Will a plane take off on a conveyor belt?

Discussion in 'Physics & Math' started by w00t, Jun 12, 2007.

  1. one_raven God is a Chinese Whisper Valued Senior Member

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    13,406
    If it can take off being dragged by the Chinook, it can be take off with the engines.
     
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  3. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    In the case of tyre scrubbing, yes.
    But since the original premise assumed quite a few technical "glitches" - perfect speed matching for one, I assumed that the tyres and belt would be in rotational contact throughout.
    I.e. an "idealised" situation.

    Sort like the old physics joke: "For the purposes of this discussion we'll assume that a cow is spherical..."
     
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  5. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    Nope, because in the case of the Chinook the helicopter is external to the belt/ plane/ wheels system.
     
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  7. 2inquisitive The Devil is in the details Registered Senior Member

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    Oli,
    Correct.
    Correct, again.
    No, this is incorrect. The opening post stated the conveyor belt speed matched the wheel speed, not the plane speed. In effect, the wheel speed would be twice the rotational speed compared with the wheel speed of a plane on a non-moving belt.
    .
    Yes, you are technically correct, I just used the example of pushing against the air to illustrate that the motion and lift of the plane is due to the engine/air interaction + wing/air interaction, not friction from the tires like the treadmill,etc., examples given by some. Also, air speed, and not ground speed, is the important parameter for lift.
     
  8. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    If the wheel speed is matched by the conveyor then the wheel isn't going to go forward relative to the ground and surrounding air. If the wheel doesn't go forward then the plane cannot, since they are a system.

    Y'know, I keep thinking I'm going to drop this thread. Maybe I should have done it before my first post.

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    Another SF endless argument.

    That's not meant in a nasty way (I could even change my mind about the result altogether). But I can't see the solution any other way.
    I'm out for a drink or three with friends tonight. Maybe the Guinness will bring an epiphany - better explanation of my view or a switch (with proof0 the other way.
     
  9. 2inquisitive The Devil is in the details Registered Senior Member

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    3,181
    This is the opening post. It is actually poorly worded as to how 'speed' is actually measured. A small airplane wheel cannot rotate at the 'same' RPM as the much larger conveyor belt, obviously (think small and large gears). The top of the conveyor belt would have a 'speed' relative to the ground, of course. Assume the top of the conveyor belt is moving at 120 mph relative to the ground. What speed would the wheels need to exactly match this speed of 120 mph? Not '0'. The wheels would need a forward speed of 120 mph relative to the ground, the same as the conveyor belt, but in the opposite direction. Understand where I am comming from now?
     
  10. andbna Registered Senior Member

    Messages:
    316
    Completely Wrong.

    The conveyer does not exert any force on the plane
    It exerts angular momentum on the wheels yes, however that cannot move the plane. Perhaps you should look up Torque and angular momentum, as it seems to elude you.

    In A car, the axel gives the wheels angular momentum, their friction on the road causes the road to move backward, but because it is much more massive than the car, it is the car who moves forward (at least as far as anyone is concerned)

    Now, in the belt, the belt gives the wheels angular momentum, but the belt is already moving backward, the resault? The plane experiances no force from the conveyer acting upon it.
    Draw a force diagram, If you can show me how a moving road in a perfect system (thus the wheel is completely free spinning, no friction of it on the axel, and infinit friction between it and the road) Then show me.

    If you looked up the forces involved in torque, you would magically discover that, no matter how much an object rotates, it doest cause any force on the axel in a given direction.

    so I say again: the belt exerts no force on the actual plane
    Therefore, if the plane activates its engines, it will take off.
    The conveyer belt will note the increase in speed of the wheels, and as I said, have a meltdown due to the fact that it not needs to reach infinite speeds, but that wont stop the pilote.

    Now, before you say that the above was wrong, I challenge you, draw a force diagram, and if you can make it disagree then post it.

    -Andrew
     
  11. Oli Heute der Enteteich... Registered Senior Member

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    11,888

    Perfect system? No friction?
    Neither were posited in the OP.
    It said a plane, a conveyor, wheels.
    I assumed real world (apart from belt speed matching and belt load capacity).

    That could be the answer.The OP needs to be more specific (which I think someone already said waaay back :

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  12. leopold Valued Senior Member

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  13. Anti-Flag Pun intended Registered Senior Member

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    3,714
    I believe your equation is flawed for a plane, but correct for a car. A car generates forward motion using friction against the ground by its wheels, and so would be directly affected by the motion of the conveyor. A plane is different, the thrust is generated against the air, presuming frictionless wheels they will rotate relative to the conveyor but independently of the planes thrust. Without the planes thrust the converyor moves - the planes free-rolling wheels spin round, where are they acting on the planes movement? Without friction the plane will not move backwards at all no matter what its thrust is doing.
    So now add the thrust.
    Edit - Incase I wasn't clear enough I should have added that the thrust of the plane against the conveyor is not the direct comparison it is the friction against the conveyor that is the opposing force, not the thrust.
     
  14. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    Umm, post #99 addressed it somewhat. And then post #108 clarified my assumptions.
    If the bearings ARE frictionless then it's a different ballgame.
    In 99 I said I wasn't considering frictionless bearings.
     
  15. leopold Valued Senior Member

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    17,455
    yes, different indeed. so much so that the plane will indeed take off. agreed?
    so, how much friction will be needed to keep the plane on the ground assuming a 200 horsepower engine? what will that friction do?

    aside,
    i would imagine the OP was/ is a hypothetical scenario.
     
  16. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    Just to be bloody-minded I'm still thinking about that. Although the inertia of the plane itself would certainly suggest so.

    Oh that's a fun one. Especially since HP has little to do with the plane's actual speed or acceleration - I'd need the plane's mass, prop rotational speed and diameter among other things...

    Of such things are discussions born...
    All these pages.

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  17. nietzschefan Thread Killer Valued Senior Member

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    holy crap you guys burnt midnight oil on this?? lmao

    Hey look, save a few pages of argueing and decide if the conveyor is frictionless(ice rink - all planes would take off like with skis on water/snow), or the conveyor belt grips the wheels perfectly. If the belt grips perfectly, you will never "beat" the weight of the A/C using lift generated from it's airfoils, even LMAO even using propwash(gimme a bloody break).

    Even in a REAL scenario a LOT of a/c would have trouble taking off with any kind of conveyor belt going against them.
     
  18. Oli Heute der Enteteich... Registered Senior Member

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    11,888

    Just call me Mr. Sad.

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  19. leopold Valued Senior Member

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    oli,
    i would love to meet you in a bar and see how much money i could weasel from you before you realized you been had.

    google the phrase "plane on a conveyor belt" and get back with me.
     
  20. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    I mentioned about 100000 million posts back that I had: and that there is no consensus on the net ANYWHERE.
    The argument is still raging on other forae.

    Maybe this is not a good time to mention the "stop a train with a ball bearing" query from my engineering student days....

    PS Leo, if I met you in a bar and I had enough money with me the drinks would be on me anyway. That's how I am.

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  21. Thoreau Valued Senior Member

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    Well, I was gonna post and say that the plane would not be able to take off... that was until i did the experiment with a treadmill and a RC plane. I was actually very shocked to see that the plane moved forward at the same velocity and speed than if the treadmill was turned off. Im quite amazed.
     
  22. leopold Valued Senior Member

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    17,455
    what say you now oli?
    let me guess, something about the above setup isn't like the OP, right?
     
  23. Oli Heute der Enteteich... Registered Senior Member

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    11,888
    Correct.

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    I'd like to see the numbers - always trust numbers.
     

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