill will accelerate backwards creating a force on the wheels. So, the way I see it, the treadmill and wheel speed will increase unbounded, but in a perfect system, this will stop the plane from moving. The belt would have to be constantly accelerating or else the plane will take off.

A lot of people argued that it is like a plane taking off on water where it just moved forward, but this is not the same situation. The belt is ACTIVELY moving backwards.

Does my analysis make sense?

[Jwink3101]

Nevermind. I thought about it more and realized that while the wheels and the treadmill speed will increase, the thing that is interesting is the force on the air and the plane will move forward.

My flaw was that if you have a stationary plane and turn on the treadmill, the wheels will turn but the plane will not in the ideal situation.

So, nevermind...

[Cyrus]

Oh dear god not this stupid question again..............

The answer, of course, is that the airplane does take off. Search the forums if you want to know why. This thread is locked.

We need a big bold disclaimer saying DONT POST THIS QUESTION ON THIS FORUM ANYMORE!

[atyy]

My tentative answer:

Consider a plane on a rigid surface with only static friction (Fr) that increases with applied force up to a maximum (Frmax).

In the case of a plane without wheels, the plane cannot accelerate unless the jet force (Fj) is greater than Frmax. In this case, the plane will take off, since the puzzle doesn't even make sense unless Fj>Frmax.

But a plane with rigid wheels can accelerate if Fj is less than Frmax, which is reasonable if we assume the wheels always roll without slipping.

Assume Fj is applied through the center of mass of the wheel of mass M, radius R and moment of inertia about its axis I.

Let Ap be the translational acceleration of the wheel relative to the ground.
Let alpha be the rotational acceleration of the wheel about its axis.
Let Ab be the translational acceleration of the belt relative to the ground.

1: Fj-Fr = M*Ap (used 'F=ma')

2: Fr*R=I*alpha (used 'torque=I*alpha')

The plane is moving forwards, the belt is moving backwards, so the translational acceleration of the plane relative to the belt is (Ap+Ab), so for rolling without slipping:

3: (Ap+Ab)=alpha*R

Solving for the friction gives:

Fr=I*(Fj+Ab*M*R^2)/(I+M*R^2)

Thus if Ab=0, then Fr<Fj, and the plane can accelerate.
But for sufficiently great but finite Ab, then Fr=Fj, and the plane cannot accelerate by rolling. It also cannot accelerate by sliding since Fj<Frmax.

[cristo]

Search the forums if you want to know why. This thread is locked.


Cyrus says it all!

[Pavel]

A friend of mine told me about this mush discussed thought experiment (http://en.wikipedia.org/wiki/Airplane_treadmill_problem). We immediately got into a debate, but we shortly reached an agreement. I wanted to run it by somebody who understands physics and tell me if we got it right.

Our consensus is that if we make an idealized and unrealistic assumption that the treadmill will instanteneously match the speed (or more appropriately force) of the airplane, the latter will NOT take off. The key word here is "instanteneously": the time between the increased jet force and the treadmill counterforce (mediated by the friction of the wheels) is absolute ZERO! However, since this is physically impossible, the plane will always take off.

Does this sound right? The only thing that bugs me is that there's an assumption in our agreement that the counterforce produced by the accelerating treadmill will increase without bound (I presume non-linearly). If this assumption is wrong, meaning there's a limit, then the force from the thrust (unlimited for the sake of the argument) will eventually overcome the counterforce from the wheels and the plane will take off even in the platonically idealized situation. What do you think? Thanks.

Pavel

[Phrak]

I can't follow your argument, but the Wikipedia article got it right.

You would be better understood stating the conditions, and leaving the argument out of it.

[atyy]

Wikipedia links to The Straight Dope, who provides two different answers, but without equations. I had a guess for the equations that roughly corresponds to each of the different answers: http://www.physicsforums.com/showthread.php?p=1864130

[Pavel]

I can't follow your argument, but the Wikipedia article got it right.

You would be better understood stating the conditions, and leaving the argument out of it.

Hi Phrak,

There's a difference between an idealized experiment, in which things are considered in principle, and an actual experiment, under certain physical conditions. When Enstein imagined what would happen if he was riding a beam of light, he was not conducting an actual experiment.

My point is that if we idealize the experiment in which the treadmill can instanteneously match the force of the airplane, it will NOT take off. However, given our experimental conditions, it's physically impossible to achieve delta time of 0. I was wondering if that was the only difference between an ideal and actual experiment that determines whether the plane will take off (assuming greater than 0 friction at the wheels).

[Vid]

http://blag.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

[Phrak]

Hi Phrak,

There's a difference between an idealized experiment, in which things are considered in principle, and an actual experiment, under certain physical conditions. When Enstein imagined what would happen if he was riding a beam of light, he was not conducting an actual experiment.

My point is that if we idealize the experiment in which the treadmill can instanteneously match the force of the airplane, it will NOT take off. However, given our experimental conditions, it's physically impossible to achieve delta time of 0. I was wondering if that was the only difference between an ideal and actual experiment that determines whether the plane will take off (assuming greater than 0 friction at the wheels).

no, no, no. I'm not getting suckered into this. There are explicitly three different problems, as given by Vids, link. Which one are you talking about?

[Jeff Reid]

Our consensus is that if we make an idealized and unrealistic assumption that the treadmill will instanteneously match the speed (or more appropriately force) of the airplane, the latter will NOT take off.The only force is the rolling resistance of the wheels. The tires would either explode or simply start sliding, if the aircraft had enough power, and the aircraft would take off.

The wheels interact independently with the ground and only provide a relatively small amount of rolling resistance. The prop interacts independently with the air which isn't signifcantly affected by the treadmill speed except for shear effects near the surface of the treadmill.

[Hootenanny]

Please note: All threads involving aeroplanes, treadmills, conveyor belts and whether they will take off, have been merged into this thread. All further discussion on this topic should be confined exclusively to this thread. Any additional threads created on this topic may be deleted or merged into this thread at the complete discretion of the mentors.

It is highly likely that any questions you may have related to aeroplanes and treadmills has already been answered. Therefore, please read this thread in it's entirety before posting a related question.

[Danger]

Jeez, Hoot... couldn't you just make this a sticky and lock it so no other idiots can contribute? :biggrin:

[cabraham]

Hi Phrak,

There's a difference between an idealized experiment, in which things are considered in principle, and an actual experiment, under certain physical conditions. When Enstein imagined what would happen if he was riding a beam of light, he was not conducting an actual experiment.

My point is that if we idealize the experiment in which the treadmill can instanteneously match the force of the airplane, it will NOT take off. However, given our experimental conditions, it's physically impossible to achieve delta time of 0. I was wondering if that was the only difference between an ideal and actual experiment that determines whether the plane will take off (assuming greater than 0 friction at the wheels).

Sorry but even if the treadmill CAN instantaneously match the speed of the airplane, it WILL take off. This has been experimentally proven, but I don't have the link.

Let's take it one step further. Suppose the stall speed of a given plane is 80 mph (129 kph). If the conveyor is moving backwards at 90 mph steadily while the airplane is just sitting on the conveyor, the plane is moving 90 mph backwards, or -90. The airplane engine is then started. The thrust from the engine accelerates the plane *forward*. Since its speed is originally -90 mph, its speed increases to -80, -70, -60, etc. Eventually it reaches 0 mph where it is stationary wrt an observer on the ground. It continues to accelerate to +10, +20, +30, etc. until it reaches +80 mph. At this point the wheels are spinning as if the plane was going 170 mph on the ground. But, at 80 mph the plane takes off.

The key to this problem is understanding that the conveyor moving backwards exerts very little force on the plane. The wheels have a small friction so that the backwards conveyor presents a slight drag to the forward moving plane. The engine provides a thrust which is forward in direction, but the drag backwards by the conveyor is that of wheel friction, too small to prevent takeoff. The forward engine thrust is much much greater than the backwards conveyor drag. The plane takes off. It just takes a slight bit longer to reach stall speed due to the small drag incurred. Draw a free body diagram and you will see that the conveyor motion exerts a torque on the wheel in the reverse direction. This results in a little friction in the bearing and a slight backwards drag.

Now I remember, it was "mythbusters" that shot a video showing takeoff. A search should turn it up. BR.

Claude

[redargon]

Now I remember, it was "mythbusters" that shot a video showing takeoff. A search should turn it up. BR.

Claude

According to me, the plane will take off as has been discussed, the wheels are not powered, etc. As much as I like mythbusters, their experiment to prove this was not very good imo. It was a little rough,and ready I mean they drag a big sheet of material behind a pickup truck and have a prop plane running in the other direction. At no point do they take wind into account, they don't keep the "treadmill" going at exactly the same speed as the plane etc etc. I love their experiments, but this one for me was a little too far fetched to be conclusive. I definitely wouldn't call it experimentally proven. Proof busted :wink: