Airplane on a treadmill!

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Bubbleawsome

Bubbleawsome

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Apr 14, 2013
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EDIT: I've seen the light, I understand it would take off now.

In the spirit of the current 48÷2(9+3) = thread we have going, let's get another debate going.

This one is about an airplane (a 747) on a (perfect) treadmill. The treadmill matches the speed of the airplane exactly. The key here is in the wording. It only matches the speed, so based on your interpretation of speed, you'll have different answers.

Many are seen here on xkcd. We're also ignoring all real-life tests because you can't have a perfect treadmill that can match a 747.

Many people (including me) seem to think the best way to interpret the issue is that the treadmill is always going fast enough to prevent the plane from moving relative to the ground, thus keeping it from acquiring enough lift to take off.

Some people think the treadmill will eventually move fast enough to cause air currents moving backwards to cause enough lift to make the plane take off.

The other theories I don't understand enough to post about, so let's debate!
 
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Red Squirrel

Red Squirrel

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Assuming the treadmill can really go that fast, I say it wont take off. The wheel bearings in the landing gear will burn up and the whole plane will go out in a massive inferno, killing everyone inside. Basically the plane will want to go forward, but to keep the rule true the treadmill will need to transfer that energy into the wheels to create enough friction to bring the plane back to original spot. That's a lot of energy in a small area.

Now a traditional treadmill going a fixed speed would not impede it much, if at all, the wheels would just have to spin faster.

Now if you really wanted to test if a plane can take off in a fixed position, could add a wall in front of each wheel. I think some planes could perhaps take off while others would not. Depends on engine/wing placement if a helicopter effect can be achieved.
 
MongGrel

MongGrel

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Lets not, as that was a Necro thread to begin with.

As this is, more or less without a link, to the one all ready in there.
 
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Red Squirrel

Red Squirrel

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Bubbleawsome said:
Can you clarify that? I can't seem to visualize that at all.
Click to expand...

Basically you can think of the wheels as just something used to create a low friction surface for the plane to sit on. In snow, skiis would work instead of wheels. With the treadmill going backwards and the thrust of the plane pushing it forward the wheels would just spin faster but the plane would still go. I imagine there would be a bit of resistance at first, since no wheel bearing is 100% perfect. So plane would probably start to go backwards a bit but then the thrust would overtake that motion and it would go forward.

That's assuming a plane that has free spinning wheels. I'm not sure if there are planes where they actually do use motorized wheels to assist take off, if there are, then those planes would probably have trouble taking off. But no matter what for a plane to work it needs to be able to provide enough thrust to create enough lift to stay in the air, so even such plane would eventually take off, just use more run way.

At least that's my guess.
 
MongGrel

MongGrel

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Have fun I guess.

2z5sirs.gif
 
Bubbleawsome

Bubbleawsome

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Red Squirrel said:
Basically you can think of the wheels as just something used to create a low friction surface for the plane to sit on. In snow, skiis would work instead of wheels. With the treadmill going backwards and the thrust of the plane pushing it forward the wheels would just spin faster but the plane would still go. I imagine there would be a bit of resistance at first, since no wheel bearing is 100% perfect. So plane would probably start to go backwards a bit but then the thrust would overtake that motion and it would go forward.

That's assuming a plane that has free spinning wheels. I'm not sure if there are planes where they actually do use motorized wheels to assist take off, if there are, then those planes would probably have trouble taking off. But no matter what for a plane to work it needs to be able to provide enough thrust to create enough lift to stay in the air, so even such plane would eventually take off, just use more run way.

At least that's my guess.
Click to expand...
Oh, I see. That makes more sense than the way I was seeing it. One heck of a debate when everyone agrees. :D
MongGrel said:
Have fun I guess.

2z5sirs.gif
Click to expand...
Come back if you enjoy shenanigans. :p
 
sandorski

sandorski

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It takes off, the speed of the treadmill makes no difference.

Discussed to death years ago.
 
Blitzvogel

Blitzvogel

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Oct 17, 2010
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No airspeed = no lift.

The treadmill doesn't mean much, as the thrust from the engines is independent of the treadmill and the wheels will just spin freely with the threadmill once rolling friction is overcome quite early into the "takeoff". Aircraft do not power their landing gear.
 
biostud

biostud

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Feb 27, 2003
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The jet engines will thrust air backwards pushing the plane forwards and the plain not having gained enough speed to take off, will crash in front of the treadmill in a giant explosion, depending on how much fuel is in the tanks.
 
Jeff7

Jeff7

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This one is about an airplane (a 747) on a (perfect) treadmill. The treadmill matches the speed of the airplane exactly. The key here is in the wording. It only matches the speed, so based on your interpretation of speed, you'll have different answers.
Click to expand...
If this is the case, then the maximum speed of the treadmill is the 747's takeoff speed. Google says that's somewhere in the general area of 170mph.
So the treadmill would be moving at 170mph.
The wheels would feel the equivalent of 340mph. If the bearings and tires can survive that, and the bearing friction force is not enough to slow the plane below takeoff speed, the plane takes off.
If the tires are indestructible and the bearings offer no friction, then the plane easily takes off.


Many people (including me) seem to think the best way to interpret the issue is that the treadmill is always going fast enough to prevent the plane from moving relative to the ground, thus keeping it from acquiring enough lift to take off.
Click to expand...
If that's the case, then the plane can't move. All you've done then is build a very complicated ground-based braking system that uses bearing friction to keep the plane stationary, to the point of utterly destroying the bearings and blasting apart the tires.
At that point, the plane's wheel hubs fall to the ground, scraping against a rapidly-moving treadmill made of an improbablium alloy. I don't have enough data on that material though to know what it will do when metal wheel hubs scrape against it at high speed.
Either way, abandoning the aircraft onto the treadmill at that point would be inadvisable.
 
J

JimmiG

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Feb 24, 2005
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Take-off speed of an airplane is relative to the air, not the ground (or a treadmill).
 
flexy

flexy

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Yeah I know, Necro of the century, but....

"provide enough thrust to create enough lift to stay in the air"

The thrust serves that a plane moves forward, as in "relative to the air".

It's the air moving across the wings which will lift a plane.

The forward thrust alone (with the plane stationary) will not lift the plane.

Ok...let's modify this.

Let's fix the plane to the treadmill.

The plane engines start, create forward thrust.

The only way for the (fixed) plane not to take off is when...
... the motorized treadmill belt is pulling back the plane (which is fixed to the belt) at the same speed.
The net result is ZERO air speed.

Without air speed --> Plane doesn't take off.
(Assuming an infinitely long treadmill here which would pull back the fixed plane as long as the experiment lasts).

So, according to THIS logic, the plane MUST take off if when its sitting freely on the belt. In this scenario the belt can not counter-act the forward thrust, it can not stop or in any way affect the plane.

So..I conclude that the plane moves forward, can create lift and then takes off.
 
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LTC8K6

LTC8K6

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Mar 10, 2004
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The treadmill is not even there, as far as a plane with free spinning wheels is concerned. It plays no part in the takeoff at all, for any practical discussion.

Also, shortly after the plane begins to move, the wings are starting to provide lift, and the weight on the wheels is getting lighter and lighter. The friction on the bearings is getting less and less, long before the plane actually lifts off.

Probably in the first 100 yards, there has already been a measurable reduction in the weight on the wheels.

A 747, with large enough pontoons, could take off upstream on the Mississippi river, and that is far more resistance than it would ever see on a giant treadmill.

Also keep in mind that a loaded 747 can take off with one engine inoperative. Even the loss of 25% of it's thrust does not prevent takeoff.

This giant treadmill is going to have to provide a lot of resistance to the rolling motion of a 747, to even start to affect takeoff.
 
sdifox

sdifox

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LTC8K6

LTC8K6

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sdifox said:
That's nothing compared to a whole space shuttle...
Click to expand...

Decent point, but that is an empty 747 rather than a 747 that is loaded to normal takeoff levels.

I don't think a normally loaded 747 could take off if you added the weight and drag of a space shuttle to it. :)

EDIT: Maybe it could with a long enough runway?
 
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