Question: Will the plane fly? (warning: nerdy)
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Originally Posted by Egan
But the problem statement was that the conveyor is moving in an opposite direction and identical speed to the plane's wheels. Therefore, the plane never achieves a positive relative speed.
If the plane does indeed go faster than the conveyor, then, yes, at some point it will achieve enough relative speed to generate lift and thus take off.
The problem statement is no different than putting the plane on stands that hold it in place. Even with the engines at max thrust, the plane is going nowhere. If you instantly take the stands away, the plane will not take flight.
If the plane does indeed go faster than the conveyor, then, yes, at some point it will achieve enough relative speed to generate lift and thus take off.
The problem statement is no different than putting the plane on stands that hold it in place. Even with the engines at max thrust, the plane is going nowhere. If you instantly take the stands away, the plane will not take flight.
Inherent to the question is the definition of velocity, which in this case must be defined relative to a common fixed frame (the earth). It wouldn't make sense to define the plane velocity relative to a moving ground below it, and then the moving ground's velocity relative to the earth. Pick a common reference point for both. But in the case of the plane, it doesn't matter anyway, it will still take off. But in the case of a car, the incorrect velocity reference causes a problem.
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Originally Posted by Egan
The problem statement is no different than putting the plane on stands that hold it in place. Even with the engines at max thrust, the plane is going nowhere. If you instantly take the stands away, the plane will not take flight.
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Originally Posted by BLegacy
Here's my opinion.
As was stated, an airplane flies when it's wing generates lift due to the difference in air pressure between the top of the wing and the bottom.
Because the wing is not moving relative to the air, nothing is acting upon the wing to lift.
I guess another scenario might explain it best.
An F1 car can literally be driven upside down in a tunnel because it generates so much downforce due to the air pressure acting on its wings. However, if you introduce it to an upside down conveyor belt that moves fast enough so that the car stays in place, there will be no increased air pressure to apply downforce on its wings; much like how there is no increased air pressure on the wings to lift it.
I guess what everyone is confused about is the difference between velocity and speed (and the theory of relativity).
As was stated, an airplane flies when it's wing generates lift due to the difference in air pressure between the top of the wing and the bottom.
Because the wing is not moving relative to the air, nothing is acting upon the wing to lift.
I guess another scenario might explain it best.
An F1 car can literally be driven upside down in a tunnel because it generates so much downforce due to the air pressure acting on its wings. However, if you introduce it to an upside down conveyor belt that moves fast enough so that the car stays in place, there will be no increased air pressure to apply downforce on its wings; much like how there is no increased air pressure on the wings to lift it.
I guess what everyone is confused about is the difference between velocity and speed (and the theory of relativity).
Now if the question said the plane was in a giant wind tunnel that somehow managed to match the plane's air speed in the opposite direction, that would be much more similar to your F1 car example and the plane would not take off since its relative air speed would be 0.
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Originally Posted by Egan
But the problem statement was that the conveyor is moving in an opposite direction and identical speed to the plane's wheels. Therefore, the plane never achieves a positive relative speed.
If the plane does indeed go faster than the conveyor, then, yes, at some point it will achieve enough relative speed to generate lift and thus take off.
The problem statement is no different than putting the plane on stands that hold it in place. Even with the engines at max thrust, the plane is going nowhere. If you instantly take the stands away, the plane will not take flight.
If the plane does indeed go faster than the conveyor, then, yes, at some point it will achieve enough relative speed to generate lift and thus take off.
The problem statement is no different than putting the plane on stands that hold it in place. Even with the engines at max thrust, the plane is going nowhere. If you instantly take the stands away, the plane will not take flight.
think of it as a your on a tread mill on roller skates, and there is a rail above you. the treadmill is designed to match the speed that you move. now using the rail above you and your arms as propulsion, would you be able to move forward...... yes. but your roller skates would be moving at twice the speed they normally would.
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Originally Posted by MechEE
The problem statement was that whatever speed the plane is moving at, the conveyor will move in the opposite direction with that same speed.
Inherent to the question is the definition of velocity, which in this case must be defined relative to a common fixed frame (the earth). It wouldn't make sense to define the plane velocity relative to a moving ground below it, and then the moving ground's velocity relative to the earth. Pick a common reference point for both. But in the case of the plane, it doesn't matter anyway, it will still take off. But in the case of a car, the incorrect velocity reference causes a problem.
Inherent to the question is the definition of velocity, which in this case must be defined relative to a common fixed frame (the earth). It wouldn't make sense to define the plane velocity relative to a moving ground below it, and then the moving ground's velocity relative to the earth. Pick a common reference point for both. But in the case of the plane, it doesn't matter anyway, it will still take off. But in the case of a car, the incorrect velocity reference causes a problem.
Then why are you talking about the plane (or car) going 200 and the conveyor going 100? If they are at the same speed, as the problem statement laid out, then the plane is not moving through space.
Explain to me how the lift is being generated from a plane that is not moving through space. How is air flowing under the wings? Maybe I'm not seeing something here.
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Originally Posted by MechEE
Many people seem to say this, but I can't understand where it comes from. How is the force imparted to a plane through a rolling interface at a wheel equivalent to holding it still with stands? Regardless of the speed at which you move the conveyor below the plane, it can only impart some finite force to the plane which will certainly be much less than the engine thrust.
Because in both cases the plane is motionless relative to space.
Thrust does not generate lift unless the vehicle is moving through space.
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Originally Posted by BlackVenom96
the origional statement says nothing about matching the wheel speed, it says it matches vehicle speed. so the plane HAS to move for the conveyor to move, at whcih point the plane is now moving forward via the thrust against the air. the wheels of the plane will spin twice as fast as they would normally against the conveyor, but the plane will continue to accelerate and take off.
think of it as a your on a tread mill on roller skates, and there is a rail above you. the treadmill is designed to match the speed that you move. now using the rail above you and your arms as propulsion, would you be able to move forward...... yes. but your roller skates would be moving at twice the speed they normally would.
think of it as a your on a tread mill on roller skates, and there is a rail above you. the treadmill is designed to match the speed that you move. now using the rail above you and your arms as propulsion, would you be able to move forward...... yes. but your roller skates would be moving at twice the speed they normally would.
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Originally Posted by Egan
Because in both cases the plane is motionless relative to space.
Thrust does not generate lift unless the vehicle is moving through space.
Thrust does not generate lift unless the vehicle is moving through space.
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Originally Posted by Egan
Then I misunderstood the problem statement. I thought he was saying that the plane was not moving, just generating thrust. My apologies.
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Originally Posted by BlackVenom96
the question was made for a reason, i have seen a few 100+ page threads on the subject, so i had a bit of a head start.
I've seen the question before (Fluid Dynamics 103B, UCD, 1992), but the question was posed a bit more clearly.
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Originally Posted by BlackVenom96
the conveyor moves relative to vehicle speed. therefor in order for the conveyor to be moving backwards at 200mph, the plane has to be moving forward at 200mph. and the wheels will be spinning at the rate they would with the plane moving 400mph on a fixed surface.
It says relative to plane speed, so depending on whether you interpret that as wheelspeed or vehicle speed..........
The 2x statement makes perfect sense, and it would take off if you're measuring the forward motion of the plane; I'd always read the question as matching wheel speed, hence the confusion.
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Originally Posted by BlackVenom96
the origional statement says nothing about matching the wheel speed, it says it matches vehicle speed. so the plane HAS to move for the conveyor to move, at whcih point the plane is now moving forward via the thrust against the air. the wheels of the plane will spin twice as fast as they would normally against the conveyor, but the plane will continue to accelerate and take off.
think of it as a your on a tread mill on roller skates, and there is a rail above you. the treadmill is designed to match the speed that you move. now using the rail above you and your arms as propulsion, would you be able to move forward...... yes. but your roller skates would be moving at twice the speed they normally would.
think of it as a your on a tread mill on roller skates, and there is a rail above you. the treadmill is designed to match the speed that you move. now using the rail above you and your arms as propulsion, would you be able to move forward...... yes. but your roller skates would be moving at twice the speed they normally would.
Originally Posted by Egan
Then why are you talking about the plane (or car) going 200 and the conveyor going 100? If they are at the same speed, as the problem statement laid out, then the plane is not moving through space.
Explain to me how the lift is being generated from a plane that is not moving through space. How is air flowing under the wings? Maybe I'm not seeing something here.
The plane (or car) moving 200 mph is relative to the conveyor belt, which is not an inertial reference frame. The absolute velocity of the plane is still defined relative to the motionless earth (like a tree planted in the ground), and is moving 100 mph. It would make no sense for you to define the velocity of the plane relative to a moving treadmill, and then the velocity of the treadmill relative to the fixed earth.
Quite simply, the plane moves, and that's how air flows around the wings and generates lift. By definition, if the plane didn't move, neither would the conveyor.
I think people are too quick to jump on what they think is the trick question (that you need airflow to generate lift), when the real trick question is that the plane still moves forward even though the ground moves backwards beneath it.
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Originally Posted by Egan
So you're just repeating something you heard on the internet?
I've seen the question before (Fluid Dynamics 103B, UCD, 1992), but the question was posed a bit more clearly.
I've seen the question before (Fluid Dynamics 103B, UCD, 1992), but the question was posed a bit more clearly.
when i first saw this on mustangworld i actually did come to my own conclusion that it would fly, but my origional resaoning was incorrect, LOL. LookingForPower origionally said it wouldnt fly.
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Originally Posted by sybir
And see, that's the whole point of the poor wording of the question.
It says relative to plane speed, so depending on whether you interpret that as wheelspeed or vehicle speed..........
The 2x statement makes perfect sense, and it would take off if you're measuring the forward motion of the plane; I'd always read the question as matching wheel speed, hence the confusion.
It says relative to plane speed, so depending on whether you interpret that as wheelspeed or vehicle speed..........
The 2x statement makes perfect sense, and it would take off if you're measuring the forward motion of the plane; I'd always read the question as matching wheel speed, hence the confusion.
This is because of the nature of the force couple at the wheel-treadmill interface. Move the treadmill backwards at some ridiculous speed (500 mph).. the plane wheels will simply skid and start to burn up, but some finite force will be generated, and it will be less than the engine thrust, so the plane will move forward, gain speed, generate lift, and take off.The fact that the velocities need to be referenced to the same frame only poses an issue in the same thought experiment with a car (such as the F1 example) or running on a treadmill.
Originally Posted by MechEE
This gets even more interesting, because in the case of the plane, even if you're matching wheel speed and not absolute speed, the plane can still take off. This is because of the nature of the force couple at the wheel-treadmill interface. Move the treadmill backwards at some ridiculous speed (500 mph).. the plane wheels will simply skid and start to burn up, but some finite force will be generated, and it will be less than the engine thrust, so the plane will move forward, gain speed, generate lift, and take off.
The fact that the velocities need to be referenced to the same frame only poses an issue in the same thought experiment with a car (such as the F1 example) or running on a treadmill.
This is because of the nature of the force couple at the wheel-treadmill interface. Move the treadmill backwards at some ridiculous speed (500 mph).. the plane wheels will simply skid and start to burn up, but some finite force will be generated, and it will be less than the engine thrust, so the plane will move forward, gain speed, generate lift, and take off.The fact that the velocities need to be referenced to the same frame only poses an issue in the same thought experiment with a car (such as the F1 example) or running on a treadmill.