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Thread: The best internet argument picture ever..

  1. #41
    Slowly improving Brenton's Avatar
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    If a plane (747) was to come in and land on the said "giant conveyer belt" that was travelling 1000kph in the opposite direction to the speed of the plane.

    1. Would the planes wheels spin REALLY REALLY fast after touching the ground?
    2. Would the plane suddenly (the moment it hits the ground) start moving backwards?
    3. Would it still need to apply airbrakes (or wheel brakes) to actually slow to a stop even though the runway is a fast moving conveyer in the opposite direction?

    Answers yes, no, yes.

    The forward speed of the powered plane is independent to the speed of the ground it is on. (once air drag and wheel friction are overcome)

    For a plane, the moment forward thrust is greater than air drag and wheel friction... the plane is going to move forward. An plane may hold in a position of equilibrium on a conveyer runway.. but when max thrust is applied (or even just > that the frictions).... the force is going to quickly overcome drag and wheel friction and the plane is going to accelerate forward (and take off).... all be it with REALLY fast spinning wheels
    Last edited by Brenton; 2nd December 2016 at 12:39 AM.

  2. #42
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    Maybe someone at home could test this out at home with a couple of upside down beltsanders some roller skates and a commonly found jet pack.

  3. #43
    digitus impudicus
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    Quote Originally Posted by Brenton View Post
    If a plane (747) was to come in and land on the said "giant conveyer belt" that was travelling 1000kph in the opposite direction to the speed of the plane.

    1. Would the planes wheels spin REALLY REALLY fast after touching the ground?
    2. Would the plane suddenly (the moment it hits the ground) start moving backwards?
    3. Would it still need to apply airbrakes (or wheel brakes) to actually slow to a stop even though the runway is a fast moving conveyer in the opposite direction?

    Answers yes, no, yes.

    The forward speed of the powered plane is independent to the speed of the ground it is on. (once air drag and wheel friction are overcome)

    For a plane, the moment forward thrust is greater than air drag and wheel friction... the plane is going to move forward. An plane may hold in a position of equilibrium on a conveyer runway.. but when max thrust is applied (or even just > that the frictions).... the force is going to quickly overcome drag and wheel friction and the plane is going to accelerate forward (and take off).... all be it with REALLY fast spinning wheels
    1. The plane's wheels would flat spot, wear through and then explode within an extremely short period of time. They take time to spin up. Any idea why they have four wheels on a little drooping bogie?
    I've had to go out and fix wheel nacelles because noob pilots landed with the brakes on. 1000kph of conveyor speed plus 240-260 kph landing speed... Pretty much instant death to aircraft wheels.
    2. No.
    2. No. Airbrakes, spoilers and reverse thrust are generally enough to slow an aircraft without using brakes. Brakes cost money and are used sparingly. Landing on a 1000kph moving runway would very likely rip the undercarriage off after the wheels detonated on touchdown. That stuff HAS happened before. Aircraft are designed within certain load criteria, after all. .

    The 1:1 speed matching mechanism of the conveyor is being totally overlooked by everyone and it's not like I haven't pointed it out continuously.

    None of you guys seem to worry about bearing friction or inertia.

    This question will have started fistfights at some places in the world.
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  4. #44
    1st Class Matrix Makeover's Avatar
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    Quote Originally Posted by KevR View Post
    Maybe someone at home could test this out at home with a couple of upside down beltsanders some roller skates and a commonly found jet pack.
    Maybe a treadmill, skateboard and desk fan?
    "I used to read, but it's faster to make up stuff" - Wally (Dilbert)

  5. #45
    Learning the Ropes
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    Quote Originally Posted by Andy! View Post
    This will turn up on facebook until the end of time now. If you want to see arguments on what is likely to happen, just post this to your facebook feed.

    But, here's a hint: ignore that it's a jet or a plane of any kind.
    Ignore that the jet pushes against the air.
    Ignore than the wheels aren't pushing the conveyor.

    Just think about how the conveyor system will work if it has to match the speed of the wheels. The speed and power of the conveyor system and that it theoretically acts instantaneously should give you something to think about.

    Attachment 19965
    The root cause of these arguments is poor definition. What is meant by "velocity of the wheel". Normally I would take that to mean velocity of the centre of gravity, which would be the velocity of the axle. But it could mean velocity of the top surface of the wheel, velocity of the bottom surface of the wheel. None of these cases pose a particular theoretical problem.

    Let Vp = velocity of the plane. Lets also assume this direction is "positive" velocity.

    Let Vc = velocity of the plane.

    Let Vw = velocity of the wheel, and Vl = velocity of the lower surface of the wheel, Vu = the velocity of the upper surface of the wheel and Va = velocity of the centre (axle) of the wheel.

    Case 1: Vw = Vl.
    Then Vl = Vc (since it is in contact) and Vw = Vc.
    But it is given that Vw = -Vc, Vc = -Vc and therefore Vc=0.
    This is just a normal take off from a stationary surface.

    Case 2: Vw = Va.
    Then Vw = Vp (assuming the axle is attached to the plane)
    and is is given that Vw = -Vc so Vp = -Vc. The conveyor belt speed is equal and opposite to the plane speed. At take off the wheels angular velocity will be twice normal but otherwise no problem.

    Case 3: Vw = Vu.
    Vw = Vu = Vp + (Vp -Vc).
    and it is given that Vc = -Vw so
    Vc = -2Vp + Vc
    And therefore Vp = 0.

    So case 3 is the only one in which the plane doesn't take off. There is no need to imagine infinities or control systems or any such thing. This interpretation of the problem has only one physical solution. This is also not what I would normally take "the speed of the wheel" to mean though.

    You could imagine what a system designed to do the impossible would do when the planes engines applied thrust, but that is not productive. Hypothetical impossible systems can do anything. Given an impossible specification a control system which does nothing at all (ie a stationary conveyor belt AKA a runway) would perform as well (or arguably better) than one which blows up.

  6. #46
    Learning the Ropes
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    Quote Originally Posted by iandall View Post
    Case 3: Vw = Vu.
    Vw = Vu = Vp + (Vp -Vc).
    and it is given that Vc = -Vw so
    Vc = -2Vp + Vc
    And therefore Vp = 0.
    Just to add a little note, if one insists that Vw = -Vc, for non-zero plane speed, then with Vp = 1/2, you can "show" 1=0. This demonstrates that the plane taking off is not just a physical impossibility (like frictionless bearings, speeds > speed of light, or infinite power), there is a logical contradiction inherent in the question. In classical logic, anything follows from a contradiction, so "plane takes off" and "plane doesn't take off" are both "correct" deductions (given the premises)!

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