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Thread: The best internet argument picture ever..

  1. #1
    digitus impudicus
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    The best internet argument picture ever..

    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.

    Name:  planetheory.jpg
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Size:  25.7 KB
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    bleeds yellow and black tigergirl's Avatar
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    that's a damn fast conveyor belt when it gets up to takeoff speed! Or am I imagining it wrong? Dunno much about planes except sitting in them lol
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    Master of Grumpiness GrahameA's Avatar
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    Which will fail/self-destruct first? The tyres on the aircraft or the conveyor system.
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    Of course the plane will take off. It's not wheel drive, its jet driven. The wheels will spin faster as the belt drives the wheels faster, thats about it.
    Last edited by xracer; 29th November 2016 at 10:01 PM.

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    Grand Master Bowman Clare Barnes's Avatar
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    Are the plane engines actually turned on or is the plane just sitting on a non-moving conveyor belt??
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    digitus impudicus
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    So, ask yourself this question.
    "Would Andy! post this apparently simple looking question that he specifically said caused all sorts of arguments, if it was so simple?"

    I also gave hints.

    But I'll ask another question.

    What if you were driving a car on a conveyor that was 10000000 horsepower and it was programmed to match the speed of the tyres.
    You stick it in top gear and flatten it.
    Obviously, you don't go anywhere.
    What happens to the revs? How fast does the conveyor belt accelerate?

    How many questions do I have to ask before someone realises that this is identical to having no load on the engine and you'd better have an RPM limiter?
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    Slowly improving Brenton's Avatar
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    digitus impudicus
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    Everyone refers to that. It actually doesn't show what would happen. It shows an imperfect conveyor.
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    Do you want me to answer as an engineer or a physicist ?

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    plane wheels aren't motorised, so the plane isn't using them to cause the forward motion it requires to reach the air speed it needs for take-off.

    The whole "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" is fairly irrelevant as we're already talking about a conveyor belt that's impossible. As big as a runway and tough enough to support a 747 at +700,000lbs... if we're already dealing with the impossible there's nothing to say it can't also do everything it needs to. You know, cause hypothetical.

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    digitus impudicus
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    Quote Originally Posted by bimble View Post
    plane wheels aren't motorised, so the plane isn't using them to cause the forward motion it requires to reach the air speed it needs for take-off.
    Yes. Everyone grabs onto that part and most can't let go of it.

    I'll just point out that if you have a toy car sitting there and you come and push it along, you are driving the wheels by the relative motion of your hand against the ground.
    So a plane drives the wheels by holding them against the ground and throwing air backwards until it can lift off them off.

    And if we have both the plane driving the wheels and also the conveyor belt trying to reach equilibrium by driving the belt, which is driving the wheels, while the aircraft is trying to roll forward and also driving the wheels... We're talking a lot of power going into those wheels. Continuously... with an instant speed matching mechanism trying to keep up with energy being put into the wheels.


    If you were running on a treadmill that instantly matched your speed, if you tried to fun forward on the belt, how long before you were running as fast as you could go?


    People totally ignore the instant speed matching feedback loop, which is putting power into the conveyor to try and match the speed of the wheels.
    Without that, the plane in the situation would do exactly what the mythbusters clip accomplished. It would slowly gain speed take off if there is enough relative lag.
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    digitus impudicus
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    Quote Originally Posted by taz00 View Post
    Do you want me to answer as an engineer or a physicist ?
    I think if you grab onto the right concept, it won't make any difference.
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    The physicist's answer is that since the linear speed of the wheels is matched by the conveyor belt then the center of the wheel has zero speed. This means that the airplane is not moving and can therefore not take off.
    The engineer's answer would be that this is a hypothetical scenario that can never take place thus the plane will take off.

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    There is no airspeed over the wing to create lift.

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    Grand Master Bowman scott p's Avatar
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    Quote Originally Posted by taz00 View Post
    The physicist's answer is that since the linear speed of the wheels is matched by the conveyor belt then the center of the wheel has zero speed. This means that the airplane is not moving and can therefore not take off.
    that was my initial answer. however, this isn't a scenario such as a car on a rolling-road dynamometer where the relative forward thrust of the wheels on the chassis can be cancelled by moving the road backwards at an equal rate. the engine thrust pushes against the air surrounding the airframe, which is *not* moving. thus the airframe moves relative to the air and airflow is generated, and the airplane takes off.

    the remaining question is how fast will the wheels be turning at the time? still thinking about that one.
    (this assumes no other real world effects like bearing drag etc.)
    Last edited by scott p; 30th November 2016 at 11:33 AM.

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    Impossible scenario..... But it won't move.

    The logic is that if you reversed the scenario to say that if a not running aircraft was on a belt going backwards at 100 kts, it would need engines started and running to provide 100 kts of fwd speed just to remain stationary relative to something not on the belt.

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    Grand Master Bowman scott p's Avatar
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    that would only be the case if the rearwards effects of the conveyor transferred absolutely to the airframe. since the wheels rotate freely, the outcome will simply be that the wheel speed will be higher than the airspeed. or, to put it another way, the airspeed will be less than the thrust of the engines would otherwise produce, by a factor of the drag coefficient required to turn the wheels on their axles.

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    *****istrator Marcus's Avatar
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    Quote Originally Posted by scott p View Post
    that was my initial answer. however, this isn't a scenario such as a car on a rolling-road dynamometer where the relative forward thrust of the wheels on the chassis can be cancelled by moving the road backwards at an equal rate. the engine thrust pushes against the air surrounding the airframe, which is *not* moving. thus the airframe moves relative to the air and airflow is generated, and the airplane takes off.
    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.
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  22. #19
    digitus impudicus
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    When I was in TAFE, I actually got given the test that said "read all the instructions before doing this test." in a class full of other new people.
    I'd already heard of it and I couldn't believe that someone had actually presented it to us.
    Apparently, I was the only person who knew. Everyone else went berserk trying to do everything within the time period.

    This is why this is the best trap question ever. People are far more willing to accept what they initially think is correct, rather than to explore where they were told to look.
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    *****istrator Marcus's Avatar
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    I would say that for the sake of the question you have to assume that there is no lag between the wheels and the belt.
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