Question

GerryRyan

Feel free to move this if it's in the wrong forum ...

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 question is:

Will the plane take off or not? Will it be able to run up and take off?

PhoenixRising

No.

The ability to takeoff is soley dependent on having a relative airflow over the wings. If the aircraft is not moving relative to the air it can't take off.

themole

the lift of a planes wing is due to air moving as different speeds on the top and bottom of the wing.

all that matters is if the wings are moving in relation to the surrounding air.
in this case they are not so the plane will not take off.

(ignoring any possibly movement of air created by the conveyor belt)

GerryRyan

Right so, was just a thought anyway. Would make airports / runways so much smaller if it was possible tho.

themole

not really dude.

most of the length of a runway has to do with landing and being able to stop before ya hit the end.

even then they are longer than they need to be for safety to give the pilot a margin for error

David19

This has been discussed a lot on the net recently and the general consensus is that the plane WILL take off. This is because a plane moves due to the thrust of its engines, which are independent of the wheels. A plane isn't driven by its wheels therefore the conveyor belt is irrelevant. The wheels will move a lot faster than usual to comepensate for the conveyor belt.

themole

ThatGuy wrote:

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).

this is the important thing.

if this is the case then the plane is not moving, relative to the surrounding air.

so no lift is generated and the plane does not take off.

David19

themole wrote:

this is the important thing.

if this is the case then the plane is not moving, relative to the surrounding air.

so no lift is generated and the plane does not take off.

The plane isn't driven by its wheels, its driven by the thrust from the engines. The wheels will just move faster than usual to counteract the movement of the conveyor belt and will probably get ripped apart. In fact I'd say a plane doesn't need any wheels to take off, it just makes it a lot easier.

themole

the wheels are there to allow the plane to move when its on the ground.

what you are missing is that with the conveyor belt moving in the opposite direction the plane is prevented from moving forward at all relative to the surrounding air

no forward motion = no motion relative to surounding air = no lift

that fact that the plane is not driven by its wheels is irrelevant. what is relevant is that the ground in effect which the plane is moving forward relative to is its self moving backwards, so the plane is not moving relative to the air

think of it this way:

if i am on a train (lets say its infinitly long) goin 2miles/hour and i walk along the train in the opposite direction then to an observer on a platform i appear to be staying in the same plane, although i am moving relative to the train.

the point is that i am not moving relavite to the outside world.

in the same way in the example the plane is not moving relative to the surrounding air

David19

Thats the point though, the plane WILL move forward. If it was a car on the conveyor belt then no the car wouldn't move. This is due to a car being driven by its wheels which are in contact with the conveyor belt. A plane doesn't work like that, its engines are independent of its wheels. The wheels are just there to reduce friction when taking off. A plane will certainly move without wheels. Think about it.

Google it.

themole

ThatGuy wrote:

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).

its got nothing to do with the wheels.

the SPEED is the point.

if the SPEED is the same in either direction then the forward SPEED of the plane= 0

themole

David19 wrote:

. A plane doesn't work like that, its engines are independent of its wheels.

they are independant.

but they are still attached to the plane.

so, if they are not moving forward neither is the plane (forget about the rotation of the wheels, i mean the horizontal motion relative to the surrounding area)

David19

themole wrote:

its got nothing to do with the wheels.

the SPEED is the point.

if the SPEED is the same in either direction then the forward SPEED of the plane= 0

Where is the force against the plane coming from? As you said the wheels are independent of the planes engines so its not from the conveyor belt. There has to be a force against the planes engines(not the wheels) for your theory to be correct.

As for the wheels being attached to the plane, they're free to move. The wheels will move as fast as necessary for take off. Since the conveyor belt is moving the wheels backwards they'll have to move a lot faster than usual to counteract the backwards motion, but they're free to do that.

themole

David19 wrote:

Imagine if you put wheels on a rocket and replaced it with the plane, do you really think the rocket wouldn't take off? The plane works in a similar way.

i presume you mean replace the plane in this case with the rocket.

if its on it side with wings to give it lift and the questions is will it rise off of the horizontal like a plane taking off then

no.

it wouldn't take off.

David19 wrote:

The wheels will move as fast as necessary for take off.

the whole point is that the questions supposes that the conveyor matches the forward movement of the plane.

so the plane can never move forward faster than the conveyor is going backwards, so relative to the surrounding air the plane does not move forward.


you can get rid of the wheels if ya like, say grease up the bottom of the plane and the result is the same.

the point is that it has nothing whatsoever to do with how the plane is driven.


you accepted that a car would not move forward, what you have to realise that all a car is is an object m,oving forward resting on the conveyer. how it is driven doesn't matter. speed is speed however the movement is attained

David19 wrote:

they'll have to move a lot faster than usual to counteract the backwards motion

the point is the problem supposes they cannot do this, as the backwards motion is kept the same as the planes forward motion

nitroboy

They should just build all planes like harrier jets, then you could just use a system of conveyer belts to 'convey' the planes around the airport and a tiny runway would only be required!

David19

themole wrote:

the point is the problem supposes they cannot do this, as the backwards motion is kept the same as the planes forward motion

The tires might get ripped apart on takeoff.

I edited my previous post while you were typing your response. You haven't told me where the force against the planes engines is coming from? As you've said the wheels are independent of the planes engines and can move freely. They're simply there to reduce friction on takeoff.


These might explain it better:

http://www.kottke.org/06/02/plane-conveyor-belt
http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

themole

the question is purely theoretical.

if taken in the sense of a physics question you have to make assumptions.

in the real world the conveyor would never be able to match speed correctly and the plane would slowly move along it and fall off the end. but there is an assmption made that this is possible in the question.

the question only asks if the plane will take off. so if the wheels do get shredded (which i think can be ignored as the reason for the question was to know whether the plane could take off with its not moving relative to the air), but if you take this into account then the wheels get shredded and the plane crashes , and hence does not take off.

i am going to read that link now, will reply back after

themole

i read a few of those comments, this is one i liked



Ricardo says:
Exactly what I was thinking. I wasnt even thinking about the wheels... just the fact the plane isn't really moving, therefore no lift force provided to make the plane actually go... up!

An example would be like sitting on those gym bikes with no wheels. You can peddle as fast as you want, but you wont feel the fresh breeze of that speed. No breeze, no wind... no lift!

a threadmill is also a good example.

if i run on a thread mill forward at the same speed that the thread mill is going backwards will i ever hit the end?

the answer of course is no.

what you don't realise is that the answer will be no for me, a remote controlled car, a rocket powered car, a plane , all the same.

Gurgle

themole wrote:

what you don't realise is that the answer will be no for me, a remote controlled car, a rocket powered car, a plane , all the same.

The thrust from the plane's engines will send it right off the conveyor belt regardless of what speed the belt is going at.

Likewise on landing, it may land on a moving belt but that won't take up the momentum, it will just shoot off the belt.

themole

Gurgle wrote:

The thrust from the plane's engines will send it right off the conveyor belt regardless of what speed the belt is going at.

explain?

in the question say the plane is moving forward at 500mph, then the conveyor is moving backwards at 500mph,

so relative to the surrounding air the plane is not moving at all

Gurgle

themole wrote:

explain?

in the question say the plane is moving forward at 500mph, then the conveyor is moving backwards at 500mph,

so relative to the surrounding air the plane is not moving at all

The plane's driving force is not through the wheels on the ground ,its thrust from the engines against the surrounding air. The conveyor will not counteract the thrust.

If the plane has a thrust which usually moves it at 500mph, it will move along the conveyor at 500mph. If the conveyor is moving in the opposite direction at 500mph, the wheels will be turning at 1000mph as the plane shoots off the edge of the conveyor at 500mph.

David19

themole wrote:

a threadmill is also a good example.

if i run on a thread mill forward at the same speed that the thread mill is going backwards will i ever hit the end?

the answer of course is no.

what you don't realise is that the answer will be no for me, a remote controlled car, a rocket powered car, a plane , all the same.

This is the gist of the problem. The reason you won't reach the end of the thread mill is because your legs are what move you forward and they are in direct contact with the thread mill. The thread mill counteracts your legs forward movement. As Gurgle pointed out a plane does not move forward through its wheels. There is no force counteracting the planes engines.

A car is not the same as a plane.

delly

Gurgle wrote:

The plane's driving force is not through the wheels on the ground, its thrust from the engines against the surrounding air. The conveyor will not counteract the thrust.

Which is true for the small bit of air directly at the engine point, but for the plane to fly, the whole wingspan must be forced against the air to generate lift. This can't be done if the plane remains in a static location.

themole

its an interesting problem.

my point of view is this.

speed is relative and measured from a point.

the plane may be moving say 500mph relative to the conveyor belt, but since the conveyor belt is moving backwards the frame of reference is moving backwards also so the plane does not move forward relative to the surrounding air.

i agree that the driving force is provided by "pulling" the plane through the air.

as i see it as the questions is worded it prohibits the plane from ever going forward faster than the conveyor bely moves backwards.

this to me means that the plane can never move along the conveyer to take off. by any means, regardless of how the propulsion is provided as this is the premise of the question.

as i see it the speed at which the wheels turn is irrelevant.

The plane's driving force is not through the wheels on the ground ,its thrust from the engines against the surrounding air. The conveyor will not counteract the thrust.

as i read the question, the conveyor matches the forward speed of the plane.


move along the conveyor at 500mph.

yes but the conveyor itself is moving backwards at 500mph, so to an outside observer the plane is not moving

themole

David19 wrote:

The reason you won't reach the end of the thread mill is because your legs are what move you forward and they are in direct contact with the thread mill.

the reason i won't hit the end is because i am in driect contact with the thread mill, as you said, but also because my forward SPEED is the same as the backward SPEED of the threadmill.

this will be true as long as i am in contact with the threadmill, but regardless of my means of propulsion, as long as i always measure speed relative to the threadmill.

Gurgle

themole wrote:

as i read the question, the conveyor matches the forward speed of the plane.

Newtons 2nd law.
In a car, the wheels are pushing against the road, the road pushes against the wheels, moving the car forward (or subjectively from the car's point of view, pushing the road backwards).
In a plane, the engines push against the air, the air pushes against the engines, moving the plane forward (we're not even talking about up now). The force is not exerted on the tarmac or in this case the conveyor, so the conveyor's motion has no effect except on the wheels.

Think wind-tunnels vs speed bowls.

delly

Gurgle wrote:

Newtons 2nd law.
In a car, the wheels are pushing against the road, the road pushes against the wheels, moving the car forward (or subjectively from the car's point of view, pushing the road backwards).
In a plane, the engines push against the air, the air pushes against the engines, moving the plane forward (we're not even talking about up now). The force is not exerted on the tarmac or in this case the conveyor, so the conveyor's motion has no effect except on the wheels.

Think wind-tunnels vs speed bowls.

But the whole point of the OP's question is regarding while it is connected to the threadmill, therefore you have to take it into account.

themole

we are talking gound speed , not air speed.

if we were talking air speed then the question is trivial as the speed of the conveyor matching the plane would make no difference as the frames of reference for speed would be different.

and in this question the gound is the conveyor

Gurgle

delly wrote:

But the whole point of the OP's question is regarding while it is connected to the threadmill, therefore you have to take it into account.

The OPs question is irrelevant, because you will not be able to keep the plane in one place with a conveyor belt.

Thrust from engine makes plane move forward, plane's motion causes airflow under wings, airflow under wings causes lift.

The wheels are just there to stop the fueslage from grinding along the ground.

delly wrote:

yes but the conveyor itself is moving backwards at 500mph, so to an outside observer the plane is not moving.

The conveyor's backward motion will not counteract the plane's forward motion.

themole

Gurgle wrote:

Did you not read the answer or not understand it?

The OPs question is irrelevant, because you will not be able to keep the plane in one place with a conveyor belt.

Thrust from engine makes plane move forward, plane's motion causes airflow under wings, airflow under wings causes lift.

The wheels are just there to stop the fueslage from grinding along the ground.

The conveyor's backward motion will not counteract the plane's forward motion.

the problem is this.

the question say that the backward SPEED of the conveyor matches the forward SPEED of the plane. to compare the speeds they must both have the same frame of reference. so this means we must compare the ground speed of the plane with that of the conveyor.

the ground speed is how long it takes an object to go from point A to point B along the ground. since we are using ground speed the plane acts like a car,ie it acts like it is being driven by its wheels.

the whole point of arguement is will the plane:
1) move along the length of conveyor (as seen from an external observer) and hence take off
2) stay in one place on the conveyor (as seen from an external observer) and hence not take off

for 1) to happen the plane must have a ground speed in the forwards direction which exceeds the reverse speed of the conveyor.

the question says the speeds are equal and hence 1) cannot occur and the case of 2) occurs.
in this case the airspeed of the plane is 0 as the air around it is not moving and the plane is not moving through it

this only works when comparing ground speed of the plane.

If you don't use ground speed, then yes the car and plane act differently and the plane will take off as normal, with the wheels spinning madly as said before.

SO, it depends on how you interpret the question.

Gurgle

themole wrote:

the question say that the backward SPEED of the conveyor matches the forward SPEED of the plane. to compare the speeds they must both have the same frame of reference. so this means we must compare the ground speed of the plane with that of the conveyor.

OK, again, the conveyor is irrelevant.

themole wrote:

1) move along the length of conveyor (as seen from an external observer) and hence take off

Yes, and the conveyor will have to be as long, if not longer than the runway it was intended to replace.

themole wrote:

1) to happen the plane must have a ground speed in the forwards direction which exceeds the reverse speed of the conveyor.

The plane will accelerate at the same rate as it would on the runway anyway, minus a bit for the extra friction effect on the wheels' bearings, as the wheels will be turning faster.

themole

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

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

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

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

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

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

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

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

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

no why is there anything wrong with that??

Gurgle

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

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

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

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

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

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

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

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

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

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