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Posers and Puzzles

Posers and Puzzles

  1. 30 Oct '06 11:39 / 1 edit
    Sorry for the double (triple) thread but this needs to be explained properly and the other threads got off topic talking irrelevant stuff.


    The easy part:
    The plane will take off.

    The hard part:
    Thought experiment.

    Imagine you are lying on your stomach on a large skateboard. Along-side you is a horizontal rail which you grab and use to pull yourself forward. Your hand pulling on the rail generates the force that pulls you forward. Nothing to do with the ground.

    Now imagine you're still on your large skateboard on a big conveyor belt. The rail is not attached to the conveyor-belt and does not move. Can you see that there is no practical speed the conveyor belt could travel that could make you move backward? Your hand holding the rail can easily overcome the friction in the bearings pulling you back, you could easily pull yourself forward as if you were on a normal floor.

    Now replace you + skateboard with a plane (I like to visualise a little Cessna) and replace your hand pulling on a railing with a propeller pulling the plane through the air. The propeller pulls air backward generating the force which moves the plane forward. Again, nothing to do with the ground.

    You should agree now that no matter what speed the conveyor belt moves the plane will pull itself through the air, accelerate to its required velocity and take off.
    The only difference between the take-off on a conveyor-belt runway and a conventional runway is that the little wheels under the plane would spin much faster on the conveyor-belt.


    NOTE 1:
    The answer is the same for a jet propelled plane except that a jet is pushing the plane forward instead of pulling it like a propeller.

    NOTE 2:
    To those sidetracked by the propeller pulling air over the wing generating lift - you are kidding yourself.

    Disagree?
    Lets do another thought experiment (location Fairyland).

    Your plane is anchored to an immovable pole by a strong cable stretching from the tail of your plane to the pole. You spin up the propellers to top speed generating sooooo much air movement over your wings that you get enough lift to lift off. Momentarily you are hovering held down only by the cable, you click a button releasing the cable and quickly accelerate forward and fly away.

    If this were possible runways would only be for landing on and the last time i flew in a propellered plane i vaguely remember rolling forward faster and faster until the massive amount of air moving over the wings (caused by forward motion) generated the lift to get us off the ground.

    I concede that with 2 very large propellers spinning extremely fast, over a specially designed wing on an extremely light plane this scenario MAY be possible but i doubt it. I don't know enough aerodynamics to explain why but i really doubt it. If anyone has evidence to the contrary, please post. I'm skilled at own-word eating.
  2. 30 Oct '06 11:45
    I don't think this explanation is easier to understand that the scientific one...
  3. 30 Oct '06 12:05 / 1 edit
    It's not the best analogy i guess but it's what jumped into my head when i explained it to my flatmate... He understood it.
    Perhaps it doesn't work so well in written form...

    Lucky i can't be sued.
  4. 30 Oct '06 14:02
    Originally posted by Marsan
    Sorry for the double (triple) thread but this needs to be explained properly and the other threads got off topic talking irrelevant stuff.


    The easy part:
    The plane will take off.

    The hard part:
    Thought experiment.

    Imagine you are lying on your stomach on a large skateboard. Along-side you is a horizontal rail which you grab and use to pull yourse ...[text shortened]... oubt it. If anyone has evidence to the contrary, please post. I'm skilled at own-word eating.
    I am amazed that this "no-brainer" should keep apparently intelligent people occupied for so long.

    1. Inless there is sufficient lift generated by airflow over the wings the plane will not fly. Let's call that critical airspeed.
    2. If the the belt moves backwards at the same speed as the wheels of the plane are moving forward the airspeed remains at zero - no flight.
    3 If the belt is slower then the plane will fly if the speed difference is greater than the critical airspeed and won't if it is less.

    That's all folks.
  5. 30 Oct '06 14:17
    Originally posted by sugiezd
    I am amazed that this "no-brainer" should keep apparently intelligent people occupied for so long.

    1. Inless there is sufficient lift generated by airflow over the wings the plane will not fly. Let's call that critical airspeed.
    2. If the the belt moves backwards at the same speed as the wheels of the plane are moving forward the airspeed remains at zero ...[text shortened]... ference is greater than the critical airspeed and won't if it is less.

    That's all folks.
    I'm amazed that some people still think a plane moves by turning it's wheels.
  6. 30 Oct '06 14:25
    Originally posted by Ian68
    I'm amazed that some people still think a plane moves by turning it's wheels.
    If there would be a lift because of turning the wheels...
    What about planes with no wheels?
    Like pontons? And... eh... what do you call those things you land on on snow with?
  7. 30 Oct '06 14:53
    Originally posted by Ian68
    I'm amazed that some people still think a plane moves by turning it's wheels.
    Irrelevant.

    Powered by prop., wheels, jet, impulse, warp, steam, thought or just the pilot's farts- that plane won't fly.
  8. 30 Oct '06 15:52
    Originally posted by sugiezd
    Irrelevant.

    Powered by prop., wheels, jet, impulse, warp, steam, thought or just the pilot's farts- that plane won't fly.
    And this is why the thread keeps going - because people keep failing to understand the explanation. The belt will not slow the plane down enough to stop it taking off.
  9. 30 Oct '06 15:58
    Originally posted by mtthw
    And this is why the thread keeps going - because people keep failing to understand the explanation. The belt will not slow the plane down enough to stop it taking off.
    If the prop. would move the plane at 100 kph om terra firma but the belt is moving backwards at the same speed - what is the speed of the air passing over the wings?
  10. 30 Oct '06 16:19
    Originally posted by sugiezd
    If the prop. would move the plane at 100 kph om terra firma but the belt is moving backwards at the same speed - what is the speed of the air passing over the wings?
    The prop would be moving the plane at 100kph with respect to the air. Propellers affect air speed, not ground speed. The belt is irrelevant.
  11. 30 Oct '06 16:22
    suqiezd please give up you are only making matters worse. Marsan i like your analogy and was hoping it would finnaly put an end to this question. Goodluck everyone!
  12. 30 Oct '06 17:11
    Originally posted by Marsan
    Sorry for the double (triple) thread but this needs to be explained properly and the other threads got off topic talking irrelevant stuff.


    The easy part:
    The plane will take off.

    The hard part:
    Thought experiment.

    Imagine you are lying on your stomach on a large skateboard. Along-side you is a horizontal rail which you grab and use to pull yourse ...[text shortened]... oubt it. If anyone has evidence to the contrary, please post. I'm skilled at own-word eating.
    i'm confused the first half of your post says that the plane will take off. In the second half you give the reason why a propeller isn't enough to create air movement under the wing and so create lift.

    Now for my 2 cents. propellers/jets/other forms of propulsion create
    ACCELERATION. Assuming that the conveyor belt is moving at a constant rate eventually the plane will be moving far faster than the conveyor and will NOT STAY IN ONE PLACE. thus it takes longer but behaves exactly like a normal runway. If on the other hand the conveyor accelerates at the same rate as the plane then the plane will not move and its velocity will remain zero thus no lift is created.

    If your gonna ask a question you should always make your conditions clear and precise. otherwise half the group answers one question the other half answers a different question and a moronic argument breaks out. I place the blame squarely at the feet of the original author of the problem.
  13. 30 Oct '06 17:55
    Originally posted by aginis
    If your gonna ask a question you should always make your conditions clear and precise. otherwise half the group answers one question the other half answers a different question and a moronic argument breaks out. I place the blame squarely at the feet of the original author of the problem.
    The question clearly stated that the plane was moving forward. You cannot blame the questioner for people who cannot understand how a plane moves and thus concluded that the plane would be stationary in relation to the surrounding air.
  14. 30 Oct '06 21:30
    Originally posted by FabianFnas
    If there would be a lift because of turning the wheels...
    What about planes with no wheels?
    Like pontons? And... eh... what do you call those things you land on on snow with?
    skis
  15. 30 Oct '06 21:31
    Originally posted by sugiezd
    If the prop. would move the plane at 100 kph om terra firma but the belt is moving backwards at the same speed - what is the speed of the air passing over the wings?
    0