Something to think about

[ouija]

This has caused senseless debate on other forums which I find most amusing Have a think about it before answering A jetplane is standing on a runway that can move (some sort of conveyor belt). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?

[Ynot]

no, as planes gain lift due to pressure differences between the underside and top of the wing if the plane is stationary relative to the air around it, then it won't gain lift

[Spartiate]

^ What he said. A plane needs air flowing across it`s wings to generate lift. If it stays motionless relative to the air, it`s not going anywhere.

[KuRoSaKi]

^ What he said. A plane needs air flowing across it`s wings to generate lift. If it stays motionless relative to the air, it`s not going anywhere. But of course giving the physics of aerodynamics, the lift must be greater than the weight and general speed of direction of the aircraft in order to generate the necessary situation that would cause the aircraft to become airborne hence it would not take off.

[Ynot]

But of course giving the physics of aerodynamics, the lift must be greater than the weight and general speed of direction of the aircraft in order to generate the necessary situation that would cause the aircraft to become airborne hence it would not take off. can you say that again....in English

[Spartiate]

can you say that again....in English My thoughts exactly . I think he was trying to prove me wrong by reformulating what I said...

[Idec Sdawkminn]

I'm in agreement with the above people. If take-off was directly linked to the speed of the propellers or propulsion forward, it could take off right away without a runway. All that the moving conveyer belt is doing is letting the plane get up to maximum power in a stationary spot. It's not any different than if it was just tied down. The air is not moving with the conveyer belt. If it was, then it could take off.

[skysaw]

Yeah, this is an "oldie-but-goodie." The plane can't take off, unless there is sufficient air pressure under the wings to lift it. No wind means no lift. Of course if there is a hurricane...

[Oneironaut Zero]

A Harrier jet could take off, under those conditions. Other than that, no, as lift requires wind resistance, as has been stated. That would be a pretty badass thing to see, though - an FA-18 Hornet at full afterburn, just sitting in one spot on a treadmill. That would be dope.

[ClouD]

The movement of the conveyor belt, wouldn't allow heavier planes to lift off. It's quite obvious. There is no actual movement. The wind does not gain speed, because of the continuation. You could lift off, but you'd crash instantly. As said, afterburner on a harrier, while stationary, would be able to take off. At EXTREME speeds though. That truly would be awesome.

[Oneironaught]

If the plane is only moving fast enough to stay in the same spot then no, it cannot take off because all it's doing is neutralizing potential forward movement. The only exception would be a plane that can take off vertically.

[KuRoSaKi]

My sentiments exactly CloudWalker.

[Goldney]

So we're all universally agreed that the plane can't take off, as it's obvious due to the laws of physics, aerodynamics blah di blah di blah. New question however: Say that there is a plane in a wind tunnel or something like that. Air is blasted over the plane's wings at a great enough velocity for the plane to take off. Would it: 1. Still take off? 2. Move anywhere if 1. is proved to be correct?

[skysaw]

So we're all universally agreed that the plane can't take off, as it's obvious due to the laws of physics, aerodynamics blah di blah di blah. New question however: Say that there is a plane in a wind tunnel or something like that. Air is blasted over the plane's wings at a great enough velocity for the plane to take off. Would it: 1. Still take off? 2. Move anywhere if 1. is proved to be correct? Most likely it would lift, and tip right over backwards. There needs to be forward thrust as well for a real "take off." Now if you combine the treadmill AND the wind tunnel, well then you've got something!

[Goldney]

That's a good point Skysaw. I just found something which contridicts the answer to the original question: Now say this aircraft could take off with the propellers facing forward. Surely it would defeat the treadmill?

[Oneironaut Zero]

The movement of the conveyor belt, wouldn't allow heavier planes to lift off. It's quite obvious. There is no actual movement. The wind does not gain speed, because of the continuation. You could lift off, but you'd crash instantly. As said, afterburner on a harrier, while stationary, would be able to take off. At EXTREME speeds though. That truly would be awesome. Well, Harriers don't actually have afterburners, and there is no speed necessary for a Harrier to take off. They can take off at a slight roll, or sitting still, as the same jet that moves them forward can be rotated downward, to provide lift. But I do have to ask: How could the jet given in ouija's example even take off? I'm lost on that one. That's a good point Skysaw. I just found something which contridicts the answer to the original question: Now say this aircraft could take off with the propellers facing forward. Surely it would defeat the treadmill? That's not a jet. It's a helicopter that turns into a prop (propeller-driven) plane.

[ClouD]

I'm wondering why this is in senseless banter. It's hardly senseless.

[Oneironaut Zero]

Hmm. I didn't realize it was in senseless. Someone must have moved it now, though, cause it's in the lounge. Wasn't me, though! [Edit: So we're all universally agreed that the plane can't take off, as it's obvious due to the laws of physics, aerodynamics blah di blah di blah. New question however: Say that there is a plane in a wind tunnel or something like that. Air is blasted over the plane's wings at a great enough velocity for the plane to take off. Would it: 1. Still take off? 2. Move anywhere if 1. is proved to be correct? Now that's an interesting concept. I think Skysaw might be right, though. It would either tip over backward, or start flying backward. There is nothing keeping it forward, so the wind resistance would have nothing to keep it from actually catching the friction of said resistance. It would lift but then move backward, crashing into the rear-wall of the wind-tunnel, probably.]

[Oneironaught]

So we're all universally agreed that the plane can't take off, as it's obvious due to the laws of physics, aerodynamics blah di blah di blah. New question however: Say that there is a plane in a wind tunnel or something like that. Air is blasted over the plane's wings at a great enough velocity for the plane to take off. Would it: 1. Still take off? 2. Move anywhere if 1. is proved to be correct? The plane will lift off, providing enough air is forced by the wind tunnel. The air flow must be as great as it would be if the plane took off normally. Also, as soon as the plane looses contact with the treadmill then it will suddenly have more forward thrust because it's no longer being pulled backwards by the treadmill's movement. This will cause it to lunge forward. If the plane's forward thrust were to be cut (because it's no longer being used to compensate for the treadmill) then the wind tunnel velocity needs to be increased to stabilise the plane or it'll just fall back down or lose control. Or, you could choose to tether the plane - with or without the treadmill factor - to prevent horizontal movement or excessive pitch or roll and have it lift off in a relatively stable manner.

[Oneironaut Zero]

The plane will lift off, providing enough air is forced by the wind tunnel. The air flow must be as great as it would be if the plane took off normally. Also, as soon as the plane looses contact with the treadmill then it will suddenly have more forward thrust because it's no longer being pulled backwards by the treadmill's movement. This will cause it to lunge forward. Ah, you're right. In my last reply I forgot that we were talking about a plane that would have a forward thrust, anyway, and was just thinking about the treadmill. If there was a wind tunnel, I think the plane could take off and, provided the wind tunnel was at a speed that matched the forward thrust of the plane, it would be up to the pilot, as to what happens. He could pull up to hard and hit the ceiling, pull down to hard and slam back into the treadmill, move side to side to hit the walls of the wind tunnel or hover in one spot like a Harrier, depending on his flight skills..

[ThePhobiaViewed]

To answer the original problem, I think the jet would move forward and be able to take off. It's pretty tricky which is why its causing so much debate but here's my view. The jets are moving the body of the plane by forcing air backwards. This is different than say a car which would stand still if the conveyor belt matched the speed of the wheels. The jets are pushing the air which causes the wheel apparatuses to move. Since the wheels are moving, the conveyor belt now matches that. As the plane keeps its jets on, it will continue to gain speed meaning that the wheels will increase speed meaning that the conveyor belt will match that speed. The wheels and conveyor belt are now in a loop where they will both keep getting faster and faster. The wheels are independent of thrust so it doesn't matter that the ground is moving the other way, this will just cause the wheels to spin faster. Let's use an example, if you put a toy car on a moving treadmill are you able to push that car forward. The answer is yes because you are providing outside force and the wheels are independent of the movement. They just spin along with the speed of the treadmill plus your movement. (I realize that the treadmill doesn't increase speed in this situation but that doesn't matter. The wheels would be constantly increasing speed and then the treadmill would increase to match that speed and both would keep getting faster if it did match the speed.) This whole question is a hypothetical question so I believe the correct answer is that the plane could take off. In real life you have things to consider such as how quickly the conveyor belt matches the speed if the wheels and what forces the wheels can take. I don't think it would make sense for the plane to be stationery because it has thrusters. Imagine looking at an airplane with its jets on and it not moving. Newtons laws tell us it will move. Basically my point is that the jets will make the jet move and that the wheels have nothing to do with that movement so they will no make a difference. These are just my thoughts and feel free to point out any flaws in my logic. Edit: I just realized that another factor is what it means by the conveyor belt matching the speed of the plane. I do not think this affects my perspective though. My brain hurts now. Edit 2: OK after searching around a bit, my final conclusion is that the wording is flawed and everyone is right Depending on how you look at it I'd say either way is correct.

[Oneironaut Zero]

Ah. I see what you're saying. The thrusters are causing the jet to move forward, not because of what's happening on the ground (the treadmill) but what is happening in the air (the body of the jet itself as opposed to the air around it). So, no matter what, the push from the afterburners would cause the jet to move forward. The treadmill would keep getting faster, to keep up with the jet's wheels' movement, but that is irrelevant to the fact that the jet body would still move forward. Wow. That's a really weird concept. Never thought about it that way.

[Oneironaught]

To answer the original problem, I think the jet would move forward and be able to take off. It's pretty tricky which is why its causing so much debate but here's my view. The jets are moving the body of the plane by forcing air backwards. This is different than say a car which would stand still if the conveyor belt matched the speed of the wheels. The jets are pushing the air which causes the wheel apparatuses to move. Since the wheels are moving, the conveyor belt now matches that. As the plane keeps its jets on, it will continue to gain speed meaning that the wheels will increase speed meaning that the conveyor belt will match that speed. The wheels and conveyor belt are now in a loop where they will both keep getting faster and faster. The wheels are independent of thrust so it doesn't matter that the ground is moving the other way, this will just cause the wheels to spin faster. Let's use an example, if you put a toy car on a moving treadmill are you able to push that car forward. The answer is yes because you are providing outside force and the wheels are independent of the movement. They just spin along with the speed of the treadmill plus your movement. (I realize that the treadmill doesn't increase speed in this situation but that doesn't matter. The wheels would be constantly increasing speed and then the treadmill would increase to match that speed and both would keep getting faster if it did match the speed.) This whole question is a hypothetical question so I believe the correct answer is that the plane could take off. In real life you have things to consider such as how quickly the conveyor belt matches the speed if the wheels and what forces the wheels can take. I don't think it would make sense for the plane to be stationery because it has thrusters. Imagine looking at an airplane with its jets on and it not moving. Newtons laws tell us it will move. Basically my point is that the jets will make the jet move and that the wheels have nothing to do with that movement so they will no make a difference. These are just my thoughts and feel free to point out any flaws in my logic. Edit: I just realized that another factor is what it means by the conveyor belt matching the speed of the plane. I do not think this affects my perspective though. My brain hurts now. Edit 2: OK after searching around a bit, my final conclusion is that the wording is flawed and everyone is right Depending on how you look at it I'd say either way is correct. But as along as the plane remains with its weight resting on the treadmill (which is hypothetically matching the forward speed of the plane) the plane is not moving through the surrounding air. So it cannot create lift. It doesn't matter how fast the wheels are spinning. If there is not sufficient air movement across the wings the plane can't generate lift. I don't think it would make sense for the plane to be stationery because it has thrusters. Imagine looking at an airplane with its jets on and it not moving. Newtons laws tell us it will move. The thrust is being negated when the surface upon which the plane rests (treadmill) is moving backwards at the same rate. And the plane on the runway will only move when forward thrust is greater than that required to break friction. Merely pushing on something doesn't make it move. Pushing hard enough is what makes it move.

[Oneironaut Zero]

The thrust is being negated when the surface upon which the plane rests (treadmill) is moving backwards at the same rate. And the plane on the runway will only move when forward thrust is greater than that required to break friction. Merely pushing on something doesn't make it move. Pushing hard enough is what makes it move. That's where I see Phobia's point, and now disagree with ya. If the forward force applied to the jet is independent of what's happening on the treadmill, I can see it moving forward. The afterburners would make the jet move forward through the air regardless of the friction on the ground, because that friction is (somewhat) negated by free-rolling wheels. (This would be different in a car, where the forward motion is caused by interaction with the ground.) The treadmill would then work to keep up with the jet's movements, because it would keep up with the speed that the wheels are turning, which would increase because of the forward motion given by the afterburners. Sooner or later, should the treadmill have no maximum speed, the jet would continue moving forward until it was moving fast enough for take off. The treadmill would simply continue to compensate. Now, if we were talking about a jet in a wind tunnel, that would be different. If the wind tunnel was pushing wind fast enough to counter the jet's afterburners, then the jet would - like I said in the previous post - be able to hover and move up, down, left or right, but not forward and backward.

[Michael]

Anyone got a few million to throw down in order to build this giant conveyer belt??? And some more to rent a plane.... and insurance n shit. I'll bet five whole dolla that the plane doesn't take off. No more cuz i'm scared I might be wrong.