Rudder Airfoil Debate....

[Pro1Foam]

It has brought up at a local field "meeting" about rudder airfoils and if they make a difference as a wing airfoil would. The majority voted "no" but i find that hard to believe. "It's just a weathervane"

[Edge 540]

AHHH did you say weathervane!?

[Pro1Foam]

Any educated brainies wanna take a crack at this?

[ilovemetro]

I think rudder airfoils kind of fit into the knife edge category, like say the fuse creates lift in a knife edge.. the rudder airfoil could change the amount of lift for the rear of the plane and could cause weird knife edge characteristics.. ?

[teryn]

Non-airfoiled surfaces (flat plates) create more drag than airfoiled surfaces; airfoils do not necessarily create more lift (or, in this case, lateral-directional stability) compared with flat plates. So it really comes down a drag issue.

Below 10 deg, the variation of lift coefficient with alpha (lift-curve slope) for a flat plate is 2*pi (for alphas < ~11 deg). Most airfoils used for subsonic aircraft have lift-curve slope values very, very close to this. This means that a flat plate is as effective at producing lift as a typical airfoil. It just produces a lot more drag for the same lift.

[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.

[wings19]

Lift is created when the air over the top of and airfoil is traveling faster than that over the bottom reducing the pressure on the top(bernoulli). Therefore, an airfoil would make more lift at the same AOA as a flat plate because the air has to travel a greater distance over the top of that surface than the bottom making a much lower pressure compared to a flat plate where the air wouldn't travel as far. When the air has to travel further it speeds up. I forget why it speeds up but I know it does because I was taught why in my ground schools but can't remember why

[Vic3D]

	Quote: Originally Posted by wings19
	Lift is created when the air over the top of and airfoil is traveling faster than that over the bottom reducing the pressure on the top(bernoulli). Therefore, an airfoil would make more lift at the same AOA as a flat plate because the air has to travel a greater distance over the top of that surface than the bottom making a much lower pressure compared to a flat plate where the air wouldn't travel as far. When the air has to travel further it speeds up. I forget why it speeds up but I know it does because I was taught why in my ground schools but can't remember why

Ditto

[Edge 540]

The flat plate is still producing lift though. Maybe not as much, but it definitely is. The flat plate and symmetrical aifroil (flat plat is symmetrical as well) both produce 0 lift at no AOA. Once a positive AOA is introduced there is lift. With the flat plate this also brings tons of drag= IDEAL FOAMY!

Disclaimer: Yes I know the " air has to travel faster to meet at the same point at the TE" is a hightly debated but extremely common explanation.

[CRG]

Why does it have to travel further over the top? What about symmetrical airfoils? What happens when you fly inverted?

There's another pesky physicist at work here also, his name is Newton....

	Quote: Originally Posted by wings19
	Lift is created when the air over the top of and airfoil is traveling faster than that over the bottom reducing the pressure on the top(bernoulli). Therefore, an airfoil would make more lift at the same AOA as a flat plate because the air has to travel a greater distance over the top of that surface than the bottom making a much lower pressure compared to a flat plate where the air wouldn't travel as far. When the air has to travel further it speeds up. I forget why it speeds up but I know it does because I was taught why in my ground schools but can't remember why

[Edge 540]

Thats why I put the disclaimer...

[gareth.ky]

This is probably a pitch sensitivity thing. Airfoiled surfaces are less, ummm, sensative to small pitch changes close to zero AoA. In cases like this it will have less drag and should still have you great control authority. So I suspect that airfoiled surfaces fly 'smoother', more precisely and require less deflection.

I, unfortunately don't own an airplane large enough the have airfoiled stabs/rudder, so I can say for sure. :-(

But hey, have you ever seen a pattern airplane without an airfoiled rudder? For 3D it wont matter much, then it really just is a weather vane because the surface is stalled.

[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...