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Thread: Will it fly?

  1. #41
    SeŮor Member Dewey Cox's Avatar
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    This riddle is nonsense.
    The jet is going to move forwards, no matter what the ground underneath is doing.
    Since the jet is going to move forward anyway, there is no way for the conveyor to keep matching the speed the wheels will spin, as it will go on to infinity.
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  3. #42
    Go Canucks Go! lone-wolf's Avatar
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    "The conveyor belt is designed to exactly match the speed of the wheels"
    Ah dang, if the conveyor belt is always matching the speed of the wheels, then I'm back to it can't take off, the wheel speed(which is the forward speed + the backwards conveyor speed) will need to be at least take off speed faster then the conveyor speed, and if the speed of the conveyor never allows that - it won't move forward for the lift, the conveyor will be going 100mph and the plane throttle will set for 50mph, the wheels will be going 150mph, the belt will speed up more, indefinitely? as it can never catch the actual speed of the wheels
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  4. #43
    Senior Member RangeBob's Avatar
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    Quote Originally Posted by lone-wolf View Post
    Ah dang, if the conveyor belt is always matching the speed of the wheels, then I'm back to it can't take off, the wheel speed(which is the forward speed + the backwards conveyor speed) will need to be at least take off speed faster then the conveyor speed, and if the speed of the conveyor never allows that - it won't move forward for the lift, the conveyor will be going 100mph and the plane throttle will set for 50mph, the wheels will be going 150mph, the belt will speed up more, indefinitely? as it can never catch the actual speed of the wheels
    Started off a bit like (b) in post #33.

  5. #44
    Senior Member DanN's Avatar
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    Quote Originally Posted by lone-wolf View Post
    Ah dang, if the conveyor belt is always matching the speed of the wheels, then I'm back to it can't take off, the wheel speed(which is the forward speed + the backwards conveyor speed) will need to be at least take off speed faster then the conveyor speed, and if the speed of the conveyor never allows that - it won't move forward for the lift, the conveyor will be going 100mph and the plane throttle will set for 50mph, the wheels will be going 150mph, the belt will speed up more, indefinitely? as it can never catch the actual speed of the wheels
    All the wheels do is keep the plane from dragging on the ground; the jet engines provide thrust that pushes on the body/wings of the plane (Newton's 3rd law) propelling it forward regardless of wheel speed. But can the bearings/wheels/tires survive?

    How about this.. let's say the conveyor is at a fixed surface speed of 100 MPH, with its surface moving towards the plane. The plane is on tarmac at the end of the runway. The pilot throttles up the engines and the nose gear wheels move onto the conveyor. Does the plane stop? Can it still accelerate forward? What happens when the main gear gets to the conveyor? Is it any different from when the nose gear moved onto the conveyor?
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  6. #45
    Go Canucks Go! lone-wolf's Avatar
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    If the conveyor is at a fixed speed, like in the mythbusters test, it'd definitely take off and the wheel recorded speed would be take off speed + 100mph of the conveyor speed while the airspeed recorded on the instruments would be just take off speed.
    The conveyor matching wheel speed which is about impossible is now what is throwing me because it can only match wheel speed if the plane has no forward momentum.
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  7. #46
    Senior Member labradort's Avatar
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    Quote Originally Posted by Doug_M View Post
    The jet has to move through the air for the wings to produce lift. The conveyor belt is preventing the aircraft from moving through the air and the intakes of the jet (and ďpullĒ of the thrusting air behind the wings) simply isnít going to be enough.
    All it said about the conveyor belt is it would match any rate of roll of the wheels. So the wheels stay stationary relative to the belt, but the belt would need to move forward along with the thrust to keep that balanced. It's just stated in a weird way and so we have different pictures of what is happening (as the problem statement).

    At first I was picturing the aircraft is tethered to the ground (not the belt) so the conveyor belt and wheels are spinning, and they were asking whether the plane would fly once the wheels spin fast enough. But that's not what it says.

    A paper airplane, glider, tree leaves, or a kite have no engines. Neither do birds. They can all fly.

    The engines in a 747 move it forward which causes air to fly past the wings, creating lift.

    According to Bernoulli's law, lift is created by wings because the curved airfoil causes air pressure over the wing to be less than the air pressure under the wing. The air pressure wants to equalize and so it pushes up against the lower surface of the wing, producing lift for the whole aircraft attached to the wings.

    The key to flight with a working wing design is to make air flow over and under the wing. Moving the wing forward is one way to do that.

    In the case of a helicopter, each blade is a thin wing. That's why they are called rotary wing and other aircraft called fixed wing. So the helicopter is able to fly up without moving forward since it fulfills the airflow over the wings requirement.

    Here's a neat thing a person can try in a PC flight simulator. Set up the weather so there is a 100 MPH steady wind coming in the same compass heading as the runway. Place the Cessna a few hundred feet or so above the runway. When the simulation is started, it is possible to fly the aircraft in a perfectly stationary position relative to the ground. It would also be possible to gently land while not moving forward relative to the ground, although keeping it down could be tricky to manage.

  8. #47
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    Assuming the conveyor reacts to wheel speed, even if its Instantaneous, the aircraft is still going to accelerate. Thrust acts on the air and applied to the aircraft through the engine mounts regardless if its a jet or propeller driven design.
    The conveyor will only cause the wheel speed to be double the ground speed, which will result in a higher than normal drag from the tire and bearings (ignoring the tire's max speed) if thrust is more than the bare minimum for a given design and weight to fly it will be able to overcome the added drag the aircraft will take off the results will be a slightly longer than normal takeoff run.
    Now in a no-wind scenario the max tire speed would be exceeded and a blowout becomes likely, which could cause a takeoff to no longer be possible.

  9. #48
    Senior Member RangeBob's Avatar
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    Traditionally, the conveyor belt rollers are stationary (other than rotating in place).
    But
    the picture looks more like tank treads.

    What if the conveyor belt moved forward under the plane, up to an incredible 190 mph ?
    So, the conveyor belt is moving forward at 190 mph relative to the eart, and the plane is coincidentally moving forward at the same 190 mph relative to the earth, and the tires are spinning backwards at 190 mph rotation but their axles are technically moving forward at the same speed as the airplane.

  10. #49
    Senior Member labradort's Avatar
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    I thought the links RangeBob posted above where a joke, but it is actually the discussions on this puzzle. I think all of them mention the problem of interpreting what the problem statement really means, before even answering anything.

    My interpretation was the wheels don't spin, but the conveyor goes ahead as the plane accelerates relative to the world and the air.

    Another interpretation is the system is trying to take the "traction" away by moving the belt backwards, like a cartoon character spinning their feet in the air and going no where. If that's the set up, this doesn't matter. There is no traction from the wheels so you can't rob it of any acceleration by running the treadmill backwards. The wheels in an airplane are passive.

  11. #50
    Senior Member DanN's Avatar
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    Just scale it down to something easier to understand.

    A jet engine exerts thrust from the exhaust, which acts on the air around it. The thrust applies force in the opposite direction, which is transferred to the wings and fuselage via the engine mounts. This is how a jet moves forward on any other given day. This is also exactly the same way a roller skate would move if you pushed it with your hand; your hand represents the force applied to the airplane by the jet engines.

    Now imagine you have a treadmill that has the purely magical ability to move its belt in the opposite direction of wheels touching it, at exactly the same speed.

    You are kneeling next to this magical treadmill (which is perfectly flat so as to not cause anything to roll off of it by gravity) and you place the roller skate on it.

    At first, nothing happens. Nothing moves. No force is being applied to move the roller skate in any direction, so the wheels don't move and neither does the treadmill belt.

    But if you reach over and give the roller skate a push (applying thrust); the roller skate moves forward, just like it would if it were sitting on the floor. Also, and at the exact same time, the treadmill rolls backward at the same speed as the wheels...

    Now this should be a runaway because as soon as the belt moves while the roller skate is being pushed, the speeds of both will accelerate wildly.. but let's assume this magical treadmill somehow manages to judge wheel speed based on relative ground speed just for giggles. That, or we have to assume the wheels and belt can turn at infinite RPM without destruction. (**OR**, the wheels and the belt are locked in static friction and the belt rolls backwards as you push the roller skate, but I've never seen a treadmill that would roll that easily. That said, this treadmill is PFM, so sure; one or the other.)

    When you pushed the roller skate it moved along the belt relative to the ground around it (and gained airspeed, and aside from drag in the wheel bearings you wouldn't even feel the relative speed of the wheels and belt, nor would that stop you from being able to push the roller skate right as far as you could reach.

    I believe that if a grown man can push a roller skate off of the end of a running treadmill, the jet can take off.
    "I don't have a firearms problem; they all work perfectly well." - Strike that. I do have a problem; the gov't has decided some are too dangerous to own.
    Membership: CCFR, SAFGC
    Govít couldnít make sense if you gave them a recipe.

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