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Thread: The best internet argument picture ever..

  1. #21
    digitus impudicus
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    It exactly matches the speed of the wheels. You may assume that it does this instantly and at all times. This is the key to the entire thing.
    Most people don't get to that point because they are looking at the red herrings to do with unpowered wheels and airspeed. The mythbusters don't account for it, so that's why they're able to take off.

    To have the instant speed matching operating at all times would take almost infinite amounts of power. Bad things happen when you have infinite power and instant feedback loops that are disturbed from equilibrium.

    You end up with an infinite loop of accelerating the aircraft tyres with infinite power at any time the aircraft tries to accelerate by adding power to the system.
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  2. #22
    Grand Master Bowman scott p's Avatar
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    Quote Originally Posted by Marcus View Post
    Airflow would not be generated because the thrust be behind the airframe. The air over the wings won't change because the belt is stopping the wings moving forward.
    I say it stays stationary.
    but the belt is turning the wheels, not moving the airframe. since the wheels can rotate and thus absorb any movement of the conveyor relative to the airframe without the airframe itself moving in space, the thrust generated by the engines is independent of the movement of the conveyor.

    so ultimately the question is impossible to answer without also defining a bunch of other physical properties of the experiment.

  3. #23
    3rd Class whoknows's Avatar
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    Attachment 19965[/quote]

    Hmm to make this Archery related...............How many points is hitting the plane worth?

  4. #24
    *****istrator Marcus's Avatar
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    Quote Originally Posted by scott p View Post
    but the belt is turning the wheels, not moving the airframe. since the wheels can rotate and thus absorb any movement of the conveyor relative to the airframe without the airframe itself moving in space, the thrust generated by the engines is independent of the movement of the conveyor.

    so ultimately the question is impossible to answer without also defining a bunch of other physical properties of the experiment.
    When they test a jet engine they bolt it to the ground, otherwise it'll fly off all around the room.
    The conveyer is just replacing the bolt down with something that prevents the wheels taking traction on the ground, and thus, it can't move. The relative forces offset each other, and without the airflow over the wings it can't get lift.

    Its also why when you run on a conveyer belt your hair doesn't flow backwards, but will bob up and down as the air can move vertically with your step.
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  5. #25
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    You are assuming a significant amount of friction can be created by the moving conveyor and wheels. While the aircraft engines do need to over come the wheel friction, it is minor compared to the force needed to overcome the inertia mass of the aircraft and accelerate it to get the necessary air flow over the wings to create lift.

    Place the aircraft on a suitable levitating magnetic field so it is floating with effectively no wheel friction. It will still take time for the force of the engines to overcome the inertial mass of the aircraft to get it moving and accelerate.

    Assuming that the conveyor belt can generate enough instantaneous friction to counter the force of the aircraft engines then it would probably also create an airflow in the opposite direction under the aircraft wings. This would create a low pressure area under the wings and also keep the aircraft from lifting off.
    Archery's a lot like Life .... a few see it as an opportunity to score cheap points; while the rest are focused on the ends that count.
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  6. #26
    So last century Sandy Hancock's Avatar
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    Assuming that the plane's jet engine is producing constant, maximal thrust in an attempt to take off, and the conveyor belt is negating the production of any movement-induced drag (apart from rolling resistance from the plane's wheels), don't we have a rough equivalent of a rocket in space? That would result in unfettered acceleration and some fairly mind-boggling conveyor belt speed.

    I envisage a fire from the friction, belts snapping and a loud explosion at some point.
    Last edited by Sandy Hancock; 30th November 2016 at 03:31 PM.
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  8. #27
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    Ok, I have my own thoughts on this, but what would happen if the conveyor was going in the opposite direction?

    Would it go twice as fast and if so, why wouldn't they use these "reverse conveyors" for short runways like on aircraft carriers?

  9. #28
    Grand Master Bowman scott p's Avatar
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    Quote Originally Posted by Marcus View Post
    When they test a jet engine they bolt it to the ground, otherwise it'll fly off all around the room.
    The conveyer is just replacing the bolt down with something that prevents the wheels taking traction on the ground, and thus, it can't move. The relative forces offset each other, and without the airflow over the wings it can't get lift.
    that's the point. the aircraft is not fixed to the conveyor. as long as the engine thrust is greater than the friction drag of the wheels on the axles, the net result will be forward movement.

    or we could assume frictionless bearings, in which case the conveyor has even less ability move the inertia of the airframe, because the motion simply translates into wheel rotation.

    Quote Originally Posted by Sandy Hancock View Post
    Assuming that the plane's jet engine is producing constant, maximal thrust in an attempt to take off, and the conveyor belt is negating the production of any movement-induced drag (apart from rolling resistance from the plane's wheels), don't we have a rough equivalent of a rocket in space? That would result in unfettered acceleration and some fairly mind-boggling conveyor belt speed.

    I envisage a fire from the friction, belts snapping and a loud explosion at some point.
    that's pretty much what I arrived at, because while the airframe is moving forward relative to the conveyor, the conveyor is going to be attempting to constantly match a wheel which is accelerating relative to it.

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  11. #29
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    Guys, the way the argument is worded the plane cannot take off.
    Maybe this sketch will help you in understanding why.

    Name:  Wheel.jpg
Views: 181
Size:  14.0 KB

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  13. #30
    Sagittarius senescentis markjam51's Avatar
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    http://www.animations.physics.unsw.e...e-conveyor.htm

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  14. #31
    Learning the Ropes
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    Andrew, where does all the energy from the thrust go?

  15. #32
    Beginner ThomVis's Avatar
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    I look at the wording.
    747 sitting on belt. Meaning wheels don't turn.
    Belt matches speed of wheels. Belt speed zero.
    All other things being assumptions (direction of belt, engines on, can you build a belt like that), I think plane is not going to take off, only on internet itself.

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  17. #33
    Master Bowman Bottom Dweller's Avatar
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    Quote Originally Posted by taz00 View Post
    Guys, the way the argument is worded the plane cannot take off.
    Maybe this sketch will help you in understanding why.

    Name:  Wheel.jpg
Views: 181
Size:  14.0 KB
    It is all relative to where you are measuring the wheel speed.

    If you measure it in the centre of the wheel hub you get a different answer.

    Now measure the speed at the bottom of the wheel where it is in contact with the conveyor belt. At this point the wheel speed will always exact match the conveyor speed because relative to the conveyor belt the wheel is always stationary at this point. So no matter how fast the conveyor belt is moving relative to the ground, relative to the bottom of the wheel it is stationary. So the plane will take off.
    Last edited by Bottom Dweller; 1st December 2016 at 06:36 AM.
    Archery's a lot like Life .... a few see it as an opportunity to score cheap points; while the rest are focused on the ends that count.
    Genesis 21:20 And God was with the boy as he grew up in the wilderness. He became a skillful archer.

  18. #34
    digitus impudicus
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    Quote Originally Posted by xracer View Post
    Andrew, where does all the energy from the thrust go?
    It goes into the wheel, where it's checked into equilibrium by the speed of the conveyor. The wheel spins up until it explodes within split seconds. It's an instantly incremental feedback loop. The thrust provides the acceleration offset which the conveyor tries to accelerate to, but because of the thrust, it can never reach equilibrium exactly.
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  19. #35
    1st Class stickman's Avatar
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    That plane isn't going anywhere if it's sitting on a conveyor belt that matches the speed of the wheels.

    WHY?? Because...

    Let's not forget gravity is pushing the plane's mass into the ground/conveyor belt so the wheels are in firm contact with that surface area.

    Since ordinarily the runway doesn't move the engine's thrust will push the plane forward causing the wheels to rotate along the runway's surface until enough speed is attained to create lift under the wings to enable takeoff.


    BUT... if the runway happened to move and match the speed of the wheels there would be no forward motion as the moving runway would be counteracting the rotation of the wheels. It wouldn't matter how much thrust you had if you can't get the wheels to move faster than the conveyor belt then you will have absolutely no forward motion.

    Much like a motor boat in a river. if you are travelling upstream against a current moving at 30kph and your propeller/engine thrust is also 30kph you will remain in one spot and not move one cm in either direction.

  20. #36
    Master Bowman Bottom Dweller's Avatar
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    Quote Originally Posted by stickman View Post
    That plane isn't going anywhere if it's sitting on a conveyor belt that matches the speed of the wheels.

    WHY?? Because...

    Let's not forget gravity is pushing the plane's mass into the ground/conveyor belt so the wheels are in firm contact with that surface area.

    Since ordinarily the runway doesn't move the engine's thrust will push the plane forward causing the wheels to rotate along the runway's surface until enough speed is attained to create lift under the wings to enable takeoff.


    BUT... if the runway happened to move and match the speed of the wheels there would be no forward motion as the moving runway would be counteracting the rotation of the wheels. It wouldn't matter how much thrust you had if you can't get the wheels to move faster than the conveyor belt then you will have absolutely no forward motion.

    Much like a motor boat in a river. if you are travelling upstream against a current moving at 30kph and your propeller/engine thrust is also 30kph you will remain in one spot and not move one cm in either direction.
    There is a big difference between how a motor boat and an aircraft move. The motor boat pushes the water for thrust. The aircraft pushes air for thrust.
    Archery's a lot like Life .... a few see it as an opportunity to score cheap points; while the rest are focused on the ends that count.
    Genesis 21:20 And God was with the boy as he grew up in the wilderness. He became a skillful archer.

  21. #37
    3rd Class whoknows's Avatar
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    Must be about time to take a poll of answers

  22. #38
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    So a jumbo sitting on the runway at the equator, facing west has to overcome the earths rotation, 1600kph, before it can take off? Can i pick myself up by my shoelaces if i try hard enough?
    Last edited by xracer; 1st December 2016 at 11:23 PM.

  23. #39
    digitus impudicus
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    The problem with this problem is that at a certain point, people mistake sarcasm for ACTUAL stupidity.
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  24. #40
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    The plane will go forward at the exact same rate, belt or no belt. The tyres will give up the ghost long before it reaches take-off speed, but it will still go forward. I think

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