Downwash, momentum

[rotateandfly]

Hi everyone!

Im currently working on a project about aerodynamics for my physics course. Everything that was taught to me in ground school is still in my head however I want to go further into detail on the subject of "downwash".

As far as I can remember, downwash is caused by the air (from the relative airflow?) which hits the airfoil at an angle and is deflected downwards, as a result the wing is lifted. Also I know that all of this has to do with momentum.

My question is, how can the relative airflow have momentum?? And how is that momentum which hits the airfoil divided up?

Any help is greatly appreciated!

-Dave

[BOAC]

Not sure how advanced this q is, Dave, but ANY change in direction of a flow requires the application of a force (F=MxA) - in this case applied by the angle of attack of the aerofoil and its camber - and ANY force has an equal and opposite reaction. This applies whether the air is moving over the wing or the wing moving through the air. This reaction can be resolved into vertical (lift) and horizontal (drag).

Is that what you are after or have I taught grandma to suck those eggs?:D

[rotateandfly]

HAHAHA, good morning to ya BOAC!

Ive already explained f=m x a in my project, but thanks anyway. Actually what Im looking for is the momentum acting on the airfoil. I believe its p (momentum) = m (mass of the aircraft) x v (speed of wing through the air). Well, the airfoil travels through the relative airflow at an angle. and the momentum from the relative airflow is deflected downward when it hits the wing, namely downwash. This "downwash" is a part of the total momentum that came with the relative airflow, and the other part is lift i believe. But what my q is, is which direction the two part momentums would go. Because if you connect them a parallel figure should occur.

Im not sure if Im asking too much or if my theory even makes sense lol. Dont hate me!

Thanks a bunch, mk?! :E

-Dave

[BOAC]

"p (momentum) = m (mass of the aircraft) x v (speed of wing through the air)." represents the momentum of the AIRCRAFT.

It is the change in momentum of the AIR you should examine. Assuming still air, it was sitting quietly minding its own business when along came this a/c and deflected it downwards, thereby changing its momentum from ZERO to something finite.

The change in 'p' depends on how quickly this deflection takes place (ie aircraft speed) and on how big a downward deflection there is eg angle of attack and effective camber.

[rotateandfly]

made my day BOAC! :ok:

Did you study physics?

Cheers! Dave

[bookworm]

I'd strongly recommend a look the aerofoils chapter in John Denker's See How it Flies (http://www.av8n.com/how/htm/airfoils.html).

In particular, when you start using words like "deflection", it's worth a look at his section 3.6_Air Is A Fluid, Not A Bunch of Bullets. I'm not suggesting that deflection is incorrect, just that it can put inappropriate physical models into your mind.

From a terminology point of view on downwash, it's worth remembering that the air bends around the aerofoil and essentially returns to its original direction over a distance of the order of a few chord lengths. The rather more persistent downwash between the wingtips is (arguably) a result of combined effect of the wingtip vortices of a finite wing and is balanced by an upwash outboard of the wingtips. For that reason, some authors tend to use the word downwash only in the context of finite wing effects (for example to calculate induced drag).

[rotateandfly]

Thanks for your post bookworm, the airfoil and airflow website is very interesting :ok:

-Dave