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daniil (775990)

daniil
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Journal of daniil (775990)

An airplane on a runway with a moving surface

[ #144821 ]
Thursday September 28 2006, @02:06PM
Robotics

It might not be Tuesday today, but still, here's an old (yet still good) physics problem for you to take a crack at:

"An airplane (jet or turboprop) is standing on a runway with a moving surface (like a conveyer belt). The surface can move in the opposite direction to the plane, that is, towards it. It has a control system that tracks the speed of the plane and adjusts the speed of the conveyer belt in such a way that the speed at which the wheels of the plane rotate would be equal (but the opposite) to the speed of the conveyer belt. Can the plane take off from this strip?"


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  • No, not unless you can get enough air moving over the wings to create lift.....

    • Correct. If the wind blowing over the wings is strong enough, a plane standing still will takeoff. Hence the need for tiedowns on light aircraft;-)
      • Somewhat Incorrect. A plane's thrust does not come from its wheels, but from an engine that moves air, either using a jet engine, or a prop.

        The only effect the conveyor belt will have on the plane will be whatever friction is inherent in the wheels when the breaks are not applied. If the wheels are sufficiently frictionless, the plane will have no trouble taking off.

        Think about it...If you stand on a largish patch of ice, and attempt to fly a kite, will you move, despite not having applied lateral force t
        • Not quite;-)

          A plane's thrust does indeed come from a prop or jet, but it can also come from anything else that moves it (ie, a catapult a la Navy aviation).

          Flight and thrust are different, however. The four forces acting on any aircraft is Lift, Weight, Thrust, and Drag. Flying occurs when the aircraft's weight is offset by it's Lift. If there is a net positive amount of Lift then the aircraft climbs. A net negative and it descends. If the Weight equals Lift, then the plane is in level flight.

          Lif

          • Lift isn't really part of the problem, except as a function of airspeed. The conveyor affects the wheels only; it's not a wind tunnel, so airspeed is unaffected.

            As long as the engines apply forward thrust, the plane will accelerate forward, regardless of how quickly or slowly the wheels move. And as the plane moves forward, lift will be generated.

            That said, taking off on a moving surface would be a pain in the butt, because wheels aren't entirely frictionless.

  • It depends on how much air is passing over the wings. You need air movement over the wings to generate lift, the forward momentum is what usually does this. If there was a 250 km/h wind hitting the plane head-on while it was on your treadmill: sure.

    • I told you to take a crack at it, not to be so goddamn clever and solve it in, like, five seconds :7

      The correct answer, of course, is that the plane will take off when the post count in the thread discussing this problem reaches 250.
  • No matter how fast the surface of the belt moves, it will have only an incidental effect on the ground speed (the non-moving ground) of the plane (minor friction through the wheels). The plane gets its "traction" from the air (propellor or jet thrust, equal and opposite reaction), not from the ground. All the moving surface will do it to make the wheels rotate faster. If the moving surface induces air flow in the direction of its motion, it will contribute to the plane's ability to take off.

    That's how it

  • The plane will take off. Moreover, if I'm reading this correctly, the control system for the conveyor belt will blow a fuse after making lots of alarm noises, and smoke will come out of the belt, in a wonderfully cartoonish way.

    The plane's forward thrust comes from the jet or turboprop, not the wheels, according to TFA:

    An airplane (jet or turboprop) is standing on a runway

    The moment it starts moving forward, the conveyor will apparently try to move backward, to match the speed of the forward rotation

  • It depends on how long until the treadmill blows a fuse and stops working. 8^) (Or how strong and what direction the wind is blowing.)

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