Airplanes and Lift

  • #1
So I have heard this around the internet before but thought is was a question worth exploring if a jet plane is sitting on a treadmill and the treadmill moves backwards at the same rate the plane is moving forward will it take off? I think no because the plane would have to be moving forward for air to go over the wings to provide lift right? unfortunately there is a lot that disagree with me and was wondering if I could get a reply from some more qualified people. (sorry if there is ignorance in this post I still haven't started under grad physics yet)
 

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  • #2
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and the treadmill moves backwards at the same rate the plane is moving forward
That is ill-defined.
Only the speed of the aircraft relative to the air matters.
 
  • #3
That is ill-defined.
Only the speed of the aircraft relative to the air matters.
basically if the airplane has zero velocity can it still achieve lift
 
  • #4
A.T.
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the treadmill moves backwards at the same rate the plane is moving forward
To be meaningful, you have to state relative to what the speeds are measured.

unfortunately there is a lot that disagree with me
Probably because everyone understands such vague question differently.
 
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  • #5
russ_watters
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basically if the airplane has zero velocity can it still achieve lift
No, it can't.

Note, this is a very easy question that is made difficult only by the fact that it is worded as a seeming self contradiction and also physically difficult to achieve:

1. You said "...the plane is moving forward..." (without stating with respect to what) when you really meant the plane is stationary with respect to the ground.

2. If the plane isn't moving forward with respect to the ground, then something unspecified is holding it in place.

We have gotten this question a bunch of times, always with similar poor wording, which is what causes the arguments/confusion. Run a search and you'll find somee...
 
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  • #6
lekh2003
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So I have heard this around the internet before but thought is was a question worth exploring if a jet plane is sitting on a treadmill and the treadmill moves backwards at the same rate the plane is moving forward will it take off? I think no because the plane would have to be moving forward for air to go over the wings to provide lift right? unfortunately there is a lot that disagree with me and was wondering if I could get a reply from some more qualified people. (sorry if there is ignorance in this post I still haven't started under grad physics yet)

The plane does not generate lift from speed; it generates lift from the speed of the air it is entering. The fast air hitting it can lift the plane.

If the plane is simply standstill relative to the air, then it won't move. This is the case in your example, the plane is moving forwards and the treadmill is moving backwards with the same speed. Relative to the moving ground on the treadmill, the plane is moving. BUT remember that to generate lift the plane needs to be moving relative to the air. So it will not lift upwards.
 
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  • #7
lekh2003
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basically if the airplane has zero velocity can it still achieve lift
Not using the correct terminology is resulting in your confusion. The plane's velocity can be 30000 m/s relative to the sun. It could also be 300000000 m/s relative to the light around it. The only thing that matters here is the velocity relative to the air. That velocity is definitely NOT zero.
 
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  • #8
Not using the correct terminology is resulting in your confusion. The plane's velocity can be 30000 m/s relative to the sun. It could also be 300000000 m/s relative to the light around it. The only thing that matters here is the velocity relative to the air. That velocity is definitely NOT zero.
but its not moving its essentially fixed to one point because of the backwards movement of the treadmill i just want to know if it will ever lift up off of the treadmill
 
  • #9
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to know if it will ever lift up off of the treadmill
What part of "No" are you not understanding.
 
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  • #10
russ_watters
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but its not moving
Then it doesn't takeoff! Simple!

However:
its essentially fixed to one point because of the backwards movement of the treadmill
That is physically impossible. If the airplane's engines are firing, the treadmill alone can't stop the plane from moving with respect to the air.

I suggest you draw yourself a diagram and label all the motions and forces. If the forces don't sum to zero, the plane has to move.
 
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  • #11
jbriggs444
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but its not moving its essentially fixed to one point because of the backwards movement of the treadmill
This partially clarifies your interpretation of the question. The airplane is stationary relative to the ground while a treadmill beneath it moves.

Further questions:

Is there any wind? A plane can take off at zero ground speed if there is sufficient wind.

Are we to neglect the wind induced by the plane's own engine(s)? I suspect that a plane could take off in its own slip-stream if one were to tether it inside a big doughnut-shaped wind tube.

What is causing the plane to move forward relative to the treadmill? One assumes that its engine is on and the prop spinning.

What is restraining the plane from moving forward relative to the ground? One assumes that its prop is only spinning fast enough to counter the rolling resistance of the tires.
 
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  • #13
Make up your mind.
it has speed but is not moving from its point due to the movement of the treadmill counteracting its forward movement
 
  • #14
I have updated the post with a diagram
 
  • #15
russ_watters
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is not moving from its point due to the movement of the treadmill counteracting its forward movement
How? What is the cause of the force in your diagram?
 
  • #17
russ_watters
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sorry for any vagueness I added a diagram that can probably explain it better than me
The diagram shows three forces. The thrust force is obvious. What is the cause of the other two?
 
  • #18
How? What is the cause of the force in your diagram?
the conveyor turns in the opposite direction of the thrust force of the plane. The plane is sitting on a conveyor belt
 
  • #19
russ_watters
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the conveyor turns in the opposite direction of the thrust force of the plane. The plane is sitting on a conveyor belt
I know the conveyor moves and the wheels spin, but motion is not force. Indeed, in constat speed motion, forces must sum to zero. So what causes the forces? Consider that the spinning wheels don't stop a plane from taking off on a runway, so ordinarily there is essentially no force exchange between runway and wheels. Why does the conveyor change that?
 
  • #20
I know the conveyor moves and the wheels spin, but motion is not force. Indeed, in constat speed motion, forces must sum to zero. So what causes the forces? Consider that the spinning wheels don't stop a plane from taking off on a runway, so ordinarily there is essentially no force exchange between runway and wheels. Why does the conveyor change that?
yeah thats where my question basically lies is if the plane is not moving there is not as much air flowing over the wings therefor less thrust is created so I was wondering if the plane could still take off even though it's wings are not gliding through the air creating that downward thrust
 
  • #21
I know the conveyor moves and the wheels spin, but motion is not force. Indeed, in constat speed motion, forces must sum to zero. So what causes the forces? Consider that the spinning wheels don't stop a plane from taking off on a runway, so ordinarily there is essentially no force exchange between runway and wheels. Why does the conveyor change that?
I'm sorry I didn't include a diagram that represents the air flow that I am referring to
 
  • #22
A.T.
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if the plane is not moving
That's a big "if", when you can't name the force that would prevent it from moving.
 
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  • #23
sophiecentaur
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if the plane is not moving there is not as much air flowing over the wings
If it is not moving through the air then there will be no lift. How many times must this be stated and an how many different ways, before you take that message on board. Whatever the wheels and the treadmill are doing will have no effect on the lift if the plane is not moving forward, relative to the air. There are no 'loopholes' to make your idea suddenly work.
PS the thrust from the engines that's required to keep the plane stationary relative to the Earth is very small - just enough to overcome the rolling friction of the wheels over the treadmill. Do not try to make your intuition govern your way of thinking here. Try to apply the laws of Physics, as have been stated in the other posts in this thread.
 
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  • #24
If it is not moving through the air then there will be no lift. How many times must this be stated and an how many different ways, before you take that message on board. Whatever the wheels and the treadmill are doing will have no effect on the lift if the plane is not moving forward, relative to the air. There are no 'loopholes' to make your idea suddenly work.
PS the thrust from the engines that's required to keep the plane stationary relative to the Earth is very small - just enough to overcome the rolling friction of the wheels over the treadmill. Do not try to make your intuition govern your way of thinking here. Try to apply the laws of Physics, as have been stated in the other posts in this thread.
Alright, thank you
 
  • #25
russ_watters
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yeah thats where my question basically lies is if the plane is not moving there is not as much air flowing over the wings therefor less thrust is created so I was wondering if the plane could still take off even though it's wings are not gliding through the air creating that downward thrust
Well, ok. At this point whether the plane takes off or not and why is really up to you. You're making assumptions in order to correct the deficiencies in the problem statement. You don't necessarily need to correct all of them, I guess, so you are left with:

Q: A plane doesn't move. Does it take off?
A: No. (Obviously)

You've decided thats the answer you want and are filling in details to make it true. That's fine. These details of why (engine is at idle now apparently...) aren't important and don't necessarily need to be developed if you don't want to.

However, my personal preference is to fix it to be a more realistic and non trick question scenario, in which case the plane takes off.

[Edit] Also, I think you will find that except as a logical trick, the presence and behavior of the treadmill is irrelevant to whether the plane takes off. In your diagram, you included a force applied by the treadmill, but the treadmill isn't capable of providing that force on its own.
 
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