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| Will the plane take off? |
| Yes it will |
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38% |
[ 10 ] |
| No it wont |
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61% |
[ 16 ] |
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| Total Votes : 26 |
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HapKi
Joined: 10 Dec 2004
Location: TALL BUILDING-SEOUL
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 Posted: Mon Dec 05, 2005 4:21 am Post subject: |
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As thrust increases, a by-product is increased wheel speed.
As the belt matches wheel speed, the plane's not going to go anywhere.
Keep increasing thrust, and the wheels and belt speed will follow.
You will have the propulsion for take-off, just not the lift from the air across the wings. |
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 4:39 am Post subject: Re: Will the plane take off? |
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| Pligganease wrote: |
| Understand this, because you obviously don't. THE PLANE IS NOT MOVING ANYWHERE RELATIVE TO THE GROUND. It is standing still. It is moving 0.0000 km/h. Going nowhere. That is the entire point of the question. If it isn't moving, there is no air flowing over the wings to create lift, because thrust=0. |
I understand what you're saying. You're saying the plane will not move forward.
You are wrong.
There is nothing to stop the plane moving forward. Nothing! Well, ok, the tyres might burst, causing the plane nose to tip forward and the engines to fly the plane into the ground. 
The fact that there is a conveyor belt trying to make the wheels go backwards does not stop the plane going forward.
IT IS A PLANE, NOT A CAR.
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| hypnotist wrote: |
| Of course the plane is moving! |
You're right. I should stop arguing this point. I will right now, because myself, Newton, or Bernoulli can't make it any more clear than it already is, and you still don't get it. |
[/quote]
Newton agrees with me, my friend. Unless you're saying the conveyor belt somehow applies a force on the plane equal and opposite to some bloody great engines, through a couple of free-spinning wheels?! |
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Swiss James
Joined: 26 Nov 2003
Location: Shanghai
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| Posted: Mon Dec 05, 2005 4:43 am Post subject: Re: Will the plane take off? |
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| hypnotist wrote: |
The plane can be moving forward at 100mph relative to the ground, the conveyor backwards at 100mph, and the wheels at a linear velocity of 200mph. It's not a car fer chrissakes!
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we're agreed that the conveyor belt is stationary on the ground right?
And that the planes wheels are touching the conveyor belt?
Then how can the plane be moving forward relative to the ground when any forward motion by the wheels is counteracted by the movement of the belt? It makes no sense to me. |
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eamo
Joined: 08 Mar 2003
Location: Shepherd's Bush, 1964.
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| Posted: Mon Dec 05, 2005 4:58 am Post subject: |
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Hypnotist,
you have to consider the need for the plane to taxi at high-speed to get the lift under it's wings. The engines of the plane will produce the thrust to move the wheels forward along the conveyor belt but as the belt speeds up to match the speed of the wheels this will render the plane stationary. Thereby producing no lift.
While the plane is in on the ground it needs the wheels to cover distance to achieve lift and take-off. The fact that the thrust comes from jet engines doesn't matter.
So no. It doesn't take off. |
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 5:54 am Post subject: Re: Will the plane take off? |
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| Swiss James wrote: |
| hypnotist wrote: |
The plane can be moving forward at 100mph relative to the ground, the conveyor backwards at 100mph, and the wheels at a linear velocity of 200mph. It's not a car fer chrissakes!
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we're agreed that the conveyor belt is stationary on the ground right?
And that the planes wheels are touching the conveyor belt?
Then how can the plane be moving forward relative to the ground when any forward motion by the wheels is counteracted by the movement of the belt? It makes no sense to me. |
Because you're thinking of a car - and crucially, the point to understanding this is that the forward motion of the plane is not the same as the forward motion of the wheels. That is true for a car, but is not true of a plane.
The plane is driven forward by its engines pushing against the air. It has wheels which support it and which move at a velocity given by (velocity of aircraft through the air - velocity of ground). Note that the linear velocity of the wheels is determined by these two values. It cannot significantly influence these values. These are free-wheels after all - they are not driven by anything. There is a small amount of friction to deal with - but little compared to the friction overcome when an aircraft lands.
Now, imagine that the plane starts to move forward at 10mph. Thus the conveyor starts to move backwards at 10mph (it has forward velocity of -10mph). Does the plane stay still? No - instead the wheels start to spin at 20mph (10 - -10). The fallacy here is to suppose that the conveyor's force acts on the plane. It does not. It only makes the wheels spin faster.
Instead of speed, think about force. How much frictional force would the bearings in the wheels need to exert on the plane to counteract the thrust of the engines - and so, how fast would the belt need to run in order to keep the plane stationary? And anyway, why would you measure the velocity of a plane by the linear velocity of its wheels, eamo?
Once again - a plane is not a car. |
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eamo
Joined: 08 Mar 2003
Location: Shepherd's Bush, 1964.
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| Posted: Mon Dec 05, 2005 6:38 am Post subject: |
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After reading the pilots forum thread linked above I seem to be equally convinced that the plane will and won't take off!!
It's crazy!! Read the thread on that link if you want to lose a bit of sanity.... |
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poker player
Joined: 27 Sep 2004
Location: On the river
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| Posted: Mon Dec 05, 2005 8:03 am Post subject: |
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OK my pilot friend emailed me back and said it will fly. The reason is posted in this forum(geez how many forums is this puzzle in?) and he says it makes sense. But then again what does he know?-he flies for Air Canada
http://www.avweb.com/news/columns/191034-1.html |
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 8:11 am Post subject: |
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| eamo wrote: |
After reading the pilots forum thread linked above I seem to be equally convinced that the plane will and won't take off!!
It's crazy!! Read the thread on that link if you want to lose a bit of sanity.... |
It seems to depend on how you define "speed of the airplane".
People are saying that the plane does not move relative to the ground. Assuming we're not dealing with the case that nothing at all is moving, the conveyor is moving relative to the ground but the plane isn't - so they're not moving at the same speed. However, the wheels are moving at the same speed as the conveyor relative to the ground. Thus, perhaps, kind of, this meets the criteria of the original question. However, it seems daft to me because a) everyone from the control tower to the cockpit will be judging the plane's speed by its air speed, not the speed of its wheels and b) the wheels would have to be spinning so damn fast (in order to generate enough frictional force to stop the aircraft) as to have melted before we get near to this point. The plane won't take off in this case.
I'm saying that the plane does in fact move relative to the ground. The conveyor speed and aircraft speed are both measured relative to the ground by a stationary observer. In this way the speed of the whole plane can be equal and opposite to the speed of the conveyor - but the plane WILL move forward, and WILL take off, with the wheels spinning twice as quickly as usual.
Now, why everyone else plumped for the first definition of 'speed' is beyond me. The second seems entirely more logical to me. It also seems to fit the original description rather better. But there we go. I'm in the minority. |
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Wangja
Joined: 17 May 2004
Location: Seoul, Yongsan
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| Posted: Mon Dec 05, 2005 3:54 pm Post subject: |
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Oh God, thank you again for all the joy of this thread, demonstrating your will that all peope are not created equal.
Back to square one:
| Quote: |
| An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast. |
That means the plane is stationary.
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stationary
* adjective 1 not moving
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The plane is not moving. Therefore, the plane's wings are not moving. With the plane's wings not moving - stationary, remember? - there is no lift generated. With no lift generated, the plane stays on the ground.
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 4:29 pm Post subject: |
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| Wangja wrote: |
| Oh God, thank you again for all the joy of this thread, demonstrating your will that all peope are not created equal. |
 Eloquent as always, Sir.
| Quote: |
Back to square one:
| Quote: |
| An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast. |
That means the plane is stationary.
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To mean this, it means one of two things:
1) You are measuring the speed of the plane with respect to the conveyor belt, but the speed of the conveyor belt with respect to the ground, OR
2) You are measuring the speed of the plane as being equal to the speed of its wheels, rather than the speed of the metal through the air (as I've said, were it a car where the driver would indeed measure the speed as reported by the speed of the wheels I'd understand immediately - but a pilot will see airspeed, which in this case will stubbornly stay as zero).
Which is it? Please enlighten a poor Tab.  |
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Swiss James
Joined: 26 Nov 2003
Location: Shanghai
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| Posted: Mon Dec 05, 2005 4:57 pm Post subject: |
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Hypnotist- if I undestand properly you're saying that although the wheels are going to spin like crazy, they'll never be able to contain the force of the thrust from the engine.
So if you were to stand level with the plane you would see the speed of the wheels increasing and increasing until they basically can't hold the plane any more, and the plane starts moving along the conveyor belt with rubber smoke everywhere.
It's like saying if the runway was iced over, the wheels wouldn't get enough purchase to turn as fast as the plane was moving forward and the bird would basically glide along the ice until it reached the critical speed and *bang* off it goes.
Right? |
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Wangja
Joined: 17 May 2004
Location: Seoul, Yongsan
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| Posted: Mon Dec 05, 2005 5:13 pm Post subject: |
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| hypnotist wrote: |
| Wangja wrote: |
| Oh God, thank you again for all the joy of this thread, demonstrating your will that all peope are not created equal. |
Eloquent as always, Sir.
| Quote: |
Back to square one:
| Quote: |
| An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast. |
That means the plane is stationary.
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To mean this, it means one of two things:
1) You are measuring the speed of the plane with respect to the conveyor belt, but the speed of the conveyor belt with respect to the ground, OR
2) You are measuring the speed of the plane as being equal to the speed of its wheels, rather than the speed of the metal through the air (as I've said, were it a car where the driver would indeed measure the speed as reported by the speed of the wheels I'd understand immediately - but a pilot will see airspeed, which in this case will stubbornly stay as zero).
Which is it? Please enlighten a poor Tab. |
Wheelspeed is irrelevant.
All that matters is airspeed - the speed with which the wings move through the air (or the air moves over and under the wings). If airspeed is zero, the wings generate no lift - lift occurs where the pressure below the wing is higher than the pressure above the wing.
If the plane is moving forward at 50 mph and the conveyor is moving in the opposite direction at 50 mph, the plane is stationary - in the same position - and the wings generate no lift. |
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 5:21 pm Post subject: |
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| Swiss James wrote: |
Hypnotist- if I undestand properly you're saying that although the wheels are going to spin like crazy, they'll never be able to contain the force of the thrust from the engine.
So if you were to stand level with the plane you would see the speed of the wheels increasing and increasing until they basically can't hold the plane any more, and the plane starts moving along the conveyor belt with rubber smoke everywhere.
It's like saying if the runway was iced over, the wheels wouldn't get enough purchase to turn as fast as the plane was moving forward and the bird would basically glide along the ice until it reached the critical speed and *bang* off it goes.
Right? |
More or less. I don't think there'd be a lot of smoke - certainly a lot less than in the scenario where the friction of the wheels has to balance the thrust of the engines!
Note that I consider the conveyor belt speed to be equal to the speen of the aircraft. Crucially, for this, the aircraft cannot be stationary.
Edit: Actually, I think the subtler point I'm making has been lost.
In a car, if the speedo says 50mph, the wheels spin at 50mph (linear velocity) and their force against the road causes the car to go forwards at 50mph. If the road is actually going backwards at 50mph, the car goes nowhere.
For a plane, the speed is unrelated to the wheel speed. The wheels react to the motion of the plane - they do not cause it. Those saying the plane does not move seem to assume that the pilot opening the throttle causes the wheels to spin faster - which can be compensated for by the conveyor moving faster. But this is not the case! The throttle does not drive the wheels of a plane. The plane's speed cannot be significantly controlled by its wheels - unlike the case of the car. The only way it could is by the conveyor spinning sufficiently fast for the frictional force felt by the plane through the bearings of its wheels to match the thrust of the engines less the plane's drag. That is why I believe those saying the plane will not move are wrong.
Last edited by hypnotist on Mon Dec 05, 2005 5:58 pm; edited 1 time in total |
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hypnotist
Joined: 04 Dec 2004
Location: I wish I were a sock
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| Posted: Mon Dec 05, 2005 5:28 pm Post subject: |
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| Wangja wrote: |
| Wheelspeed is irrelevant. |
That's what I have been saying all along... 
| Quote: |
| All that matters is airspeed - the speed with which the wings move through the air (or the air moves over and under the wings). If airspeed is zero, the wings generate no lift - lift occurs where the pressure below the wing is higher than the pressure above the wing. |
Sure. My argument is based entirely around my belief that the plane will move forward. So far, I'm yet to see an argument as to why it won't. You seem to find it self-evident, but as I stated in my post above, it seems to imply one of two interpretations of the question - neither of which are satisfactory to me.
| Quote: |
| If the plane is moving forward at 50 mph and the conveyor is moving in the opposite direction at 50 mph, the plane is stationary - in the same position - and the wings generate no lift. |
WHY is it stationary? You and Pligganease restate this as if it's obvious from the question, but I still don't accept that it is 
The plane can be moving forward at 50mph relative to the ground and the conveyor moving at 50mph in the other direction relative to the ground and the wings have lift (the wheels in this case turning as if the plane was doing 100mph along the ground). There's nothing in the original question that says this can't be the case, is there?
Or, if you still don't understand why I don't understand, just answer this: If the plane is stationary, how is the plane moving forward at 50mph? |
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Swiss James
Joined: 26 Nov 2003
Location: Shanghai
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| Posted: Mon Dec 05, 2005 5:59 pm Post subject: |
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well this link
http://www.avweb.com/news/columns/191034-1.html
that poker player posted postures that the plane would be stationary, and it would still take off because the air speed would increase as the thrust from the engines increase.
It still seems counter-intuitive to me, but then the same article says that:
If the take off speed of a plane is 45 mph and you can get the plane to stand still on the runway in a 45 mph head wind- (with the brakes off, so I guess the engines would have to be going some just to prevent the thing moving backwards), the plane could take off without moving forward relative to the ground at all.
This makes sense to me. |
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