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Airplane: Does it take off?

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  • Airplane: Does it take off?

    Here's a lively debate I grabbed from another forum. There is no consensus there either, so don't try and cheat.

    The question is:

    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 opposite direction).

    Does the plane ever take off, and why or why not?
    How'd I get so white and nerdy?

  • #2
    Re: Airplane: Does it take off?

    Um. Plane lift depends on the movement of air, not the speed of the wheels on the ground (or belt). If the plane is effectively sitting in one spot, it's not going to take off.

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    • #3
      Re: Airplane: Does it take off?

      Pracitically not a safe or effective way to launch an airplane.

      But I suspect as long as the airplane can increase it's speed such that it can takeoff, it will takeoff, however the question is will that takeoff speed will be the same as the usual takeoff speed when using a static runway, or will be it twice as much or half as much?

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      • #4
        Re: Airplane: Does it take off?

        Originally posted by pzarquon
        Um. Plane lift depends on the movement of air, not the speed of the wheels on the ground (or belt). If the plane is effectively sitting in one spot, it's not going to take off.
        I agree that with no wind there will not be lift off, there will be nothing "pushing" against the airlerons and stuff to enable it to lift off. I guess it's like running on a treadmill, you don't feel any "wind".

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        • #5
          Re: Airplane: Does it take off?

          Originally posted by pzarquon
          Um. Plane lift depends on the movement of air, not the speed of the wheels on the ground (or belt). If the plane is effectively sitting in one spot, it's not going to take off.
          What he said^. That plane aint taking off no matter how fast that conveyor belt is going.

          Reminded me of something that comedian Steven Wright pondered: "A car is travelling at the speed of light. The driver turns on the headlights. Does anything happen?

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          • #6
            Re: Airplane: Does it take off?

            So this is a question of ground speed vs. air speed. Of course there are many variables of physics to consider (wind speed, direction, weight of aircraft, etc), but this is one argument worth considering it WILL TAKE OFF...

            The operative part of this otherwise self-contradicting question is that the "plane MOVES in one direction". So it's already determined that the plane has movement or better said, propulsion.

            No matter what counter-directional speed this "conveyor belt runway" is attempting to hamper a take-off, the thrust from the engines has already given the aircraft enough forward propulsion to begin its attempt to increase air speed for take-off, regardless of ground speed.

            The landing gear's wheels may be spinning twice as fast due to the counter-moving runway, but has been pointed out, the speed of the wheels aren't a factor. An aircraft's ability to take off lies strictly in the amount of airflow it's able to move over and under the wing surface. If the aircraft's propellor or jet engine(s) thrust can provide enough air speed required (approx. 55 - 150 knots, depending on aircraft) it WILL TAKE-OFF, regardless of ground speed.

            A factor that might be of consideration are the working condition of the landing gear's wheel bearings. In this scenario, the only thing that's hampering a take-off are the counter-rotation of the landing wheels on the conveyor belt runway. If there's any type of friction in the wheel bearings, the thrust from the aircraft's engine required to generate enough air speed to take off will be that much greater.

            Apply wheel brakes, and you've got yourself a backward moving airplane headed for an imminent catastrophe.
            sigpic The Tasty Island

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            • #7
              Re: Airplane: Does it take off?

              Wow!

              If the conveyor belt is going the opposite way at the same speed as the plane trying to take off... the ground speed is zero! It's as if the plane was at a standstill. Unless it's a Harrier Jet or there is a headwind that is strong enough to create lift that plane isn't going nowhere.

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              • #8
                Re: Airplane: Does it take off?

                Originally posted by Pomai
                If the aircraft's propellor or jet engine(s) thrust can provide enough air speed required (approx. 55 - 150 knots, depending on aircraft) it WILL TAKE-OFF, regardless of ground speed.
                What? Under the scenario, the plane will be stationary, no matter how fast those engines turn. I'm not sure how a plane with a "ground speed" of zero could have a different "air speed," particularly if it never leaves the ground.

                I suppose if the thrust were directed downward moreso than aft, the plane might get off the ground in some fantasyland, but certainly not due the conventional ways airplanes fly.

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                • #9
                  Re: Airplane: Does it take off?

                  Here's one way to explain my theory: Make pretend you're (you are the airplane) running on a treadmill with a rope tied (air pressure) to a wall in front of it. If you pulled on that rope with your arms (arms being the propeller/jet engine), technically and with considerable skill you could pull yourself forward, regardless of how the treadmill (conveyor belt runway) affected your feet in its feeble attempt to run in place or get pulled backward. The treadmill machine itself has ZERO ground speed, yet the counter-rotating belt could reflect the same speed as whatever it is you attempt at pulling yourself forward. But it doesn't matter. You still have that rope to grab onto (ability to take off).

                  What's most important is the the ability of your arms energy (thrust) to negate the force of the counter-rotating speed of the tread belt (conveyor belt runway) by means of creating forward propulsion (air speed) through pulling on that rope (converting air pressure into propulsion). Also, the less able your legs are to keep up with that energy, the less likely you will succeed. Hence my consideraton of landing wheel bearings.

                  Now if you're talking Harrier Jump Jet (VSTOL), that would be the equivalent of simply jumping off the treadmill. "Jump Jet". Which needs downward thrust.. as in "legs", not "arms".
                  Last edited by Pomai; December 28, 2005, 11:57 PM.
                  sigpic The Tasty Island

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                  • #10
                    Re: Airplane: Does it take off?

                    But your adding a variable that's not part of the scenario: the ability that the plane can go faster than the belt. The original problem states that belt is going as fast as the plane. Like someone noted above its like running on a treadmill... if it can be programmed to speed up when you run faster, you are still in the same spot; or zero ground speed.
                    Last edited by cezanne; December 29, 2005, 12:42 AM.

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                    • #11
                      Re: Airplane: Does it take off?

                      One thing everybody has eluded to is the fact that the airplane's mass on a conveyor belt has not changed due to motion (or lack of it). However forward motion can happen quite suddenly if the conveyor belt stops abruptly catapulting the airplane forward and also killing all aboard from the sudden positive g's placed on their bodies.

                      That is THE ONLY WAY a conveyor belt static runway could launch a plane.

                      If the point of this theory is to reduce the length of a runway to the footprint of the plane then the only way is to create a strong enough headwind against the front of the plane's wings to allow necessary lift.

                      Unlike the Harrier Jump Jet which forces engine exhaust downward to push the plane upward, if you could create wind speeds proportional to the rotational speeds for liftoff in front of a plane, the plane would seemingly float. But as soon as the wheels leave the ground the onboard engines have to fire up to push the plane into the headwind with controlled thrust otherwise the plane would be blown backwards.

                      Basically you need continuous hurricane force headwinds directed at the nose of the plane to allow for lift off.

                      On a conveyor belt there is no frontal headwinds generated because the mass of the plane hasn't changed relative to the mass of the air surrounding it. Thus a conveyor belt runway cannot work. Sorry laws of physics deny this theory from working.
                      Life is what you make of it...so please read the instructions carefully.

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                      • #12
                        Re: Airplane: Does it take off?

                        Well, another local forum will try to reenact the question with a RC airplane and a treadmill. The problem is, that no one has a RC plane.

                        And this would be a good science project.
                        How'd I get so white and nerdy?

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                        • #13
                          Re: Airplane: Does it take off?

                          I think finding a R/C plane is the least of their worries. I don't think there are any treadmills out there that can match speed in a timely manner. Also if the plane is electrical powered over time it's thrust is going to decrease not increase if it was gas powered (and adjustable throtle).

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                          • #14
                            Re: Airplane: Does it take off?

                            Maybe you should submit the problem to Myth Busters on Discovery Channel and save yourselves a lot of time.

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                            • #15
                              Re: Airplane: Does it take off?

                              Originally posted by doc1456
                              The problem is, that no one has a RC plane.


                              yer right; i did hear; RC planes dissappeared completely off da face of the earth!

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