Plane and conveyor belt...

In order to gain the advantage of a bit of free airspeed and so reduce the amount of fuel the engines have to burn to accelerate the plane from zero GROUNDspeed to take-off AIRspeed.

They also land into the wind as well so that the touchdown GROUNDspeed can be a bit lower while still having enough AIRspeed to keep the aircraft aloft. This reduces wear on expensive tyres and brakes, as well as reducing the fuel used in reverse thrust.
 
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Where will that ground speed come from if the aircraft is stationary to it's realitive surroundings?

Wotan
 
I was answering your question asking why aircraft take off into the wind.

If the wind is blowing at 20mph and a particular aircraft is generating enough lift to take off when it reaches an AIRspeed of 150mph, then by taking off into wind it only needs to accelerate from a standing start to 130mph GROUNDspeed. If it took off with a tail wind it would need to reach a GROUNDspeed of 170mph in order to have the AIRspeed of 150mph necessary to become airbourne.

As has been said, if the wind were to be blowing at a higher speed than the take-off speed of a particular aircraft then IN THEORY the aircraft could lift off vertically with ZERO GROUNDspeed. GROUNDspeed, or speed relative to an oppositely moving runway, is IRRELEVANT to flight, only AIRspeed matters. In practice, the aircraft would be extremely difficult to control in such a vertical take-off and would almost certainly crash if any pilot were suicidal enough to try it.

I've seen it done successfully with radio control models, though.
 
What intrigues me about this puzzle is that it can separate a group of reasonably intelligent people into two opposing groups of thought.... (not so chuffed about the abuse and patronising comments i get - it's not even my question, i'm the messenger ffs !!).

But where are we all (sorry, half of us!) going wrong?

I think the problem is two separate problems.... i think we'd all agree that Newton's 3rd would suggest that a plane's thrust against the air would cause the air to push the plane in the opposite direction?..

All the stuff about airflow over the wing creating lift is also a given - no doubt there.

Where the problem is for me, is the odd situation of the conveyor (floor) counteracting the rotation of the wheels. Some are saying that the wheels spin twice as fast, well then so does the conveyor. What if it were skates on ice? The crux of this for me, is what force or scientific constant the conveyor is supposed to represent. Is it introducing an impossible science (similar to perpetual motion)?

Changing the focus slightly, if the question was "we have a shopping trolley (instead of a plane), on a conveyor that matched the wheel speed etc. yet i was pushing it.... i would need to walk on the conveyor to provide thrust, so to emulate the original question, i'm on a bridge straddling the conveyor.... would i be able to move the trolley forward by pushing it?
 
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Changing the focus slightly, if the question was "we have a shopping trolley (instead of a plane), on a conveyor that matched the wheel speed etc. yet i was pushing it.... i would need to walk on the conveyor to provide thrust, so to emulate the original question, i'm on a bridge straddling the conveyor.... would i be able to move the trolley forward by pushing it?

Think about a luggage trolley and one of those moving walkways they have at airports.

You walk on the solid ground next to the moving walkway whilst the trolley you are pushing is on the walkway. (I know, that means you'll be leaning awkwardly over the handrail of the conveyor to push the trolley - but lets just go with it).

If you are walking in the opposite direction to the way the walkway is going then the trolley will be harder to push than normal, and the wheels will spin fast - but you'll still be able to push it.

If you're walking with the direction of the walkway then it'll be easier to push the trolley than normal and the wheels will be spinning slowly - or possibly not at all if you are precisely matching the walkway speed.

Either way, you can STILL push the trolley forwards at walking pace.
 
Those who make 'em said:
...Jet reaction is definitely an internal phenomenon
and does not, as is frequently assumed, result from
the pressure of the jet on the atmosphere.
In fact, the
jet propulsion engine, whether rocket, athodyd, or
turbo-jet, is a piece of apparatus designed to
accelerate a stream of air or gas and to expel it at
high velocity. There are, of course, a number of ways of doing this, but in all instances the resultant reaction or thrust exerted on
the engine is proportional to the mass or weight of air
expelled by the engine and to the velocity change
imparted to it. In other words, the same thrust can be
provided either by giving a large mass of air a little
extra velocity or a small mass of air a large extra velocity. In practice the former is preferred,
since by lowering the jet velocity relative to the atmosphere a higher propulsive efficiency is obtained...

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snico said:
This is the original question that was asked

"A plane is standing on a 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 the opposite direction). Can the plane take off?

The question has been asked at least twice. Here:

//www.diynot.com/forums/viewtopic.php?t=54168

and again here:

//www.diynot.com/forums/viewtopic.php?t=134493

They are essentially the same question but note the word "wheels" in each of them, which is missing from snico's version. The critical question is this: Which parts of the wheels are the OP's referring to? (Please excuse my use of a preposition to end a sentence with. :LOL: :LOL: :LOL: )

If it's the speed of the axles then that's the same as the speed of the plane (as per snico). The engine pushes the plane forward. The belt runs backwards at the same speed, causing the wheels to spin twice as fast as they would normally do. Assuming that they don't disintegrate, the plane will take off. A little extra thrust will be needed to overcome the increased rolling resistance but it will take off anyway. :) :) :)

If, on the other hand, it's the speed of the tyre then there's a big problem. Any wheel rotation will be exactly matched by the belt such that the contact point remains stationary. The plane cannot move. :( :( :(

But what about the engine thrust? :confused: :confused: :confused: Although the plane isn't going anywhere, its wheels' angular velocity is increasing and this requires a pair of forces: one acting forwards at the axles and the other acting backwards at the tyres. The belt provides the force at the tyres while the engine provides the force at the axles. As I said earlier, all the thrust will be consumed in accelerating the wheels. Maybe I should have said "-- in making the wheels spin faster". :) :) :)
 
Any wheel rotation will be exactly matched by the belt

Thats impossible to acheive though if you mean rpm on a proper conveyor.
 
... If, on the other hand, it's the speed of the tyre then there's a big problem. Any wheel rotation will be exactly matched by the belt such that the contact point remains stationary. The plane cannot move... :( :( :(

Ummmm, conveyor moves from nose to tail, wheel contact point moves same direction - how can contact remain stationary - if conveyor accelerates?
:D :D :D
BTW. The original question gave no rotational or linear limitations so we have to work with that.

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