Plane on a Treadmill

[Yume]

As long as the treadmill continues to cancel out the movement that would be generated from the plane's thrust, it will not "take off" or do much of anything. End of topic.

quoted for the sake of quoting

The question I got from the video wasn't wether it would leave the ground, it was wether the plane would move forward off the treadmill. I don't think the treadmill causing some kind of airflow over the wings was ever part of the equation.

Snow likes to bring up non-sequiturs.

[kkay]

This is a very dumb question. Treadmill automatically cancels horizontal movement, meaning the only way for the plane to move would be to completely lose contact with the surface. Since the plane isn't moving, there is no flow of air over the wings which would normally exist due to the plane's movements, except for the obvious natural flow of air separate from the plane-treadmill system. Even if you assumed that there was some interaction between treadmill and air and thus between air and plane, or some other stupid force like the thrust heating the air blah blah, none of these forces have nearly the magnitude to lift a plane to completely lose contact with the ground.

Now if you're talking about some crappy paper plane or other stupid model where the plane spins at some infinite speed and the plane is some infinitesimal mass, I would have no idea, because there's so many random forces that actually matter when you consider such extremes that you may as well pretend your breath is a hurricane and lifts up the plane.

[Ichron]

It's obvious the plane will stay still and do nothing. A plane can drive on the ground all day. You're going to have to turn that puppy upwards to the sky to start it to take off, and I don't believe that's ever mentioned in the video.

That and the whole no lift thing because there's no air flow underneath the plane since it's stationary.


I think the REAL question to answer is, if Doc Brown's Delorean hits 88mph on a treadmill, does it go back in time?

[shiv_]

I think the REAL question to answer is, if Doc Brown's Delorean hits 88mph on a treadmill, does it go back in time?

I just shit myself

[Yume]

This is a very dumb question. Treadmill automatically cancels horizontal movement, meaning the only way for the plane to move would be to completely lose contact with the surface.

o rly? I think it's a great question because people don't actually read or think about it and therefore claim an answer based on assumptions that don't have any physics behind them.

[Ichron]

I'm no engineer, but I would say from the simple fact of there being no real air flow to create lift would wreck the idea completely. Go outside for a run, feel a small breeze of air flowing around you? Now go run on a treadmill. No breeze. Same idea, no air flow because the plane is stationary, there's no air flow around it save for the normal air current, which isn't enough to lift it off.

Right?

[Yume]

I'm no engineer, but I would say from the simple fact of there being no real air flow to create lift would wreck the idea completely. Go outside for a run, feel a small breeze of air flowing around you? Now go run on a treadmill. No breeze. Same idea, no air flow because the plane is stationary, there's no air flow around it save for the normal air current, which isn't enough to lift it off.

Right?

The point is the treadmill doesn't do anything. People assume the airplane is like a car or you running on the ground in which the ground moving opposite of you actually has an effect on you moving relative to the world. But for a plane the treadmill doesn't put any force on the airplane to counteract the engines. Yes the plane has to move to fly but it does move.

[kkay]

Airspeed is a relation of movement though. You can't have speed relative to air if the air is stationary relative to the earth, and you're also stationary relative to the earth. If you wanted air speed, then either the air or the plane would have to be moving relative to each other. Obviously the air isn't moving unless planes somehow create a force causing the air to move in a direction opposite of the direction that the force is in. The other option is the plane is moving, parallel to the movement of the air which is horizontal. I assumed that the wheels would always be rolling, without skid, therefore there can't be horizontal movement.

Which way is the plane moving the way you see it Yume? It' can't move horizontally by definition of the problem.

[kkay]

NM NM NM. I read it as matching the speed of the wheels, not of the plane, hence the no skid assumption. The way you worded it does make it a bit more complex.

Actually, what type of engines? Propellers like on huge commercial planes function by sucking in air I believe, but if there's a thruster that instead burns fuel to push a force out the back, it's different. Can you clarify? If it's a propeller, then it would move due to sucking in the air to create its air speed I would think, and thus would move in your example. However if it's a thruster like on a rocket, it wouldn't move.

[manx]

I still cant get past the fact that its impossible even in a perfect world for the treadmill to match the speed of the plane.

[Lemmiwinks]

NM NM NM. I read it as matching the speed of the wheels, not of the plane, hence the no skid assumption. The way you worded it does make it a bit more complex.

Actually, what type of engines? Propellers like on huge commercial planes function by sucking in air I believe, but if there's a thruster that instead burns fuel to push a force out the back, it's different. Can you clarify? If it's a propeller, then it would move due to sucking in the air to create its air speed I would think, and thus would move in your example. However if it's a thruster like on a rocket, it wouldn't move.

You're overthinking the problem.

Go back to day 1 of Physics 101 and remember what your prof preached ad nauseam: draw a free body diagram.

Operate under the standard physical world: no friction, wind resistance, etc, unless specified.

Ignore the Y-axis as it is irrelevant. Assume an infinitely long lolmill.

Here's your FBD:

<----Thrust [Plane]

The treadmill exerts zero force to oppose the thrust force. What happens when imbalanced forces exist? Motion occurs. Motion, in this case, means air movement over an air foil. Which, assuming Bernoulli is still correct, generates lift.

Regarding the engine differences, the primary function of both a turboprop and a turbojet is to move the wings through the air. The only way the prop differs from the jet is that it causes an effect called false lift due to localized pressure differences over the wing directly behind the prop compared to the rest of the plane.

Jet engines suck air in only to burn fuel to accelerate air out of the rear of the turbine. Props "pull" the wing through the air.

[Androk]

You're overthinking the problem.

Go back to day 1 of Physics 101 and remember what your prof preached ad nauseam: draw a free body diagram.

Operate under the standard physical world: no friction, wind resistance, etc, unless specified.

Ignore the Y-axis as it is irrelevant. Assume an infinitely long lolmill.

Here's your FBD:

<----Thrust [Plane]

The treadmill exerts zero force to oppose the thrust force. What happens when imbalanced forces exist? Motion occurs. Motion, in this case, means air movement over an air foil. Which, assuming Bernoulli is still correct, generates lift.

Regarding the engine differences, the primary function of both a turboprop and a turbojet is to move the wings through the air. The only way the prop differs from the jet is that it causes an effect called false lift due to localized pressure differences over the wing directly behind the prop compared to the rest of the plane.

Jet engines suck air in only to burn fuel to accelerate air out of the rear of the turbine. Props "pull" the wing through the air.

Exactly.


The fucking wheels on the plane are doing NOTHING, but opposing the forces exerted on it by the treadmill. Like lemmi said, draw a free body diagram. You've got 3 relevant forces. The treadmill, the wheels, and the thrust. Since the treadmill and the wheels are cancelling eachother out, you're left with one unbalanced force, so it has to move. Air movement over the wings means nothing for this problem, because it is asking if the the plane will move forward off the treadmill not fly into the air. A prop or a turbine does not need air movement to exert a force. They just need air to suck in on one side and air to push against on the other side.

[Snow]

Here's another follow up teaser:

Now suppose the planes engines were off and that it was loaded with cargo in the front of the plane. The planes captain takes all the cargo and moves it to the very back of the plane. An observer standing next to the treadmill watchs. Does the observer see the plane move?

[Yume]

Here's another follow up teaser:

Now suppose the planes engines were off and that it was loaded with cargo in the front of the plane. The planes captain takes all the cargo and moves it to the very back of the plane. An observer standing next to the treadmill watchs. Does the observer see the plane move?

The way I see it is the cargo has to achieve some sort of velocity to get from the front to the back. If the cargo were pushed (frictionless) from the front of the plane at a certain velocity and then had a nonelastic collision with the back wall of the plane, the plane would have a force pushing it forward meaning that if it were on a runway it would move forward then stop (net kinetic energy gain = 0, net potential energy gain = 0).

But instead of sitting on a runway it's sitting on a treadmill that moves in the opposite direction of the plane. However we've previously determined that any motion of the treadmill has zero force on the plane because the plane still uses ball bearing wheels. So the only forces on the plane are pushing it forward as the cargo heads to the back and then pushing it back as the cargo hits the back of the plane meaning that the plane is displaced relative to the observer, starting still and ending still.

[kkay]

NM NM NM. I read it as matching the speed of the wheels, not of the plane, hence the no skid assumption. The way you worded it does make it a bit more complex.

Actually, what type of engines? Propellers like on huge commercial planes function by sucking in air I believe, but if there's a thruster that instead burns fuel to push a force out the back, it's different. Can you clarify? If it's a propeller, then it would move due to sucking in the air to create its air speed I would think, and thus would move in your example. However if it's a thruster like on a rocket, it wouldn't move.

You're overthinking the problem.

Go back to day 1 of Physics 101 and remember what your prof preached ad nauseam: draw a free body diagram.

Operate under the standard physical world: no friction, wind resistance, etc, unless specified.

Ignore the Y-axis as it is irrelevant. Assume an infinitely long lolmill.

Here's your FBD:

<----Thrust [Plane]

The treadmill exerts zero force to oppose the thrust force. What happens when imbalanced forces exist? Motion occurs. Motion, in this case, means air movement over an air foil. Which, assuming Bernoulli is still correct, generates lift.

Regarding the engine differences, the primary function of both a turboprop and a turbojet is to move the wings through the air. The only way the prop differs from the jet is that it causes an effect called false lift due to localized pressure differences over the wing directly behind the prop compared to the rest of the plane.

Jet engines suck air in only to burn fuel to accelerate air out of the rear of the turbine. Props "pull" the wing through the air.

Ah, but a FBD by itself may indicate motion, but it does not exactly mean that the object has to have movement relative to the system. In your description, assuming we changed it from a plane to a simple block on a treadmill, the force of "thrust" would create acceleration and movement in the block, which is then transferred 100% to the treadmill, meaning the treadmill would accelerate relative to the force. The block experiences no net movement.

I didn't know how jet engines worked exactly, but I figured it was a force applied against the pressure of the air right behind the engine, sorta like if you attach a balloon full of air to a straw with a thread through it, the balloon will move opposite of the direction the air inside is moving due to differences in pressure. Hence why I asked how the engine worked. If I had thought of it as moving in regards to how a propeller plane would move then I would have had a different answer.