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OT: Will the plane take off?

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  • OT: Will the plane take off?

    This made for some interesting debate on another forum I'm on, hopefully we can get some good discussion going on here.

    A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

    The question is:

    Will the plane (747 passenger jet) take off or not?
    You never learn anything by doing it right.

  • #2
    Forget the wheels and think about teflon coated skis on a teflon belt.

    Teflon has a lower coefficient of friction than wet ice. Teflon skis on a teflon belt would have about the same friction as wheels. The coefficient of friction for teflon on teflon is 0.04, the lowest known for a solid substance. The coefficient of friction for rubber tires rolling on concrete is about 0.035 (side note: The friction coefficient for tapered roller wheel bearings is 0.0018, rubber sliding on pavement is .75)

    Planes take off with skis all the time, at least around here this time of year.

    By eliminating the wheels from the idea we eliminate a source of confusion. The skis have the same friction characteristics as the wheels but obviously cannot be used to propel the aircraft (also no brakes).

    With skis on the plane the teflon belt could be turned on in the backward direction with the plane engine off and the plane would just sit there at first. The inertia of the plane would prevent it from accelerating backward with the belt immediately. Only a very small force would be required in the forward direction to hold it in place, even with the belt at full speed backward.

    This force could be provided by the aircraft engine at idle which is enough to cause a plane to taxi.

    With the belt running at full speed under the skis and the engine idling the plane remains at rest on the moving conveyor since the small amount of thrust from the idling engine is enough to overcome the very small friction of the skis on the belt. In effect, the plane is still parked as if the belt were motionless. We have no bearings to consider and no wheel rpms to think about, just the skis slip-sliding on the moving belt.

    So, here we have a plane sitting on a moving belt. The plane remains in the same place even if the belt is moving because there isn't enough friction between the teflon skis and the teflon belt to drag it backward against the thrust of the idling engine.

    Now we apply full power to the airplane engine.

    What happens?
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    • #3
      Get enough air over the wing and the plane will lift, not enough and it will not. Simple aerodynamics JRouche
      My old yahoo group. Bridgeport Mill Group


      • #4
        Heh, you guys are good. Better than the ones on the last forum. I was all like no, of course not, then someone said yeah it will, and then I was like duh, I'm an idiot.
        You never learn anything by doing it right.


        • #5



          • #6
            The way I sees it------

            IF------- 100% of the jet engines' energy were mechanically driving the aircraft wheels against the conveyor (Zero exhaust forces to rear of craft) the whole mess would sit still, providing the thrust of the driven wheels against the conveyor always equalled the thrust of energy provided by the conveyors' power supply.

            BUT--------the jet engines thrust is against the surrounding air and not the conveyor, so the plane will proceed to lift off.

            HOWEVER-----With a robust conveyor system which will keep a speed equal to the plane, them wheels on the plane are probably gonna explode somewhere before liftoff.



            • #7
              Airspeed not ground speed enables a plane to fly,if I read the post correctly the plane would be standing still.



              • #8
                You haven't defined the problem adequately. "The plane is sitting on a runway", then suddenly "the plane moves in one direction" - by what agency?

                If it's being moved by its engines, what the runway does is of little consequence. Lift is a function of, among other things, air speed, not ground speed. The plane will take off normally. There are boundary layer effects but in this case they're negligible.

                So where's the "interesting" part?


                • #9
                  Also, the jet engines do not "act on the surrounding air", they pump it through the engine producing thrust.
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                  • #10
                    The only consequence of this configuration is that the wheels will be spinning at twice the normal speed when the plane lifts off.

                    If it were a train it would never leave the station but planes are not driven-wheel machines.


                    • #11
                      If I read the original question correctly the plane starts moving because of thrust from the engines. The conveyer belt runway starts moving in the OPPOSITE direction matching the speed of the plane so the plane actually is not moving forward thus no lift. Kind of like running on a treadmill. You run forward on a belt moving in the opposite direction. You are running in place.


                      • #12
                        My first thought was yes, of course it will take off, and quicker BUT ...

                        It could also become sort of an engine test stand because while the engines are pulling forward, due to zero body velocity the wings may never get enough volume and velocity of "incoming" air.



                        • #13
                          I am not sure you worded the question correctly. I assume that you realize that there is a difference between groundspeed and airspeed? Airspeed is what is critical for takeoff. You can have a groundspeed of 100 knots and a tailwind of 50 knots and a net airspeed of 50 knots. If a plane needs an airspeed of 135 knots to get airborne, then the preceding example requires the groundspeed to increase to 185 knots. That is why aircraft take off into the wind.

                          Regardless, the aircraft is a jet and the propulsion system is not through the aircraft’s wheels, but the hot expansion of gases that produce thrust for the engines. The thrust will still propel the aircraft forward regardless of what speed the wheels are spinning. In the real world I would doubt that the friction induced by the wheel bearings would be enough to prevent takeoff. This assumes that the question you cited claims that the conveyer will match the groundspeed of the aircraft relative to the Earth and not the conveyer belt. Takeoff speed for a normal 747 is about 140 knots, so the wheels would be rotating at about 280 knots at takeoff, which is not outside the realm of reality for that aircraft.

                          Incidentally, the aircraft will leave the ground at about the same point it would down a normal runway minus any effects from wheel bearing friction.


                          • #14
                            Pat Miles has stated it correctly. The aircraft develops lift by air flowing over the wings. In this scenario there is no airflow, the aircraft is motionless as it sits on the "treadmill". If you had a stepladder. you could climb right up on the wing as it is sitting there (just don't get near the engine intake). What is taking place at the landing gear level is irrelevant, only the airflow over the wing produces lift - not the rotation of the wheels.


                            • #15
                              What if there were no hypothetical questions?