Mythbusters will finally test...

[Bobblehead]

That Straight dope had a good analogy, and it was referred to here.

Think of it like you are wearing rollerblades on the treadmill and holding a rope. the treadmill turns on and you start pulling on the rope. The initial jerk might be a bit of a jolt, but it is pretty easy to pull yourself forward. It almost makes no difference that you are on a treadmill, it is pretty easy to move forward by pulling on the rope.

the only difference in this case is instead of pulling on a rope the plane is pushing against the air, exactly the same way it does when it is taking off or flying.

The treadmill will have negligible effect on the plane, it will takeoff without a worry.

[photon]

Originally Posted by kermitology

Funny you should mention a Learjet 36..

This is the plane that I'm working with (currently in Little Rock)

Heh actually I mentioned it because I saw your previous post about it (had to pick a jet).

Ask your pilot how much thrust from the engines goes into overcoming the friction and rolling resistance of the wheels percentage wise. 1% or less?

And I'm jealous

[Barnes]

Originally Posted by Bobblehead

That Straight dope had a good analogy, and it was referred to here.

Think of it like you are wearing rollerblades on the treadmill and holding a rope. the treadmill turns on and you start pulling on the rope. The initial jerk might be a bit of a jolt, but it is pretty easy to pull yourself forward. It almost makes no difference that you are on a treadmill, it is pretty easy to move forward by pulling on the rope.

the only difference in this case is instead of pulling on a rope the plane is pushing against the air, exactly the same way it does when it is taking off or flying.

The treadmill will have negligible effect on the plane, it will takeoff without a worry.

That is a good analogy. Wheel spin would likely double but there is absolutly no reason why it would not take off. Save operator error.

[Fozzie_DeBear]

I say plausible, whatever surface the plane is launching from is irrelevant as long as it doesn't cause too much drag. The only thing that should stop a plane from taking off is insufficient airspeed over the wings and I don't see how a conveyor belt would do that...if the power was coming from the wheels (like a car) it would be a no brainer...

[Hack&Lube]

This really depends on how fast the conveyer belt is moving isn't it? If the conveyer belt is moving fast enough to negate the thrust of a propeller aircraft, gravity will keep the plane on the belt and airspeed over the wings won't be sufficient to cause enough lift to take-off.

When you are a running on a treadmill, do you feel any air resistance? No. It's because you are not moving through the air, you are basically stationary. The entire key is getting the conveyer belt to move fast enough and somehow get wheels that could survive something like that and we'll see.

This is another myth that takes some time to wrap your head around, like the one where the pigeons in a truck start flying - does the truck get lighter? (no)

[QuadCityImages]

This is really a great topic. At first I, like many others, were like, "of course it won't fly. It will stay stationary on the treadmill and not generate lift by air flowing over the wings."

Then I thought about it some, and realize that I was unconsciously comparing the plane to a car, and imagining the wheels turning to move the plane forward. Once I realized I was making that wrong assumption, it became clear that the plane would fly. The wheels will have to turn at 200 knots, or whatever twice as fast as normal is, but wheels on various fast cars do that all the time.

The rollerskates with a rope explanation really is the best. The wheels really have nothing to do with anything. All a planes wheels do is keep it from scraping the paint on the runway and allow it to steer. Assuming the wheels can turn fast enough, the conveyor could be going 5x faster than the takeoff speed and it wouldn't matter.

[sureLoss]

I think a better analogy is flying a kite.

When you were a kid flying a kite in the park, you had to run(fast) to get the kite to fly up in the air. However imagine trying to get that kite up in the air while running on a treadmill.

Don't think it will work.

[photon]

Originally Posted by sureLoss

I think a better analogy is flying a kite.

When you were a kid flying a kite in the park, you had to run(fast) to get the kite to fly up in the air. However imagine trying to get that kite up in the air while running on a treadmill.

Well the analogy is flawed for two reasons.. first the kite flies because the airspeed is high enough without the person moving (wind), and second the running only happens to get the kite up above the area close to the ground where the wind is slow up into the air where it's faster and more constant.

Plus the kid running is using friction with the ground (running) to provide the forward thrust. A better analogy would be a kid on a skateboard with a jet on it trying to launch a kite.

[Barnes]

Originally Posted by sureLoss

I think a better analogy is flying a kite.

When you were a kid flying a kite in the park, you had to run(fast) to get the kite to fly up in the air. However imagine trying to get that kite up in the air while running on a treadmill.

Don't think it will work.

This analogy is very flawed. It would be like a kid on a treadmill with a kite but the kite had engines. The treadmill and the kid are irrelevant.

[JohnnyB]

Originally Posted by photon

A better analogy would be a kid on a skateboard with a jet on it trying to launch a kite.

His analogy may be flawed, but you just made it a lot cooler.

I admit my mind has been changed since reading the start of this thread and thinking it was implausible.

[Regorium]

What if I had an infinitely fast conveyor belt? Using the rope on a treadmill analogy, if the wheels on my rollerblades were completely ideal, it would be pretty easy for me to pull myself forward no matter how fast the belt was moving.

But, if there is any semblance of friction, there will reach a point where the treadmill is going so fast, and I cannot pull myself forward anymore due to the backwards motion. ie. if I had an infinitely fast treadmill that can counteract the thrust of the plane, keeping it stationary, would it still fly? <--- in my opinion, that is what the myth is really asking.

I must be missing something.

[BlackArcher101]

Originally Posted by Regorium

What if I had an infinitely fast conveyor belt? Using the rope on a treadmill analogy, if the wheels on my rollerblades were completely ideal, it would be pretty easy for me to pull myself forward no matter how fast the belt was moving.

But, if there is any semblance of friction, there will reach a point where the treadmill is going so fast, and I cannot pull myself forward anymore due to the backwards motion. ie. if I had an infinitely fast treadmill that can counteract the thrust of the plane, keeping it stationary, would it still fly? <--- in my opinion, that is what the myth is really asking.

I must be missing something.

That's true... however, for the treadmill to be going fast enough for friction to negate thrust, the planes speed would be outrageous since the faster the plane goes, the faster the conveyor goes. If the plane had to travel that fast, then it could have taken off a long time before friction matches thrust.

[photon]

Originally Posted by Regorium

What if I had an infinitely fast conveyor belt? Using the rope on a treadmill analogy, if the wheels on my rollerblades were completely ideal, it would be pretty easy for me to pull myself forward no matter how fast the belt was moving.

But, if there is any semblance of friction, there will reach a point where the treadmill is going so fast, and I cannot pull myself forward anymore due to the backwards motion. ie. if I had an infinitely fast treadmill that can counteract the thrust of the plane, keeping it stationary, would it still fly? <--- in my opinion, that is what the myth is really asking.

I must be missing something.

If you had an infinitely fast treadmill, or one that was at least fast enough to counteract any thrust from the airplane, then in theory yes you are correct. Though that's not really what the myth is about IMO.

In practice if you had a treadmill that went that fast, the tires would blow out long before you'd get enough to counteract the thrust of the airplane. Well maybe a really small airplane with a really small engine you might be able to do it.

If you had tires that couldn't blow (solid or whatever), then you'd probably spin the wheels fast enough to cause the bearings to fail, the wheels would stop turning, and you'd either shear the landing gear off or cause the plane to flip.

And even if it was built well enough for none of that to happen, you'd still cause the airplane to flip because while the force from the engines goes through the plane's centre of gravity, the force coming from the landing gear isn't so it's going to cause the airplane to rotate the nose into the treadmill.

[VanCity Cowboy]

Originally Posted by photon

If you had an infinitely fast treadmill, or one that was at least fast enough to counteract any thrust from the airplane, then in theory yes you are correct. Though that's not really what the myth is about IMO.

In practice if you had a treadmill that went that fast, the tires would blow out long before you'd get enough to counteract the thrust of the airplane. Well maybe a really small airplane with a really small engine you might be able to do it.

If you had tires that couldn't blow (solid or whatever), then you'd probably spin the wheels fast enough to cause the bearings to fail, the wheels would stop turning, and you'd either shear the landing gear off or cause the plane to flip.

And even if it was built well enough for none of that to happen, you'd still cause the airplane to flip because while the force from the engines goes through the plane's centre of gravity, the force coming from the landing gear isn't so it's going to cause the airplane to rotate the nose into the treadmill.

if you have an infinitely fast conveyor belt, it would increase the wheel bearing fiction.

If you have the wheel friction decrease to approach zero, then the conveyor belt would have smaller and smaller effect.

Now we've all pushed a heavy car, so we've all seen that wheels are generally pretty easy to rotate and there's not a lot of energy lost to the friction of the wheel.

If the wheels inner bearing friction were enough to stop the plane from moving forward would the plane even need brakes to land? wouldn't this huge amount of friction be able to stop the plane?

[Regorium]

Originally Posted by VanCity Cowboy

if you have an infinitely fast conveyor belt, it would increase the wheel bearing fiction.

If you have the wheel friction decrease to approach zero, then the conveyor belt would have smaller and smaller effect.

Now we've all pushed a heavy car, so we've all seen that wheels are generally pretty easy to rotate and there's not a lot of energy lost to the friction of the wheel.

If the wheels inner bearing friction were enough to stop the plane from moving forward would the plane even need brakes to land? wouldn't this huge amount of friction be able to stop the plane?

It was just a theoretical question using the extremes just to figure out the concept. It's obviously impossible to have an infinitely fast conveyor belt, with such well built mechanical parts to generate enough friction to stop a jet engine.

Though I think I understand the spirit of the myth being more about how the plane does not need to interact with the ground to accelerate, unlike humans or cars. Instead of whether or not a plane can lift off with zero ground speed.

[photon]

Originally Posted by Regorium

Though I think I understand the spirit of the myth being more about how the plane does not need to interact with the ground to accelerate

Totally agree.

[metal_geek]

What would happen in the opposite situation... a plane landing on a conveyor belt moving the same speed at the plan in the opposite direction? I would think the plane would land on the belt and land the same as normal minus the distance caused by the breaking friction. The only difference would be the "Speed" of the wheels would be double that of normal landing speed on contact..
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Ford puma picture

[BlackArcher101]

We have LIFTOFF!!!! as per this page http://www.kottke.org/08/01/mythbust...-conveyor-belt

Now to wait for the video.

[photon]

[Bobblehead]

And the official Mythbusters forum goes crazy with people who still don't believe the plane would fly. Even the Executive Producer posts that they did it correctly and explains why. I don't think it will matter to those who refuse to believe.

http://community.discovery.com/eve/f...m/f/9401967776