[LUconn]

I've been reading this debate raging on another messageboard for sometime now that is like over 17 pages and still running. I came into it with one opinion, but I think I've changed my mind. I'll pose it here knowing that it's based in physics and none of you are physicists. The hypothetical situation is:

Could a plane take off on a giant runway/conveyor belt that is running against the direction in which the plane is trying to go? And I guess to condense things up, it is a treadmill that can go faster than a plane.

The Mythbusters have actually finished taping a show that includes this, but it hasn't aired yet.

[RagingTireFire]

It's a simple answer. No. The plane's speed against the air is what provides lift. It doesn't matter how fast the plane's wheels are going.

[adam42381]

I would lean towards the conclusion that it could take off. Isn't the point of the runway takeoff to get the engine revved up to the point where it can lift off the ground. I would think that it would still lift since the speed the wheels are moving would be equivalent to the the speed the plane would be moving if it were traveling down the runway. The force on the engine should be the same. I could be way off here.

[Innocent Bystander]

It's a simple answer. No. The plane's speed against the air is what provides lift. It doesn't matter how fast the plane's wheels are going.

This is the correct answer

[HenryGale]

It's a simple answer. No. The plane's speed against the air is what provides lift. It doesn't matter how fast the plane's wheels are going.

This is the correct answer

yup

[LUconn]

It's a simple answer. No. The plane's speed against the air is what provides lift. It doesn't matter how fast the plane's wheels are going.

I think you're half right. It doesn't matter how fast the wheels are going. But it does take off. The force pushing the plane forward isn't coming from the wheels spinning, it's coming from the engines on the wings, or on the nose. So I don't think it'll slow down the plane one bit. It will just cause the free spinning wheels, to spin tiwce as fast as they normally would

[JDUB]

i'm gonna say no, because in order for it to lift off the ground the air has to hit the flaps or whatever they're called to propel it up. if it isn't moving, the jets do not push up, they only push straight ahead. so there would be no air under the flaps to lift the plane off the ground

[RagingTireFire]

It's a simple answer. No. The plane's speed against the air is what provides lift. It doesn't matter how fast the plane's wheels are going.

I think you're half right. It doesn't matter how fast the wheels are going. But it does take off. The force pushing the plane forward isn't coming from the wheels spinning, it's coming from the engines on the wings, or on the nose. So I don't think it'll slow down the plane one bit. It will just cause the free spinning wheels, to spin tiwce as fast as they normally would

The principle of flight is based on air moving under the wings at a faster speed than the air moving over the wings. At takeoff, the engines are there to provide the thrust necessary for that to happen, not necessarily to make the plane go faster. In this scenario, there is nothing to make the air move any faster so, for all intents and purposes, the plane may as well be sitting still. What you're describing is a very fast bus.

[LUconn]

I don't understand why the air wouldn't move any faster. The plane is still going from point A (the back of the runway) to point B (the other end) just as fast as it normally would, the ground under it just happens to be moving.

[vastrightwinger]

It has everything to do with the airspeed over the wings. This is why WWII planes had to be strapped down to Aircraft carriers b/c they would just lift off the deck when the ship was at full speed. Also, when a hurricane is approaching, all the plane owners down hear scramble to secure their wings because they will lift off the ground by force of the wind. So no, a treadmill will have no effect on the planes ability to lift off.

[Ed Dantes]

You guys listen to Neal Boortz too much. If you're on a treadmill, you're stationary and there is no air running across the wings. You can't generate lift if no air runs across the wings.

[adam42381]

Mythbusters is airing the episode on January 30th.

http://dsc.discovery.com/tv-schedules/s ... x&start=10

[Ed Dantes]

I understand how some are going to say that the propeller acts as an air screw and pushes the air over the wings, but the propeller does very little in this regard. In fact, many planes don't even have propellers in front of the wings!

Where are they on this baby?

[Ed Dantes]

I don't understand why the air wouldn't move any faster. The plane is still going from point A (the back of the runway) to point B (the other end) just as fast as it normally would, the ground under it just happens to be moving.

But you're not going from point A to point B; you're staying stationary somewhere between A and B.

[LUconn]

what? I thought we were in agreement over this. I swear I had read this entire thread and you were in agreement with me, but now I've gone back and read your posts and you're not. I call shennanigans! You must have changed your post.

[LUconn]

I don't understand why the air wouldn't move any faster. The plane is still going from point A (the back of the runway) to point B (the other end) just as fast as it normally would, the ground under it just happens to be moving.

But you're not going from point A to point B; you're staying stationary somewhere between A and B.

That's not the case though. In the case of your jet pictured above, the engines on the tail are pushing the plane forward. Lets assume for a second that you start out on sold ground. As soon as the plane crosses over onto the moving treadmill, the plane will continue to move forward at that same rate but the wheels on the tread will now move twice as fast. The ground movement doesn't affect the engines pushing it.

[Ed Dantes]

I don't understand why the air wouldn't move any faster. The plane is still going from point A (the back of the runway) to point B (the other end) just as fast as it normally would, the ground under it just happens to be moving.

But you're not going from point A to point B; you're staying stationary somewhere between A and B.

That's not the case though. In the case of your jet pictured above, the engines on the tail are pushing the plane forward. Lets assume for a second that you start out on sold ground. As soon as the plane crosses over onto the moving treadmill, the plane will continue to move forward at that same rate but the wheels on the tread will now move twice as fast. The ground movement doesn't affect the engines pushing it.

If I'm running full speed onto a down escalator, which is going as fast down as I'm going up, my legs aren't moving twice as fast to keep me in that position.

The fact is, you need to have a net gain of airspeed in order to take off. All things being equal, you don't have that if the plane cannot overcome the treadmill.

(And yes, I did change my post -- after talking with a pilot).

[LUconn]

that's because your legs are providing the thrust which is pushing off the ground (which is now moving). If you had a force that was pushing you that was not using the ground, you wouldn't slow down.

[adam42381]

what? I thought we were in agreement over this. I swear I had read this entire thread and you were in agreement with me, but now I've gone back and read your posts and you're not. I call shennanigans! You must have changed your post.

I thought I was losing it. I opened this thread 3 times and each time Ed had a different take on things. I thought I was losing it because I didn't see where it was edited (of course, I might be thinking of another board which tells if the post has been edited or not).

[ALUmnus]

(And yes, I did change my post -- after talking with a pilot).

That's weak. Take it like a man.

[Ed Dantes]

that's because your legs are providing the thrust which is pushing off the ground (which is now moving). If you had a force that was pushing you that was not using the ground, you wouldn't slow down.

If I had a parachute behind me as I was running, the parachute would not fill up with air because there is no net of air as I'm running up the stairs. It'd be limper than that Red Sox poster on prom night.

[Ed Dantes]

(And yes, I did change my post -- after talking with a pilot).

That's weak. Take it like a man.

Sorry for speaking to an expert on the subject. Geez!

[Ed Dantes]

That's not the case though. In the case of your jet pictured above, the engines on the tail are pushing the plane forward. Lets assume for a second that you start out on sold ground. As soon as the plane crosses over onto the moving treadmill, the plane will continue to move forward at that same rate but the wheels on the tread will now move twice as fast. The ground movement doesn't affect the engines pushing it.

Seriously, since when did wheels dictate whether or not a plane could take off?

[LUconn]

They dont! That's my point.

[Ed Dantes]

The net force of a plane going forward and the conveyor belt going backward is zeroed out. At that point, you're left with a plane on runway, not going anywhere. Those don't fly, you know.