Rudder Airfoil Debate....

[Pro1Foam]

OK, so how would different rudder airfoils act different? From the post I gather it would be pitch, cough cough, I mean yaw sensitivity.

[CRG]

I wasn't taking a shot at you.

In my experience many CFI's have a disappointingly poor knowledge of even basic aerodynamics, and it gets passed on.

Gareth's on the right track. A properly airfoiled control surface will exhibit lower drag and improved control response, including less deadband around neutral. There are tons of variables though, on both models and full size, and speed/cost/ease of construction are driving forces on many full scale aircraft that use flat tail surfaces.

Craig.

	Quote: Originally Posted by Edge 540
	Thats why I put the disclaimer...

[Vic3D]

	Quote: Originally Posted by Edge 540
	The flat plate and symmetrical aifroil both produce 0 lift at no AOA. Once a positive AOA is introduced the air still travels faster over the top on the falt plate, thus the negative pressure- lift.

If they produce no lift, then how can planes fly straight?

Maybe I misread your post...

[CRG]

The wing still has some (small) angle of attack, and lift is produced.

	Quote: Originally Posted by Vic3D
	If they produce no lift, then how can planes fly straight? Maybe I misread your post...

[Edge 540]

Yep, the symmetrical airfoils-rotor blades etc produce no lift an zero AOA.
Get a heli, put collective at 0 and you can spin those blades as fast as you want and you still aren't going anywhere. The lift curve for symmetrical airfoils looks like and X^3 graph. (symmetric about the origin)

Most under cambered airfoils on the other hand actually produce lift for at least several degrees negative AOA.

[as722]

What if the airfoil used on the fin was "flat bottom" will it do better K.E. one way than the other ?

[T. Bob]

Thanks alot Albert
That was a waste of time.

[GremlinX]

	Quote: Originally Posted by Edge 540
	Yep, the symmetrical airfoils-rotor blades etc produce no lift an zero AOA. Get a heli, put collective at 0 and you can spin those blades as fast as you want and you still aren't going anywhere. The lift curve for symmetrical airfoils looks like and X^3 graph. (symmetric about the origin) Most under cambered airfoils on the other hand actually produce lift for at least several degrees negative AOA.

Can you explain what camber in a wing is? Thanks. This is an interesting read..

[Edge 540]

	Quote: Originally Posted by as722
	What if the airfoil used on the fin was "flat bottom" will it do better K.E. one way than the other ?

Assuming you are talking about the stab and not the fin-
Usually the tail is providing a downward force to counteract the pitching moment of the wing. This is why flying wings need reflexed airfoils (turn up at the TE)This type does not need any tail loading due to the low pitching moment. (obviously a benefit if your plane is aaaa tailless!)

Gremlin.. imagine a line going from the leading edge of the airfoil to the trailing edge. It stays halfway between the top and bottom surfaces. This is the mean camber line. In a symmetrical airfoil it is staight. With an uncerchambered airfoil it is curved down.

going with the picture now-
The mean camber line, which splits the area of the airfoil would be curved on the first two in this case higher in the middle, but straight on the symmetrical airfoil. (It looks a little off to me, but we'll call it symmetrical)

[Edge 540]

FYI semi-symmetrical is an ambiguous term that is usually NOT recognized as a category by anyone except modelers. So dont go asking an AE about the semi symmetrical foil you're putting on your plane. lol

[teryn]

	Quote: Originally Posted by Edge 540
	FYI semi-symmetrical is an ambiguous term that is usually NOT recognized as a category by anyone except modelers. So dont go asking an AE about the semi symmetrical foil you're putting on your plane. lol

This is correct. "semi-symmetrical" means that the camber is some non-zero number.

[teryn]

	Quote: Originally Posted by Pro1Foam
	So which would have more lift at the same AOA, a flat plate or a fully symetrical airfoil. or is that a fair question seeing that indicate a specific airfoil.

It depends on the thickness of the airfoil. But they are going to be close.