That sounds like a paradox, if the airplane is actually changing its position then it has a speed, therefore the treadmill will match it and the plane will stop moving, but since it will match it again then the treadmill will stop too.
Airplane on a treadmill
- Thread starter Scn64
- Start date
That sounds like a paradox, if the airplane is actually changing its position then it has a speed, therefore the treadmill will match it and the plane will stop moving, but since it will match it again then the treadmill will stop too.
unclehobart
New Member
K62
New Member
Well, obviously if you put a harrier on a treadmill it would have no problem taking off because they can rely on the vectored thrust to lift the aircraft up.
If we are just talking about "conventional" aircraft (Cessna to a 747 for example) They rely on their wings to generate lift. They could not take off from a tread that matched it's speed, because it's position does not change.
The way I see it, any aircraft that could take off in a vacuum could take off on a treadmill. Those that couldn't, couldn't. (Ok, not a good example, hmm.. lets pretend the engines didn't require oxygen either)
K62
New Member
The CAN just point and shoot, but they do not often because of the massive amount of fuel it consumes. They will usually keep the thrust vectored slightly downward and take off like a conventional aircraft. This way they do not totally rely on the thrust to lift them. They will USE their wings to generate lift also.
Lol, sorry.. I thought we were talking about conventional aircraft here. It's a given that a harrier could take off in this situation.
*edit*
The example was a 747 which might have a take off speed of... 200 knots? (That's just a guess) So, you would need 200 knots flowing around the wings to create enough lift to allow it to leave the ground.
If the wind is moving fast enough, yes. But, then the tread mill is really irrelevant. It could just be sitting there if it was windy enough. It would have to be a pretty windy day!
chcr
Too cute for words
With rockets, tonks. a JATO assisted takeoff is a rocket takeoff, not an airplane takeoff. You're right though, it's pretty impressive. I think that's outside the parameters of this thought experiment though.
Another note, tonks. The engines of most jet or prop planes are not designed to generate any lift at all. In fact, they're rather carefully designed to send all their thrust forward through the center of mass of the airplane.
Unc, your buffer lift idea has merit but I think if it would work at all, it'd only work on a light plane. There really isn't enough data on the treadmill parameters to even make a guess how much of how much buffer lift would be available but I think it unlikely to be several tons. I know that a 747 needs over 360kph of airspeed to lift off. If the treadmill is matching that speed in the opposite direction, I think you'd find that any buffer lift generated by the speed of the treadmill whizzing by would be insignificant. The answer to the question is still no, IMO.
K62s example points it out rather nicely. On an aircraft carrier, they turn into the wind and then shoot the plane off the ship with a very powerful catapult. This is because airplanes need speed through the air to fly. The treadmill experiment is designed specifically to eliminate that.
chcr
Too cute for words
BeardofPants
New Member
Think of it this way: the plane on the treadmill is stationary. Can a stationary plane take off? No. If that's still giving you hernias, try this: tickle rob next time he's running on a treadmill - does that generate enough force to trip you over? No. Now, tickle rob whilst he's jogging down the road - unless you're a really good tickler, you're gonna get trampled/tripped/whatever. This question assumes that the force of the treadmill will match the force of the plane. That's the key. It is stationary.
Scn64
New Member
The answer is, yes it would take off. The plane would move forward because the jet engines push against the air, not the ground. It doesn't matter how fast the treadmill is moving. It could even be moving faster than the plane. The wheels would spin at twice the speed the plane is actually moving.
Think of this example: A man is strapped to a skateboard that's on a moving treadmill. You are standing behind the man (not on the treadmill) and you have a stick against his back. Even if the treadmill is moving, it wont push the man back because the stick is holding him in place. Now, if you push forward with that stick, the man will move forward too because you and the stick are not affected by the moving treadmill, just as the jets are not affected by the moving ground because they "push" against the air.
Think of this example: A man is strapped to a skateboard that's on a moving treadmill. You are standing behind the man (not on the treadmill) and you have a stick against his back. Even if the treadmill is moving, it wont push the man back because the stick is holding him in place. Now, if you push forward with that stick, the man will move forward too because you and the stick are not affected by the moving treadmill, just as the jets are not affected by the moving ground because they "push" against the air.
K62
New Member
The question says that the tread mill matches the speed of the airplane, not just the wheels!!!!! The Airplane can not move forward because the treadmill moves backward in the exact same speed!
BeardofPants
New Member
And that's what people seem to keep missing. It has nothing to do with either the wheels or the engine. The total movement of the plane will be matched EXACTLY by the movement of the treadmill. Read the question again.
Scn64
New Member
Yes, the treadmill matches the speed of the plane but that doesn't matter. As I said, the treadmill could be going even faster than the plane and it would still take off. The wheels are not what moves the plane forward. The jets pushing against air move it forward. The wheels would spin "freely" which means they'd spin at the combined speed of the plane and treadmill.
BeardofPants
New Member
BeardofPants
New Member
It doesn't have anything to do with wheels/friction/engines. A plane will not fly without lift. You can supply all the thrust you need, but if the treadmill is negating that thrust, you won't get lift. It's simply. Plane = +v, threadmill = -v, thus velocity is equal to zero. Zero velocity = zero lift.
Scn64
New Member
It doesn't have anything to do with wheels/friction/engines. A plane will not fly without lift. You can supply all the thrust you need, but if the treadmill is negating that thrust, you won't get lift. It's simply. Plane = +v, threadmill = -v, thus velocity is equal to zero. Zero velocity = zero lift.
I'm saying that the plane WILL move forward and, therefore, create the necessary lift. The treadmill is working against the wheels, not the engines...and it's the engines that move the plane forward. The wheels just provide a way to reduce friction against the plane, and in this case they would be doing their job very well by spinning at the combined speed of the treadmill and plane.
K62
New Member
Relative to the ground and air you are stationary because the treadmill is moving the same speed going the opposite direction of you!
If you are talking about the treadmill moving the same speed of the wheels ONLY, this may be different. The question says "which turns in the opposite direction that the plane is facing, is capable of precisely matching the speed of the airplane." You say the plane moves forward, but it can not because the treadmill moves backward the same speed.
BeardofPants
New Member
The question doesn't SAY wheels, it says against the WHOLE plane. There's a distinction, man. If the threadmill matches the WHOLE plane's speed, then the wheels have nothing to do with it.
BeardofPants
New Member
Um, wot he said. 