#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: 1039
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: 31597
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: 31597
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: 31597
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: 31597
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: 31597
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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Brad B.
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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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Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.
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#270974 - 29/11/2005 18:40
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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John's point number 3 was also incorrect. An airplane could take off stationary to the ground if there was a strong enough wind blowing towards it. In the case of this puzzle, the wind is still, but the same principle applies. The plane must move relative to the air molecules in order to generate lift. And since that motion isn't governed by the ground or the wheels, the plane's gonna move as soon as it starts generating thrust against that air.
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#270975 - 29/11/2005 18:45
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 19/01/2002
Posts: 3584
Loc: Columbus, OH
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Quote:
Which means John's point number 4 was incorrect, not number 3. That's what I was missing.
John's point number 3 was also incorrect.
Ok. I'll concede that. So the question becomes what speed is the runway moving for the plane to move at all? Would it not become infinity? At that point, wouln't the wheel bearings be melted or frozen?
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#270976 - 29/11/2005 18:46
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:
John's point number 3 was also incorrect. . . The plane must move relative to the air molecules in order to generate lift. . .
Right, but if the airplane isn't moving and the molecules aren't moving, then the plane isn't moving relative to the air molecules and so it won't lift no matter how much thrust it has (unless the direction of the thrust is such that it pushes the plan off the ground- I assume a horizontal thrust).
_________________________
-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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#270977 - 29/11/2005 18:49
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
So the question becomes what speed is the runway moving for the plane to move at all? Would it not become infinity? At that point, wouln't the wheel bearings be melted or frozen?
I just started thinking along these lines. It's interesting.
My first stab at it is this:
At takeoff, the conveyor belt would be moving at twice the speed of the airplane's standard takeoff speed. This would be fine, assuming the plane's tires can handle that much friction. I think the tires are probably rated for that.
I'm probably wrong, it's probably one of those things that requires the conveyor belt and the tires to reach infinity speed. I never understand those physics problems.
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#270978 - 29/11/2005 18:50
Re: Will the plane fly?
[Re: JBjorgen]
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pooh-bah
Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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That is also how I understood it. Effectively, once the engine starts thrusting, the belt would move backwards at the speed just enough for rolling friction of wheels to cancel the thrust. That's the only way to satisfy description of the problem. The airplane does not fly (and its landing gear doesn't last long, either  )
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#270979 - 29/11/2005 18:52
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
Right, but if the airplane isn't moving and the molecules aren't moving, then the plane isn't moving
I never said the airplane wasn't moving. I just said the conveyor belt was irrelevant to the moving of the airplane because the plane's thrust doesn't act on the ground, it acts on the air. That's the bit that I didn't make clear in my earlier posts.
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#270980 - 29/11/2005 18:53
Re: Will the plane fly?
[Re: bonzi]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
once the engine starts thrusting, the belt would move backwards at the speed just enough for rolling friction of wheels to cancel the thrust
No: the friction doesn't increase with the speed of the belt. So either the thrust can overcome that fixed amount of friction (in which case the plane can take off from the belt, unless its wheels fail), or it can't (in which case the plane couldn't take off even on a normal stationary runway).
Peter
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#270981 - 29/11/2005 19:00
Re: Will the plane fly?
[Re: JBjorgen]
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pooh-bah
Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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Quote:
Quote:
Which means John's point number 4 was incorrect, not number 3. That's what I was missing.
John's point number 3 was also incorrect.
Ok. I'll concede that. So the question becomes what speed is the runway moving for the plane to move at all? Would it not become infinity? At that point, wouln't the wheel bearings be melted or frozen?
No, you were right (if I interprete poorly worded problem correctly). There is no wind, and I assume that the problem author wants us to ignore the draught caused by the belt.
The situation is physically possible only if we assume rolling friction of wheels growing with rotational speed. Then, as I already said, the belt wil 'use' that friction to keep the plane stationary.
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Q#5196
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#270982 - 29/11/2005 19:01
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
So the question becomes what speed is the runway moving for the plane to move at all?
That sort of depends on how rapidly the control system for the belt, reacts to the speed of the plane's wheels. Certainly once the plane starts moving forwards, the belt will find itself permanently behind the wheels' speed, and will accelerate to its maximum speed as fast as its control system allows.
Quote:
Would it not become infinity? At that point, wouln't the wheel bearings be melted or frozen?
OK, so the belt is allowed to go infinitely fast but the wheel bearings aren't allowed to be frictionless? 
If the belt is perfectly frictionless and massless but the wheels aren't, then the wheels will burn out and the plane won't fly. If, on the other hand, the wheels are frictionless but the belt isn't, the plane will fly. If neither belt nor wheels is idealised -- if both are real-world objects -- then the answer depends on whether or not the wheels can survive the belt's maximum speed, i.e. on the engineering of the belt and the wheels.
I agree with JeffS, though, that this is probably not the physics question that the originator was trying to ask.
Peter
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#270983 - 29/11/2005 19:02
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:
I never said the airplane wasn't moving.
What you said was that the following was wrong:Quote:
3. Air will never move over the wings unless the plane makes forward progress on the runway.
However, since the runway and air molecules are relative to one another, in order for the plane to move across the molecules it would have to also move across the runway.
Quote:
I just said the conveyor belt was irrelevant to the moving of the airplane because the plane's thrust doesn't act on the ground, it acts on the air.
Right, which means that the plane has to move foward down the runway despite the efforts of the moving belt, which it will because of precicely the reasons you state.
I agree with you- I'm just pointing out why I didn't see it on your first post.
_________________________
-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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#270984 - 29/11/2005 19:10
Re: Will the plane fly?
[Re: peter]
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pooh-bah
Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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Quote:
Quote:
once the engine starts thrusting, the belt would move backwards at the speed just enough for rolling friction of wheels to cancel the thrust
No: the friction doesn't increase with the speed of the belt. So either the thrust can overcome that fixed amount of friction (in which case the plane can take off from the belt, unless its wheels fail), or it can't (in which case the plane couldn't take off even on a normal stationary runway).
I agree, but then the main premise of the problem is impossible.
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Q#5196
MkII #080000376, 18GB green
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#270985 - 29/11/2005 19:12
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
However, since the runway and air molecules are relative to one another
My point was that they're NOT. If the runway is a conveyor belt, it's moving independently of the air molecules.
Quote:
in order for the plane to move across the molecules it would have to also move across the runway.
Not if the runway (conveyor belt) moves independently of the air molecules. Depending on how you interpret the question, the runway is either moving the same direction of the plane's travel, or opposite the plane's travel, but either way, it's independent of the air. The plane is either not moving with relation to the runway/belt, or moving twice as fast with relation to the runway/belt, but again, that's completely independent of its real-world (relative to the air) motion.
I know we're just arguing semantics. I just didn't make clear that the plane really DOES move.
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#270986 - 29/11/2005 19:14
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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#270987 - 29/11/2005 19:23
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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#270989 - 29/11/2005 19:25
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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Don't you love silly arguments? Pressing onward: Quote:
If the runway is a conveyor belt, it's moving independently of the air molecules.
I disagree. The conveyor belt itself is stationary*, as are the air molecules. As long as the plane is stationary relative to the belt, it is stationary relative to the air molecules.
Of course, the plane will move relative to both because the speed of the belt cannot offest the friction and so the plane will take off. But it still has to move relative to the stationary belt and air molecules to do so.
*by saying the belt is stationary, I mean that the plane "moving down the runway" implies we view the runway as a stationary object. If we view the runway as a moving object then the plane was already "moving down it" the moment the wheels started turning.
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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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#270990 - 29/11/2005 19:33
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Yeah, I see where semantics could have tripped us up there.
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#270991 - 29/11/2005 19:46
Re: Will the plane fly?
[Re: JeffS]
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old hand
Registered: 17/07/2001
Posts: 721
Loc: Boston, MA USA
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glad you guys are 'enjoying' this little question. The other forum I saw this on has over 1700 replies in about 1 day.
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#270992 - 29/11/2005 19:48
Re: Will the plane fly?
[Re: tfabris]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Quote:
Depending on how you interpret the question, the runway is either moving the same direction of the plane's travel, or opposite the plane's travel, but either way, it's independent of the air.
The belt is moving OPPOSITE to normal wheel rotation. Says so upstairs there.
As the plane attempts to move forward down the belt the belt moves forward with it in order to cancel wheel rotation.
The plane is still moving forward down the runway and through the air, and past the control tower, the terminal, and the parking lots. Each increase in speed and attempt to turn the wheels forward yields an equal increase in speed of the magical belt.
At Vr the pilot pulls back on the stick and the plane takes off- giant treadmill or no. The only thing saved in the scenario is the tires and bearings on the plane.
End of story.
Honest.
The problem is supposed to help the students understand displaced reference frames.
Like running inside a box car in a moving train. You run 10mph relative to the box car, but relative to the ground you're doing You+Train.
In the example, the student has to dissociate themselves from standing on the belt watching for tire motion. Stand in the parking lot and everything is copacetic.
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#270993 - 29/11/2005 19:48
Re: Will the plane fly?
[Re: JeffS]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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How do you suppose that the belt and the atmosphere are moving at the same speed?
That is, you're saying that relative to the plane, both the belt and air are stationary. But as viewed from a remote place, they could all be moving 200MPH. How do you expect that the air is moving 200MPH? And, if it is, where is the transistion between the 200MPH near the airplane and the near-zero MPH near the remote observer?
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#270995 - 29/11/2005 19:56
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
That is, you're saying that relative to the plane, both the belt and air are stationary. But as viewed from a remote place, they could all be moving 200MPH.
No- viewed from a remote place the belt speed and the wheels are moving 200MPH, but the plane and atmosphere would both appear stationary.
Of course, as previously stated, at 200MPH the plane would not remain stationary relative to the atmosphere because the friction would not prevent the foward movement and the plane would take off. But it is this foward movement that must take place for the take off to happen.
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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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#270997 - 29/11/2005 21:13
Re: Will the plane fly?
[Re: tfabris]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Quote:
Oh, and also, by the way:
http://www.avweb.com/news/columns/191034-1.html
Nice explanation, but I call foul (on a tedious, inconsequential side note-tangent)!
Quoth your article:
Quote:
The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is shown: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"
Quoth our thread starter:
Quote:
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?
The former suggests the belt moves opposite to the airplane's motion, the latter suggests the belt moves opposite to wheel rotation.
The bottom of the wheel (where it contacts the ground) is moving opposite to the plane's direction already.
Either way, though, and blah blah blah...
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#270998 - 29/11/2005 21:35
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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Well, that's somewhat more precisely worded, but less accurately, I'd say. It explicitly states that the airplane is moving forward. That's all we need to know to know that it will take off.
The problem with all of these restatements is that it's obviously an exercise in relativity, but none of them ever give frames of reference.
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#270999 - 29/11/2005 22:01
Re: Will the plane fly?
[Re: tfabris]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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Okay, it isn't quite taking off, but my father once landed a passenger plane backwards because the windspeed was higher than the landing speed for that aircraft. The passengers were quite amused apparently
_________________________
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MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
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#271000 - 29/11/2005 23:37
Re: Will the plane fly?
[Re: frog51]
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carpal tunnel
Registered: 23/08/2000
Posts: 3826
Loc: SLC, UT, USA
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I've had to do that in my paraglider a couple of times on the coast. Airspeed of the glider is around 23-25mph at trim... and you can add a few mph by pushing on a speed bar which pulls down on the front risers decreasing the angle of attack. So if you find yourself going backwards in the air, you can bet that landing is going to be a good time.
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#271001 - 30/11/2005 00:02
Re: Will the plane fly?
[Re: ithoughti]
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pooh-bah
Registered: 19/09/2002
Posts: 2494
Loc: East Coast, USA
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Brilliant, the Navy and Air Force finally got with the times: A free distributed think-tank, simply by posting a few qustions. We'll think up their micro air-craft carrier / portable runway system in no time.
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#271002 - 30/11/2005 02:10
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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This thread has been a hoot to read!.
Quote:
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 conveyor 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?
The question "Can the plane take off?" implies that an attempt to take off should be presumed.
Working from that we go on to the consturct of a runway sized conveyor that is "designed to exactly match the speed of the wheels at any time, moving in the opposite direction of rotation".
An aircraft does not depend on it's wheels to get it moving. Nor does it care if the ground(conveyor) under it decides to start moving forward or backward.
If the conveyor is moving forward, at the same speed as the wheels then, while the wheels are not turning, they are indeed moving forward, same as the aircraft.
If the conveyor is moving backward, the wheels still move forward with the aircraft, they just spin faster. Either way the aircraft still moves forward, the wings move through the air and lift is generated. The airplane takes off.
The rest, is just barffelgabb.
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Glenn
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#271003 - 30/11/2005 02:27
Re: Will the plane fly?
[Re: gbeer]
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addict
Registered: 23/12/2002
Posts: 652
Loc: Winston Salem, NC
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And the answer, my friends, is 42. That's all you need to know.
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#271004 - 30/11/2005 03:42
Re: Will the plane fly?
[Re: gbeer]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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This line causes a bit of a problem for me. Quote:
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
This is impossible. As soon as the wheels start moving forward a little, the forward thrust has overcome inertia and friction, and the conveyor belt rolls into action. Unortunately, by moving against the plane, it has caused the wheels to turn FASTER, and as such, the belt must roll faster. Hrmmmm, if they go faster, the wheels will go FASTER. If we assume the quoted statement to be true, the belt must assume infinite speed instantly in order to match the plane's wheel's speed. Which, in "reality" could never happen since the speed of the conveyor belt would always be (speed of conveyor belt) + (speed of plane) relative to the ground.
In other words, it's akin to the paradox of having to always go half the distance to reach something, and therefore never making it. The Zeno Tort.
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#271005 - 30/11/2005 05:28
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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Quote:
Now I wanna see them doing this one on Mythbusters.
Say... isn't there a member of this BBS that has an RC plane? One "pca", in particular? Maybe we can convince him to build a conveyor belt...
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#271006 - 30/11/2005 17:53
Re: Will the plane fly?
[Re: gbeer]
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pooh-bah
Registered: 15/01/2002
Posts: 1866
Loc: Austin
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Quote:
If the conveyor is moving forward, at the same speed as the wheels then, while the wheels are not turning, they are indeed moving forward, same as the aircraft.
That's the first thing that I thought. That is, that the wheels won't actually be spinning at all.
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#271007 - 30/11/2005 18:49
Re: Will the plane fly?
[Re: RobotCaleb]
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pooh-bah
Registered: 19/09/2002
Posts: 2494
Loc: East Coast, USA
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Then the conveyor belt will not be moving because the wheels are not moving.
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#271008 - 30/11/2005 18:54
Re: Will the plane fly?
[Re: FireFox31]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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But if the conveyor belt doesn't move, then the wheels will start moving, at which time the conveyor belt would have to start up. I guess both the wheels and the conveyor belt would each be moving at half the speed of the plane.
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Bitt Faulk
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#271010 - 01/12/2005 00:26
Re: Will the plane fly?
[Re: Robotic]
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pooh-bah
Registered: 15/01/2002
Posts: 1866
Loc: Austin
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Quote:
No ... airplane motion.
Relative to the conveyor belt.
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#271011 - 01/12/2005 02:11
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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Quote:
But if the conveyor belt doesn't move, then the wheels will start moving, at which time the conveyor belt would have to start up. I guess both the wheels and the conveyor belt would each be moving at half the speed of the plane.
The wheels are attached to the plane they cannot move at any speed except that of the plane. Don't confuse the rpms of the wheel with it's net motion.
Edited by gbeer (01/12/2005 02:12)
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Glenn
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#271012 - 01/12/2005 03:27
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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Truly though the question as stated is really more in the nature of finding out something about the person(s) willing to attempt an answer.
Look back through this thread. Some of the responses show how a person tackles problems. Others posts illustrate the writers grasp of mechanics and kinematics. Some writers take a stand then endlessly validate it with practical trivia, avoiding the the fact that the provided info is ambiguous. Assumptions run rampant. Some just like a good arugment or generate one by themselves not clearly writing what they mean. (Sort of like the original question.) As an assesment tool it it can tell you a lot about people. Even if they recognize it as such, it still says something about them.
Fun abounds.
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Glenn
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#271013 - 01/12/2005 07:38
Re: Will the plane fly?
[Re: RobotCaleb]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Quote:
Quote:
No ... airplane motion.
Relative to the conveyor belt.
To clarify:
No wheel motion.
Belt motion.
Airplane motion (relative to the air/airport/rest of world).
The 'No' referred only to the first item. The second and third items were simple results of what logically would follow.
Ok- seriously, this thread and me are done.
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#271014 - 01/12/2005 07:48
Re: Will the plane fly?
[Re: lectric]
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Anonymous
Unregistered
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lectric and JBorgen are right. It´s impossible to build such a coveyor belt.
Conveyor belt speed must equal the wheel speed, according to this problem.
Those two speeds can only be equal when the air speed of the plane is 0.
Once the engines start up, the air speed will be greater than 0, which means the wheels must turn faster than the conveyor belt.
So the only way the conveyor belt can match the speed of the wheels is when everything is stationary (belt speed, wheel speed, and air speed all must equal 0). The plane will fly, but it´s impossible for the belt to match the wheel speed.
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#271015 - 01/12/2005 14:25
Re: Will the plane fly?
[Re: gbeer]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Quote:
The wheels are attached to the plane they cannot move at any speed except that of the plane. Don't confuse the rpms of the wheel with it's net motion.
True. Maybe.
It says that conveyor belt moves opposite the direction of rotation at the speed of the wheels. If what you're saying is right, then it moves at the speed of the wheels (that is, the same speed as the plane), but that means that the wheels wouldn't be rotating, so I think your way of reading it isn't internally consistent. Of course, the real answer may be that no real-world situation actually matches the criteria of the question.
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#271016 - 01/12/2005 16:18
Re: Will the plane fly?
[Re: Robotic]
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pooh-bah
Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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Ah, then I missread the problem. In this case, of course, those who said that the plane would take off a bit earlier (because the engine does not have to oversome the wheel rotational friction) are right.
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Q#5196
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#271017 - 01/12/2005 20:49
Re: Will the plane fly?
[Re: bonzi]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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No, the problem was just poorly worded. I've now seen variants on the puzzle that word it more clearly. The idea is that the conveyor belt is supposed to try to stop the plane from taking off, not help it to do so. The idea of the problem is to make you (briefly) think that the plane won't move, until you realize that planes don't use their wheels for power.
The problem is really a lot simpler than what we've made it out to be in this thread. Its only goal was to make you go "Oh, duh, right, the propeller thrusts against the air, not the ground."
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#271018 - 01/12/2005 20:59
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
The problem is really a lot simpler than what we've made it out to be in this thread. Its only goal was to make you go "Oh, duh, right, the propeller thrusts against the air, not the ground."
It's a bit more subtle than that; it's also asking "Does the force due to friction increase with speed?". If yes, then there's a belt speed at which the thrust is balanced-out, and the plane doesn't take off. If no, the plane takes off.
And in fact the answer is that friction doesn't increase with speed, but that's a slightly surprising fact, and plenty of people's "folk" understanding of physics won't encompass it.
Peter
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#271019 - 01/12/2005 21:01
Re: Will the plane fly?
[Re: peter]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Well, that's not entirely true, as it's very possible for the axle components to be made of materials that expand when heated, and it would be possible for it to freeze as the heat due to friction increases.
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#271020 - 01/12/2005 21:55
Re: Will the plane fly?
[Re: tfabris]
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Anonymous
Unregistered
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Quote:
The problem is really a lot simpler than what we've made it out to be in this thread. Its only goal was to make you go "Oh, duh, right, the propeller thrusts against the air, not the ground."
I think it´s a little more complicated then that, because the problem states an impossible premise - that the conveyor belt can match the speed of the wheels.
In order for the belt speed to match the wheel speed, the plane must remain stationary (Imagine running on a tread mill). But the thrust is applied to the air and so the plane moves forward, forcing the wheels to turn faster than the conveyor belt.
So the conveyor belt will never even move. The only possible speeds it can match the wheel speed at are 0 and infinity.
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#271021 - 01/12/2005 22:03
Re: Will the plane fly?
[Re: peter]
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Anonymous
Unregistered
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Quote:
"Does the force due to friction increase with speed?". If yes, then there's a belt speed at which the thrust is balanced-out, and the plane doesn't take off. If no, the plane takes off.
I don´t think that´s true. Once the engines apply thrust to the air, the plane will move forward. And once the plane has moved forward at all, the belt has been defeated as the wheels haved turned faster.
That is unless the belt can go from a standstill to infinity miles per hour instantaneously.
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#271022 - 02/12/2005 02:06
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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Quote:
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
This part of it is entirely self consistant, though it seems otherwise. Neither half of the statment says anything to counter the other, as long as it is accepted that the first half of the statment is talking about net speed (linear) while the second half is talking about rotational speed. Two compleatly different things.
The belt can move to do both, match the speed of the wheel (linear), and move to oppose the rotation of the wheel at the same time. It's the same motion for the belt. The net effect is that the wheel is never allowed to spin.
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#271023 - 02/12/2005 02:54
Re: Will the plane fly?
[Re: ]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
But the thrust is applied to the air and so the plane moves forward, forcing the wheels to turn faster than the conveyor belt.
Not true, I think. Just because the plane is moving foward doesn't necessarily mean the wheels are turning faster. I've skidded out on ice in my car enough to know that the wheels don't have to turn to get foward motion. Once the plane has overcome the friction (not hard), the conveyor belt will no longer have to speed up because the wheels won't turn any faster.
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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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#271024 - 02/12/2005 05:10
Re: Will the plane fly?
[Re: JeffS]
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Anonymous
Unregistered
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You are talking about the belt moving in the opposite direction of the plane, right?
Think of a car on an oversized treadmill. If you set the treadmill to 25 mph, and set the cruise control on your car to 25 mph, then you´re not going to move anywhere (air speed will equal 0 with no wind). If you hit the gas and drive 30 mph then you´ll move forward and your air speed will be 5 mph, and the wheels will be turning 5 mph faster than the belt.
So the only time that the wheel speed and belt speed are equal is when the air speed is zero. When the thrust of the car is applied to the wheels the belt can always keep up and the airspeed will remain at zero. But when you switch on the JATO rocket attached to the back bumper of your car, then the thrust is applied to the air which of course makes the airspeed (and earth-ground-speed) increase, forcing the wheels to turn faster than the belt. If the belt tries to keep up by spinning faster, then it´ll only further increase the speed of the free-rolling wheels.
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#271025 - 02/12/2005 11:15
Re: Will the plane fly?
[Re: ]
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carpal tunnel
Registered: 19/01/2002
Posts: 3584
Loc: Columbus, OH
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Correct...untill the wheels reach their limit, at which point the plane will begin to drag its wheels accross the runway if the engine is strong enough.
Think of the wheels being locked in place so they don't spin. That's all this problem does. It removes the turning of the wheels from the equation completely. If a plane can take off with its wheels locked in place, then it can take off on a conveyor belt runway.
_________________________
~ John
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#271026 - 02/12/2005 14:02
Re: Will the plane fly?
[Re: JBjorgen]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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not exactly - some aircraft may have trouble taking off if they had locked axles which prevent the wheels turning...due to friction. All aircraft with wheels should suffer no ill effects taking off from a conveyor belt (excluding possible axle or wheel failure at high rotation speeds)
_________________________
Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock
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#271027 - 02/12/2005 14:48
Re: Will the plane fly?
[Re: ithoughti]
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new poster
Registered: 21/11/2001
Posts: 40
Loc: Maine, USA
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Quote:
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
My interpretation of this is that the linear belt speed must match the linear speed (forward motion) of the wheels (the planes ground speed) but in the opposite direction of rotation.
It does NOT mean that the conveyor must match the ROTATIONAL speed of the wheels, as the resulting linear belt speed would be dependant on the ratio of the diameters of the planes wheels and the belts wheels.
So, we know that the conveyor belt is attached to the ground, and the plane is on the belt.
Assume the plane accelerates to some ground speed x, before the belt turns on.
The planes wheel is moving forward with the plane at the same speed x.
The planes wheel is rotating in a clock-wise (CW) direction.
The planes wheel is also moving at some rotational velocity proportional to its diameter (both of which are irrelevant).
With the plane moving at speed x, the belt turns on (see attached diagram).
The conveyor belt wheels are rotating counter clock-wise (CCW) opposite to the rotation of the planes wheels. The linear speed of the belt is the same as the planes ground speed x.
In conclusion:
The only difference between this takeoff, and takeoff from a normal runway, is that the planes wheels will be rotating twice as fast as normal. Yes, the plane will take off.
-Jeff
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#271028 - 04/12/2005 01:16
Re: Will the plane fly?
[Re: insightful]
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Anonymous
Unregistered
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The only possible speeds that the belt can match the wheels are zero and infinity. There´s no inbetween, so it´s pointless to talk about friction.
The belt would have to accelerate from 0 mph to infinity instantaneously. Once the engines start, then the plane will move forward. If the plane is moving forward then the wheels are moving faster than the belt.
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#271029 - 04/12/2005 02:18
Re: Will the plane fly?
[Re: ]
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carpal tunnel
Registered: 19/01/2002
Posts: 3584
Loc: Columbus, OH
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Quote:
The only possible speeds that the belt can match the wheels are zero and infinity. There´s no inbetween, so it´s pointless to talk about friction.
Wrong. I know I was saying the same thing earlier in the thread, but it's not true. The belt always matches the wheels. The wheels have a mechanical limit to how fast they will spin due to friction, etc... The two speeds are 0 and the limit of the wheels.
_________________________
~ John
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#271030 - 04/12/2005 04:09
Re: Will the plane fly?
[Re: ]
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old hand
Registered: 15/02/2002
Posts: 1049
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I don't think this is true. As the plane accellerates (relative to the ground or stationary air), the conveyor-belt runway will accellerate in the same direction as the plane, causing the wheels to remain motionless (relative to the plane and conveyor-belt). The whole conveyor-airplane "assembly" will accellerate; the plane due to thrust, and the conveyor because its controller is programmed to negate wheel motion.
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#271031 - 04/12/2005 18:58
Re: Will the plane fly?
[Re: TigerJimmy]
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addict
Registered: 25/06/2002
Posts: 456
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Quote:
the conveyor-belt runway will accellerate in the same direction as the plane, causing the wheels to remain motionless (relative to the plane and conveyor-belt).
This is the confusion due to the extremely poorly-worded question.
- One group of people think the question means the conveyer belt moves with the plane, keeping the wheels stationary (or rather, non-rotating).
- The other group thinks the question means the belt moves against the plane, in an attempt to keep the center of mass of the plane stationary.
Your analysis falls into the first group.
Almost everyone else on this thread has been analyzing this from the second perspective.
I think it is pretty obvious for "Group A" people that the plane takes off and the wheels don't spin.
It is for all the people who interpreted this question in the "Group B" style that this discussion is mostly about.
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#271032 - 04/12/2005 20:51
Re: Will the plane fly?
[Re: tfabris]
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addict
Registered: 24/07/2002
Posts: 618
Loc: South London
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Quote:
The problem is really a lot simpler than what we've made it out to be in this thread. Its only goal was to make you go "Oh, duh, right, the propeller thrusts against the air, not the ground."
Well I'll agree with you Tony if nobody else will, I think people here are reading far too much into this "problem"! 
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#271034 - 05/12/2005 02:18
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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Quote:
Wrong. I know I was saying the same thing earlier in the thread, but it's not true. The belt always matches the wheels. The wheels have a mechanical limit to how fast they will spin due to friction, etc... The two speeds are 0 and the limit of the wheels.
Assuming, the point, Consider that the conveyor runs on wheels too. Far far more of them than the plane. The conveyor is going to fail far sooner than the plane's wheels.
_________________________
Glenn
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#271035 - 05/12/2005 02:51
Re: Will the plane fly?
[Re: JBjorgen]
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carpal tunnel
Registered: 12/11/2001
Posts: 7738
Loc: Toronto, CANADA
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Wow, I was going to comment on this thread when it was only at one page but I thought it would quickly die out once people realized it was just a bit of a mind teaser (the wheels have absolutely nothing to do with the answer).
If a plane crashed on the Canadian-US border, in which country would the survivors be burried?
Imagine a toy car, being held by your hand, with its wheels touching the surface of a motorized treadmill. Now imagine that treadmill moving as slowly or as fast as it can (or you want) in either direction (it doesn't matter). You can hold the car in one place, move it forward or move it backward. The force of your hand controlling the position of the car is analogous to an engine on a plane creating thrust. It moves independent of what its wheels or the treadmill are doing. It doesn't matter what color the car is, nor whether it has plastic bushings or bearings for its axles. That falls completely outside the simple scope of the original question.
I didn't read all the pages, but did anyone confirm if the plane was a private affair with an empeg installed?
Bruno
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#271036 - 05/12/2005 08:40
Re: Will the plane fly?
[Re: music]
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addict
Registered: 23/12/2002
Posts: 652
Loc: Winston Salem, NC
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Quote:
The other group thinks the question means the belt moves against the plane, in an attempt to keep the center of mass of the plane stationary.
This is how I interpreted the question. In order to take off, the plane must have some sort of airflow under the wings. With no wind present, the plane must create thrust in order to achieve air flow. If the purpose of the conveyor belt is cause the plane from thrusting forward (independent of the wheels or friction or whatever)--in other words, if the backward speed of the conveyor belt is inversely proportionate to the forward thrust of the plane--I don't think the plane would take off.
Ultra-simplified example:
Airplane’s forward speed (500 mph) - conveyor belt’s inverse speed (500 mph) = the plane is essentially stationary.
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#271037 - 05/12/2005 09:06
Re: Will the plane fly?
[Re: Cybjorg]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
If the purpose of the conveyor belt is cause the plane from thrusting forward (independent of the wheels or friction or whatever)--in other words, if the backward speed of the conveyor belt is inversely proportionate to the forward thrust of the plane--I don't think the plane would take off.
But the forward thrust is a force, and the backward speed of the conveyor is a speed -- they can't be directly compared.
Quote:
Ultra-simplified example:
Airplane’s forward speed (500 mph) - conveyor belt’s inverse speed (500 mph) = the plane is essentially stationary.
The plane's engines don't cause "speed" directly, they cause force. Like Newton said, force is mass times acceleration. The forces acting on the plane are thrust (forwards), friction (backwards), weight (downwards), and lift (upwards). If the forwards thrust exceeds the friction, the net forwards force is nonzero and so the acceleration is also nonzero. The plane moves relative to the rest of the airport, and so also relative to the belt. The belt simply can't apply enough force to the plane to counteract the forwards force of the engines, especially as the effective friction force on the plane does not increase with the speed of the belt. As the plane speeds up, the lift also increases until it exceeds the weight, at which point the net vertical force is upwards and the plane takes off.
Peter
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#271038 - 05/12/2005 11:02
Re: Will the plane fly?
[Re: Cybjorg]
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carpal tunnel
Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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Quote:
in other words, if the backward speed of the conveyor belt is inversely proportionate to the forward thrust of the plane--I don't think the plane would take off.
The problem states that the conveyor would match the speed of the WHEELS, not the thrust of the plane. The wheels don't have to turn at all for the plane to move foward. The only reason wheels turn is due to friction, but once there is enough force the place is going to move foward no matter what is going on with the wheels. At this point the belt will be matching the speed of the wheels, but not the speed of the plane. The plane moves foward and it takes off.
_________________________
-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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#271039 - 05/12/2005 18:28
Re: Will the plane fly?
[Re: JBjorgen]
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Anonymous
Unregistered
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Quote:
Quote:
The only possible speeds that the belt can match the wheels are zero and infinity. There´s no inbetween, so it´s pointless to talk about friction.
Wrong. I know I was saying the same thing earlier in the thread, but it's not true. The belt always matches the wheels. The wheels have a mechanical limit to how fast they will spin due to friction, etc... The two speeds are 0 and the limit of the wheels.
Well I figured if we have a magical conveyor belt that can accelerate to infinity then the airplane wheels could be frictionless.
The point is that the conveyor belt is impossible to build. Either something will break (the belt or the airplane wheels) or the wheels will turn faster than the belt, which means the belt failed its task.
The whole problem is a catch-22.
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#271040 - 06/12/2005 02:10
Re: Will the plane fly?
[Re: Cybjorg]
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carpal tunnel
Registered: 12/11/2001
Posts: 7738
Loc: Toronto, CANADA
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Only if the plane was bolted to the conveyor. Since it isn't, and since a plane is not propelled by its wheels, the conveyor would have no effect on the plane because it would have no effect on the thrust of its engines and therefore no effect on it forwarrd movement through the air.
It's just a simple question with the wheel business thrown in to make people forget that planes are not wheel-driven.
Direct from South Park: Nothing to see here, move along.
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#271041 - 06/12/2005 11:27
Re: Will the plane fly?
[Re: hybrid8]
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Anonymous
Unregistered
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Quote:
It's just a simple question with the wheel business thrown in to make people forget that planes are not wheel-driven.
There´s no question that the plane will gain air speed and take off due to the thrust from the engines. The confusing part is the impossibility of the conveyor belt matching the speed of the wheels.
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#271042 - 06/12/2005 11:30
Re: Will the plane fly?
[Re: hybrid8]
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veteran
Registered: 25/04/2000
Posts: 1525
Loc: Arizona
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Quote:
Only if the plane was bolted to the conveyor. Since it isn't, and since a plane is not propelled by its wheels, the conveyor would have no effect on the plane because it would have no effect on the thrust of its engines and therefore no effect on it forwarrd movement through the air.
It's just a simple question with the wheel business thrown in to make people forget that planes are not wheel-driven.
Direct from South Park: Nothing to see here, move along.
The problem is not that the plane is not wheel-driven, but whether or not the plane would have any forward velocity with respect to the surrounding air. The only reason a plane flies is because of the airflow around the airfoils (wings), which creates the pressure differential, which creates lift. With no velocity relative to the air, lift cannot be created. The whole problem boils down to, does this conveyor belt keep the plane stationary with respect to the air?
- Tim
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#271043 - 06/12/2005 13:05
Re: Will the plane fly?
[Re: Tim]
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carpal tunnel
Registered: 12/11/2001
Posts: 7738
Loc: Toronto, CANADA
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The conveyor does nothing. It's a fictitious monkey wrench. The conveyor would have to move faster to match the rotation of the wheels, but would also be adding to the rotation speed as it moved. So the effective ground "speed" (the amount of conveyor passing under wrt to the plane) would be insane. But, since the air over the wings and the plane thrust are completely independent of the ground and wheels, it will still have forward velocity and will therefore take off. The conveyor is not moving the plane backwards (and negating its propulsion/movement).
As mentioned, forget about the conveyor and the wheels - they're just there to get people down the wrong path. As can be seen by most of the posts in the thread and no doubt the dozens in the other forums where this has been posted. Reading too much into the question.
Quote:
The whole problem boils down to, does this conveyor belt keep the plane stationary with respect to the air?
The simple answer to this is NO. So we shouldn't even bother to think about seized wheels, hub friction or anything else. This is not the same as having no wheels on the plane where it rests on the conveyor. Nor having a pontoon plane on a fast current that's moving the plane backwards (so it would have to produce enough thrust to compensate for its reverse movement before it even generates positive/forward movement wrt to standing air.
I think I'll send this question out to a few dozen people at work on my last day...
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#271046 - 06/12/2005 17:29
Re: Will the plane fly?
[Re: ithoughti]
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addict
Registered: 09/06/1999
Posts: 559
Loc: Newfoundland, Canada
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Seems so simple to me that I can't believe there is this much debate going on.
My father's Cessna has a set of skis and floats as well as wheels. The floats and skis don't rotate whatsoever but the plane will still take off. If it was sitting on a convevor belt travelling the same speed as the skis/floats then it would be that much easier for the plane to take off as the skis/floats wouldn't have any drag (other then air resistance) to hold back the plane.
In this situation I see it as being basically the same as locking up the brakes on the wheels. If the conveyor belt is to match the speed of the wheels then the wheels do not need to be rotating. The belt will simply speed up at the same rate as the plane does with it's thrust provided by the prop.
I think the question is tricking people to some degree (duh!). The runway is moving in the opposite direction of rotation compared to the wheels... however, the wheels would not actually be rotating but will still gain forward speed.
Fun question!
Rene
_________________________
12 gig empeg Mark II, SN: 080000101 30 gig RioCar SN: 30103114 My blog
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#271047 - 06/12/2005 23:15
Re: Will the plane fly?
[Re: ShadowMan]
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addict
Registered: 25/06/2002
Posts: 456
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Quote:
Seems so simple to me that I can't believe there is this much debate going on.
[...]
If it was sitting on a convevor belt travelling the same speed as the skis/floats then it would be that much easier for the plane to take off as the skis/floats wouldn't have any drag
Again, people have interpreted this badly-worded question in two ways.
You are in the first group (i.e., you think the conveyer moves with the plane).
I think that EVERYONE who interprets the question in the way you do agrees with each other:
I.e., "the question is simple, there is no debate, the plane takes off, it is totally obvious, I know planes are driven forward by their engines and not their wheels, please don't waste my time."
It is for the other group of people that this question is even remotely interesting.
I.e., the people who believe the question is asking what happens if the conveyer moves against the plane. Those are the people who can get into debates about infinitely fast belts, wheel friction, etc.
If you are not in the second group then the question is so ridiculously obvious to everyone within about 5 seconds that there is no point even talking about it.
If you are in the second group, then you have to make various assumptions about whether or not this magical belt moving against the plane could somehow "hold back" the plane against the force of the plane's engine.
Some of the noise of this debate is due to the fact that not everyone is attempting to answer the same question!
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#271048 - 06/12/2005 23:52
Re: Will the plane fly?
[Re: music]
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pooh-bah
Registered: 12/02/2002
Posts: 2298
Loc: Berkeley, California
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I'm with bruno on this one. The plane takes off.
Unideal plane, unideal conveyor belt - the belts going to fall to pieces before it has a chance of increasing the rolling resistance enough to slow the plane down. Ideal plane, ideal conveyor belt, the belt has no effect, as the plane obviously has frictionless wheels. The only way the plane doesn't take off is if you've got an unideal plane, and an ideal conveyor belt, which just isn't fair.
Matthew
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#271049 - 07/12/2005 00:00
Re: Will the plane fly?
[Re: music]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
If you are in the second group, then you have to make various assumptions about whether or not this magical belt moving against the plane could somehow "hold back" the plane against the force of the plane's engine.
In the other versions of the question, the ones that are worded more clearly, the wheel speed isn't even mentioned, nor is wheel rotation. They just say that the belt moves in the opposite direction of takeoff, at the same speed as the plane. (Making it obvious what they meant).
In that case, even taking into account friction, it's not a big deal because the relative speed between plane and belt doesn't need to approach infinity. The relative speed ends up being twice the plane's takeoff speed, and the plane's wheels are most likely rated for that speed because sometimes it's gotta land pretty close to that fast.
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#271050 - 07/12/2005 17:59
Re: Will the plane fly?
[Re: tfabris]
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addict
Registered: 25/06/2002
Posts: 456
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Quote:
In the other versions of the question, the ones that are worded more clearly, the wheel speed isn't even mentioned, nor is wheel rotation. They just say that the belt moves in the opposite direction of takeoff, at the same speed as the plane. (Making it obvious what they meant).
Tony, I like that version much better!
It doesn't have the ambiguities of the version here, and it avoids the need for infinities and concerns about friction and wheel rotation.
Best of all, it has one clear and unambiguous answer.
And that answer is YES. 
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#271051 - 08/12/2005 11:04
Re: Will the plane fly?
[Re: tfabris]
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Anonymous
Unregistered
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Quote:
even taking into account friction, it's not a big deal because the relative speed between plane and belt doesn't need to approach infinity. The relative speed ends up being twice the plane's takeoff speed
Tony, you´re wrong on that point. If the plane has a positive earth-ground speed, then the wheels are rotating faster than the conveyor belt.
The conveyor belt can be seen as a giant wheel attached to the earth and the plane wheels are of course attached to the plane, so we have two sets of wheels rolling against eachother. If both sets of wheels turn at the same speed (not RPM´s, but the speed of the outside diameter of the wheels) in opposite directions, then the objects they are attached to will remain stationary in relation to eachother. So that means the earth and the airplane must be stationary in relation to eachother for the plane wheels and the conveyor belt to move at the same speed in opposite directions. But of course, as we all agree, the plane WILL move forward in relation to the earth (and the air), due to the thrust from the engines, so it´s impossible for the conveyor belt to match the speed of the plane wheels.
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#271052 - 08/12/2005 11:23
Re: Will the plane fly?
[Re: ]
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Anonymous
Unregistered
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CBS = conveyor belt speed
WS = wheel speed of the airplane
AS = airplane speed in relation to the earth (which we´ll assume is the same as the air-speed, since there is no wind)
The following is a stipulation of the original problem; the conveyor belt is designed to match the speed of the wheels in the opposite direction:
CBS = -WS
The conveyor belt is what drives the free-rolling airplane wheels, so for every inch the conveyor belt moves, the wheels move an inch in the opposite direction. The wheels are also fixed to the landing gear of the airplane and the conveyor belt is fixed to the earth, so for every inch the plane moves in relation to the earth, the wheels will move forward one inch on the conveyor belt. So:
WS = -CBS + AS
So if we substitute the conveyor belt speed (CBS) with the first equation, then we have:
WS = -(-WS) + AS
or
WS = WS + AS
So if the earth/air speed of the plane (AS) is not zero, then you can clearly see that the above equation is impossible.
The impossibility comes from the equation to determine the conveyor belt speed (CBS = -WS) which was a stipulation of the original problem -- that the conveyor belt is designed to match the speed of the wheels at all times. So this conveyor belt is impossible to build, unless you can build it so that it accelerates from zero to infinity instantaneously, since zero and infinity are the only valid values for WS and CBS.
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#271053 - 08/12/2005 12:43
Re: Will the plane fly?
[Re: ]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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Billy, He's saying that the question is worded so that the belt matches the PLANE's speed relative to the earth, not the wheels' speed. In that case, the wheels would simply be moving twice as fast.
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#271054 - 08/12/2005 13:25
Re: Will the plane fly?
[Re: lectric]
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Anonymous
Unregistered
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Ah, I see. Well then. I'd like to find whoever worded the original question and punch him in his stupid face.
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#271055 - 08/12/2005 13:47
Re: Will the plane fly?
[Re: ]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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That being said, the question as it was worded HERE is talking about the wheels of the plane, in which case you'd be right.
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#271056 - 09/12/2005 05:14
Re: Will the plane fly?
[Re: ]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
Tony, you´re wrong on that point. If the plane has a positive earth-ground speed, then the wheels are rotating faster than the conveyor belt. (...) so it´s impossible for the conveyor belt to match the speed of the plane wheels...
And as I said before, in the properly-worded versions of the question, the unambiguous ones, they never talked about the conveyor having to match the wheel speed. Just the plane's speed. In that version of the question, the plane's wheels only have to spin at twice normal takeoff speed before it lifts off. My only point for bringing this up is so that the friction - infinity camps don't have to keep debating those issues.
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#271057 - 09/12/2005 06:09
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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Quote:
My only point for bringing this up is so that the friction - infinity camps don't have to keep debating those issues.
Ah... what about the camp which figures that, as long as we have such a magical conveyor belt, we might as well also assume that other magical physics property of it being frictionless, as well?
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#271058 - 09/12/2005 16:04
Re: Will the plane fly?
[Re: canuckInOR]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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I'm saying those discussions are moot when the question is worded properly.
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#271059 - 06/02/2006 03:32
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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#271060 - 06/02/2006 04:04
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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Fabulous, We finally hear from an expert! IE someone from more than 50 miles away with a briefcase. Or would that be since it's written on the web, it must be right.
_________________________
Glenn
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#306717 - 31/01/2008 21:08
Re: Will the plane fly?
[Re: ithoughti]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Mythbusters did this in the most recent episode. Their experiment confirmed our conclusion: that it had little to no effect on the plane's takeoff.
_________________________
Bitt Faulk
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#306732 - 01/02/2008 13:53
Re: Will the plane fly?
[Re: wfaulk]
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addict
Registered: 11/01/2002
Posts: 612
Loc: Reading, UK
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I am repeatedly stunned by the amazing confusion about basic physics that so many people display in this kind of analysis.
If this level of science confuses people then it does help me to understand the popularity of 'faith' as a means of explanation of the world around us.
_________________________
LittleBlueThing
Running twin 30's
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#306737 - 01/02/2008 14:08
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 08/03/2000
Posts: 12338
Loc: Sterling, VA
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Mythbusters did this in the most recent episode. Their experiment confirmed our conclusion: that it had little to no effect on the plane's takeoff. The only problem I had with their tests is that they didn't replicate the circumstances. In their small and large-scale tests, the plane was not matching the speed of the belt, did it? Otherwise it wouldn't move forward relative to the ground until it took off, right? I'm sure I'm missing something.
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Matt
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#306738 - 01/02/2008 14:14
Re: Will the plane fly?
[Re: Dignan]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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The plane exerts virtually no force against the ground. The wheels spin freely (other than friction coming from the rotation around the axle). The vast majority of the force that the plane expends is against the air. Thus, the speed of the ground in relation to the plane has virtually no bearing on anything. It's only the speed of the air passing around it. And, in fact, if there was a headwind that matched the plane's takeoff speed, it wouldn't move forward, but it would still leave the ground. (Theoretically. I'm sure that getting the wind to blow in that consistent a direction at that consistent a speed is virtually impossible. Maybe in a wind tunnel.)
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Bitt Faulk
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#306739 - 01/02/2008 14:35
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 08/03/2000
Posts: 12338
Loc: Sterling, VA
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Gotcha. Makes sense.
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Matt
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#306740 - 01/02/2008 14:44
Re: Will the plane fly?
[Re: LittleBlueThing]
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old hand
Registered: 17/01/2003
Posts: 998
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I am repeatedly stunned by the amazing confusion about basic physics that so many people display in this kind of analysis. I too could not understand why they even did this test. The outcome just seemed so obvious. However I'm sure my wife, (who is very intelligent, but not when it comes to physics) would have been confused. In this episode they killed bugs with radiation, in San Francisco. I wonder how may protesters they had at their door the next day.
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#306741 - 01/02/2008 14:44
Re: Will the plane fly?
[Re: Dignan]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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In their small and large-scale tests, the plane was not matching the speed of the belt, did it? Otherwise it wouldn't move forward relative to the ground That's the root of the puzzle! No matter how hard you try, you can't make *that* happen! As soon as the plane throttles up, it's gonna move forward. Doesn't matter how hard you pull backwards on that conveyor belt, the plane goes forward at more or less the same speed that it would whether the conveyor belt was there or not. You can only keep the plane from taking off in two ways: 1. Applying its brakes. 2. *Not* throttling up. But those would keep the plane from taking off whether there was a conveyor belt or not. Here's another visualization of it that Vixy mentioned to me as we were watching the episode last night: What if we took flight out of the puzzle completely? What if the plane were just taxiing? The puzzle ends up being exactly the same, with the same result. If you throttle the plane up to the place where it would normally taxi at, say, 10mph, and you pull back on that conveyor belt at 10, 20, 30, or even 50mph, the plane will still move forward at 10mph.
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#306745 - 01/02/2008 15:35
Re: Will the plane fly?
[Re: wfaulk]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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And, in fact, if there was a headwind that matched the plane's takeoff speed, it wouldn't move forward, but it would still leave the ground. (Theoretically. I'm sure that getting the wind to blow in that consistent a direction at that consistent a speed is virtually impossible. Maybe in a wind tunnel.) Not so rare, actually. This is why light planes are tied down at airports. Airplanes take off and land into the wind, of course, so that less space is required. WWII aircraft carriers sailed full speed into the wind for aircraft launches for this very reason. What's more, it's perfectly possible to have negative ground speed when flying into a headwind- all you need is a mountain pass to funnel the air up to high velocity (or higher than the flying speed of the aircraft). I'm reminded of a story told by Richard Bach about flying a biplane across the country. I can't remember which book it was in- perhaps Nothing By Chance. He had to cross such a pass at low levels where the wind speed was less (wind speed is slower close to ground due to friction with the ground). He scared the heck out of a few drivers who happened to be on the road, but it was his only way through. At higher altitudes the wind drove him backwards. 
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#306751 - 01/02/2008 16:37
Re: Will the plane fly?
[Re: canuckInOR]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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The bicycle pushes on the treadmill. The airplane pushes on the air. There is NO power being transmitted to the wheels.
Hm. CanuckInOR's post to which this was a reply has vanished.
Edited by wfaulk (01/02/2008 16:41)
Edit Reason: undefined antecedent
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Bitt Faulk
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#306752 - 01/02/2008 16:44
Re: Will the plane fly?
[Re: wfaulk]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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Damn, Bitt... you're too fast on the reply -- you got me while I was in the middle of deleting my post because I realized my mistake, which, sadly, was exactly the same mistake described in the straight dope, only with a bicycle, instead of a car.
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#306753 - 01/02/2008 16:50
Re: Will the plane fly?
[Re: canuckInOR]
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old hand
Registered: 17/01/2003
Posts: 998
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Iwas just about to post this as well.
If you put the bike on the treadmill, tied a role to the wall in front of the bike, then turn the treadmill on, it would not matter how fast the treadmill was turning you could still pull the bike forward. The force you use to pull it forward would only be slightly greater than if the treadmill was not running. The only extra force required is the fiction force from your wheel bearings.
The airplane example is the same thing only the force it uses is pushing against air, not pulling a rope.
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#306754 - 01/02/2008 16:56
Re: Will the plane fly?
[Re: Redrum]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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I am repeatedly stunned by the amazing confusion about basic physics that so many people display in this kind of analysis. I too could not understand why they even did this test. The outcome just seemed so obvious. However I'm sure my wife, (who is very intelligent, but not when it comes to physics) would have been confused. Having just been one of the confused (though I got it the first time round), may I suggest that it has less to do with lack of knowledge about physics, and more to do with just not fully understanding how planes are powered, and, consequently, trying to relate it to a powered vehicle that they do know about? Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped.
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#306755 - 01/02/2008 17:01
Re: Will the plane fly?
[Re: Redrum]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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Iwas just about to post this as well.
If you put the bike on the treadmill, tied a role to the wall in front of the bike, then turn the treadmill on, it would not matter how fast the treadmill was turning you could still pull the bike forward. The force you use to pull it forward would only be slightly greater than if the treadmill was not running. The only extra force required is the fiction force from your wheel bearings.
The airplane example is the same thing only the force it uses is pushing against air, not pulling a rope.
Heh, yeah... what confused me was Vixy's suggestion to take flight out of the question. And, FWIW, I'll not delete posts ever again... Thank heavens it's Friday... now I just have to hide long enough 'til I can go home from work. :-p
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#306756 - 01/02/2008 17:04
Re: Will the plane fly?
[Re: canuckInOR]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped. I suppose the people who make this mistake assume that the plane puts power to its wheels until the moment it needs to get airborne? In other words, vroom drive down the runway really fast and then LEAP into the air at which point you start flapping your wings? 
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#306757 - 01/02/2008 17:04
Re: Will the plane fly?
[Re: canuckInOR]
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old hand
Registered: 15/02/2002
Posts: 1049
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I am repeatedly stunned by the amazing confusion about basic physics that so many people display in this kind of analysis. I too could not understand why they even did this test. The outcome just seemed so obvious. However I'm sure my wife, (who is very intelligent, but not when it comes to physics) would have been confused. Having just been one of the confused (though I got it the first time round), may I suggest that it has less to do with lack of knowledge about physics, and more to do with just not fully understanding how planes are powered, and, consequently, trying to relate it to a powered vehicle that they do know about? Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped. I think that's exactly what's happening. Have you ever set up a computer for grandparents who, to underline something, use the backspace key and delete what they just created? That's how you do it on a typewriter.
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#306760 - 01/02/2008 18:13
Re: Will the plane fly?
[Re: tfabris]
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carpal tunnel
Registered: 13/02/2002
Posts: 3212
Loc: Portland, OR
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Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped. I suppose the people who make this mistake assume that the plane puts power to its wheels until the moment it needs to get airborne? Yeah, why not? If you don't know how the mechanics work, then it's a reasonable assumption to make. A wheeled vehicle moves forward by having the engine turn the wheels. A plane is a wheeled vehicle with wings. Therefore... In other words, vroom drive down the runway really fast and then LEAP into the air at which point you start flapping your wings? I'd flap my wings as I vroomed down the runway -- like a duck taking off from a lake!
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#306761 - 01/02/2008 18:20
Re: Will the plane fly?
[Re: TigerJimmy]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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Okay so it is the opposite direction, but a few years back my dad landed a Britten Norman Islander (9 seater, twin engined, high wing, go anywhere plane) slightly in reverse onto the apron, immediately adjacent to the gate. Of course this was in Orkney, which is considerably windier than the rest of the UK.
He said it was dead easy. Wind speed of 90 knots meant it was inevitable.
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MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
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#306762 - 01/02/2008 18:41
Re: Will the plane fly?
[Re: frog51]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Wow that's a pretty big plane to be landing backwards. Must have been a sphincter factor of 9.5 on that one.
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#306763 - 01/02/2008 18:49
Re: Will the plane fly?
[Re: canuckInOR]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped. You're also going to be stumped if you forget (or never knew) that the friction forces on the wheels don't increase as the wheels' speed increases -- which is a whole lot less intuitive, IMO. The dialogue "Does the plane take off?" -- "No" -- "Yes it does" is, in a way, a very roundabout version of the dialogue "Is there a factor for relative speed in the equations of friction?" -- "Yes" -- "No there isn't". Peter
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#306764 - 01/02/2008 18:50
Re: Landing backwards
[Re: tfabris]
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carpal tunnel
Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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I've done that kind of landing with an R/C model. The biggest problem was dealing with the air turbulence. The plane wanted to dart up and down.
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Glenn
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#306765 - 01/02/2008 18:52
Re: Will the plane fly?
[Re: frog51]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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He said it was dead easy. Wind speed of 90 knots meant it was inevitable. 90 knots? That's a category 2 hurricane! I bet plenty of airport buildings in the UK wouldn't have survived that... Peter
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#306766 - 01/02/2008 19:48
Re: Will the plane fly?
[Re: tfabris]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Wow that's a pretty big plane to be landing backwards. Must have been a sphincter factor of 9.5 on that one. LOL Wow- landing backwards! That's one for the log books. That Britten-Norman Islander sure does remind me of an Aero Commander. Very similar in appearance and size. I'm sure I would have mistaken an Islander for an Aero Commander, not ever having heard of Britten-Norman before. Interesting that John Britten, one of the aircraft's designers, has a namesake known especially to motorcycle enthusiasts.
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#306767 - 01/02/2008 19:48
Re: Will the plane fly?
[Re: peter]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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That's only relevant if you'd think that the wheels couldn't turn twice as fast as normal takeoff speed. Seems unlikely that an airplane would be engineered with tolerances that low, though I could be wrong.
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Bitt Faulk
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#306770 - 01/02/2008 20:31
Re: Will the plane fly?
[Re: canuckInOR]
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carpal tunnel
Registered: 10/06/1999
Posts: 5916
Loc: Wivenhoe, Essex, UK
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Yeah, why not? If you don't know how the mechanics work, then it's a reasonable assumption to make. A wheeled vehicle moves forward by having the engine turn the wheels. A plane is a wheeled vehicle with wings. Therefore...
Only if you are too lazy to think on to what happens to keep it in the air once the wheels leave the ground !
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Remind me to change my signature to something more interesting someday
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#306771 - 01/02/2008 20:55
Re: Will the plane fly?
[Re: andy]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Only if you are too lazy to think on to what happens to keep it in the air once the wheels leave the ground ! Fairy dust and happy thoughts? 
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#306773 - 01/02/2008 21:27
Re: Landing backwards
[Re: gbeer]
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old hand
Registered: 20/07/1999
Posts: 1102
Loc: UK
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I've managed this on several occasions with a ducted fan powered glider with about a 2 metre wingspan. Our flying field is fairly high up and has quite laminar wind flow patterns if the wind is coming from the right direction and is fast enough. It's perfectly possible to have the thing land completely vertically from several hundred feet up, while maintaining an ordinary level flight angle. It looks almost impossible, the plane simple sinks gently down until it touches the ground. It's a very good example of the difference between ground speed and air speed. The plane is, of course, doing some 25-30 miles per hour relative to the air, so is flying completely normally, but the air is doing exactly the same speed in the opposite direction relative to the ground so the plane just hangs there in much the same way bricks don't pca
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Experience is what you get just after it would have helped...
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#306780 - 02/02/2008 07:27
Re: Will the plane fly?
[Re: Robotic]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Fairy dust and happy thoughts? Nope. Lift demons.
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#306781 - 02/02/2008 07:53
Re: Will the plane fly?
[Re: wfaulk]
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addict
Registered: 11/01/2002
Posts: 612
Loc: Reading, UK
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And, in fact, if there was a headwind that matched the plane's takeoff speed, it wouldn't move forward, but it would still leave the ground. (Theoretically. I'm sure that getting the wind to blow in that consistent a direction at that consistent a speed is virtually impossible. Maybe in a wind tunnel.) I can offer personal testimonial that this works Go to the top of a windy hill. Strap yourself to a hang glider. Have some (experienced) people hold ropes tied to the glider. Angle the glider into the wind. Rise vertically into the air. Practise all the banking, climbing and descent controls in safety just a few feet off the ground. Great fun!
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LittleBlueThing
Running twin 30's
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#306796 - 02/02/2008 16:09
Re: Will the plane fly?
[Re: Redrum]
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carpal tunnel
Registered: 12/11/2001
Posts: 7738
Loc: Toronto, CANADA
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I gave almost the same example in December 2005 in this very thread. Instead of a bicycle I proposed a toy car on a treadmill held in your hand. The first comment I made in that post sums it all up though. In a plane crash, where do you bury the survivors? You don't. Does anyone know the Mythbusters episode name that featured this? I want to search for it on my PVR.
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#306800 - 02/02/2008 18:54
Re: Will the plane fly?
[Re: wfaulk]
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pooh-bah
Registered: 20/01/2002
Posts: 2085
Loc: New Orleans, LA
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I have an extremely intelligent friend that simply cannot wrap his head around this problem. He is just having a hard time separating airspeed from ground speed. For some reason he's having a mental block about the air above the belt not moving WITH the belt.
Another thing I thought was very interesting was that the pilot they hired to test the myth believed the plane would sit there "like a brick". This guy is a stunt pilot. Huh?! How could he not intuitively know this?
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#306801 - 02/02/2008 19:05
Re: Will the plane fly?
[Re: hybrid8]
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carpal tunnel
Registered: 08/03/2000
Posts: 12338
Loc: Sterling, VA
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Instead of a bicycle I proposed a toy car on a treadmill held in your hand. That's exactly what the Mythbusters did. Episode name? I'm pretty sure it's titled "Airplane on a Conveyor Belt."
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#306811 - 02/02/2008 21:46
Re: Will the plane fly?
[Re: Dignan]
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addict
Registered: 11/01/2002
Posts: 612
Loc: Reading, UK
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I've just been through this with Mrs lbt and she thought it wouldn't take off. Anyway, after explaining that she should have known better and she was, therefore, a plank; I am now out of bed and in the study typing <sigh>.  OK - so that got me thinking... given there was some ambiguity about the wording of the question : what should the 'correct' question have been?
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LittleBlueThing
Running twin 30's
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#306812 - 02/02/2008 23:28
Re: Will the plane fly?
[Re: LittleBlueThing]
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carpal tunnel
Registered: 12/11/2001
Posts: 7738
Loc: Toronto, CANADA
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I'm not even mentioning this bloody riddle to Erin. We're supposed to be getting married in April and I don't want it getting in the way. On Nikoncafe.com the same subject has now gone to 70 pages - all within the past two weeks or so. Heh.
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#306833 - 03/02/2008 21:03
Re: Will the plane fly?
[Re: peter]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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Heh - 90 knots is pretty normal windspeed back home. It isn't as destructive as high winds in England for a few reasons:
Trees - none. Or almost none. We have some 40 year old oaks that are 7 feet tall, as that is the height of the wall.
Gusts - none. The wind just blows. From west to east.
Buildings - blimming solid construction!
They are good aircraft - I used to get to fly one round the mountains in the Falklands. And they can land on anything - beaches, mud, grass, runways... And Dad was a bit of a mental pilot (was fleet air arm - Buccaneer pilot!) so would go for any manoevre that seemed fun. He managed a few vertical takeoffs too!
Back on topic - when I used to fly r/c gliders I used to lie on a steep west facing slope and hold a glider at arm's length. A decent 2.5m wingspan glider would happily climb from my hand with a windspeed of anything over about 30 knots (well, less than that at 1m off the ground I suppose.) Landing back into the hand was much trickier - only really simple with a glider with airbrakes/spoilers.
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MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
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#306842 - 04/02/2008 07:25
Re: Will the plane fly?
[Re: frog51]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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I used to get to fly one round the mountains in the Falklands. Cool! When were you down there?
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#306847 - 04/02/2008 09:04
Re: Will the plane fly?
[Re: julf]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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Between '82 and '87 I was there a fair bit, and I went back a few times until around '91. Dad set up the Falkland Islands Government Air Service, which up until then consisted of two Beaver floatplanes, and as he was then Loganair's chief pilot and had a fair bit of time in that sort of environment he organised the airfields and procedures etc. It's an amazing place. I keep trying to work out cost effective ways to take SWMBO there on holiday.
Have you managed to visit the islands?
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock
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#306854 - 04/02/2008 12:33
Re: Will the plane fly?
[Re: lectric]
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veteran
Registered: 25/04/2000
Posts: 1525
Loc: Arizona
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I have an extremely intelligent friend that simply cannot wrap his head around this problem. He is just having a hard time separating airspeed from ground speed. For some reason he's having a mental block about the air above the belt not moving WITH the belt. Instead of a conveyor belt, explain it as taking off on a sheet of ice. It is the same principle as the conveyor belt without all the extra red herrings of friction, belt speed, etc thrown in.
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#306867 - 04/02/2008 15:36
Re: Will the plane fly?
[Re: Tim]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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I have an extremely intelligent friend that simply cannot wrap his head around this problem. He is just having a hard time separating airspeed from ground speed. For some reason he's having a mental block about the air above the belt not moving WITH the belt. Instead of a conveyor belt, explain it as taking off on a sheet of ice. It is the same principle as the conveyor belt without all the extra red herrings of friction, belt speed, etc thrown in. Oh! I like that twist! Then you can ask the question "How long is the landing rollout on ice?" Answer: the same length as if you didn't touch the brakes on a normal landing. lol
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#306879 - 04/02/2008 18:43
Re: Will the plane fly?
[Re: frog51]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Have you managed to visit the islands? Not yet, but some of my friends have. And I have some interesting "artifacts" from the war. The most interesting is a circular circuit board, designed to fit inside a sewer pipe. It has a radio receiver/detector that detects a combination of 2 audio carrier tunes, and after receiving that special carrier (from, let's say, an airplane with a suitable transmitter that flies past, it counts down for one week (suitable time period to make everything untraceable / deniable) and then issues a detonation signal... Seems teh argentinians left a bunch of them in sewer pipes, along with a fair bit of TNT...
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#306883 - 04/02/2008 19:31
Re: Will the plane fly?
[Re: julf]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Speaking of airplanes and Argentina... Want/slobber drool
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#306898 - 05/02/2008 02:12
Re: Will the plane fly?
[Re: Robotic]
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pooh-bah
Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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Just found this video clip of a sea plane taking off from a trailer behind a truck. Who needs a conveyor belt? edit: and another of a nifty glider takeoff.
Edited by Robotic (05/02/2008 02:19)
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10101311 (20GB- backup empeg)
10101466 (2x60GB, Eutronix/GreenLights Blue) (Stolen!)
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#306902 - 05/02/2008 06:58
Re: Will the plane fly?
[Re: julf]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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Yes - we ended up with a few souvenirs after they had been defused. The most interesting were some mines, a bazooka and two rifles, but the only ones we got back to UK were the mines. Luckily we didn't find anything booby trapped!
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock
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#306903 - 05/02/2008 07:45
Re: Will the plane fly?
[Re: frog51]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Do you need a deactivation certificate for the mine?
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#306919 - 05/02/2008 17:47
Re: Will the plane fly?
[Re: julf]
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pooh-bah
Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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oh yes:-) They were very strict on that, for my antipersonnel and my antitank mine.
_________________________
Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock
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