# Aerospace Momentum transfer?

1. Oct 2, 2008

### jason.bourne

momentum transfer???

when aircraft is in motion, it has momentum.

Since it has momentum, there is momentum transfer taking place between the aircraft and the air particles. During this momentum transfer what exactly is happening between the wings and fluid particles that are present right around wings? what are the consequences of gaining momentum from aircraft?
as they gain momentum its obvious that they are set into motion relative to the aircraft. does this turbulent flow occurs due to this momentum transfer? if yes then what are the other things happening?

2. Oct 3, 2008

### Cyrus

Re: momentum transfer???

I don't understand all of your question because I think some of it does not make any sense. So, I'll just say what's going on:

The fluid particles right at the wing are stagnant (stationary). There is a boundary layer around the wing that builds up as the velocity goes from zero, all the way up to whatever the local steady state velocity is at that point along the wing.

The fluid particles approaching the wing at the free stream velocity $$V_\infty$$ has some momentum. When they approach the wing they can go either above the wing, or below the wing. If they go below the wing, they are going to be deflected downward. This changes the momentum of the particle to a new direction, and as we know a change in momentum produces a force via newtons 3rd law equal and opposite on the wing. If, on the other hand, the particle of air goes above the wing, it will become accelerated and by bernoullis equation will reduce the static pressure at the top surface of the wing.

So the wing has more pressure at the bottom, less pressure at the top causing lift. As the same time, it's also turning the flow downwards which also produces lift. To say that it's bernoulli or flow turning that causes lift really makes no sense becasue the two go hand in hand. You can't do one without the other.

3. Oct 4, 2008

### LURCH

Re: momentum transfer???

The main consequence is mechanical motion. The air particles being impacted by the weighing are pushed aside, causing friction heating and changes in pressure. These changes in pressure generally have the effect of causing air particles to move back in the direction of the original position after the aircraft has passed. However, the forces applied are not equal (as Cyrus mentioned), and so this "return to position" as many imperfections and is far from smooth. This results in a turbulent mass of air being left behind the aircraft, similar in many ways to the wake of a boat.
I think it would be more useful to say that they gain momentum relative to the surrounding air. The air particles start out with a large amount of momentum relative to the aircraft. After the collision, the air particles is moving in the same direction as the aircraft, so the difference of momentum between the two has actually diminished. However, relative to the surrounding air particles, the individual particle in question starts out stationary, and is then set in motion by collision with the aircraft. The difference in momentum between this particle and the surrounding "stationary" air particles has increased.

4. Oct 5, 2008

### jason.bourne

Re: momentum transfer???

hey Cyrus, m sorry, m German so i have problem with my English. I knew i didn't put my question in a right way. But Lurch got it what i meant to ask.

5. Oct 5, 2008

### jason.bourne

Re: momentum transfer???

are these wake that is left by the moving aircraft, stationary or moving, suppose if any observer is observing them standing on the ground?

i have read this many times in different textbooks but i still don't understand why they will be moving "in the direction of aircraft". coz suppose if m standing on the wing of an moving aircraft (just assumption) i see the particles collide with the aircraft, there will be momentum transfer and particles will move in the opposite direction. i guess m weak with this relative theory. till now i assumed that particles move in the direction of aircraft due to the back flow that occurs due to separation. i know m wrong, can you please make this thing clear i mean why in the direction of aircraft the particles move after the collision?

i hope you got my question. god i hate my english.

6. Oct 5, 2008

### djeitnstine

Re: momentum transfer???

When the air is split by the wing there is a vacuum and the air molecules tend to want to move from a higher pressure to a lower pressure. Since the vacuum is always being created in the direction the aircraft is moving....you can imagine the rest.

7. Oct 5, 2008

### LURCH

Re: momentum transfer???

In fact, your two questions are actually two forms of the same question. That should be encouraging; you only need to find half as many answers as you thought you did!

It is true that if you are standing on the wing of the aircraft, you see air particles colliding with the leading edge of the wing, gaining some slight lateral movement, but mostly moving toward the trailing edge. The situation is very similar to what you would experience if you are standing on the hood of a moving automobile. It is the motion of the air as seen from the vehicle's point of view. However, you see a very different situation if you are standing near a roadway when an automobile passes by. Assuming there is no wind, you will see the air particles standing still. Then, as the auto passes by, you would see those air particles get disturbed in a very chaotic fashion. But then, eventually, one certain direction would be seen to dominate; the same direction that the auto was moving.

The same happens with the aircraft. The air gets jumbled up in a way that might look random at first, but over all, it ends up beign "pulled along" or sucked after the aircraft. So, compared to the sarounding air, these particles start out stationary, and then (through transfer of momentum) are set in motion in the direction the plane is moving. So the difference between their velocity and the plane's velocity has decreased, while the difference between their velocity and the velocity of the sarounding air has increased (from "zero" to "something").

BTW: I think your english is pretty good. Certainly a lot better than my German!

8. Oct 5, 2008

### jason.bourne

Re: momentum transfer???

thank you Lurch, i got it. thank you so much.

thanks all of of you guys.