airplane on convayor

[Isair]

Hm I have run into this question, and I want to see what you think people?

Question:
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 opposite direction).<br style="font-style: italic;"><br style="font-style: italic;">The question is:<br style="font-style: italic;"><br style="font-style: italic;">Will the plane take off or not? Will it be able to run up and take off?

Now I hope you will exercise your brains, and come up with right answer, as well if someone could simulate this in Proe

 

[2ms1]

of course it will take off! The only thing the conveyor does is double the angular velocity of the wheels relative to plane. Ignoring friction forces related to wheels, it will takeoff in exactly same time, distance, etc as if no conveyor

 

[vlad1979]

If you can match these speeds then it will take off:


http://www.aerospaceweb.org/question/performance/q0088.shtml


But I don't think it will take off because you say the conveyor moves in the opposite direction so it will act like a break, so the plane willbe standingwith the engines running.


And if I remember right (I could be wrong) the plane is taking off because of speed and also because there will be a difference of pressure on the wings, the shape of the wing will cause the flow of air to be speed up on top of the wing (because of it's shape) and the plane will take off.


[url]http://www.aerospaceweb.org/question/aerodynamics/q0005.shtm l[/url]


http://www.allstar.fiu.edu/aero/flight32.htm


So in conclusion:


-I don't think it will take off because you don't have enough air speed to cause a difference of pressure. The plane is not moving.


-But if you can somehow produce a flow of air that willmatch the speeds above (maybe in a wind tunnel) I think it should take off.


These are all my thoughts, I'm not an aerospace engineer.

Maybe someone who works in the field could give an accurate answer.

 

[vlad1979]

But if it's a vertical take off plane then it would definitely take off.

http://www.technologystudent.com/culture1/harr2.htm

 

[wsylvester]

I agree ...no airspeed..no lift


but then


http://www.ornithopter.ca/index_e.html

 

[prohammy]

Isair,


Think of it this way. If the plane was on ice, with the wheels spinning and giving no traction. The plane won't take off. No forward motion = no take off.


My two cents


Kev

 

[AHA-D]

There is no way the conveyor belt can stop the plane, except when the brakes are on ! A plane is not equal to a car !


In a car you make the wheels turn with the engine, the wheels exert friction on the road and so push the road from under the wheels. Since inertia of the road is much larger than inertia of the car, the car moves.


A plane is freely standing on its wheels. The engines of the plane exert a reaction force. The trust of the engine in one direction is "translated" in a reaction force to the opposite side. So there's no way to keep the plane from rolling, no matter how hard you drive the conveyor belt (forgetting all internal friction and resistance for a moment).


Other thing : if a carrier were to able to cruise at 300 km/h it wouldn't even need a runway to lift the planes in the air.


Alex

 

[2ms1]

You guys are all joking right?

 

[wsylvester]

it's got wheels... I thought it was standing...had legs

 

[magneplanar]

My comments:

If the conveyor keeps the plane from gaining enough forward momentum then the plane will not have enough lift to take off for a standard aft thrust configuration. A plane does not rely on its wheel as a friction force against the ground for take off, but it does need them to allow motion up to the minimum take off speed. At that point the wing shape is producing enough lift from the difference in pressure. You can run the engines all day and the plane won't take off if the brakes are on. The conveyor belt would be like having the brakes on if it keeps the plane from moving forward with a matching opposite motion. Friction losses and delay in speed compensation would allow some movement in this case.

To fly you have to overcome the forces acting on the body. Overcome gravity with lift or thrust (including VTOL, JATO, redirected thrust, ie: harrier), and you go up. Overcome drag and you go forward. The control surfaces modify these actions to go in a particular direction.

If the goal is to take off in a short distance with a typical aircraft then a directed stream of air in front of the plane could provide this. Get that right and you wouldn't need wheels. You could take off in that stream of air and then land by re-alligning with that stream of air. If my calculations are correct, you would only need fifty-five polititions to produce enough air volume.

Cheers...

M

 

[wensink]

It wont take off.


I have flow 40 hours and i can speak with some expierence. Ground speed is not important with flying. Air speed is. No airspeed no lift. So if you have to take of with 80 knots and you have a tail wind of 80 you wont get of the ground.


1 of the fun things about this is. when you fly with 80 knots and you have a headwind of 85. you travel 5 knots backwords (seen from the ground)

 

[Isair]

Uh huh

I was sure that plane wouldn't take off, but now I don't know.
The confusion is the speed of airplane. If airplane moves, then it have speed, and then conveyor have the same speed.
But then from which point is measure speed of airplane;
if airplane speed is regard speed of conveyor, then airplane in regard of earth is standing still.

But if airplane moves in regard of earth, then it moves on conveyor and have speed, and can take off.

 

[magneplanar]

Israir,

Wensink made a good point. To produce lift the airplane has to move and/or have enough relative wind passing over it as Wensink described.

In your description, the conveyor balances out the forward motion of the airplane. So if a person was standing next to the conveyer they would not see much movement of the airplane, even though they saw the conveyer moving and the tires spinning. Since you did not mention any extra wind there would not be enough lift and no take off would occur.

Just remember that the wings produce the lift and not the movement. Movement forward allows for enough air passing over the wing to produce enough lift if there is no additional wind. If there is a headwind then that combination will be reached sooner.

M

 

[johral]

It will take off - you are using the air to get your thrust, not the ground (like a car does) - the wheels will see double the actual speed, but they won't keep it from taking off. It would be like asking if a plane could take off from wet ice - yes it can, but a car would just sit there with its wheels spinning.


Think about it in reverse - if the plane lands on a conveyor in the above scenario will it stop immediately? Nope - the wheels will spin up to twice the air speed, but the plane will still travel down the conveyor until the brakes are applied.


The prop or jet will propell the plane regardless of what the conveyor is doing below it. The air is what makes the plane move in the first place - the wheels just keep the plane off the ground. Thrust moves the plane, and that is due to air, not wheels.


Try it with a wheeled rocket - do all you want with the conveyor, but it will still launch.
Edited by: johral

 

[pjw]

The plane would take off for sure. It is irrelevant what is happening between the wheels and the ground since the force required to push the aircraft forward is produced solely from the thrust of the engines against the air. Think of it this way:


If the aircraft was on ice; and the wheels were locked up solid with the brakes - would thewheels be able to counteract the thrust produced by the engines? Or would they just skid (assuming frictionless contact between tyres and ice?).


If anything, having a rolling runway, adjusting for the speed of the wheels which were in contact with it would ensure that the aircraft took off even sooner than it normally would. There would be no energy lost due to friction from tyre and runway (since the relative speed between the two would be zero) and there would be no energy lost due to overcoming friction due to elasticity in the wheel bearings.


Always think thatthe forces from the 'reaction' are generated entirely by the 'action' and then consider what is causing that action. Rotational speed of the wheels relativeto the runway is irrelevant unless the forward motion is produced through the wheels.


If thedrive required by the aircraft for take off was generated through the wheels and the ground (as in the case of a car) then the aircraft would never leave the ground.


I think a goodidea for aircraft manufacturers would be toprovide some method of accelerating landing gear wheels to the approximate landing speed of the aircraft prior to landing to prevent the 'skid' of the tyres against the tarmac. Would save on tyre wear on short-haul flights for sure.


PJW

 

[magneplanar]

Hi Johral and PJW,

I like this thinking process, I have a different view and I want to explain that, but I also like hearing your points of view. It is good for discussion and I hope the replies are constructive.

It seems you are saying that the wheels would be like skids on ice on this conveyor, which indicates low friction. If I understand the proposed problem, the conveyor would change speeds with wheel speed which is worse than ice, actually having an infinite friction. At some point, on ice the plane would slide regardless of the wheels, and the wheels no longer matter anymore than just a point to slide on (tried it, its wild and crazy). At that point the aircraft could move forward just like planes on skids do.

However, The conveyor is compensating for the initial forward motion so the plane would not appear to move, just like having the brakes on. The first movement is critical, you have to overcome that friction before you can move fast enough to fly. This conveyor would in theory never reach that point. Bear in mind one detail. The thrust is rarely close to the weight of the aircraft . That means too much friction would make it hard to reach take off airspeed over the wings, even if you are sliding. Only some special built light weight planes use skids and have high power to weight ratio engines.

example:

747-400 max take off weight: 875,000 lb (396,890 kg)
max thrust: PW 63,300 lb (281.57 kN) (x4)= 253,300 lbs
thrust and wing lift=flight

not nearly enough thrust to overcome the weight, even if it is sliding. So how would you get the added lift unless there is forward movement? (the engines provide little or no air for the wings to produce lift, even in a single engine, they primarily provide thrust)

The more thrust you added to overcome the lack of movement, the more the conveyor would spin to match the wheel speed. Since max thrust is still less than the aircraft weight you would never move.

In a real world situation the conveyor would not be able to keep up and/or the wheels would reach a max speed and lock up. You would then eventually break free and slide. Whether you drag, slide, or roll you still have to get enough movement of air over the wings. I do not see enough of this forward movement happening so I still say it would not take off.

cheers...

M

 

[SW]

As has been explained, its all about the
airspeed. All the conveyor does is alter the ground speed. Assuming the wheel
friction is relatively low, which it would be in practice, you can roll the
conveyor backwards as fast as you like and the relative ground speed increases
but the airspeed stays the same. Lift is proportional to the square of the
airspeed, how fast the wheels are spinning doesn't come into the job.


Look at it this way:

1. <li style="">The pilot is sitting in his aeroplane finishing his sandwiches on
   a calm, windless day without realising that someone had replaced the
   runway with a conveyor.<li style="">The conveyor starts rolling backwards.<li style="">He is a bit surprised as his plane begins to move backwards, but
   he screws the lid on his flask and releases the wheel brakes.<li style="">The friction in the wheel bearings means the plane continues to
   move backwards.<li style="">The pilot has a bite of his chocolate biscuit and pushes the
   throttle forward till the forward thrust is equal and opposite to the
   friction in the wheel bearings. The aeroplane is now standing on the conveyor,
   which is rolling backwards while the planes wheels are spinning round. The
   ground speed is now negative but the airspeed is still zero.<li style="">The conveyor speed increases and if we assume the friction in
   the wheels also increases then pilot needs to increase the throttle a bit.
   He doesn

 

[AHA-D]

This entire discussion is pointing to "fixed-mind-logic". Most people are thinking "car" and not "plane". A car needs ground and friction to move, a plane doesn't. (well, correction, it needs ground to be supported when the wings aren't doing so) When seeing a plane taxi towards the runway you might be tempted to think it's the same as a car driving, but it is not. There is no engine driving the wheels, the plane is moving forward on a relatively low amount of thrust from the jet engines.


Magneplanar mentions weight and thrust. Based on your figures you see that the engines of the 747 can almost deliver 64% of the lifting weight. Make the plane a little lighter and it can take off vertical ! Compare this (it's impossible due to the units in use, but still ...) with a car that easily weighs 12000 N and when it has a powerfull engine that engine will develop 400 N/m and you'll understand that a car has nowhere near the power of a 747. That's why your car doesn't get to 900 km/h, which is a piece of cake for a 747.


The only thing that can keep the plane from lifting in the air is that the conveyor belt woult reach a speed so fast that the wheels of the plane desintegrate due to centrifugal forces. But the average conveyor belt I know doesn't travel at 300 km/h, so it's a fair bet to asume that the plane is long gone before the belt is at that speed.


Alex

 

[trancepose]

i think we should first check relativity. as far as ı know, airplanes do not move by means of friction between land or any interfase it is in touch. they are forced by a thrust engine or propellers, both of which uses aerodynamic thrust forces. that's why our relation should be set up with respect to absolute position w.r.t. ground. that means wheels do not push the plane.
now there are several cases,
1. for non activated brakes, no friction and no slip at wheel-conveyor interface: if conveyor begins carrying plane before engines are started, and give an initial velocity of plane's max. speed, then airplane will stand still( intertia!!!) remember magicians pulling tablecloths below glasses without even spilling a single drop of water. as the engine is started, thrust force will work perfectly
2. for active brakes, no friction and no slip at wheel-conveyor interface: plane will never move an inch on conveyor
3. for real cases: plane will take off. intertial forces may or may not work but thrust force makes remaining forces negligible.

 

[magneplanar]

I just figured the first movement would be tough to get, but I know the wheels are free spinning...If the aircraft moves forward, I can see that it would take off, even if the wheels were spinning completely opposite to the forward motion. Once the plane is rolling the wheels provide no significant breaking force unless the crazy copilot is riding the brakes.

AHA-D, its more like 32% ( did you use the kg value?) but a vertical take off 747 would be nice to see

SW- who served the pilot with catering before take off? that could explain the bumpy lift off, or was the flight attendant working the controls...heh

thanks for the discussion, now what was this 3d stuff we were trying to figure out?