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  1. Join Date
    Jul 2006
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    3,601
    #61
    Quote Originally Posted by Alpha_One View Post
    The speed which the runway runs, or even if it's going forwards or backwards, has absolutely nothing to do with the wind on the plane's wings!

    Good night sir.
    Actually, it does. Can you walk? How do you walk? You exert force on the ground. What if the ground played a trick on you and instead of being hard and firm, it moved at the exact opposite direction you're walking, instantly, and matched your speed accurately? Let's say you were able to find handrails to guide you, but the ground still matched your speed nonetheless (maintaining the original condition of the question). Will you get anywhere?

    Here, and in Niky's statement, the wind = handrails. Your hands = engines.

  2. Join Date
    Aug 2004
    Posts
    22,710
    #62
    *mbeige: The belt matches the airplane's speed. What Alpha_One is saying is that the movement doesn't cancel the plane's momentum, it just makes the wheels turn faster.

    Seriously, do the treadmill test... it's the only way to visualize it so you can understand...

    Ang pagbalik ng comeback...

  3. Join Date
    Oct 2005
    Posts
    115
    #63
    the lift is not created by the engine. the lift is created by the wind interacting with the wings. the engine just provides the forward motion. the higher the forward motion the greater is the lift. thats why after take off and the plane has enough velocity the flaps are pulled back because the higher velocity can create enough lift already. that's the reason also why planes take off most of the time against the direction of the wind, to save power creating higher wind velocity on the wings.

  4. Join Date
    Jul 2006
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    3,601
    #64
    Niky, I understand your point of view completely (You too Alpha one). And you two have very good arguments, this is a good discussion.

    However, the point remains that whatever the plane's speed becomes, the conveyor belt will match it in the opposite direction. If this happens, the plane will not move, and no lift will occur.
    Last edited by mbeige; November 9th, 2006 at 08:08 PM.

  5. Join Date
    Aug 2004
    Posts
    22,710
    #65
    If you guys won't believe us, maybe you'll listen to actual scientists:

    http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

    You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.

    Ang pagbalik ng comeback...

  6. Join Date
    Oct 2002
    Posts
    2,703
    #66
    iirc ... problems like this are easily solved with a free-body diagram, showing only the forces acting on the plane

  7. Join Date
    Dec 2004
    Posts
    1,310
    #67
    Quote Originally Posted by mbeige View Post
    Actually, it does. Can you walk? How do you walk? You exert force on the ground. What if the ground played a trick on you and instead of being hard and firm, it moved at the exact opposite direction you're walking, instantly, and matched your speed accurately? Let's say you were able to find handrails to guide you, but the ground still matched your speed nonetheless (maintaining the original condition of the question). Will you get anywhere?

    Here, and in Niky's statement, the wind = handrails. Your hands = engines.
    If the ground moved in the exact opposite direction and tried to match my speed, I'd have to pace twice as fast to get the same speed as before. The ground isn't going to move if I don't. The ground moving effectively halves my my speed, but it CANNOT cancel it out because I must be moving for it to be moving. It doesn't stop me from doing whatever speed I can do before providing I can pace twice as fast.

    Hell, from a completely logical perspective, this problem is either trivial or paradoxic. In this case it's the former.

    You're right, in Niky's statement, wind=handrails, hand=engines. If you pull against the handrails, you move forward, no matter what the treadmill does with your legs and feet. So when the engine pulls against the wind, it'll go forward, no matter what the runway tries to do with the wheels.

  8. Join Date
    Aug 2004
    Posts
    22,710
    #68
    Again... the plane's speed generated by the engines is relative to the surrounding air... NOT the ground. The plane generates thrust against the air, not the ground. Otherwise, 747s would gouge great big holes in the runway with their wheels as they take off. (Incidentally... no, they don't).

    Thus, the plane's thrust is directed at the air behind it, which pushes it forward relative to everything else. The speed of the conveyor acts only on the wheels of the plane.

    I made a mistake in my previous post... you would actually have to have the conveyor moving thousands of times faster than the plane to actually cause enough friction for anything, and it still wouldn't make a difference, specifically because of the example above. You could have bloody stumps where your legs used to be from friction with the treadmill, but you can still pull yourself forward... :hihihi:

    EDIT: BWISIT KA OTEP!!! I hate this question!!! Tuwing nakikita ko ito sa internet, may away!!! :hysterical: Bwahahaha!!!
    Last edited by niky; November 9th, 2006 at 08:17 PM.

    Ang pagbalik ng comeback...

  9. Join Date
    Dec 2004
    Posts
    1,310
    #69
    Quote Originally Posted by boybi View Post
    because the handrails are attached to the treadmill. the air is not 'attached' to the ground or the conveyor.
    However, the air is "attached" to the engine! Which pulls on the air, causing the plane to move forward.

  10. Join Date
    Oct 2005
    Posts
    115
    #70
    lift on wings = wind velocity on wings x area of wing x lift coefficient of wing.
    0 velocity of wind on wings = 0 lift, cannot take off.

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