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747 on a treadmill conundrum..
#31
RE: 747 on a treadmill conundrum..
How about we put the wheels on train tracks and convert them into train wheels? no exploding tires, there. and no treadmill required, just... errr... a near-infinite rail track being pulled back, somehow.

Or, put the plane on water with the appropriate "shoes" (can't remember the proper name for those things) and have the plane trying to lift off when the water current moves backward, matching the speed of the plane.... that ruins the free-spinning detail in wheels...

Or put two airplanes, one of them upside-down and facing the opposite direction, with the wheels touching each other's and have equal thrust go into both planes.

Or put the plane in a wind tunnel and put the treadmill in there too.... see if the plane lifts off without moving.

Thought experiments are fun!
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#32
RE: 747 on a treadmill conundrum..
Any zero friction case the plane takes off fine, and the same with any sensibly programmed treadmill.
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#33
RE: 747 on a treadmill conundrum..
But I ran on my treadmill today and didn't take off.
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#34
RE: 747 on a treadmill conundrum..
To my mind the real question is if a plane has thrust from its jet engines but no forward movement through the air, can it take off? The treadmill is a silly way to ask this question because the plane's wheels have nothing to do with it's forward momentum except to reduce friction. In other words keeping a jet plane stationary with a tread mill is not really possible.

You might try to slow the plane down by adding friction breaks to the wheels or scoops on the floats of a float plane but even in the case of locked wheels scoops sufficient to slow the plane to a stand-still the plane would probably either just skid along the runway and/or treadmill or tear the floats off the plane.

If you tie the plane in place you've restricted it's ability to lift for reasons having nothing to do with the thought experiment, i.e you've tied it in place, though you could measure the amount of upwards pull on the ropes and thus get at the heart of the question which is whether the plane can take off with the jets running but no air flow over the wings from forward motion. Would the jet engines produce air flow over the wings without forward motion such as to cause lift? They do have to suck in air in order to propel air backwards.
If there is a god, I want to believe that there is a god.  If there is not a god, I want to believe that there is no god.
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#35
RE: 747 on a treadmill conundrum..
Yep. But I want to go back to the original thought experiment. The forward thrust of the jet or propeller would move the plane forward relative to the air. The moving runway is probably a red herring. That plane will be moving forward through the air and relative to fixed objects on the ground regardless of what the conveyor belt runway is doing. But I find it hard to imagine just what effect the moving runway would have. Perhaps it would create a drag along the ground in which case I could imagine the plane nosing into the ground as a result. But perhaps the moving runway accommodates the forward motion while eliminating the need of the tires to rotate. Not sure.
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#36
RE: 747 on a treadmill conundrum..
(September 6, 2014 at 9:33 am)whateverist Wrote: Yep. But I want to go back to the original thought experiment. The forward thrust of the jet or propeller would move the plane forward relative to the air. The moving runway is probably a red herring. That plane will be moving forward through the air and relative to fixed objects on the ground regardless of what the conveyor belt runway is doing. But I find it hard to imagine just what effect the moving runway would have. Perhaps it would create a drag along the ground in which case I could imagine the plane nosing into the ground as a result. But perhaps the moving runway accommodates the forward motion while eliminating the need of the tires to rotate. Not sure.

I can't see what effect a moving runway would have at all. It shouldn't affect the plane's speed in any significant way. I don't see how it would affect air movement either. I really can't see what the speed of the tires has to do with anything.
If there is a god, I want to believe that there is a god.  If there is not a god, I want to believe that there is no god.
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#37
RE: 747 on a treadmill conundrum..
Yeah, me neither. As others have noted, the tires are free spinning. So it is hard to see how they could result in any drag.

Looking at the Mythbusters video I don't think their moving tarp does a good job of producing no tire rotation. They'd just about have to drag the tarp in the opposite direction to get that. It still should have no effect.
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#38
RE: 747 on a treadmill conundrum..
They will result in some drag because their bearings are not totally frictionless. But wheel drag will be negligible next to the thrust of the engines, or the aerodynamic drag on the aircraft resulting from forward motion. So the wheels will have little effect whether it is rolling on the treadmill or a concrete runway.

The rated speed of the tires are a concern. Airplane tires, like car tires, have a rated speed. Putting the plane on a treadmill will cause them to roll twice as fast as normal and likely exceed rated tire speed. I don't know how long they will last and whether they will survive one complete take off run. But I don't think this kind of detail is what the original puzzle was getting at. The original puzzle was about pulling the leg of people who don't much about how aerodynamic flight work in principle.
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#39
RE: 747 on a treadmill conundrum..
(September 6, 2014 at 10:23 am)Chuck Wrote: They will result in some drag because their bearings are not totally frictionless. But wheel drag will be negligible next to the thrust of the engines, or the aerodynamic drag on the aircraft resulting from forward motion. So the wheels will have little effect whether it is rolling on the treadmill or a concrete runway.

The rated speed of the tires are a concern. Airplane tires, like car tires, have a rated speed. Putting the plane on a treadmill will cause them to roll twice as fast as normal and likely exceed rated tire speed. I don't know how long they will last and whether they will survive one complete take off run. But I don't think this kind of detail is what the original puzzle was getting at. The original puzzle was about pulling the leg of people who don't much about how aerodynamic flight work in principle.
Or what causes the plane's forward propulsion for that matter. I thought about this this morning in terms of the Kitty Hawk planes that were pedal powered, but since the peddles turned the prop not the wheels, the problem remains the same with regard to the treadmill.

It's speed relative to the air not the ground that matters for flight. If you don't believe it all you have to do is think about how an airfoil kite flies. It's not going anywhere relative to the ground, but wind-speed lifts it and keeps it aloft and drag from the string hold the angle of attack. http://en.wikipedia.org/wiki/Foil_kite
If there is a god, I want to believe that there is a god.  If there is not a god, I want to believe that there is no god.
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#40
RE: 747 on a treadmill conundrum..
(September 6, 2014 at 1:55 am)SteelCurtain Wrote: All of this is moot, though. The plane's wheels have effectively zero effect on the plane's ability to create thrust and move forward on the runway. If the treadmill is always set to match the forward velocity of the aircraft, then the 747's wheels will always be rotating at 2x the indicated airspeed. The plane will take off just fine.

So we could make really short aircraft carriers?



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