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Post by nemesis on Feb 13, 2008 1:08:58 GMT 1
I just watched that video and I don't know what they're cheering about because it did NOT work, lol. The conveyor belt didn't keep up so the aircraft moved forward down the runway. It was not stationary when it took off.
Who they tryin to kid, lol?
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Post by nm on Feb 13, 2008 1:45:44 GMT 1
Remember airflow can come from the engines: props and high-lift devices, eg the Fiesler Storch; and the Airbus military transport (also, for example) uses props for low-speed performance.
I think.
NM
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Post by afkmatrix on Feb 13, 2008 2:05:32 GMT 1
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Post by Mark on Feb 13, 2008 3:34:01 GMT 1
I would say that whoever asked the question simply does not understand aircraft.
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Post by Mike B on Feb 13, 2008 5:46:08 GMT 1
Hi AFK, Watching the mythbusters video, you can see that the plane moves forward past the cones, therefore the conveyor belt's reverse speed is not matching the planes forward speed, otherwise the plane would stay alongside a cone?
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Post by Mark on Feb 13, 2008 8:39:52 GMT 1
Assume that the plane is moving forward at 70 knots, and this mythical conveyor is hence moving in the opposite direction at 70 kts. The plane is still moving forward at 70 kts as the thrust is provided by the prop, acting on the air. The poor wheels are spinning at 140 kts, but this doesn't make any difference as the wheel just freewheel.
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Post by afkmatrix on Feb 13, 2008 10:37:55 GMT 1
Yep Mark is right, basically just forget about the wheels. The wheels do not power the aircraft at all. About the only thing that can happen is that the wheels can't take the speed and blow up lol.
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Post by nemesis on Feb 13, 2008 11:05:08 GMT 1
That second video shows exactly what we're trying to say. That lil aircraft was sat on that treadmill at a registed speed of ten miles an hour, more than enough to lift it off the ground if it were travelling forward at that speed. Because it was stationary with no actual airflow, it happily stuck to the ground.
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Post by eric on Feb 13, 2008 13:15:27 GMT 1
if the speed required to lift the aircraft off the ground was comming from the wheels, like a car for example, then , no it would never be able to take off.....but because the engines are thrusting air out the back of the engines, the aircraft will move forward through the air REGARDLESS of what the wheel are doing.
Even if the aircraft was moving backwards on the treadmill at 200 mph prior to engine start, once the engines went up to full thrust, the aircraft would be able to move forward and take off, as it's all about the engines pushing against the air....not the wheels.
the aircraft carrier is able to assist the aircraft into the air by steaming full speed into the wind, so the aircraft already has lift being generated over the wings......and the wheels aren't even moving, so wheel speed is imaterial.
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Post by nm on Feb 13, 2008 15:54:31 GMT 1
What about flying backwards? Used to be a trick in a de Havilland Tiger Moth on a windy day: if the minimum required airspeed was less that the wind speed, you could go backwards while flying forwards!
NM
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Post by Mike B on Feb 13, 2008 19:00:02 GMT 1
I still maintain that the aircraft will not fly. Suppose you could paint a molecule of air above the plane blue and there was zero wind. Forgetting about the speed of the wheels: If the thrust of the engine counteracted the reverse speed of the conveyor belt, the plane would keep it's position under the blue molecule of air. Thus there would be no airflow over the wings to generate lift, ergo, the plane would not fly!
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Post by dirtydozen on Feb 13, 2008 19:29:08 GMT 1
MikeB.Thats more or less what i was going to post but you beat me to it. It doesnt matter how powerful the engines are a wing needs airflow to pass over it to work.
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Post by eric on Feb 13, 2008 20:56:53 GMT 1
Ok, who's for rigging up a test......I know an Airline Pilot, so has anybody got a 737 or 747 laying around?
Then we need somebody to rig up a extremly long conveyer belt (I'm thinking Don would be perfect for this one!)
And then we could let rip........and all those saying it wouldn't take off would have to buy the rest of us a round of drinks, (mine's a Guiness) when she tears off down the runway and into the air!!!
'Cos yer WRONG!!!!!!!!!
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Post by nm on Feb 13, 2008 21:16:55 GMT 1
Ah, but your 737 or 747 is only a jet! Get a prop-pulled (piston-engined or turboprop) aircraft and the engines blast an airflow over the wings.
NM
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Post by swanrail on Feb 13, 2008 22:15:05 GMT 1
I do not think that I can build you a suitable treadmill, according to my calculations, it would have to move at a speed of over 600 knots to match the aircraft thrust. Even if this were possible, the air on the surface of the treadmill, would due to friction, produce enough lift for the aircraft to take off!!! And before you say so, if the conveyor belt had no friction on its sufrace, the wheels would not grip anyway - eg ice on a road!!! -fait accompli!! ;D ;D ;D
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