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View Poll Results: Will an airplane take off on a treadmill??
Airplane Hour
(Weds., December 12 at 9 PM ET/PT)
Adam and Jamie find out if either of them can safely land a Boeing 747-400 on a runway in varying weather conditions. Meanwhile, Kari, Tory and Grant risk life and limb to investigate skydiving myths regularly featured in Hollywood action films. Is it possible to catch up with someone in freefall if that person jumps out a plane before you do? Can you really hold a conversation during freefall? And would you survive if you opened your parachute only a few feet off the ground? Finally, Adam and Jamie carefully navigate their way through a myth that has baffled everyone from web bloggers to pilots. If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction, can the plane take off? Extensive small-scale testing with a super treadmill and a nearly uncontrollable model airplane don't completely resolve the myth, so our flight cadets supersize the myth with help from a willing pilot and his Ultralight flying machine.
Yes it will. Imagine a car under water in neutral, on a treadmill, with a propeller in the back. Propeller turns on as does the treadmill, car will move forward. As long as the plane moves fast enough to build a pressure system it will take off. Ways it would fail- A) too much friction for given conveyor belt length B)too much friction for engine size. As long as both are met it will move forward fast enough to build positive pressure thus lift. Remember, the plane is being propelled through means other than the wheels (the air) and thus the wheels just create friction that must be overcome. Ive already stated the two ways this would not happen and as long as both are met it will take off.
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The plane wouldn't care about the conveyor belt.
It just means the wheels would be moving at double speed.
The conveyor belt would also have to be as long as the plane's roll-out before rotation.
Now, move the conveyor belt in the same direction as the plane and the wheels would be standing still, but the speed of the plane would be unaffected.
(All of this ignores the internal friction in the wheel bearings which is negligible on a 4000 ton jumbo jet.)
I saw that episode when it came out a few weeks ago and I didn't see this myth covered, just the skydiving and flight sim parts.
The plane will only take off if the air that the wing is moving through is going fast enough, relative to the wing, to generate lift equal to the weight of the plane.
We assume that the jet's engines are generating enough thrust to at least match the resistance of the oncoming airstream. More is better, an even match means you're on the edge of a stall. That's bad.
So, if the airplane is motionless relative to the air, but moving at takeoff speed relative to the conveyor belt, no, it will not take off. If the air is moving at the same speed as the conveyor belt, then yes, it will take off. That is essentially a normal take-off.
So, if the airplane is motionless relative to the air, but moving at takeoff speed relative to the conveyor belt, no, it will not take off. If the air is moving at the same speed as the conveyor belt, then yes, it will take off. That is essentially a normal take-off.
The conveyor belt has no effect one way or the other. The only mitigating factor is friction in the wheel bearings and, relative to the massive inertia of a jumbo jet, this effect is completely inconsequential.
Now, if the plane were on skis or pontoons so that it is, effectively, fixed to the conveyor, then you have the whole "aircraft carrier" situation in simulation.
The conveyor has no effect on the air flow.
The plane will move at the same speed relative to the air that surrounds it whether or not there is a conveyor belt present.
Planes are not driven by their wheels. The engines have no idea that there is a motive force at the wheels.
If you could have perfect bearings and tires, the plane would sit there motionless when the conveyor is started and stopped. Only the engines (and their thrust) would move it.
Since the plane is so massive relative to its rolling resistance, its inertia will render the friction of the rolling stock a non-entity for this demonstration.
The way I see it, in the opposite direction conveyor belt/treadmill setup, the airplane is using all of its thrust simply to hold its original position relative to the earth and, therefore, the earth's atmosphere. As a result, there will be no airflow over the wings, because the airplane is still essentially static.
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Last edited by ScudRunner; 01-07-2008 at 06:59 PM.
Static and Jethro are wrong - Maniac is (of course) right.
You are thinking of the wheels as providing thrust, like a car with a wing on top.
The plane just uses those wheels to keep itself off the ground - they have nothing to do with the equation. The engine's thrust forces back, and the plane itself goes forward with "an equal and opposite" force.
Better make sure there's nothing at the end of that conveyor belt, because there's a jumbo jet coming!
(Oh, and yes, re-post!)
S
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^Well, if you stick the plane on a conveyor belt and then accelerate it forward to takeoff speed, it absolutely might take off. But, as soon as it broke ground, it would fall to the ground if the engines weren't producing enough thrust to maintain the airflow generated by the conveyor belt's motion.
I mean, that's essentially the theory behind the aircraft carrier catapault.
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Last edited by ScudRunner; 01-07-2008 at 06:59 PM.
Lift is generated by the difference in velocity between the solid object and the fluid( in this case air). There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion.
No Motion,, No Lift.
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^Well, if you stick the plane on a conveyor belt and then accelerate it forward to takeoff speed, it absolutely might take off. But, as soon as it broke ground, it would fall to the ground if the engines weren't producing enough thrust to maintain the airflow generated by the conveyor belt's motion.
I mean, that's essentially the theory behind the aircraft carrier catapault.
No, an aircraft carrier catapult attaches to the plane. If you slid the deck (or the entire ship for that matter) forward or backward with the plane unattached to the boat, the plane would stand still relative to the ocean/wind/sky/whatever.
Quote:
Originally Posted by Easy_E1
No Motion,, No Lift.
Yes, but the conveyor has nothing to do with the motion of the plane because it is on wheels.
Take a large book and balance a ball on it. Now move the book horizontally, perfectly parallel to the ground.
What does the ball do? It attempts to stay in place.
This is exactly what would happen if you moved a conveyor underneath of a plane in the orientation of the wheels, regardless of direction or speed of the plane or anything else.
Do the same thing with a bowling ball and you will see even less reciprocal motion because the friction of the interface relative to the mass of the ball is even less.
Now, imagine 4000 tons of plane resting on 10, well balanced wheels?
^Right, but in this case the plane is attached to the conveyor belt/aircraft carrier by the wheels. You can see this phenomenon in a helicopter sitting on a deck of a boat moving forward at 15-20 knots, where the rotor system will push through what is known as ETL because the relative "wind" generated by the motion of the boat is creating airflow over the rotor system. Granted, we had the parking brake on, and thus the wheels weren't rolling and were holding us to the deck. So, I guess you could set the brakes on the big airplane, then accelerate it on the conveyor belt and it would work. But, that goes back to what you said earlier about needing a big *** conveyor belt.
Either way, it won't work with a conveyor belt operating in the opposite direction of intended flight.
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Last edited by ScudRunner; 01-07-2008 at 07:19 PM.
If the plane is attached to the conveyor belt and the belt is moving the plane forward, sure it has a chance to lift off. If belt speed is sufficient to replace drag with lift.
Until you run out of conveyor belt.
Basically it's acting as a catapult as your describing.
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The bitterness of poor quality remains long after the sweetness of low price is forgotten.
I guess it depends on your definition of "attached." In the plane with the parking brakes set scenario, the now non-rolling wheels are anchoring the plane to the conveyor belt, right up to the point where the wings produce enough lift for the plane to take off.
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The only pain I've got time for is the pain I'm puttin' on fools who don't know what time it is.
Either way, it won't work with a conveyor belt operating in the opposite direction of intended flight.
NO! The conveyor belt has no effect if it is not attached to the plane.
A heli is on skids. It has friction holding it to the carrier/belt.
A plane on a catapult is not pushed by the wheels, its thrown by a sabot of sorts that is attached to the non-rolling portion of the gear apparatus.
If you just had a plate under the wheels, the catapult would just shoot out from under the plane and it would still be standing there.
The conveyor can move in either direction at any speed and it will have zero effect on the aircraft at all.
Am I really the only one that sees how obvious this is? This is 7th grade physics.
It would have to have a release mechanism to let it go when it obtained lift. And then it would have to have enough thrust to maintain that lift and speed it is moving.
But if a plane is sitting on a conveyor belt and the belt is moving in the opposite direction that the plane will be traveling , then it's going to sit there and do nothing.
To many variables in the equation.
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