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Post by BlueDolphin on Sept 25, 2007 19:55:37 GMT -5
Say we have a very long treadmill. So long that you can effectively ignore the ends. You put a plane on it and tell the pilot to start to take off. As his engines speed up, you turn the treadmill to run in the opposite direction matching the speed of the plane as measured from the ground if it was on a regular runway.
Can the plane take off?
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Post by bezzerkker on Sept 27, 2007 3:00:27 GMT -5
I'd say no. The wind resistance a plane experiences as it moves forward helps it in take off, not simply the engines running and the wheels moving.
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Post by BlueDolphin on Sept 27, 2007 23:00:58 GMT -5
That was my first answer, but now I'm more unsure. Can the plane move with respect to ground even with a treadmill underneath it?
A car or a person cannot move on a treadmill because they push off the ground to move. But planes move by pushing the air. The wheels are supports only. They can spin freely on the ground.
I think that since it pushes the air, it will end up moving the same speed more or less with respect to ground and thus the air too (assuming the wheels have negligible friction pulling the plane backwards).
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Post by bezzerkker on Sept 28, 2007 3:41:24 GMT -5
it'd move forward some due to its engines pushing air back, but I doubt it would ever get enough power to lift. And, the treadmill would compensate for any forward motion the jet engines would add, yet the lack of air rusing under its wings would prevent lift.
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Post by BlueDolphin on Sept 28, 2007 12:24:44 GMT -5
I would depend on the friction the wheels are pulling the plane back. At the moment of "takeoff" the treadmill will be spinning backwards at the same rate the plane moves forward. The plane will still be moving in relation to the air however. The wheels will just be spinning at twice the speed as usual. If this spinning takes out too much energy from the engines, then it can't fly, but otherwise, it should take off as usual I think.
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Post by bezzerkker on Sept 30, 2007 18:19:01 GMT -5
It's like trying to get a kite to fly without wind. You need the wind to lift the plane up. If the plane is not moving, but staying the same spot on the treadmill, it will not have enough air resistance. Jet engines move a plane forward, not up. There is a part on the wings that changes angle as the pilot pushes down or pulls up on the steering wheel. This part directs the air flow, changing the plane's lift. During take off, this part of the plane is angled downward so the air resistance pushes upwards against it as the plane is moving at a high speed down the runway. This high speed is necessary in order for there to be the proper amount of resistance. There is no resistance, barring wind, storms, hurricanes, tornadoes, if the plane is not moving in relation to where is started. Here's something you should try. Sprint as fast as you can. You'll hear air rushing past your ears and pushing against you. Now, run the same speed on a treadmill. You don't have the air pushing against you.
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Post by BlueDolphin on Sept 30, 2007 20:40:35 GMT -5
It's like trying to get a kite to fly without wind. You need the wind to lift the plane up. If the plane is not moving, but staying the same spot on the treadmill, it will not have enough air resistance. What I'm trying to say is that the plane might not remain in the same spot on the treadmill even when it is spinning. You have to push off of the treadmill to move on it which is why you can't move while on a treadmill. This is different from airplanes since they push off the air to move. This means that it might be possible for a plane to move in relation to the air even with the treadmill spinning underneath it trying to stop it. Like think of a wagon. If you pull the wagon while you are standing on the treadmill, it won't move relative to the ground. Say you pull at 1m/s while the treadmill goes at -1m/s you end up staying still relative to the ground. But if you pull the wagon alongside the treadmill at 1 m/s, the treadmill won't be able to stop its movement going at -1m/s. All this does it cause the wheels to spin backwards at 2m/s, but the wagon moves forward (and experiences wind) nonetheless.
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Post by Evilduck on Oct 9, 2007 20:05:58 GMT -5
I have to agree with Jeff. The treadmill could keep a car stationary by moving in the opposite direction, but not a plane.
The plane pushes against the air, not the ground. So if the wheels of the plane just spun freely, they could soak up anything the treadmill did and allow the engines to push it forward.
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Post by Arachis on Oct 10, 2007 7:03:53 GMT -5
the problem I think, is that the airplane wheels dont spin freely. There is friction between the wheels and the axis. Hence, the plane would be moving backwards a bit on the treadmill, and it would be a question of how fast the treadmill is going versus how fast the plane can push.
Also, could the plane take off in reverse if it didnt have wheels by hitting on the breaks?
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Post by Evilduck on Oct 10, 2007 23:44:11 GMT -5
I would say that a bit of rotational friction is negligible compared to the thrust from a jet engine. And I don't think planes can fly in reverse.
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Post by Random on Oct 12, 2007 18:57:39 GMT -5
yea because of the way wings work a jet could not take off backwards Basically because of As his engines speed up, you turn the treadmill to run in the opposite direction matching the speed of the plane as measured from the ground if it was on a regular runway. the plane will never take off. The only way it would be able to in this situation is if it out-ran the treadmill by the same margin that is needed for it to take off normally.
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Post by BlueDolphin on Oct 12, 2007 19:48:15 GMT -5
But in the wagon analogy, this might not matter. Running a treadmill backwards at 1m/s while you pull forwards 1m/s causes the wheels to turn backwards at 2m/s, but the wagon moves nonetheless. Since engines provide airspeed, running an engine to move the plane at 1m/s would make the plane move through the air at 1m/s regardless of what the ground is doing underneath. The wheels would just have to spin faster to compensate.
Maybe the wording isn't very good, but I would think that since the engines don't push off of the ground, it really would outrun the treadmill every time no matter how fast the treadmill spun backwards.
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Post by Random on Oct 12, 2007 21:25:07 GMT -5
I'm not 100% sure but I'm fairly certain thats not how physics work.
I'm having trouble phrasing what I mean but pretty much what the plane is pushing off of doesn't matter I don't think. All a treadmill does is give things a negative velocity, otherwise nothing magically changes when compared to a normal situation.
Also in that last example, thats not how jets/jet engines work. For a jet on a treadmill that is moving backwards at 1 m/s, it would have to get 2 m/s (relative to the ground) to effectively have the amount of lift provided by moving through air at 1 m/s. If the turbines gave the jet enough speed to normally move 1 m/s relative to the ground, and the treadmill was moving 1 m/s as well, the jet would not be moving at all relative to the air/ground and thus would generate 0 lift.
The only way a jet could take off like you guys are talking about is if its engines somehow shot wind and extremely high speeds over the wings I think and I don't even know if that works.
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Post by BlueDolphin on Oct 12, 2007 22:52:50 GMT -5
I'm not 100% sure but I'm fairly certain thats not how physics work. I'm having trouble phrasing what I mean but pretty much what the plane is pushing off of doesn't matter I don't think. All a treadmill does is give things a negative velocity, otherwise nothing magically changes when compared to a normal situation. I am in agreement here. My answer does rely on this assumption but with a small difference. It does matter what you are pushing off of. If you are running on a treadmill, you are moving with respect to the ground since your feet are pushing off of the ground. Relative speeds do matter. However, treadmills do not move the air backwards. They only move the surface. If you run on a treadmill, you don't feel the wind blowing past you because it doesn't force the air backwards as well. A jet engine moves the plane with respect to the air. If a treadmill is moving backwards at 1m/s the air around it is still at rest with respect to the ground. This means the engine will move the airplane at pretty much the same speed with respect to the ground as without a treadmill. Here is another interesting thought experiment. Imagine that your goal is to keep the wagon from moving. You simply hold onto the handle. Now the treadmill moves at -1m/s while the wagon obviously cannot move with respect to the ground. However if treadmills really canceled out movement, the wagon ought to be going the other way at -1m/s since you are not giving it 1m/s velocity to "cancel it out" However this is an impossibility. Since it cannot be going backwards while you are holding onto it.
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Post by Random on Oct 13, 2007 1:34:42 GMT -5
A jet engine moves the plane with respect to the air. If a treadmill is moving backwards at 1m/s the air around it is still at rest with respect to the ground. This means the engine will move the airplane at pretty much the same speed with respect to the ground as without a treadmill. I'm not completely sure where you guys are coming from here with the airplane being able to move as though the treadmill wasn't there, but the jet would be moving back through the air as well. The air doesn't move to match the jet's velocity. Its the same as if you were in a pool with a treadmill on the bottom that you were standing on. As long as you're still standing on and being moved by that treadmill you're going to have to put more effort into swimming to achieve the same speed. Wheels don't make a difference. Here is another interesting thought experiment. Imagine that your goal is to keep the wagon from moving. You simply hold onto the handle. Now the treadmill moves at -1m/s while the wagon obviously cannot move with respect to the ground. However if treadmills really canceled out movement, the wagon ought to be going the other way at -1m/s since you are not giving it 1m/s velocity to "cancel it out" However this is an impossibility. Since it cannot be going backwards while you are holding onto it. Also, theres a massive problem with this and I don't know the word for it. Wheels. As soon as you prevent the wagon from moving in the first place it loses the -1 m/s, and the wheels start moving. I can't remember enough physics to say what exactly it gives the wheels, but, yea. I think this whole thread is a massive piece of evidence that overthinking is bad.
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