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jet plane on conveyor — Parallax Forums

jet plane on conveyor

skylightskylight Posts: 1,915
edited 2012-09-02 20:25 in General Discussion
I'm not sure if this long running question has been asked before on this forum but here goes.

You have a jet plane sitting on a runway which happens to be a conveyor belt, the wheels of the jet plane has encoders that relay wheel speed to the conveyor runway which instantaneously matches the speed but in the reverse direction to the way the jet plane is facing, Ignoring any friction in the wheel bearings would the jet be able to take off? And no it's not a VTOL


For those just joining this thread an amended scenario has been posted in post #135

http://forums.parallax.com/showthread.php?142067-jet-plane-on-conveyor&p=1122309&viewfull=1#post1122309

Please refer to this before posting.
«1345678

Comments

  • mindrobotsmindrobots Posts: 6,506
    edited 2012-08-27 10:31
    You mean so that there was no airflow over the wings except for prevailing winds?

    No. Wings don't lift without air flowing over them. No lift, no takeoff. Ladies and gentlemen, our departure will be delayed......
  • lardomlardom Posts: 1,659
    edited 2012-08-27 10:31
    No wind, no lift.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 10:40
    I will give the official answer after a few more have posted.
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-08-27 10:41
    I don't think it's possible for the conveyor belt to match the wheel speed. The jet will move forward due to the engine thrust, which has nothing to do with what the wheels are doing.
  • ctwardellctwardell Posts: 1,716
    edited 2012-08-27 10:42
    The wheels don't move the plane, thrust from the engines does, so yes, it will still move forward and take off.

    The only thing I see happening is the wheels will spin twice as fast as normal.

    C.W.
  • mindrobotsmindrobots Posts: 6,506
    edited 2012-08-27 10:42
    Wind over the wings: That's why a few navies around the world have this incredible amusement ride.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 10:48
    Ok this is hypothetical so the runway does match the speed. also it's a very calm day there is no wind speed.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-27 10:55
    Yes, the jet would take off -- no differently than from a stationary runway, except that its wheels would be spinning faster. The wheels are not powered but basically just idlers. The jet engines are what do the work against the plane's inertia, and that's all that counts.

    OTOH, if you asked the same question about a truck pulling a glider to launch it, the answer would be no, since its wheels are powered, and it would basically be on a treadmill.

    -Phil
  • skylightskylight Posts: 1,915
    edited 2012-08-27 10:59
    Well someone is right so far :smile:
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-08-27 11:01
    If the runway matches the wheel speed then the jet is not moving by definition. This violates Newton's laws of motion. The thrust from the jet engines exerts a force on the jet, which will cause it to accelerate. Therefore, you have presented an impossible scenario.
  • mindrobotsmindrobots Posts: 6,506
    edited 2012-08-27 11:01
    Steering could be interesting. Hope you can lock your nose wheel and your thrust is perfectly balanced.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:06
    mindrobots wrote: »
    Steering could be interesting. Hope you can lock your nose wheel and your thrust is perfectly balanced.
    Yes and yes
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-27 11:08
    Dave makes an interesting point. OTOH, even a stationary runway "matches the wheel speed" if there's no skidding. IMO, the whole conveyor-belt thing is just a red herring to divert attention from the actual mechanics involved.

    -Phil
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:10
    Ok there seems to be enough doubters and giving the answer without the reasoning behind it so that the arguments against can still be offered, the official answer is yes the plane would take off
  • PaulPaul Posts: 263
    edited 2012-08-27 11:13
    http://mythbustersresults.com/episode97

    BUSTED! (The plane took off normally)
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-08-27 11:15
    skylight wrote: »
    Ok there seems to be enough doubters and giving the answer without the reasoning behind it so that the arguments against can still be offered, the official answer is yes the plane would take off
    But then your basic premise that the treadmill matches the wheel speed is violated.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:19
    I was aware of the mythbusters having done the experiment but thankfully the answer at that link doesn't really explain why, If anyone finds a better explanation on the web I'd ask if you could keep it to yourself for a bit longer just to allow counter arguments.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:20
    Dave Hein wrote: »
    But then your basic premise that the treadmill matches the wheel speed is violated.
    Why is that?
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-08-27 11:26
    Read post #11. If the treadmill matches the wheel speed the jet can't be moving. If the jet is moving the treadmill can't match the wheel speed.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-27 11:30
    Again, if there's no skidding, the belt matches the wheel speed, since "no skidding" has to be the definition of "matching the speed." A belt running at any speed -- forward or backward -- meets this criterion, so long as the wheels are in contact with it and just idling in sync. The belt really has nothing to do with the problem except to serve as a diversion.

    -Phil
  • mindrobotsmindrobots Posts: 6,506
    edited 2012-08-27 11:34
    st soldier with a keen interest in birds: Where'd you get the coconuts?
    King Arthur: We found them.
    1st soldier with a keen interest in birds: Found them? In Mercia? The coconut's tropical!
    King Arthur: What do you mean?
    1st soldier with a keen interest in birds: Well, this is a temperate zone
    King Arthur: The swallow may fly south with the sun or the house martin or the plover may seek warmer climes in winter, yet these are not strangers to our land?
    1st soldier with a keen interest in birds: Are you suggesting coconuts migrate?
    King Arthur: Not at all. They could be carried.
    1st soldier with a keen interest in birds: What? A swallow carrying a coconut?
    King Arthur: It could grip it by the husk!
    1st soldier with a keen interest in birds: It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.
    King Arthur: Well, it doesn't matter. Will you go and tell your master that Arthur from the Court of Camelot is here?
    1st soldier with a keen interest in birds: Listen. In order to maintain air-speed velocity, a swallow needs to beat its wings forty-three times every second, right?
    King Arthur: Please!
    1st soldier with a keen interest in birds: Am I right?
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:36
    @ Dave Hein I did read your reply and indeed the wheel is in contact with the runway and I know where you are coming from with your argument. This question although simplistic once you know the reasoning behind the answer never fails to turn peoples opinions as more and more answers are given, there are strong arguments for and against but one camp is flawed in their thinking process.
    All I will say is I have seen this cause heated arguments on other forums so I will say if all could keep it civil everyone is entitled to give their explanation no matter how flawed,

    edited to include name reply was intended for as other comments were posted before I replied
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:38
    @ mindrobots I just love that film :smile:
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-08-27 11:43
    Phil Pilgrim (PhiPi) wrote: »
    Again, if there's no skidding, the belt matches the wheel speed, since "no skidding" has to be the definition of "matching the speed." A belt running at any speed -- forward or backward -- meets this criterion, so long as the wheels are in contact with it and just idling in sync. The belt really has nothing to do with the problem except to serve as a diversion.

    -Phil
    The OP goes through some pains to describe a conveyer belt that moves at the same speed and in the opposite direction that the wheels are spinning. This can happen only if the jet is not moving. If the jet moves the wheel must rotate at a slightly faster speed than the treadmill is moving, which violates the conditions of the problem. Actually, the only correct answer is that under the stated conditions of the problem the jet cannot take off, that is, it is not allowed to move at all, which means that the engines cannot be exerting any thrust.

    skylight, can you explain how the plane can take off without violating the conditions you presented in your first post?
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-08-27 11:45
    Its a trick question.

    Since the plane is "sitting" on the runway. It is not moving so the conveyer is not moving either.

    The answer is Nothing happens because nothing is happening!

    Duane J
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-27 11:48
    Dave Hein wrote:
    The OP goes through some pains to describe a conveyer belt that moves at the same speed and in the opposite direction that the wheels are spinning. This can happen only if the jet is not moving.
    A stationary runway "moves at the same speed and in the opposite direction that the wheels are spinning" when a jet is taking off. Otherwise, there would be skid marks.

    -Phil
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-08-27 11:50
    Say the frictions of the wheels only reduces the planes speed by 1%
    The conveyer belt would have to go 99 times faster than max thrust of the planes engine to stop the plane from moving forward
    So nearly unlimited speed of the conveyer belt is needed.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:52
    Dave Hein wrote: »
    The OP goes through some pains to describe a conveyer belt that moves at the same speed and in the opposite direction that the wheels are spinning. This can happen only if the jet is not moving. If the jet moves the wheel must rotate at a slightly faster speed than the treadmill is moving, which violates the conditions of the problem. Actually, the only correct answer is that under the stated conditions of the problem the jet cannot take off, that is, it is not allowed to move at all, which means that the engines cannot be exerting any thrust.

    skylight, can you explain how the plane can take off without violating the conditions you presented in your first post?
    Your argument was exactly the argument I gave on another forum before I could see the correct solution, as I said this question can make you think all kinds of scenarios to try to explain your thinking but once you are on the wrong road it's very hard to be convinced otherwise.

    I'm not trying to be cruel or condescending in any way but can I leave the explanation a bit longer so those that doubt it would take off can have a chance to elaborate on their thinking.
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:54
    Duane C. Johnson wrote: »
    Its a trick question.

    Since the plane is "sitting" on the runway. It is not moving so the conveyer is not moving either.

    The answer is Nothing happens because nothing is happening!
    Duane J
    Ah but the plane takes off suggesting that the plane did indeed move forward
  • skylightskylight Posts: 1,915
    edited 2012-08-27 11:56
    tonyp12 wrote: »
    Say the frictions of the wheels only reduces the planes speed by 1%
    The conveyer belt would have to go 99 times faster than max thrust of the planes engine to stop the plane from moving forward
    So nearly unlimited speed of the conveyer belt is needed.
    Just for this hypothetical question as i stated before please ignore any friction in the wheel bearings they are frictionless!
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