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#270974 - 29/11/2005 18:40 Re: Will the plane fly? [Re: JeffS]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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]
JBjorgen
carpal tunnel

Registered: 19/01/2002
Posts: 3582
Loc: Columbus, OH
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]
JeffS
carpal tunnel

Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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).
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#270977 - 29/11/2005 18:49 Re: Will the plane fly? [Re: JBjorgen]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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]
bonzi
pooh-bah

Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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]
peter
carpal tunnel

Registered: 13/07/2000
Posts: 4172
Loc: Cambridge, England
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]
bonzi
pooh-bah

Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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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#270982 - 29/11/2005 19:01 Re: Will the plane fly? [Re: JBjorgen]
peter
carpal tunnel

Registered: 13/07/2000
Posts: 4172
Loc: Cambridge, England
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]
JeffS
carpal tunnel

Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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.
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#270984 - 29/11/2005 19:10 Re: Will the plane fly? [Re: peter]
bonzi
pooh-bah

Registered: 13/09/1999
Posts: 2401
Loc: Croatia
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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#270985 - 29/11/2005 19:12 Re: Will the plane fly? [Re: JeffS]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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#270987 - 29/11/2005 19:23 Re: Will the plane fly? [Re: tfabris]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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#270988 - 29/11/2005 19:23 Re: Will the plane fly? [Re: tfabris]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
Now I wanna see them doing this one on Mythbusters.
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#270989 - 29/11/2005 19:25 Re: Will the plane fly? [Re: tfabris]
JeffS
carpal tunnel

Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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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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]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
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]
ithoughti
old hand

Registered: 17/07/2001
Posts: 721
Loc: Boston, MA USA
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]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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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#270994 - 29/11/2005 19:50 Re: Will the plane fly? [Re: Robotic]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Quote:
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.

Now assume that the train is travelling at the speed of light.
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#270995 - 29/11/2005 19:56 Re: Will the plane fly? [Re: wfaulk]
JeffS
carpal tunnel

Registered: 14/01/2002
Posts: 2858
Loc: Atlanta, GA
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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#270996 - 29/11/2005 20:50 Re: Will the plane fly? [Re: wfaulk]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
Quote:
Quote:
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.

Now assume that the train is travelling at the speed of light.

...through a dark tunnel...and then it turns on its headlights! Can you see anything?
What if you point a flashlight backwards down the path you've taken at lightspeed- are those photons 'moving' at zero velocity?
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#270997 - 29/11/2005 21:13 Re: Will the plane fly? [Re: tfabris]
Robotic
pooh-bah

Registered: 06/04/2005
Posts: 2026
Loc: Seattle transplant
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]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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]
frog51
pooh-bah

Registered: 09/08/2000
Posts: 2091
Loc: Edinburgh, Scotland
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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#271000 - 29/11/2005 23:37 Re: Will the plane fly? [Re: frog51]
loren
carpal tunnel

Registered: 23/08/2000
Posts: 3826
Loc: SLC, UT, USA
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]
FireFox31
pooh-bah

Registered: 19/09/2002
Posts: 2494
Loc: East Coast, USA
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]
gbeer
carpal tunnel

Registered: 17/12/2000
Posts: 2665
Loc: Manteca, California
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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#271003 - 30/11/2005 02:27 Re: Will the plane fly? [Re: gbeer]
Cybjorg
addict

Registered: 23/12/2002
Posts: 652
Loc: Winston Salem, NC
And the answer, my friends, is 42. That's all you need to know.

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