Question...

Discussion in 'Off Topic' started by bigguy, May 2, 2007.

  1. bigguy

    bigguy Retired

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    If you put a jet plane on a conveyor belt that was going in the oposite direction as the plane was and matched the speed of the jet exactly when the jet sped up...Would it take off. ??? :dunno:
     
  2. t2t2t

    t2t2t New Member

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    It would, because engines will get time to go on full power and when jet starts rising its nose, the power that is made by engines will be used straightaway to go up.

    Took me a toilet break to come up with above.
     
  3. Livewire

    Livewire Abuse Compliance Officer Staff Member

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    My thinking would be no, because air needs to me moving over the wings to cause enough lift to actually make the aircraft fly.

    Sure it's buzzing down the conveyor belt at 150mph and up, but the conveyor belt is keeping it stationary, so no air is moving over the wings at all.

    You could pull back on the flight stick or whatever its called to try and lift the nose, but with no air moving all that'll happen is the ailerons will change positions and make noise.



    Tested with an RC airplane some time last year, cept I had it on a treadmill instead of a conveyor belt. When a good gust of wind came along it got airborne, but until air was moving over the wings all it did was empty the fuel tank.


    @t2t2t: Good theory, but the problem is getting the nose to rise. I think you're probably right though; if the nose could get lifted, it'd probably take off, but how is the nose going to get airborne if the ailerons or elevator (back side of the horizontal part on the tail of a plane) don't have air moving over them? You could push it, but that'd be cheating ;)
     
  4. swirly

    swirly Active Member

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    Yea, the way a plane or bird or whatever flys is air flowing over the wings to give it lift. we did an experiment once to simulate it.
     
  5. Chris Z

    Chris Z Active Member

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    I think it would stay in the exact same place. :p
     
  6. Woolie

    Woolie Member

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    A thing to remember is, that a plane on the runway does not gain speed by powering the wheels, it gains it by the engines forcing air through. If a plane was on a conveyor belt that matched the planes speed, as soon as the plane starts positively accelerating, the conveyor belt would have no effect on the plane as the wheels are simply free turning.

    There are many variations of this. If the conveyor belt exerted more "friction" like force on the plane than the engines were producing thrust, the plane would continue to move backwards with the belt, until the force of the engines overbalanced the force of the conveyor belt, the planes -ve speed would then reduce and would increase in a positive direction.

    Another thing to note is that a plane will only be able to take off when at a certain air speed. For example, lets say the plane can gain enough lift to take off when it reaches a speed of 240 knots. If the plane was in a headwind (wind coming towards the front) of 100 knots, then the plane would only need to accelerate to a ground speed of 140 knots before it could lift off. Likewise, if there was a tail wind of 60 knots, then the plane would have to accelerate to a ground speed of 300 knots in order to reach the desired air speed.

    Just to clear up a few other things, when a plane is climbing, the engines are providing more of the lift than the wings as the aerofoil shape is inefficient at that angle of attack. The engine is providing the power to climb.

    To get a planes nose to rise, there needs to be enough air flow of the tail of the plan, as this is where the elevators are located. To bank a plane, the tabs (ailerons) on the end trailing edge of the wing move accordingly and disrupt the flow of air. When a propeller plane is flying with its nose high, and with its throttle fully open it will be flying slowly, however it can raise and lower the nose fast as there is air flowing over the tail caused by the propeller. Its banking is sluggish, because there is little air flowing over the ends of the wings.


    Sorry, I haven't checked what I've written, but if there's something you want to know, let me know.
     
  7. Joker Boy

    Joker Boy New Member

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    lol of course it's the engines... all the wheels do is allow the plane to maneuver on land while moving at insanely slow speeds or even stopped. (it couldn't maneuver on the ground without wheels, right?). But the engines still push the plane forward so the wheels still turn the same speed as if they were powering the movement. It doesn't matter where the power is coming from. There could be an RC Micro Car pushing the plane at 240 knots and the wheels would still stay spinning the same as if the engines were powering it.

    So that should not be a factor in taking off... (where the power comes from)

    EDIT:

    Sorry, I read that wrong. You're right about that, but the plane is so heavy that the sheer pressure of the several hundred ton plane (passengers?) on the wheels would cause them to resist to rolling friction, and pushing it on the conveyor belt would only slow it down.

    If you wanted to simulate this effect, you could put a gigantic fan in front of the plane and pump it up to 240 knots, but that would cancel out the variable of wind/air molecules flowing over the wing.
     
    Last edited: May 2, 2007
  8. bigguy

    bigguy Retired

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    Remember that whatever speed the plane gets to the conveyor belt matches it.
     
    Last edited: May 2, 2007
  9. Woolie

    Woolie Member

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    Oh my, we could do with a physicist for this one.
     
  10. mikel2k3

    mikel2k3 New Member

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    surely the speed of the the conveyer belt would simply cancel the speed of the plane out...

    If two trains are heading towards each other at 100Mph each, they would collide at a force of 200Mph...

    Soo if they are bother travelling in the same direction at 100Mph, they wouldnt get closer to each other, or further away.

    So id say the plane wouldnt move or take off at all :)
     
  11. Joker Boy

    Joker Boy New Member

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    True, but the base of the question is actually: What determines the takeoff of a plane? The force of the engines, or the speed of the plane?

    In other words, could a plane take off with just engines forcing it upward, or would it have to be moving?

    -----------------------------------------------------

    Another point is that people refer to planes with takeoff speeds. "747 takes off at around 240 -320 knots, etc." Is this because it has to be going that fast or would it take that long before the engines have enough power to lift the plane out of the air?
     

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