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United States24751 Posts
On February 21 2009 13:20 ieatkids5 wrote:Show nested quote +On February 21 2009 12:58 micronesia wrote:
Let me propose an alternate #1 perhaps...
#1b: A plane is trying to take off. The speed that the plane normally travels to take off is 500 miles per hour. There is a steady 500 mph headwind. Will the plane be able to take off even though it is initially stationary in the horizontal direction? it seems like the plane should just move vertically, since the way the wings are designed causes higher pressure below the wing, forcing the plane up. but since the plane isnt continuing in a forward motion, the plane will only stay in the air as long as there is a 500 mph wind. and the plane wont move forward or backward either, only up or down (assuming the 500mph wind doesnt push the plane).
Be very careful about the part I put in bold above. It is a common misconception that the reason why a plane flies is because there is higher pressure below the wing than above it due to the shape of the wing and type of airflow around it. I'm not sure if this is what you were thinking or not, but the best way to think about it is that there is a life force (up force acting on the wings/plane) and a drag force (backwards force since the wind can't only push the plane upwards).
In the case of the plane taking off normally... the engines are applying a forward force on the plane in order to create a speed of 500 mph after taking into consideration the drag force associated with the wing configuration.
In the case of the 500 mph headwind... the same thing should happen where the engines are pulling hard enough to overcome the drag... and I guess the plane would have vertical controls identical to the regular take-off (the life force would be a function of the same things which don't depend on wind speed relative to the ground).
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On February 21 2009 13:18 Archaic wrote:
The treadmill version also suggests that the wheels have a very high friction coefficient. For example, since the plane's velocity is created being unrelated to the wheels, the treadmill had no area to pull the plane back except for the wheels.
Given the high speeds, the treadmill would simply turn the wheels, but not actually move the plane back significantly, assuming the plane was moving forward from the engines.
I retract my statement about it not working. The plane would take off.
The plane would not take off.
Ok think about it this way, how fast is the plane actually travelling while it is on the treadmill?
It's going the same as humans on treadmills, 0 MPH, a human running to his house on a treadmill will never get there.
So that means it's going to take off and be going how fast? 0-5 MPH or something like that as the rear wheels leave the treadmill, so now we have this plane in midair, travelling at an initial speed of 0 and it's going to just rocket up to the necessary speed for a plane to normally maintain flight??
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United States24751 Posts
On February 21 2009 13:20 BanZu wrote:Show nested quote +On February 21 2009 13:18 micronesia wrote:On February 21 2009 13:17 BanZu wrote:On February 21 2009 13:13 micronesia wrote:On February 21 2009 13:02 BanZu wrote:On February 21 2009 12:55 micronesia wrote:On February 21 2009 12:12 BanZu wrote:
#1
An airplane is on a treadmill and they move in opposite directions. When the airplane's engines are running at full power the airplane's speed is the same as the treadmill's speed. Will the airplane be able to take off? Or will it simply stay in place? This doesn't make sense to me. The plane's motion is dictated by the engines... the wheels have nothing to do with it... if the engines are running full blast... the plane will move at the same speed relative to the air, regardless of if the ground is moving backwards or not. I thought that if an engine was running at full power it still isn't enough to lift the airplane. Therefore the plane could only move forward on it's wheels like a car. And since it's running at the same speed as the treadmill it stays in the same place relative to the treadmill/Earth, which means there's no airflow under the wings to lift it up. ??? I don't totally understand your confusion, but let me point out that a car uses engine power to turn wheels... and that is what makes the car go forward. Unlike a car, a plane uses engines to pull air through them and propel the plane forwards. Yea, but I thought that the wind pulled by the engine itself wasn't enough. Is it? If you think the plane applies a torque the wheels, which helps to accelerate the plane, then you are not correct. No, I know that's not how a plane works. So what you're saying is that if a giant clamp was holding a plane in mid-air and only let go when the engines were at full power the plane would automatically be able to fly because there's enough air flow under the wings?
No I don't think that would work. The plane would start to fall immediately. I guess if you rotated the plane down into a nosedive you could slowly pitch the plane back up if you didn't stall the engines in the process (depends on the type of plane).
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You'd have a better chance of talking someone into the helicopter taking off, in fact the helicopter would take off if the cockpit area was designed to give flight when it spun, among other modifications.
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Micronesia can you explain again how the plane would be able to take off?
Also, what is the force that pushes an airplane up?
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On February 21 2009 13:29 inReacH wrote:
You'd have a better chance of talking someone into the helicopter taking off, in fact the helicopter would take off if the cockpit area was designed to give flight when it spun, among other modifications.
I thought that since the blades stood still relative to the Earth that there would be no airflow and therefore nothing to push up?
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On February 21 2009 13:30 BanZu wrote:Show nested quote +On February 21 2009 13:29 inReacH wrote:
You'd have a better chance of talking someone into the helicopter taking off, in fact the helicopter would take off if the cockpit area was designed to give flight when it spun, among other modifications. I thought that since the blades stood still relative to the Earth that there would be no airflow and therefore nothing to push up?
First, the helicopter would be thrown off the turn table as soon as it started getting fast enough to be thrown, which is not nearly as fast as helicopter blades spin.
If that didn't happen somehow, no it wouldn't take off because as you said the blades are not moving relative to the earth.
As I was saying before, if the cockpit area was designed in such a way that when it spun it could provide lift, then it would lift because the cockpit area would be spinning indeed.
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United States24751 Posts
On February 21 2009 13:29 BanZu wrote:
Micronesia can you explain again how the plane would be able to take off?
Also, what is the force that pushes an airplane up?
In which case do you want me to explain it. #1 from the OP or my #1b?
The lift force (upward force) is the force acting on an airplane that allows it to go up and stay up (oppose gravity). It is caused by several factors. I believe the most predominant one is this:
Airplane wings are designed to redirect air down. You can try this with your hand and see the same effects... stick your hand out the window of a moving car (obviously after checking to be sure it's safe to do so) and tilt the hands forwards and backwards. They experience a force opposite the direction you redirect the air (Newton's Third Law of Motion). My dad refers this as 'aeropalmics'.
The shape of the wing also has an effect. The wing is more curved on the upper surface than on the bottom surface. This causes the air to bend into a somewhat circular path as it travels along the exposed surface of the wing. The 'center' of the circle is below the wing, so the air is said to be accelerating down. If the plane wing is pulling the air down, then it is experiencing a force up (Newton's Third Law).
If anyone cares, I believe the reason why the air actually follows the surface of the wing instead of just continuing in a straight line is because of entrainment. The air molecules in between the straight line path of the wind, and the curved surface get pulled into the current, which creates a lower pressure in between the straight line path of the wind, and the wing surface. This causes the wind to tilt towards the surface to even out the pressure.
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On February 21 2009 13:32 inReacH wrote:Show nested quote +On February 21 2009 13:30 BanZu wrote:On February 21 2009 13:29 inReacH wrote:
You'd have a better chance of talking someone into the helicopter taking off, in fact the helicopter would take off if the cockpit area was designed to give flight when it spun, among other modifications. I thought that since the blades stood still relative to the Earth that there would be no airflow and therefore nothing to push up? Wait actually.. I think the helicopter does lift off, although I might just be pulling an Archaic. The blades don't stand still relative to earth, they stand still relative to the turn table. Reguardless, they are still interacting with the atmosphere, the thing is the helicopter would crash immediately since the ENTIRE THING is moving at the speed of it's propeller. In fact, it wouldn't take off, but only because it would be thrown off the turntable almost immediately as the turn table start moving so fast.
Yes, the blades stand still relative to the Earth because wherever the blades turn, the turntable rotates the opposite direction.
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On February 21 2009 13:39 BanZu wrote:Show nested quote +On February 21 2009 13:32 inReacH wrote:On February 21 2009 13:30 BanZu wrote:On February 21 2009 13:29 inReacH wrote:
You'd have a better chance of talking someone into the helicopter taking off, in fact the helicopter would take off if the cockpit area was designed to give flight when it spun, among other modifications. I thought that since the blades stood still relative to the Earth that there would be no airflow and therefore nothing to push up? Wait actually.. I think the helicopter does lift off, although I might just be pulling an Archaic. The blades don't stand still relative to earth, they stand still relative to the turn table. Reguardless, they are still interacting with the atmosphere, the thing is the helicopter would crash immediately since the ENTIRE THING is moving at the speed of it's propeller. In fact, it wouldn't take off, but only because it would be thrown off the turntable almost immediately as the turn table start moving so fast. Yes, the blades stand still relative to the Earth because wherever the blades turn, the turntable rotates the opposite direction.
Yes, sorry I edited my post as you wrote that.
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On February 21 2009 13:38 micronesia wrote:Show nested quote +On February 21 2009 13:29 BanZu wrote:
Micronesia can you explain again how the plane would be able to take off?
Also, what is the force that pushes an airplane up? *long post*.
Do you mind giving a full run-down of what happens, step-by-step, in situation #1?
I want to find out the answer once-and-for-all
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wait, i don't get why the plane would lift
why do planes obtain lift to begin with? i could be wrong, but i was under the impression that the engines move it forward so that the air relative to the wings moves in such a way as to provide lift due to pressure difference or whatever. then, if the treadmill negates the forward movement of the airplane, such that the air relative to the wings does not move, how could there be lift? O_o
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United States24751 Posts
On February 21 2009 13:49 BanZu wrote:Show nested quote +On February 21 2009 13:38 micronesia wrote:On February 21 2009 13:29 BanZu wrote:
Micronesia can you explain again how the plane would be able to take off?
Also, what is the force that pushes an airplane up? *long post*. Do you mind giving a full run-down of what happens, step-by-step, in situation #1? I want to find out the answer once-and-for-all
I think my response to the following will help.
On February 21 2009 13:49 JeeJee wrote:
wait, i don't get why the plane would lift
why do planes obtain lift to begin with? i could be wrong, but i was under the impression that the engines move it forward so that the air relative to the wings moves in such a way as to provide lift due to pressure difference or whatever. then, if the treadmill negates the forward movement of the airplane, such that the air relative to the wings does not move, how could there be lift? O_o
A treadmill could not prevent wind from passing over the wings. You are imagining this to work like a car... which an airplane does not.
If a car is on a treadmill... then if the speeds match, then there is no wind if you stick your hand out the window.
But the way an airplane works, the engines make the plane move forward relative to the wind... not the ground.
I think you guys are thinking of an alternate scenario where a plane is tied to a wall while gunning its engines. If you cut the string, can the airplane take off right then?
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so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no?
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Haven't ready the thread, but the plane would take of since the engines push against the air, not the ground. The plane's wheels would be spinning twice as fast as either the plane's/treadmill's speed.
The helicopter would not take off because it is spinning its blades at the same speed as it is spinning, so the blades would be stationary. In this case, the helicopter is pushing off of the turntable, not the air.
Look at it this way. On a frictionless surface, would a plane and a helicopter be able to take off? (one can)
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Yes.
And wtf...? Yes? (fuck rotational, rectilinear ftw)
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#1 Yes you will get liftoff
#2 no idea
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United States24751 Posts
On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no?
If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air.
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On February 21 2009 14:26 micronesia wrote:Show nested quote +On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no? If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air.
okay then i'm back to my original query. since the air and ground are stationary relative to each other, for the wings to be moving relative to air it must be moving relative to the ground as well, no? since the treadmill forbids that, how can there be lift? it seems that there is assumption that even with the treadmill, the plane is still moving forward relative to the ground. i thought the purpose of the treadmill was specifically to stop this from occurring? else, what the heck is the treadmill doing
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EPT
