Can someone answer this question?

S

Scottwax

New member
A plane is standing on 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 opposite direction).



The question is:



Will the plane take off or not? Will it be able to run up and take off?
 
Setec Astronomy said:
No, it won't, because it's airspeed is zero.
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Dead on. The wheels may be spinning but without proper airspeed over the wings it will never generate lift and therefore never leave the ground.



Similarly, in theory, if you tethered the plane so it couldn't move and placed in front of it a fan large enough to move the needed air over the wings to generate lift, the plane would get off the ground yet still not move due to a lack of forward thrust.
 
Another question. If you had a big-*** wing on your car and were going fast enough, could you drive on an inverted road?
 
Black240SX said:
Another question. If you had a big-*** wing on your car and were going fast enough, could you drive on an inverted road?
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if you had enough power and were light enough. F1 cars are thought to be able to do it. But they are super light and have lots of power and down force.
 
lol



I originally thought that it would not take off, but after reading some of the 14 pages on the subject at the physics forum, I've changed my mind. It will take off!
 
After rereading the question and taking a look at the forum, I was under the wrong impression. I didn't understand that the plane would be using engine power and because of that the wheel speed doesn't matter. Yes, it will take off.
 
The fact that the "ground" is moving the the opposite direction is moot. The airplane is reacting to the fact that the jet is creating thrust out the engines. The wheels are not powered and therefore do not need to interface with the ground in the smallest amount. In essence, the wheels only hold the plane up and reduce the friction of it as it moves down the runway.



Think about it this way.... The plane is already airborne. This eliminates the entire effect of the "moving runway." Does it fall out of the sky? Of course not because the thrust created by the expulsion of the hot gasses out the back of the engines are greater than the resistance to move the plane forward. Newton's law: "For every action, there is an equal and opposite reaction" is what drives the plane forward. A moving runway has nothing to do with the scenario.



I have to go to this physics forum and see the arguments. It should be interesting.
 
DARKSTI said:
if you had enough power and were light enough. F1 cars are thought to be able to do it. But they are super light and have lots of power and down force.
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A F1 car would have no problems but the difficult part would be getting enough speed for the aerodynamics to come into play. The mechanical grip of an F1 car is not that high
 
I say NO, it would not take off. I've already read two pages of that thread and I will read the rest too. Here's my input.



If the belt is moving at the same speed in the opposite direction of the plane's movement, the airplane's wings are generating zero lift because the airflow under the wings is zero.

Some of the guys are saying that the plane WILL take off because of the thrust generated by the engines. Let's say the engine on a typical plane on a normal runway requires X pounds of thrust from its engines in order to take off. If this plane on the conveyor belt acheived X pounds of thrust, it would be able to fly away right? Well, if the plane could somehow jump off the belt, the plane would have to instantaneously go from 0mph (air speed) to whatever speed planes typically take off at in order to INSTANTLY acheive enough lift under its wings to remain airborne.



So the moral of the story... the plane would never be able to leave the belt without gaining enough lift to separate its tires from the belt. There is zero airflow under the wings, therefore the tires would just spin there on the belt until they exploded.
 
Somewhere in the 14 pages of responses, is a video link of a fan (propeller) sitting on a skateboard (airplane) riding along on a sheet of paper placed under the skateboard. As the video shows, even if you yank the paper out from under the wheels of the skateboard, the fan continues to pull it forward even as the paper is still under the wheels and moving backward. This movement had little impact to the forward motion of the skateboard. The fan pulls it against the [/I] stationary air while the impact of the movement of the paper (conveyor belt) only serves to speed up the rotation of the wheels.



If you assume that the original proposition was that the conveyor moves at the opposite speed that the plane moves RELATIVE to a fixed point not on either the plane nor the conveyor, the wheels will spin at ~twice the speed of the plane's motion forward . If you assume that the plane's wheels will generate enough frictional forces to resist the force of the engines and it therefore will not move in relation to this fixed point, then you are correct, it will not fly. However, unlike modern cars, I believe most aircraft can generate enough thrust to overcome their brakes and drag even locked wheels forward. The plane will move forward and will fly.
 
Black240SX said:
lol



I originally thought that it would not take off, but after reading some of the 14 pages on the subject at the physics forum, I've changed my mind. It will take off!
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:D



I've read that thread to the end, and I'm so confused now, that the only way to convince me one way or the other is to show me an airplane on a conveyor belt taking off!



But I still believe that no matter how fast the belt is moving against the speed of the plane, there is ZERO airflow under the wings. Without that airflow, the plane will never leave the ground. The only possible airflow would be from air moving toward the area of low pressure near the exhaust.
 
plenty of details are missing from the original question, but no chance. the plane will not fly, no matter the speed of the engine or the speed of the conveyer. wheels have nothing to do with it other than being the only point of resistance between the plane and the conveyer, neither of which have anything to do with the speed at which the wings are moving through air.



that said, planes fly and remain aloft because of lift created by movement of the plane's wings through air. since we can't control the speed or direction of air on this scale, we accelerate the plane through the air until the aerodynamic effect of the wings creates lift. if the air's not moving over the wings / if the wings aren't moving through the air, no lift is created and conventional flight won't occur.



here's an example... I'm a cyclist and train indoors on rollers in the winter. rollers are a simple machine that allows you to ride a bike unharnessed and remain stationary. two cylinders in back cradle the rear wheel, one cylinder in front centered just slightly in front of the front wheel's center. one cylinder in back is connected to the front cylinder by a band so all the cylinders are moving when you begin to pedal. pedal and move the wheels fast enough for the physics involved (kinda like a gyroscope or spinning top), and you find balance and pedal away as if you were on normal ground. now, I get hotter and sweat a lot more training indoors than out. know why? no wind to speed evaporation. why? I'm not actually traveling forward through the air. if I'm pedaling the equivalent of 20 mph, sweat doesn't fly backward at 20 mph. it falls straight down because I'm not actually moving. neither is the plane.



in the case at hand, I picture myself standing beside the experiment. let's say the wind's blowing at 5 mph where I'm standing. the plane's engine(s) could accelerate to an air speed equivalent of 600 mph (offset of course by the conveyer), and air speed over the wings is still 5 mph. that just ain't gonna make it if we're looking for flight, and in my mind, the plane can't take off.



by the way, remember what happens when an aircraft is flying and air speed is reduced drastically? disasters occur, and we call it wind shear. if you've ever been caught in a plane experiencing wind shear, horizontal or vertical, I'll bet you agree with me that the plane in this experiment is grounded. the only time this happened to me, even the lawyer sitting behind me fell silent. absolutely the scariest flight I've ever taken.
 
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