Question

themole · 17-02-2006 07:49PM

your point of view is only true if talking about air speed.

and i have agreed that the conveyor is irrelevant if the air speed of the plane is matched to the ground speed of the conveyor

i was talking about ground speed.

Gurgle · 17-02-2006 08:17PM

themole wrote:

your point of view is only true if talking about air speed

lol, theres no such thing as a point of view in newtonian physics

David19 · 19-02-2006 05:09PM

I read a good analogy for this the other day.

Imagine your on a thread mill wearing roller skates and there's a rope attached to the wall infront of you. Now set the speed of the thread mill to match your speed. What happens when you pull on the rope? You move forward of course. You could set the thread mill to 3 times your speed and you'd still move forward. You could of course get someone to push you forward on the thread mill instead of pulling on the rope. That would mimic the forward thrust of the plane.

Gurgle · 20-02-2006 10:41AM

David19 wrote:

Now set the speed of the thread mill to match your speed. What happens when you pull on the rope? You move forward of course. You could set the thread mill to 3 times your speed and you'd still move forward.

Excellent analogy

nitroboy · 20-02-2006 03:41PM

Excellent analogy indeed? great minds think alike.....

you forgot the bit about the tread mill matching the speed of the wheels. ur goin nowhere buddy, even if you had a rocket strapped to your back if hypothetically speaking the conveyer will match the speed of the wheels. you might fall over though.

I'm going to stop reading this thread before you convince me otherwise

David19 · 20-02-2006 08:25PM

Nowhere does it say the conveyor belt matches the speed of the wheels. It says it matches the speed of the plane. The wheels will go twice as fast as the conveyor belt.

On a side note, if the conveyor belt matches the speed of the wheels it would mean the wheels would be effectively fixed. A plane might actually still take off with fixed wheels. It would depend on the power of the engines. It would certainly move forward. Anyway, that's nothing to do with this.

phantom_lord · 20-02-2006 08:49PM

nitroboy wrote:

I'm going to stop reading this thread before you convince me otherwise

I'd take that as meaning he gives up...

Capt'n Midnight · 20-02-2006 10:09PM

WTF !

Aircraft Carriers are fast ships. They could save a fortune on construction, engines and fuel if they only needed to be as fast as most other warships. So why are they usually capable of 30+ Knots and even then head into the wind when planes are taking off ?
Just one reason, to increase the speed of air over the planes wings.

If a conveyer could get planes into the air then it would make sense to use them than a steam catapult. 20,000Kg to 73.9m/s in 2 seconds takes about 5 MegaWatts, more than enough to drive a conveyer as fast as you could want.

Planes can take off vertically if the wind speed is high enough even if the wheels don't move. The Storch can do it in a moderate breeze.

If the conveyer induced a breeze by drag then the plane could take off, but any wind would do conveyer or not.

Gurgle · 21-02-2006 03:49PM

nitroboy wrote:

ur goin nowhere buddy, even if you had a rocket strapped to your back if hypothetically speaking the conveyer will match the speed of the wheels. you might fall over though.

hehe, you haven't done the junior cert yet have you?

nitroboy · 21-02-2006 06:36PM

no why is there anything wrong with that??

Gurgle · 22-02-2006 12:11PM

nitroboy wrote:

no why is there anything wrong with that??

nothing whatsoever, except that you're arguing physics with people with degrees, masters, even PhDs here.

delly · 22-02-2006 12:14PM

Gurgle wrote:

nothing whatsoever, except that you're arguing physics with people with degrees, masters, even PhDs here.

You would think that with all those letters floating about, that some agreement could be formed :rolleyes:

Civilian_Target · 22-02-2006 01:04PM

delly wrote:

You would think that with all those letters floating about, that some agreement could be formed :rolleyes:

Nah - because some of them are engineers

nitroboy · 22-02-2006 03:58PM

Gurgle wrote:

nothing whatsoever, except that you're arguing physics with people with degrees, masters, even PhDs here.

I know I'm argueing physics but I dont think this particular subject is the sole domain of the intellectual elite (excluding engineers !)

but lets get back to the original question

will the plane take off or not?

I would think NOT, and anyone with a decent degree masters or PhD will agree, thats one of the reasons you dont find planes sitting on conveyer belts in airports.

and as for the analogy with the person on the conveyer belt with roller skates being pulled by a rope:
if the person goes forward then the wheels MUST go faster than the opposing speed of the conveyer belt, if this happens then the conveyer belt is not adjusting to the forward speed therefore you have a different scenario in place. therefore the analogy is not relevent to the original posting.

Spear · 22-02-2006 05:34PM

nitroboy wrote:

will the plane take off or not?

I would think NOT, and anyone with a decent degree masters or PhD will agree, thats one of the reasons you dont find planes sitting on conveyer belts in airports.

It will take off. The conveyor belt does not impede the forward movement of the plane. The contact of the wheels against the conveyor is not the source of acceleration, the engines are. The speed of the plane relative to the conveyor will be twice the groundspeed.

/disclaimer: I did crappy in my physics degree.

planck2 · 22-02-2006 08:10PM

if there is no relative motion between the plane and the conveyer belt then the plane cannot take off. This is a necessary condition for the plane to be able to take off, but not a sufficient one. It also needs to have the correct speed in order to have a proper airflow around the wings.

David19 · 23-02-2006 03:43AM

planck2 wrote:

if there is no relative motion between the plane and the conveyer belt then the plane cannot take off. This is a necessary condition for the plane to be able to take off, but not a sufficient one. It also needs to have the correct speed in order to have a proper airflow around the wings.

Umm the plane will take off. The plane will take off as normal to an outside observer. There is absolutely no reason the plane will not take off. There is absolutely no force against the planes engines. The plane will easily attain the necessary speed for take off. For anyone who thinks it won't take off could you name the force against the planes engines?

David19 · 23-02-2006 03:45AM

nitroboy wrote:

and as for the analogy with the person on the conveyer belt with roller skates being pulled by a rope:
if the person goes forward then the wheels MUST go faster than the opposing speed of the conveyer belt, if this happens then the conveyer belt is not adjusting to the forward speed therefore you have a different scenario in place. therefore the analogy is not relevent to the original posting.

The wheels can go as fast as they need to. There's nothing in the question restricting the speed off the wheels.

Gurgle · 23-02-2006 10:10AM

nitroboy wrote:

will the plane take off or not?

The conveyor is irrelevant to that question. The conveyor will have no effect whatsoever on the planes motion.
The engines will start, the plane will move forward until it reaches take-off speed and take off.
Doesn't matter if its on tarmac, water, snow, ice, a conveyor belt, an oil slick, an aircraft carrier, the roof of the Dail. If the plane can achieve takeoff speed within the available distance (before it falls off the edge or crashes into something) then it will take off.

The distance required will not change.

Watch Top Gun (an old movie from the 80s, you may not have heard of it). On the aircraft carriers, theres no conveyor belts, they use vertical 'walls' behind the planes to increase the force. And if conveyor belts would have helped the US Navy would have them. I doubt if they ever tried though, as they spend enough money to hire people with physics degrees.

delly · 23-02-2006 11:20AM

Maybe this picture would help? (and no, i don't have photoshop skills)

planck2 · 23-02-2006 06:27PM

Please ignore my original post. The original question states that the plane moves in one direction with velocity v and the conveyor belt moves in the opposite direction with velocity v. The relative velocity between the plane and the conveyor belt is always zero, to a ground observer that is. However, the plane still moves with respect to the ground with velocity v , so there is an airflow over the wings, therefore it will take off.

elurhs · 08-03-2006 05:48PM

I know I'm resurrecting an old thread, but I've just seen it, and it's messed with my head for an hour, so I had to stick in my two cents.

First I was totally sure the plane wasn't taking off. But the more I thought about it, the more I saw that it WOULD take off.
This is my take. A car moves by exerting a force from it's engine on the ground/belt through it's wheels. The wheels push off the ground, moving the car forward. When the belt is moving, it pushes back with the same force, so the car is stationary.

The key here, as pointed out before, is that the plane moves by exerting a force on the AIR, not the GROUND. As the wheels exert no force on the ground normally, the conveyer belt won't make much of a difference. The plane will move forwards and take off.

(And I have a degree in physics, although in fairness I was never that hot at Newtonian mechanics):D

delly · 08-03-2006 05:50PM

elurhs wrote:

I know I'm resurrecting an old thread, but I've just seen it, and it's messed with my head for an hour, so I had to stick in my two cents.

First I was totally sure the plane wasn't taking off. But the more I thought about it, the more I saw that it WOULD take off.
This is my take. A car moves by exerting a force from it's engine on the ground/belt through it's wheels. The wheels push off the ground, moving the car forward. When the belt is moving, it pushes back with the same force, so the car is stationary.

The key here, as pointed out before, is that the plane moves by exerting a force on the AIR, not the GROUND. As the wheels exert no force on the ground normally, the conveyer belt won't make much of a difference. The plane will move forwards and take off.

(And I have a degree in physics, although in fairness I was never that hot at Newtonian mechanics):D

NO NO NO!!! Let this thread die :mad:

Nietzschean · 09-03-2006 05:37AM

elurhs wrote:

The key here, as pointed out before, is that the plane moves by exerting a force on the AIR, not the GROUND. As the wheels exert no force on the ground normally, the conveyer belt won't make much of a difference. The plane will move forwards and take off.

Mainly just due to the force being exerted being independent of friction with the ground, the only way the conveyor belt could have an effect is if there is a huge ammount of friction in the wheels, though even then as pointed out before they would probally just crumple(though the then increased ammount of friction by the planes body would probally cause it to rip appart instead of taking off). Though i'd be think'n friction with the wheel's bearings must be well lower than any threshold to cause that....

Please ignore my original post. The original question states that the plane moves in one direction with velocity v and the conveyor belt moves in the opposite direction with velocity v. The relative velocity between the plane and the conveyor belt is always zero, to a ground observer that is. However, the plane still moves with respect to the ground with velocity v , so there is an airflow over the wings, therefore it will take off.

Thats actually wrong, the conveyor belt may have velocity v in one direction, but if we take the thrust given by the planes engines gives it also a velocity v, then the actual motion of the plane forward will be v, but the planes wheels will be moving at 2v

nitroboy wrote:

I would think NOT, and anyone with a decent degree masters or PhD will agree, thats one of the reasons you dont find planes sitting on conveyer belts in airports.

Will a degree in maths do to oppose you then?(well in like 3months time..)

OP wrote:

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The OP states the conveyor belt will match the planes velocity, but thats entirely irrelevent, that will just mean the wheel speed will increase, it will have only a marginal if any effect on the actual planes motion.
So yes, the plane would take off, but the reason we don't have them in airports is that it just wouldn't provide any benefit.

ApeXaviour · 09-03-2006 01:06PM

Nietzschean wrote:

So yes, the plane would take off, but the reason we don't have them in airports is that it just wouldn't provide any benefit.

As Nietzshean said it wouldn't be of any benefit as the conveyor belt would have to be as long as a runway and would actually hinder it from taking off. Why? Cos the extra friction in the wheels would slow it down with respect to the air (which is what matters, not the ground). Infact if you insisted on a conveyor belt, it would be more efficient to have it run in reverse (ie the direction the plane is taking off in) giving the plane extra velocity with respect to the air and helping it take off.

But if you want a plane to take off stationary like, you're better off putting a big-ass fan infront of it. It would have to be a ridiculous fan though, completely impractical.

Gurgle · 10-03-2006 04:11PM

ApeXaviour wrote:

But if you want a plane to take off stationary like, you're better off putting a big-ass fan infront of it. It would have to be a ridiculous fan though, completely impractical.

That would give the lift to take off, but not the airspeed to fly. As soon as it got out of the fan's airstream it would drop like a rock.:D

ApeXaviour · 10-03-2006 04:40PM

Gurgle wrote:

That would give the lift to take off, but not the airspeed to fly. As soon as it got out of the fan's airstream it would drop like a rock.:D

lol true true...

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