Airplane and Conveyor Belt Debate

[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 don't 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.
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 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 that's 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?

 

[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 wouldn't 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 wouldn't 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 that's 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?

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.

 

[russ_watters]

As I posted earlier, there are two different versions of this; one that's 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!