Physics Question

[911paramedic]

OK, a friend of mine frequently posts brainteasers for us on our site. (400+ members, all veterans or active duty) This is his latest question and we are deeply divided on the answer, just about down the middle. I would like to put this out there so I can get a very simple answer to explain it to him. (I am sure of my answer, but dont want to post it for fear of contamination) Here is his 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 the opposite direction). Can the plane take off?"

Thanks in advance. (no, its not homework, you guys know me...duh)

 

[imported_LoKe]

No, the plane can not take off. Even though it's moving at the proper speed, the plane is not actually moving so it can not manipulate the air.

Of course, I'm probably wrong.

 

[TuxDave]

I would say no since you don't get the airspeed over the wings to get the necessary 'lift'. But then again I'm an EE an not an aerospace engineer.

 

[pray4mojo]

No it can't. Bernoulli says so.

 

[JoeFahey]

42

 

[edro]

Of course not. Lift is not generated by the ground VS plane relationship... it is caused by air movement over and under the wing. If there is no wind movement, there is no lift.

Another look at it:

When you run on a treadmill... do you feel wind in your face?

 

[KillerCharlie]

I've seen this come up before... usually the question is not stated very clearly. What I'm assuming is that the conveyor moves at velocity X in one direction relative to a fixed reference frame and the airplane is moving at velocity X relative to the CONVEYOR. In other words, the airplane is not moving relative to the fixed reference frame--the airplane is not moving relative to the air. If the airplane is not moving relative to the air, then it will not fly.

To sum it up: if there's no air flowing over the wings, the plane ain't flyin'. The speed of the wheels doesn't mean anything.

I guess if it was an F-35 though, it could still fly

 

[dbot]

Originally posted by: LoKe
No, the plane can not take off. Even though it's moving at the proper speed, the plane is not actually moving so it can not manipulate the air.

Of course, I'm probably wrong.

Actually i think you're right. Even though the wheels are moving at the right speed, the airplane isn't. Which means the wings are not generating any lift.

 

[Kalmah]

There is no wind moving over the wings to create lift. It will not take off.

 

[swtethan]

not unless you had a helicopter like plane

 

[giantpinkbunnyhead]

No, the plane will NEVER take off. No matter what happens beneath the plane, the true airspeed is ZERO until a situation occurs to produce airflow over the wings.

<---is a pilot

 

[jonessoda]

Of course it can.

Assuming it's a V22 Osprey.

But then it would probably crash as soon as it got up.

 

[911paramedic]

Thank you. I said that the net velocity was zero, so the plane was stationary and without air over the airfoils there is no lift generated.

The question itself is indeed a bit vague but I think we all understand its meaning. An even simpler (or more complex?) answer may be needed to prove to the "flyers" that this does not happen. I tried the hamster wheel thing, the net speed, newtons laws (2 and 3, not sure which this is covered by) and a few others, but the group is still divided just about down the middle. Now that there are some answers up, I was a firm "no way Jose" guy, its all net velocity.

Thanks.

P.S. Keep 'em coming, I need all I can get until they understand this.

 

[letdown427]

"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 the opposite direction). Can the plane take off?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

 

[hytek369]

Originally posted by: LoKe
No, the plane can not take off. Even though it's moving at the proper speed, the plane is not actually moving so it can not manipulate the air.

Of course, I'm probably wrong.

i like that, i agree

 

[911paramedic]

Originally posted by: letdown427
"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 the opposite direction). Can the plane take off?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels ar emoving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

OMG, here we go again...

Sucking the air and creating movement is still movement, which the wheels keep up with. That means the conveyor matches that speed exactly, and from somebody say, in the air control tower, sees a plane sitting still with its wheels spinning and the conveyor whirring.

You lose.

 

[DrPizza]

Hmmmm...
A. If the plane is not moving relative to the air, it cannot take off.

B. The idea of a conveyor moving the plane backwards wouldn't work. The plane is providing thrust against the air behind it. i.e. when you turn on the engines, the plane moves relative to the air; not necessarily the ground. If the ground were perfectly frictionless, the tires on the plane would not spin; yet the plane could still accelerate down the runway and take off. Thus, the conveyor moving in the opposite direction relative to the direction the plane is moving would have this effect: The planes wheels would spin faster; the conveyor realistically could *not* keep the plane stationary.

edit: changed the word "land"?? to "take off"

 

[giantpinkbunnyhead]

Originally posted by: letdown427
"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 the opposite direction). Can the plane take off?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels ar emoving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

Nope. The engines dont do a damn thing to keep a plane in the air. Only airflow does. The engines are only a method to accelerate a plane to a true airspeed where the wings have enough airflow to generate lift greater than the weight of the plane. It doesn't matter if you have your engines generating full thrust at max takeoff power. If that airplane does not have airflow over the wings, it WILL NOT FLY. A plane on the conveyor belt, assuming the belt always moves the exact same speed in the opposite direction, will have a groundspeed compared to the conveyor belt but the true airspeed is zero. True airspeed is simply the speed of an aircraft THROUGH THE AIR. If that plane is sitting still, by whatever means, and there is no wind... it's airspeed is zero. Similarly, if a parked plane encounters wind of 50 knots, it will behave as if it's moving at 50 knots of speed. Some small airplanes can take off at this speed, so in such a situation you can actually see a plane lift itself off the ground. The plane doesn't know it's not moving down the runway at 50 knots.

 

[giantpinkbunnyhead]

Originally posted by: letdown427

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

OK, didn't catch your edit. You're introducing wind into the picture. Again... wings... and by extension the airplane... can only fly if there is enough airflow over the wings. Whether this airflow is oncoming wind due to forward motion of the airplane or wind provided by a huge fan matters not. All a plane needs to fly is a sufficiently fast current of air past the wings.

 

[LostWanderer]

It's hard to believe there's that many people that would argue the plane actually could take off. It can't because there's no air flowing over the wings. The argument that the engines are sucking air is probably valid, but that airflow is through the thing, not over the wings which give it lift. Any lift generated by the suction of the engines is incidental and due to inefficient intake, i.e. spill over that goes over the wings.

Well that's my 2 cents anyway.

 

[KillerCharlie]

Originally posted by: letdown427

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

No, you are wrong. The engine thrust is only overcoming the resistence of the wheels enough to match the velocity of conveyor. The engines give you force FORWARD, but wings give you force UPWARD. If there's no airflow over the wings, you're not going up.

Originally posted by: giantpinkbunnyhead
No, the plane will NEVER take off. No matter what happens beneath the plane, the true airspeed is ZERO until a situation occurs to produce airflow over the wings.

<---is a pilot

You're 99.999% right, but I'm going ton bust you on a technicality .

I believe it COULD be possible for an aircraft to take off in this situation. This would occur at very very low Reynolds number - highly viscous flow. Imagine the airplane is very tiny... say, smaller than a fly. When this conveyor starts moving with respect to the fixed reference frame, it drags along the air with it. Air (except on the molecular scale) cannot scrape against a surface - it has to move with the surface. This gives way to the boundary layer. In the case of air flowing over a wing, there is a thin region right on the surface of the wing where the velocity goes from 0 (at the surface of the wing since it can't scrape against the wing) to the rest of the air.

In this conveyor case, the air at the surface of the conveyor has to move with the conveyor. However, just a little bit above the conveyor, the air is not moving. This results in a velocity gradient - a boundary layer. With this boundary layer it is now possible to get air to flow over the tiny vehicle.




<---is an aerodynamicist

 

[DrPizza]

letdown was close... read my post. It's an impossible situation because a conveyor CAN'T keep the plane from moving, unless there is a great deal of friction between the wheels and the plane.

A car moves itself forward by exerting a force against the road in the opposite direction. A plane doesn't exert this force against the runway. The runway/tire interface creates some resistance to the plane moving forward, but I seriously doubt this force is even close to the force the plane exerts on the air.

edit: spelling

 

[giantpinkbunnyhead]

Originally posted by: DrPizza
Hmmmm...
A. If the plane is not moving relative to the air, it cannot land.

B. The idea of a conveyor moving the plane backwards wouldn't work. The plane is providing thrust against the air behind it. i.e. when you turn on the engines, the plane moves relative to the air; not necessarily the ground. If the ground were perfectly frictionless, the tires on the plane would not spin; yet the plane could still accelerate down the runway and take off. Thus, the conveyor moving in the opposite direction relative to the direction the plane is moving would have this effect: The planes wheels would spin faster; the conveyor realistically could *not* keep the plane stationary.

It could, actually. This stems back to the airflow issue. While wheels aren't the source of propulsion for an aircraft, they are a source of friction. As airflow increases, lift increases... which causes the wheels to be less effective as the wings become more effective. As long as the true airspeed is zero... a plane on wheels is no different than anythign else on wheels. The entire weight of the craft is borne by those wheels. Such a setup can indeed remain stationary if a conveyor underneath rotates the opposite direction at the same speed. Granted in a frictionless environment this would not work... if the plane had enough lift to be off the ground it would not work. But neither of those apply here.

 

[911paramedic]

OMFG, now there is a website posted saying it CAN take off...

http://www.straightdope.com/columns/060203.html

The "straight dope" is smoking dope.

 

[giantpinkbunnyhead]

Originally posted by: LostWanderer
It's hard to believe there's that many people that would argue the plane actually could take off. It can't because there's no air flowing over the wings. The argument that the engines are sucking air is probably valid, but that airflow is through the thing, not over the wings which give it lift. Any lift generated by the suction of the engines is incidental and due to inefficient intake, i.e. spill over that goes over the wings.

Well that's my 2 cents anyway.

You are correct. With jets, virtually NO airflow over the wings is added to by the engines. But on prop-driven, multi-engine, non-centerline thrust aircraft, the propwash does flow around the wings. It creates more lift than a jet would, but still nowhere NEAR enough to be beneficial for anything.