[Guest]

This is a very highly debated topic, even though scaled tests have been conducted. The popular Mythbusters show on discovery did it full scale, but some still dispute the way they did it. You can probably find thousands of forums that host this same question, and I wouldn't be suprised if it has even been posted here before. So here goes!

A large passenger jet sits on a tredmill the size of a normal runway that they would normally take off on. When the jet starts up, it climbs in speed slowly. The tredmill matches the jet's speed EXACTLY. For every mile an hour the jet climbs, the tredmill goes in REVERSE that exact amount as-well.

The grand question: Will the plane take-off?

Assume that:

The wind is at 0 MPH

The Jet is powered by turbines

Edited September 6, 2008 by RedRum

[unreality]

I don't think AAA understands a simple fact:

the airplane wheels are FREE-SPINNING, and have no effect on the speed of the plane. They're just for smooth landings and take-offs, and aren't connected to anything. They're just free-spinning wheels

[Yoruichi-san]

Okay, I agree with AAAsn888s. Given that there is 0mph wind as stated in the opening, the plane will not take off.

I respect Taliesin's pilot background, but I have a background in fluid dynamics (flow of fluids, including AIR as in this case), and here is what I know:

Defining "taking off" as lifting in the air, the reason a plane takes off is that there is because the MOVING AIR traveling above the wing more slowly than the MOVING AIR traveling below the wing. The shape of airplane wings is designed so the bottom is straight (shorter distance between front of wing and back of wing) and the top is curved (longer distance). This means that the air going below the wing gets to the back of the wing faster, which creates more air pressure under the wing and allows the wing (and the plane) to lift.

As I've tried to emphasize, the key is MOVING AIR. In order to take off, planes need a runway to accelerate to get the (relatively still) air moving around the wings (relative to the plane) fast enough so that there is enough of a velocity/pressure gradient (difference) created to lift the heavy plane.

If the plane, and most importantly, the wings, are not moving relative to the air, then there is no MOVING AIR around the wings, so there is no velocity/pressure gradient created, so the plane will not lift off.

[unreality]

Ysan, the plane DOES move forward through the air, regardless of the treadmill. You're right that IF the plane was standing still, it couldn't get up the acceleration to lift off. But the plane does move relative to the air, since the treadmill doesn't affect the jets on the back, just the free-spinning wheels. Thus the plane takes off

Edited September 6, 2008 by unreality

[Yoruichi-san]

Ysan, the plane DOES move forward through the air, regardless of the treadmill. You're right that IF the plane was standing still, it couldn't get up the acceleration to lift off. But the plane does move relative to the air, since the treadmill doesn't affect the jets on the back, just the free-spinning wheels. Thus the plane takes off

No...the way it says in the opening, the plane is not moving relative to the ground, and the wind is 0mph relative to the ground...so the plane and the air are still relative to each other...

[unreality]

No...the way it says in the opening, the plane is not moving relative to the ground, and the wind is 0mph relative to the ground...so the plane and the air are still relative to each other...

It never says that, it says the treadmill matches the airplane's speed - however all this does is double the wheel's speed without affecting the airplane speed

[Yoruichi-san]

Well, thinking about it more, I suppose the plane may be able to take off, depending on the coefficient of friction/drag of the treadmill...the friction from the moving treadmill will induce some motion in the air, by "dragging" the air with it. So I suppose if the treadmill went fast enough, there could be enough induced air flow to lift the plane...

[Guest]

No...the way it says in the opening, the plane is not moving relative to the ground

Care to point out where that's said?

[Yoruichi-san]

It never says that, it says the treadmill matches the airplane's speed - however all this does is double the wheel's speed without affecting the airplane speed

It's not the airplane's own speed that matters, it's the airplane's speed relative to the air. If the treadmill is matching the plane's speed, than the plane's speed relative to the air if 0mph. But that is neglecting friction, which I discuss in my previous post.

[Yoruichi-san]

Care to point out where that's said?

Isn't that what it means for the treadmill to match the plane's speed exactly? I.e. the plane is going 0mph relative to the ground, and the wind (which is measured relative to the ground) is 0mph...

[Guest]

Well, thinking about it more, I suppose the plane may be able to take off, depending on the coefficient of friction/drag of the treadmill...the friction from the moving treadmill will induce some motion in the air, by "dragging" the air with it.

This really makes no sense. Whether the surface of the treadmill has frictionless (they won't cause the wheels to spin at all) or have enough friction where there's no slip between the wheels and the material is irrelevant. The thrust of the plane's engine(s) will push it along the surface of the treadmill.

So I suppose if the treadmill went fast enough, there could be enough induced air flow to lift the plane...

The treadmill goes the same speed as the plane in the opposite direction. The plane takes off.

[unreality]

If the treadmill is matching the plane's speed, than the plane's speed relative to the air if 0mph.

That is where you go wrong, Ysan. The wheels are free-spinning. If the plane was turned off and the treadmill was still going, the airplane would stay in one place since the wheels would just turn (assuming friction is minimal)

Now if the airplane turns on, it moves forward. The treadmill just succeeds in moving the wheels, which doesn't affect anything.

Think of a plane on an icy lake - can it take off? Yes, because the speed comes from air thrusters, not wheels

Edited September 6, 2008 by unreality

[Yoruichi-san]

Uh...well, for Unreality's point, the opening needs to be clarified...if the plane is actually moving relative to the ground, then my answer is yes, the plane can take off, but I thought the point of the question was if it wasn't...

As for the friction I'm talking about, I'm not talking about the coefficient of friction between the treadmill and the wheels, I'm talking about the "drag" coefficient, the coefficient of friction between the air and the treadmill, which would induce air flow around the wings.

[Guest]

If the treadmill is matching the plane's speed, than the plane's speed relative to the air if 0mph.

Wrong. The plane will take off just as it did on Mythbusters and of course the plane's speed relative to the air is not 0.

Isn't that what it means for the treadmill to match the plane's speed exactly? I.e. the plane is going 0mph relative to the ground, and the wind (which is measured relative to the ground) is 0mph...

When the planes jet engines provide enough thrust, it pushes the still air and moves forward relative to the ground, whether or not it's on a treadmill. So no, the plane is not going 0mph.

The treadmill moving backwards and matching the speed of the plane has no noticeable effect on take-off.

I still am not following where you got "it says in the opening, the plane is not moving relative to the ground" from. Saying that the treadmill will match the speed of the plane is not the same as saying the plane will not move relative to the ground. That's just the incorrect conclusion some have come up with and believe the plane can't become airborne. It can.

[dms172]

im just throwing this out there if the plane is still going forard because of thrusters or engines and the wheels are free spinning whats the point of a plane being on a treadmill

[unreality]

dms: exactly! It doesn't make a difference except for doubling the speed of the wheels. The airplane's speed relative to the ground & the air still increases and it goes forward and then takes off

[Guest]

Uh...well, for Unreality's point, the opening needs to be clarified...if the plane is actually moving relative to the ground, then my answer is yes, the plane can take off, but I thought the point of the question was if it wasn't...

Whether or not the plane ends up moving relative to the ground is not part of the OP. The point of the OP is for the riddle solver to conclude whether or not the plane will end up moving relative to the ground. Those that believe the plane will take off do, those that believe it won't take off don't. If the OP made mention of the plane moving or not moving in relation to the ground, there wouldn't be anything to figure out.

As for the friction I'm talking about, I'm not talking about the coefficient of friction between the treadmill and the wheels, I'm talking about the "drag" coefficient, the coefficient of friction between the air and the treadmill, which would induce air flow around the wings.

You don't believe that minimal airflow caused by the plane moving twice as fast in relation to the treadmill under it as it would have on a normal runway would stop the plane from taking off, do you?

[Brandonb]

Ok, I'll admit that I haven't read the entire thread.

It's impossible. An airplane does not fly because of its engines. It flies because of the lift it gets when the wings "ramp up" the high speed air moving underneath them.

No matter how fast the engines can make it go in a prone position, it is not possible for '0' wind resistance underneath the wings to elevate the airplane.

[Yoruichi-san]

Okay, in lieu of further clarification from the author, here are my final conditional answers:

If the point of the treadmill was NOT to have the plane be not moving relative to the ground, then yes, the plane will take off, the thrusters just have to provide twice the speed.

If the point of the treadmill was to have the plane be not moving, then:

1) Assuming a frictionless treadmill (no drag), the plane would not take off because there is no moving air (no velocity/pressure gradient).

2) Assuming there is a non-zero coefficient of drag, then the treadmill would "drag" air along with it, and induce air flow around the wings, which, if the plane and hence the treadmill goes fast enough, could cause a high enough velocity/pressure gradient to cause the plane to lift off.

[Guest]

Ok, I'll admit that I haven't read the entire thread.

It's impossible. An airplane does not fly because of its engines. It flies because of the lift it gets when the wings "ramp up" the high speed air moving underneath them.

No matter how fast the engines can make it go in a prone position, it is not possible for '0' wind resistance underneath the wings to elevate the airplane.

The treadmill only exists to move the aircrafts wheels, the aircraft moves, it takes off

edit to add to Yoruichi-san's reply - what if it was a glider on a very hot day, well greased wheels and a rising thermal ???

I believe you have covered your basis and that has given us all a 'lift'

Edited September 6, 2008 by Lost in space

[Guest]

Ok, I'll admit that I haven't read the entire thread.

It's impossible. An airplane does not fly because of its engines. It flies because of the lift it gets when the wings "ramp up" the high speed air moving underneath them.

No matter how fast the engines can make it go in a prone position, it is not possible for '0' wind resistance underneath the wings to elevate the airplane.

[moderating]

You don't think it's fair to have others explain why you're wrong because you didn't want to bother with reading the explanations already given, but you didn't mind bothering to chime in with the wrong conclusion, do you? Please read the explanations others went already went through the trouble of giving and then post counter-arguments. Thanks.

[/moderating]

[Guest]

What about friction?

I mention that in one of my earlier posts. I didn't want to mention it again to make the explaination more confusing

[Guest]

Very well explained, but completely inaccurate - in the wind tunnel - assuming it is large enough the plane WOULD take off - it takes wind (moving air) over the wings to create lift - groundspeed is irrelevant.

Theoretically - in a steady 300 MPH headwind - I could hover in a jet with a 0 MPH groundspeed...

This is what I said..

[Guest]

Okay, in lieu of further clarification from the author, here are my final conditional answers:

What clarification would you like? The premise is pretty straight forward.

If the point of the treadmill was NOT to have the plane be not moving relative to the ground, then yes, the plane will take off, the thrusters just have to provide twice the speed.

What action the treadmill has on the plane (or doesn't have) is not dependent of "what the point of the treadmill" is. The treadmill moves exactly the speed of the plane but in the opposite direction. What the OP or anyone else claims is "its point" is irrelevant.

If the point of the treadmill was to have the plane be not moving, then:

I really don't know what you're thinking here. The treadmill has no point. It does what the OP promises it theoretically will do.

1) Assuming a frictionless treadmill (no drag), the plane would not take off because there is no moving air (no velocity/pressure gradient).

The plane takes off. There is moving air because the jet engines push air and propel it forward. How fast the wheels spin or don't spin is irrelevant.

Why would you assume a frictionless treadmill anyway?

2) Assuming there is a non-zero coefficient of drag, then the treadmill would "drag" air along with it, and induce air flow around the wings, which, if the plane and hence the treadmill goes fast enough, could cause a high enough velocity/pressure gradient to cause the plane to lift off.

Oh brother. The plane takes off for the same reason every other plane takes off. Do you think the plane will be standing still and air flow caused by a rapidly moving treadmill will make it airborne?

[Guest]

Say that the plane is already flying in the air, and for some reason has its wheels out. If the wheels were spinning at the same speed as the plane due to some force(not wind), that would not affect the speed of the plane because the wheels are not what give the plane speed. The turbines are. That is essentially what is happening here.

Edit: While friction may have some minor impact, it would be incredibly small, and of little importance to this question(if any).

Im pretty sure some aircraft do this when coming in to land. They use the air to spin the wheels up so than on touch down is reduces the effect on inertia. This doesn't help in the discussion though..

[Guest]

Phrased another way - according to Scraff's assertions, if the plane just sat on the runway and the runway started moving in the opposite direction the plane would stand still because the wheels are "free spinning" - well, it wouldn't. It would require a force equal to the total force moving it backward to keep it in a stationary position relative to a fixed object sitting next to it on the tarmac. The only way the plane advances is if the force advancing it increased, thereby causing it to move FASTER than the runway - again breaking the premise...

PS - hope there is no offense Scraff - I am enjoying the conversation - just not agreeing...

This is correct. But things lke inertia come into it. Think about pulling a table cloth off a table, whith plates/cups still on.