Airplane and Conveyor Belt Debate

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In the discussion, participants debate the scenario of a 747 landing on a treadmill moving in the opposite direction at the same speed. The consensus is that the plane would roll off the treadmill without stopping, as the wheels would spin faster due to the treadmill's motion. Concerns are raised about the potential overheating of the landing gear bearings, which are not designed for such high speeds. It is emphasized that the plane's speed relative to the ground does not affect its airspeed, which is crucial for lift. Overall, the argument concludes that the treadmill does not provide any advantage in stopping the aircraft compared to a regular runway.
  • #31
russ_watters said:
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...)
 
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  • #32
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?
 
  • #33
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.
 
  • #34
New member with a ?

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 really 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
 
  • #36
So perhaps we could see the initial question...? The wording of the initial question makes all the difference here.
 
  • #37
russ_watters said:
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.
 
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  • #38
brewnog said:
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.
 
  • #39
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!
 
  • #40
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.
 
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  • #42
plane and a conveyor belt

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?
 
  • #43
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.
 
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  • #44
lol mythbusted, I heard what you said in the end of that video...ahem...
 
  • #45
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.
 
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  • #46
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.
 
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  • #47
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...
 
  • #48
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)
 
  • #49
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.
 
  • #50
Maybe you guys can help settle a little argument here.

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?
 
  • #52
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.
 
  • #53
Airplane On Conveyor Belt?

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?
 
  • #55
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.
 
  • #57
Air

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.
 
  • #58
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.
 
  • #59
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?
 
  • #60
I have riddle type of question

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
 
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