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Posers and Puzzles

Posers and Puzzles

  1. Donation mwmiller
    RHP Member No.16
    03 Sep '15 15:54
    This one has been kicked around a few times but it's been a while. Is it time to dust it off and see where it goes? Here is the original statement and question.

    "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"
  2. 03 Sep '15 16:14
    Originally posted by mwmiller
    This one has been kicked around a few times but it's been a while. Is it time to dust it off and see where it goes? Here is the original statement and question.

    "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direc ...[text shortened]... onveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"
    Yes - and the Mythbusters proved not only that it should, but that it does.

    The reason, by the way, is that a plane taking off is not driven by its wheels - those are in free spin. No matter how fast the conveyor belt goes, all that happens is that the wheels spin faster and the rest of the plane barely feels the effect. The effect of the propellor or jet exhaust, meanwhile, does propel the plane forward until it takes off.
  3. 11 Sep '15 16:15 / 1 edit
    Originally posted by mwmiller
    This one has been kicked around a few times but it's been a while. Is it time to dust it off and see where it goes? Here is the original statement and question.

    "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direc ...[text shortened]... onveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"
    This point needs clarification:

    " The plane moves in one direction..."

    Relative to what? The ground? The air?

    I'd expect it doesn't move relative to either (and the air doesn't move relative to the ground.)

    How is lift generated?
  4. Donation mwmiller
    RHP Member No.16
    11 Sep '15 19:58
    Originally posted by JS357
    This point needs clarification:

    " The plane moves in one direction..."

    Relative to what? The ground? The air?

    I'd expect it doesn't move relative to either (and the air doesn't move relative to the ground.)

    How is lift generated?
    The plane would move in one direction relative to the runway/conveyor.
    LIft would be generated by air flowing over and under the wings.
  5. 11 Sep '15 20:40 / 1 edit
    Originally posted by mwmiller
    The plane would move in one direction relative to the runway/conveyor.
    LIft would be generated by air flowing over and under the wings.
    OK.

    edit: the plane's wheels rotate at a faster rate (2x) than they would normally. That's all.
  6. Subscriber sonhouse
    Fast and Curious
    07 Oct '15 19:40
    Originally posted by JS357
    OK.

    edit: the plane's wheels rotate at a faster rate (2x) than they would normally. That's all.
    But the bearings might not be able to take that many RPM's and the wheel falls off and it never gets off the ground
  7. Subscriber divegeester
    Don’t feed the troll
    07 Oct '15 20:25
    Originally posted by sonhouse
    But the bearings might not be able to take that many RPM's and the wheel falls off and it never gets off the ground
    Unlikely. The plane would still reach lift velocity in the same time and as it do the relative weight on the wheels would reduce so the wheels and bearings would have negligible increased force on them.
  8. Subscriber Suzianne
    Misfit Queen
    24 Nov '15 10:31 / 3 edits
    Originally posted by mwmiller
    The plane would move in one direction relative to the runway/conveyor.
    LIft would be generated by air flowing over and under the wings.
    Wait.

    Lift is generated by air movement over the wings, like all airfoils. But where is this supposed air movement coming from? The plane's not moving, because the conveyor is matching any speed the plane puts on. The plane could be going 600 mph (I understand that lift velocity occurs at far less speed than this), but so is the runway in the other direction. 600 + -600 = 0. So where's the supposed lift coming from if there is no air movement (it's not moving through the air at 0 speed - relative to the air, it's not moving at all - 0 thrust (air movement) equals 0 lift)?

    It's the same as if the plane were on a regular runway but blocked from moving by an immense wall in front of it or if it's held back by a massive chain. No air movement equals no lift.
  9. Standard member wolfgang59
    Infidel
    24 Nov '15 21:50
    Originally posted by Suzianne
    Wait.

    Lift is generated by air movement over the wings, like all airfoils. But where is this supposed air movement coming from? The plane's not moving, because the conveyor is matching any speed the plane puts on. The plane could be going 600 mph (I understand that lift velocity occurs at far less speed than this), but so is the runway in the other di ...[text shortened]... se wall in front of it or if it's held back by a massive chain. No air movement equals no lift.

    The wheels are not powered - they are just free-spinning.
  10. Standard member lemon lime
    blah blah blah
    25 Nov '15 02:05 / 1 edit
    Originally posted by Suzianne
    Wait.

    Lift is generated by air movement over the wings, like all airfoils. But where is this supposed air movement coming from? The plane's not moving, because the conveyor is matching any speed the plane puts on. The plane could be going 600 mph (I understand that lift velocity occurs at far less speed than this), but so is the runway in the other di ...[text shortened]... se wall in front of it or if it's held back by a massive chain. No air movement equals no lift.
    Air movement would be coming from the propellers... but what if the plane is jet propelled? Apparently forcing air to move back away from the plane (propeller or jet) pulls the air in front across the wings, and this is what provides the lift.
  11. Donation mwmiller
    RHP Member No.16
    25 Nov '15 18:52
    Here's how it basically works.
    The engine pulls or pushes the plane forward, depending on if it is a jet or propeller engine.
    The jet pushes it, while the propeller pulls it forward through the air.
    The forward movement caused air to flow across the surfaces of the wing as it moves forward through the air.
    The shape is such that the distance the air travels over the wing surface is not the same for the upper and lower part of the wing. The difference in that distance that the air travels creates a lifting pressure on the wing.
    As the speed through the air increases, the lifting force is also increased.
    At a certain speed there is enough lift to cause the plane to become airborne.
    All the wheels do is allow the body of the plane to roll easily on the surface of the ground.
    The conveyor belt is not relevant.
  12. Standard member lemon lime
    blah blah blah
    25 Nov '15 20:05
    Originally posted by mwmiller
    Here's how it basically works.
    The engine pulls or pushes the plane forward, depending on if it is a jet or propeller engine.
    The jet pushes it, while the propeller pulls it forward through the air.
    The forward movement caused air to flow across the surfaces of the wing as it moves forward through the air.
    The shape is such that the distance the air tra ...[text shortened]... dy of the plane to roll easily on the surface of the ground.
    The conveyor belt is not relevant.
    I was under the impression the conveyor belt kept the plane stationary in relation to the ground. And if there is no motion of air in relation to the ground, then the conveyor band is relevant in that there should be no motion of air across the wings due to forward motion of the plane...

    In relation to the ground there would be no forward motion. So the only force in play causing air to flow across the wings would have to be air either being pushed or pulled from front to back due to propellers or jets.
  13. Donation mwmiller
    RHP Member No.16
    25 Nov '15 20:20
    Originally posted by lemon lime
    I was under the impression the conveyor belt kept the plane stationary in relation to the ground. And if there is no motion of air in relation to the ground, then the conveyor band is relevant in that there should be no motion of air across the wings due to forward motion of the plane...

    In relation to the ground there would be no forward motion. So th ...[text shortened]... would have to be air either being pushed or pulled from front to back due to propellers or jets.
    The engine moves the body of the plane forward through the air.
    The conveyor belt is not able to stop that.
  14. Standard member lemon lime
    blah blah blah
    25 Nov '15 21:11
    Originally posted by mwmiller
    The engine moves the body of the plane forward through the air.
    The conveyor belt is not able to stop that.
    If the conveyor belt isn't stopping forward motion of the plane then what exactly is it doing? If the conveyor is preventing forward motion by causing the plane to remain stationary (in relation to the ground) then what is causing air to move over the wings? In the OP it says the air is calm... I take that to mean the air is also not moving (in relation to the ground).

    I may have misunderstood what the moving conveyor belt accomplishes... is the plane moving forward in relation to the ground, or is it only moving forward in relation to the surface of the conveyor?
  15. Standard member lemon lime
    blah blah blah
    25 Nov '15 22:12 / 1 edit
    Originally posted by mwmiller
    The engine moves the body of the plane forward through the air.
    The conveyor belt is not able to stop that.
    I read the OP again, and the forward motion of the plane is cancelled out by an equal backward motion of the conveyor. This means there is no forward motion of the plane through the air... something is pushing/pulling air toward the front of the plane.

    Edit: this is hardly worth mentioning, but the motion of the conveyor band could also be adding a bit of force to the air flow... but I imagine any contribution from that would range from almost nothing to negligible.