UPSB v4
Off-topic / Can a plane on a treadmill fly?
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Date: Sat, May 7 2011 23:19:44
A plane is standing on a runway that can move (like a giant conveyor belt). 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). Will the plane be able to take off?
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Date: Sat, May 7 2011 23:32:54
No. The lift that makes the plane fly is cause by air speed. Plane not moving horizontally=No air movement over and under the wing=no flight.
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Date: Sun, May 8 2011 00:23:09
Yes, mythbusters proved it will take off
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Date: Sun, May 8 2011 00:56:19
yes, because it doesnt use motors in the wheels, it uses push from the jet engines of propellers to move forward. IE a car will not move forward since it gets motion FROM THE WHEELS. whereas a plane gets its motion from the propellers or jets. the wheels on a plane are only so that it doesnt slide on the ground when it moves.
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Date: Sun, May 8 2011 01:16:52
No, a plane needs wind for its wings to generate lift. If the plane and the treadmill were at the same speed, the plane would only stay at one spot > no moving air. Regarding how the mythbusters did it, I think they did it wrong. After watching their full-scale video, the plane was still "moving" on the conveyor belt, which is not what the myth implies. The conveyor belt has to match the plane's speed in the opposite direction exactly, which is what the mythbusters failed to do.
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Date: Sun, May 8 2011 01:37:39
Loanshark wrote: No, a plane needs wind for its wings to generate lift. If the plane and the treadmill were at the same speed, the plane would only stay at one spot > no moving air. Regarding how the mythbusters did it, I think they did it wrong. After watching their full-scale video, the plane was still "moving" on the conveyor belt, which is not what the myth implies. The conveyor belt has to match the plane's speed in the opposite direction exactly, which is what the mythbusters failed to do.
They made a super shitty belt for it, too. -
Date: Sun, May 8 2011 02:01:51
@King you are right. nuf said
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Date: Sun, May 8 2011 02:16:35
a plane DOES NOT generate speed from its wheels, so regardless of how fast the belt is moving, the planes engines will provide forward motion and lift off. the wheels just spin around pointlessly.
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Date: Sun, May 8 2011 02:20:46
The answer is yes, the plane will move forward on the belt, since the propulsion is from the engine/propeller and not a motor connected to the wheel. Think of the plane moving forward and the wheels getting only getting dragged along the ground. The thing is that the plane will in fact move forward. The engine/propeller pushes the air back, which by newtons third law, pushes the plane forward. Note that the plane is being pushed forward against the air and not against the ground. Anyone wanna link me to the mythbusters episode? I remember they tested this but I dont remember anything about it.
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Date: Sun, May 8 2011 02:50:57
just watch mythbusters...they proved it.
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Date: Sun, May 8 2011 02:56:19
Yeah it could. You guys might say that the MythBusters didn't match the plane's speed exactly in the opposite direction (which would thus produce a net movement of 0) but the conveyor belt isn't moving the air around the plane in the opposite direction at the same speed of the plane's thrust. These two forces (the treadmill and plane engines) are completely independent of each other. The treadmill moving in the opposite direction just makes the plane look like it's not going anywhere since but it really is.. just in a more cyclical path of the belt.. if you take the belt away, the plane would still move forward at x mph/kmph and can still take off at that speed.. the surface that the plane rests on isn't really relevant.
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Date: Sun, May 8 2011 03:08:22
Lol the problem with this hypothetical is that the set-up is impossible. Because the propulsion is from the jets in the back of the plane, it will go forward no matter what the ground beneath it is doing, the wheels will just rotate freely beneath it That's why the mythbusters said it could take off, because it WOULD take off because the conveyor belt wouldn't ever be able to stop it from moving forward.
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Date: Sun, May 8 2011 03:15:24
upward force that allows plane to take off is determined by thrust and propulsion generated by plane engines and the lift force created by relative flow of air along aerofoil shape of the wing. this will be impossible if fw speed = bw speed because there'd be no fw resultant force (thrust generated by engine) there is no possibility for plane to take off that way (UNLESS lift force is sufficient to do so, which is virtually impossible) .
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Date: Sun, May 8 2011 03:36:40
Lol you guys are all missing the point that the plane CAN'T STAY STILL ON A TREADMILL. Consider a force diagram. In the forward direction there's thrust from the plane engine. In the backward direction, there's NOTHING. So the plane moves forwards. It's a basic physics question asked in a strange way that trips people up.
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Date: Sun, May 8 2011 05:02:02
IF talking about a front engine one man propeller airplane and the engine is on, the plane will move faster then the conveyor belt and if given enough space, the plane will take off but the amount of space needed would be to immense to simulate. But if you mean that as the speed and acceleration of the airplane is matched by the conveyor belt, the lift that is required for a standard airplane will not be satisfied i.e; the plane will not fly.
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Date: Sun, May 8 2011 06:18:59
strat1227 wrote: Lol you guys are all missing the point that the plane CAN'T STAY STILL ON A TREADMILL. Consider a force diagram. In the forward direction there's thrust from the plane engine. In the backward direction, there's NOTHING. So the plane moves forwards. It's a basic physics question asked in a strange way that trips people up.
Everyone needs to shut up and listen to this guy. -
Date: Sun, May 8 2011 07:25:49
Retro-spectre wrote: A plane is standing on a runway that can move (like a giant conveyor belt). 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). Will the plane be able to take off?
With the above condition, the plane shouldn't move at all on the conveyor belt. No displacement, how can the plane fly at all. Mythbusters did it wrong. Yes, horizontal thrust is not generated by the wheels. But according to the Bernoulli's principle, in an aerofoil, a horizontal air flow is needed to produce a pressure difference onto the aeroplane wing. The pressure difference will then lift the aeroplane, enabling it to take off.
If the Mythbusters are doing it correctly, the plane shouldn't be moving so there will be no airflow generated onto the aerofoil. No air flow equals no take off.
The plane shouldn't be able to take off.
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Date: Sun, May 8 2011 07:28:35
Retro-spectre;92132]A plane is standing on a runway that can move (like a giant conveyor belt). [B]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).[/B] Will the plane be able to take off?[/QUOTE] [QUOTE=strat1227 wrote: Lol you guys are all missing the point that the plane CAN'T STAY STILL ON A TREADMILL. Consider a force diagram. In the forward direction there's thrust from the plane engine. In the backward direction, there's NOTHING. So the plane moves forwards. It's a basic physics question asked in a strange way that trips people up.
According the the post by Retro-Spectre, 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). Meaning if the plane is accelerating, the conveyor belt accelerates as well. THEREFORE, in the backward direction, there's an equal opposing force. SO THE PLANE STAYS AT THE SAME SPOT FOREVER DAMMIT. -
Date: Sun, May 8 2011 07:38:13
KTrinh93 wrote: if you take the belt away, the plane would still move forward at x mph/kmph and can still take off at that speed.. the surface that the plane rests on isn't really relevant.
Problem is nobody's taking the belt away from you. In this equilibrium without any exterior interference the plane will stay where it is forever. -
Date: Sun, May 8 2011 08:46:50
Nice editing.
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Date: Sun, May 8 2011 16:57:05
loonwern93 wrote: According the the post by Retro-Spectre, 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). Meaning if the plane is accelerating, the conveyor belt accelerates as well. THEREFORE, in the backward direction, there's an equal opposing force. SO THE PLANE STAYS AT THE SAME SPOT FOREVER DAMMIT.
lol false. The wheels spin freely under the plane, so no matter how fast the conveyor belt moves it won't affect the plane. the "staying still on a conveyor belt" only works if the forward force is generated from the ground (like a human running or a car driving). A plane's forward motion is independent from the ground (propeller or jet engines don't push on the ground, they push on the air). So unless you got the AIR AROUND THE PLANE to move at the same speed and not just the conveyor belt, then it'll still move forward. it's nice to see all the amateurs throw opinions around lol, but facts are facts. -
Date: Sun, May 8 2011 17:05:04
loonwern93;92307]With the above condition, the plane shouldn't move at all on the conveyor belt. No displacement, how can the plane fly at all. Mythbusters did it wrong. Yes, horizontal thrust is not generated by the wheels. But according to the Bernoulli's principle, in an aerofoil, a horizontal air flow is needed to produce a pressure difference onto the aeroplane wing. The pressure difference will then lift the aeroplane, enabling it to take off. http://reniyoung.files.wordpress.com/2010/11/bernoullis-principle.gif If the Mythbusters are doing it correctly, the plane shouldn't be moving so there will be no airflow generated onto the aerofoil. No air flow equals no take off. The plane shouldn't be able to take off.[/QUOTE] This picture is incorrect in explaining how a wing works, as the wing isn't shown to have an angle of attack. And how a wing works is all about its angle of attack. A wing pushes air down with it's angle of attack, which makes the air push the plane up (Newton's third law). Airfoils only help to make wings more efficient. An airfoil shaped wing without an angle of attack (like the one in the picture) would not be able fly. (xkcd ranting about this misconception). That being said, it's still correct that a wing needs air to move across it to generate lift, as else there would be no air for it to push down. And atleast according to how the thought experiment is intended there should not be any air moving across the wings, except maybe (if talking about a prop plane) what little wind the propellers might produce. So Mythbusters must have done something wrong somewhere, else what they have done is to disprove Newtonian physics. [QUOTE=strat1227 wrote: lol false. The wheels spin freely under the plane, so no matter how fast the conveyor belt moves it won't affect the plane. the "staying still on a conveyor belt" only works if the forward force is generated from the ground (like a human running or a car driving). A plane's forward motion is independent from the ground (propeller or jet engines don't push on the ground, they push on the air). So unless you got the AIR AROUND THE PLANE to move at the same speed and not just the conveyor belt, then it'll still move forward. it's nice to see all the amateurs throw opinions around lol, but facts are facts.
Yea, I guess that's correct, although couldn't the friction from the treadmill below theoretically be so vigorous that it would cancel out the forward force from the engine, thereby causing plane to stand still. That's what I understood the thought experiment to really ask, namely can a plane take off while not moving forward. -
Date: Sun, May 8 2011 18:15:11
The setup was that the belt would match its speed but in the opposite direction, which for most vehicles would keep it still. With that setup, the plane would NOT stay still. As far as your setup, the speed that the conveyor belt would have to go to match the force of a JET ENGINE with only rolling frictional force would be so crazy it would literally rip the wheels off of any plane. @The Onion
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Date: Sun, May 8 2011 18:33:51
strat1227 wrote: The setup was that the belt would match its speed but in the opposite direction, which for most vehicles would keep it still. With that setup, the plane would NOT stay still. As far as your setup, the speed that the conveyor belt would have to go to match the force of a JET ENGINE with only rolling frictional force would be so crazy it would literally rip the wheels off of any plane. @The Onion
Yes, I know that, wheels are by their very nature not made to create a lot of friction. But it's still only a thought experiment. And you could also imagine a case where the brakes might be on. But in the end it must all come down to the friction then. Is the friction the same as, or a significant fraction of, the forward pushing force of the engines, the plane will not take off. However if, as is most likely with free spinning wheels, the friction of the wheels is only a tiny fraction of the force from the engines, the plane will still take off. -
Date: Sun, May 8 2011 18:36:37
You're changing the thought experiment though (because you understand the original one won't work) Go re-read the original post. It says the conveyor belt is going the same speed, in the opposite direction. Then it makes the false assumption that it will stay still because of this. Obviously you and I understand that it will not stay still, so you're changing the thought experiment to something that it obviously won't take off from
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Date: Sun, May 8 2011 19:01:27
strat1227 wrote: You're changing the thought experiment though (because you understand the original one won't work) Go re-read the original post. It says the conveyor belt is going the same speed, in the opposite direction. Then it makes the false assumption that it will stay still because of this. Obviously you and I understand that it will not stay still, so you're changing the thought experiment to something that it obviously won't take off from
Well, maybe, or maybe the thought experiment is just ambiguous by its very nature, which is why it fosters so much debate. (More xkcd). -
Date: Sun, May 8 2011 19:15:15
Lol that's what I've been saying, the setup is inherently flawed, one way or another
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Date: Sun, May 8 2011 19:29:28
it might work if the conveyor is not completely flat...
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Date: Thu, May 19 2011 20:57:48
Retro-spectre wrote: A plane is standing on a runway that can move (like a giant conveyor belt). 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). Will the plane be able to take off?
I doubt it unless is a harrier jet because if a conveyor is matching the speed of the plane in the opposite direction it means the plane is not moving forward at all and therefore wouldnt get any air to achieve any lift. it would be the same as trying to fly a plane that's not moving.