#270944 - 29/11/2005 14:31
Will the plane fly?
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old hand
Registered: 17/07/2001
Posts: 721
Loc: Boston, MA USA
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This particular question has been the subject of a hot debate over on another forum (well, several really). I want to know how you brainiacs feel about it? Quote:
the teacher threw this one at the class today:
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
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//matt
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#270945 - 29/11/2005 14:36
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 08/03/2000
Posts: 12338
Loc: Sterling, VA
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I haven't read any of the other threads, but my feeble knowledge of planes is saying no. If there is no wind moving over the wings, it won't take off, right?
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Matt
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#270946 - 29/11/2005 14:59
Re: Will the plane fly?
[Re: ithoughti]
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pooh-bah
Registered: 25/08/2000
Posts: 2413
Loc: NH USA
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Yes. The plane would most definitely take off.
The thrust applied by a plane is not transmitted via the wheels, rather directly against the surrounding air via the prop or jet thrust. Since the wheels provide only a nominal rolling resistance, their rotational speed will have no effect on the forward velocity of the plane. The only resistance is the air drag and interial load of the plane's mass. The plane will take off as normal.
Zeke
Edit: Further clarification: it is the reltive velocity of the air around the wings that provides lift. Were the air speed greater than or equal to the stall speed, the plane will fly, regardless of velocity relative to the ground. If the flightspeed of your plane is say, 80mph, and you're flying into a headwind of 80mph you will not have any forward ground speed, but you will be able to fly up, down, sidways etc.
Edited by Ezekiel (29/11/2005 15:03)
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#270947 - 29/11/2005 15:56
Re: Will the plane fly?
[Re: Ezekiel]
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enthusiast
Registered: 31/05/2002
Posts: 352
Loc: santa cruz,ca
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wait a sec... does this 'plane' have a big type R sticker on it?
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#270948 - 29/11/2005 16:04
Re: Will the plane fly?
[Re: Ezekiel]
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addict
Registered: 23/12/2002
Posts: 652
Loc: Winston Salem, NC
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Despite wind speed (or in this case, the speed of the air passing under the wings), if this "plane" turns out to be a glider or some sort of winged aircraft incapable of generating its own thrust, its airborn time might be short lived.
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#270949 - 29/11/2005 16:32
Re: Will the plane fly?
[Re: Cybjorg]
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pooh-bah
Registered: 25/08/2000
Posts: 2413
Loc: NH USA
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In which case, it the glider would not fly by sitting on a runway by itself regardless whether the runway were tarmac or a conveyor belt or a moving river of jellyfish. Its flight time would be zero, not short lived.
What's far more germain is if the plane is an African or European swallow, er plane.
-Zeke
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#270950 - 29/11/2005 17:04
Re: Will the plane fly?
[Re: Ezekiel]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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I don't get this. The plane is effectively stationary, right? So where is it getting the forces and the lift that the wings require to get into the air?
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#270951 - 29/11/2005 17:11
Re: Will the plane fly?
[Re: CrackersMcCheese]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Quote:
I don't get this. The plane is effectively stationary, right? So where is it getting the forces and the lift that the wings require to get into the air?
Not true- the belt only acts to cancel the wheel rotation.
The plane is moving through the air down the runway, as I read it.
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#270952 - 29/11/2005 17:13
Re: Will the plane fly?
[Re: Ezekiel]
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old hand
Registered: 01/10/2002
Posts: 1038
Loc: Fullerton, Calif.
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Quote:
The thrust applied by a plane is not transmitted via the wheels, rather directly against the surrounding air via the prop or jet thrust. Since the wheels provide only a nominal rolling resistance, their rotational speed will have no effect on the forward velocity of the plane. The only resistance is the air drag and interial load of the plane's mass. The plane will take off as normal.
Actually, the plane would take off more quickly. In the Cessna I fly, those tiny wheels are a huge source of resistance. It's about all the plane can do to do 70mph on the ground and instantly goes to 90mph as soon as the wheels leave the ground. It should achieve 70 much faster without the wheel resistance.
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#270953 - 29/11/2005 17:15
Re: Will the plane fly?
[Re: Robotic]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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Hmm I'm reading it as the conveyer belt is countering the plane rolling forward so velocity = zero :S
Edited by Phil. (29/11/2005 17:17)
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#270954 - 29/11/2005 17:15
Re: Will the plane fly?
[Re: Robotic]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Quote:
conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation
That would mean zero ground speed, and, therefore, effectively zero wind speed. I can't see how it could possibly take off. Well, unless there was a hurricane.
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#270955 - 29/11/2005 17:20
Re: Will the plane fly?
[Re: Robotic]
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carpal tunnel
Registered: 19/01/2002
Posts: 3584
Loc: Columbus, OH
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Quote:
The plane is moving through the air down the runway, as I read it.
The problem is that the conveyor belt would have to move at infinite speed.
1. The plane will never leave the ground until the wings provide lift.
2. The wings will never provide lift without air moving over them.
3. Air will never move over the wings unless the plane makes forward progress on the runway.
4. The plane cannot make forward progress on the runway if the runway is moving at the same speed beneath the plane in the opposite direction.
The problem is that this is impossible. You are essentially providing thrust with 0 resistence, causing the plane to accelerate to infinite speed, which is canceled out by the infinite speed of the conveyor runway. It's a silly thing to argue about because it cannot happen.
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~ John
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#270957 - 29/11/2005 17:25
Re: Will the plane fly?
[Re: Robotic]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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But don't the wings need flowing air to go anywhere? I still can't see where this air is coming from. Its not moving compared to the surroundings? Or is it?
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#270958 - 29/11/2005 17:26
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 20/12/1999
Posts: 31594
Loc: Seattle, WA
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Quote:
Air will never move over the wings unless the plane makes forward progress on the runway.
Incorrect. The plane must make forward progress through the air, not on the runway.
If the plane has thrust, it will take off, regardless of what the conveyor belt is doing. Because the thrust acts on the air, and the wings act on the air.
I'm not clearly understanding which way the conveyor belt is moving, but it's either going to reduce rolling resistance or increase it. But in the end, unless it burns out the plane's tires in the process, the plane's thrust will overcome the rolling resistance and the plane will move forward pretty quickly.
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#270959 - 29/11/2005 17:30
Re: Will the plane fly?
[Re: wfaulk]
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enthusiast
Registered: 31/05/2002
Posts: 352
Loc: santa cruz,ca
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the wheels don't propel the plane. if the belt could move in the opposite direction to counter the wheel speed I think the wheels would have to spin at twice the normal rate compared to air speed, but would have little or no effect on the planes ability to take off.
if it was a car with wings, then sure, it would sit still.
Edited by lastdan (29/11/2005 17:33)
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#270960 - 29/11/2005 17:31
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 19/01/2002
Posts: 3584
Loc: Columbus, OH
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Quote:
Incorrect. The plane must make forward progress through the air, not on the runway
And tell me how this "forward progress through the air" is going to occur with a stationary plane and no wind?
I know what you're thinking....you're thinking that thrust is being generated and the plane must move somewhere right? Wrong. Gravity is pulling downward on the plane. Until it has the groundspeed to get some enough lift, it cannot take off. It cannot get groundspeed if stationary. Hence the infinite wheel speed problem.
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~ John
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#270961 - 29/11/2005 17:36
Re: Will the plane fly?
[Re: lastdan]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Oh, now I see what you're saying. The thrust comes from the engines and the wheels have nothing to do with it. A conveyor belt as described would actually just have the effect of transferring the device that overcomes ground friction from the wheels to the belt. In other words, regardless of the conveyor belt, the plane will have forward movement compared to the wind, since it's the air that it's pushing against anyway.
Edited by wfaulk (29/11/2005 17:37)
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#270962 - 29/11/2005 17:39
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 20/12/1999
Posts: 31594
Loc: Seattle, WA
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Quote:
And tell me how this "forward progress through the air" is going to occur with a stationary plane and no wind?
Sure, as soon as that plane's propeller (or jets) begin to generate thrust, the conveyor belt is almost irrelevant. (Except for altering the amount of rolling resistance, a minor factor compared to air resistance.)
Planes take off with no wind all the time. You don't need wind to take off. You just need thrust. And the thrust acts against the air, not against the ground.
Quote:
Gravity is pulling downward on the plane. Until it has the groundspeed to get some enough lift
Bzzt, wrong again. Groundspeed does not generate lift. Airspeed does.
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#270963 - 29/11/2005 17:40
Re: Will the plane fly?
[Re: wfaulk]
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pooh-bah
Registered: 14/01/2002
Posts: 2489
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Ahhhh I see now! I was thinking of powered wheels as opposed to jet power.
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#270964 - 29/11/2005 17:40
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Actually, once you realize what's going on, it's an incredibly awkward way of describing it, since there wouldn't actually be any wheel rotation at all, and that's what tricks us into thinking about it as if it were a car. It would be much more correct to say that the conveyor belt is moving at the same speed as the airplane.
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#270965 - 29/11/2005 17:42
Re: Will the plane fly?
[Re: JBjorgen]
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enthusiast
Registered: 31/05/2002
Posts: 352
Loc: santa cruz,ca
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if the runway had an enormous fan blowing from behind- the plane could still move forward, and would do so with greater ease, but the wings could not create lift.
the rolling belt under the plane would still have little or no effect.
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#270966 - 29/11/2005 17:47
Re: Will the plane fly?
[Re: lastdan]
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carpal tunnel
Registered: 20/12/1999
Posts: 31594
Loc: Seattle, WA
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Quote:
if the runway had an enormous fan blowing from behind- the plane could still move forward, and would do so with greater ease, but the wings could not create lift.
Correct.
Quote:
the rolling belt under the plane would still have little or no effect.
Well, little effect. Rolling resistance is a factor. If I'm understanding the way the question was posed, the plane would actually take off easier because the belt would eliminate rolling resistance.
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#270967 - 29/11/2005 17:49
Re: Will the plane fly?
[Re: CrackersMcCheese]
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carpal tunnel
Registered: 20/12/1999
Posts: 31594
Loc: Seattle, WA
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Quote:
Ahhhh I see now! I was thinking of powered wheels as opposed to jet power.
No airplane uses powered wheels, that I know of. An airplane that depended upon powered wheels would only fly in very short hops. 
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#270968 - 29/11/2005 18:24
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
Quote:
Air will never move over the wings unless the plane makes forward progress on the runway.
Incorrect. The plane must make forward progress through the air, not on the runway.
Right- but how will the plane make progress through the air if it is not moving foward down the runway? The air is stationary (no wind), so the plane must move in order for the air to pass over the wings, right? It seems to me that the plane won't move as long as the ground shifts to compensate for whatever thrust is being generated, assuming the thrust is being generated parallel to the ground.
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Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.
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#270969 - 29/11/2005 18:26
Re: Will the plane fly?
[Re: lastdan]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
the rolling belt under the plane would still have little or no effect.
Exactly. The friction from the belt, acting on the plane, is independent of the belt's speed (classically; in reality it probably varies slightly, but not enough to affect this particular experiment). Once the thrust from the plane's engines exceeds that friction, the plane will move forwards, and take off, unless the belt's speed is sufficient to burn out or destroy the plane's landing gear.
That speed is probably not all that big (200mph?), and it seems technically feasible to make a conveyor belt that fast. If, as in the original statement of the question, the conveyor belt is specifically designed to carry on increasing its speed to try and match the plane's wheels, which it can't do other than by destroying them, then perhaps it is designed to destroy the wheels.
On the other hand, a conveyor belt the size of a runway moving at 200mph will cause a substantial draught of air all by itself. Depending on the exact flow this generates, it will either help the plane take off (by acting as a headwind) or hinder it (by moving air more quickly under the wings only, that the plane's motion can move air over the wings).
Peter
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#270970 - 29/11/2005 18:29
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 20/12/1999
Posts: 31594
Loc: Seattle, WA
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Quote:
how will the plane make progress through the air if it is not moving foward down the runway?
It will move forward down the runway as soon as its propeller starts turning with enough thrust. No matter what the conveyor belt does, it's not gonna stop that from happening. If the conveyor is trying to stop the plane from moving, it's acting on the wrong part of the plane. To stop the plane, you need to act upon the air, not upon the wheels.
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#270971 - 29/11/2005 18:32
Re: Will the plane fly?
[Re: peter]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
Once the thrust from the plane's engines exceeds that friction, the plane will move forwards, and take off, unless the belt's speed is sufficient to burn out or destroy the plane's landing gear.
Ah, this would be the answer- I thought I was missing something and this is it. The plane DOES have to move foward to take off (otherwise no airflow), but the increasing speed of the belt won't be able to prevent this from happening. I think discussions of destroying the landing gear are probably overreaching the exercise.
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Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.
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#270972 - 29/11/2005 18:34
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 05/01/2001
Posts: 4903
Loc: Detroit, MI USA
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Quote:
Actually, once you realize what's going on, it's an incredibly awkward way of describing it, since there wouldn't actually be any wheel rotation at all, and that's what tricks us into thinking about it as if it were a car. It would be much more correct to say that the conveyor belt is moving at the same speed as the airplane.
Exactly. Whether intentional or not, this question is very poorly worded. There is more debate about what the question is actually asking than there is about whether or not the plane could fly.
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#270973 - 29/11/2005 18:36
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
It will move forward down the runway as soon as its propeller starts turning with enough thrust.
Which means John's point number 4 was incorrect, not number 3. That's what I was missing.
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-Jeff
Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.
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