Kutta Condition

[RobByers]

I noticed there is nothing in the 12 plans about sharpening the trailing edges. It just says adjust the bend by hand if neccessary and use your fingers to squeeze along the length of the bend until the skin lies flat against the underlying structure. Has anybody tried to crease their trailing edges using the piano hinge method suggested in other kits? I am looking at the TE of my rudder skin. It is firmly against the ribs, but the radius of curvature on the TE sure is big. I hate to waste all that aeronautical engineering education by not establishing the Kutta condition on my plane!

 

[scsmith]

Mr Kutta is happy

Hi Rob,

Somewhere off the surface, a little downstream, the flow will close and make a 'Kutta condition'.
Square-cornered bases are best for this, because the separation points are fixed. The small base area actually can do some interesting, sometimes beneficial things.
A radiused trailing edge is less desirable, since the separation point(s) are vaguely defined, and can move around. In bad cases, it can cause a bi-stable, slightly deflected control surface that will sit just slightly to one side or the other of neutral. This is annoying.
I think the bulk of experience with RV's is that the surfaces work fine as long as the trailing edge is something like 3/16" ( 3/32" radius) or less. Others might say 1/8" radius is OK.
The RV-7,9,10 went to a nice sharp TE with a wedge-shaped filler and double-flush rivets. I'm not familiar with the -12 specifically, but if it is like all the other models, the above guide should be good enough.

I found the limit on bending the skins was the point where the stiffeners hit the opposite surface - I wish I had made the angled trim line on the stiffeners a shallower slope so I could bend the skins just a bit farther before the stiffeners hit.

 

[PCHunt]

Say WHAT?

Had to look it up. Still in the dark.

http://en.wikipedia.org/wiki/Kutta_condition

I am guessing, that working backward from the Kutta Condition explanation, this is somehow related to why we pinch trailing edges of ailerons to alter heavy wing issues???????

 

[Alex]

Paging Dr. Kutta

Somewhere off the surface, a little downstream, the flow will close and make a 'Kutta condition'.
Square-cornered bases are best for this, because the separation points are fixed. The small base area actually can do some interesting, sometimes beneficial things ...

I agree. Trailing edge profile is going to have a much bigger effect on control force deadzone, trim, and hinge moment than on overall efficiency of the surfaces. Rob gets bonus points for name-dropping the Kutta condition, which unfortunately is almost never mentioned in lay explanations of why airfoils produce lift.

 

[RobByers]

Hi Rob,

Somewhere off the surface, a little downstream, the flow will close and make a 'Kutta condition'. Square-cornered bases are best for this, because the separation points are fixed. The small base area actually can do some interesting, sometimes beneficial things.

[Ah, yes. I had forgotten about the flat trailing edge. So I might be better off flatening the backside of the TE if I can't crease it or reduce the radius enough? I wonder if this radius is flat enough to be effectively the same thing.]

A radiused trailing edge is less desirable, since the separation point(s) are vaguely defined, and can move around. In bad cases, it can cause a bi-stable, slightly deflected control surface that will sit just slightly to one side or the other of neutral. This is annoying. I think the bulk of experience with RV's is that the surfaces work fine as long as the trailing edge is something like 3/16" ( 3/32" radius) or less. Others might say 1/8" radius is OK.
The RV-7,9,10 went to a nice sharp TE with a wedge-shaped filler and double-flush rivets. I'm not familiar with the -12 specifically, but if it is like all the other models, the above guide should be good enough.

[The 12 rudder does not have wedge filler to help close the TE. Can't speak to the wings. Not that far yet.]

I found the limit on bending the skins was the point where the stiffeners hit the opposite surface - I wish I had made the angled trim line on the stiffeners a shallower slope so I could bend the skins just a bit farther before the stiffeners hit.

[The rudder doesn't have skin stiffeners. Again, can't speak to the wings. I did, however, run into the problem you describe with the angled trim line on the stiffiners in the practice kit. It had a riveted TE with the AEX wedge sticking out the back. Bottom line is, I feel I need to sharpen this TE, the AE in me won'e let me NOT do it. However, I don't want to crease it without somehting under it like an AEX wedge if it is going to weaken my skin.]

Note: Some of my comments are imbedded in the quote above = []

 

[RobByers]

I agree. Trailing edge profile is going to have a much bigger effect on control force deadzone, trim, and hinge moment than on overall efficiency of the surfaces. Rob gets bonus points for name-dropping the Kutta condition, which unfortunately is almost never mentioned in lay explanations of why airfoils produce lift.

That's precisely why I am concerned over the shapness of the rudder TE in the 12. This rudder is HUGE compared to the overall surface area of the tail and has a HUGE moment arm due to the fuselage length driven by the size limitation of the HS. The effect of deadzone, trim, and hinge moment on the yaw axis is likely to be pretty big if the stagnation point drifts from side to side on a rounded surface. I don't want to have to use my feet that much!

 

[Ironflight]

Flashbacks!

Aaahhhhh!!!!!

This thread is taking me back 30 years to a classroom in Minnesota....my favorite professor, Jack Moran (who had been a design engineer with Northrup on the flying wing - the EARLY flying wing...) was teaching Computational Fluid Dynamics (the computers were barely more than rocks, strings, and levers....) and explaining how we closed the flow at the back end using the Kutta condition. My favorite quote of all time - "We do this because, well...... because it works!"

The thing about the 12 is, I personally would not try to over think it. you're not really building an experimental, you're trying to copy the airplane that Van's has built as closely as possible (unless, of course, you're building ELSA, in which case, disregard all....).

Paul

 

[Gandalf]

I noticed there is nothing in the 12 plans about sharpening the trailing edges. It just says adjust the bend by hand if neccessary and use your fingers to squeeze along the length of the bend until the skin lies flat against the underlying structure.

Hi Rob... On the flaperons I used my wife's baker's rolling pin! I placed a soft towel over the surface and placed a reverse bend in the flaperon trailing edge using one hand to hold the leading edge and one hand to roll the rolling pin. I then washed the rolling pin and replaced it before she could notice it was gone

Jeff

 

[scsmith]

Too sharp a radius bend will crack

That's precisely why I am concerned over the shapness of the rudder TE in the 12. This rudder is HUGE compared to the overall surface area of the tail and has a HUGE moment arm due to the fuselage length driven by the size limitation of the HS. The effect of deadzone, trim, and hinge moment on the yaw axis is likely to be pretty big if the stagnation point drifts from side to side on a rounded surface. I don't want to have to use my feet that much!

Hi again Rob,
I meant to mention before, that even if the stiffeners don't touch the opposite surface ( no stiffeners in a -12?) that if you try to make that skin fold to too small a radius, the skin will likely crack there sometime in the future. So its best to stick to what has been found to work. If you get a 3/32 radius you will not have any hunting problems on the rudder - I think the vast majority of RV-s are in that catagory. I think what happens is that the boundary layer thickness at the TE determines what an effective radius needs to be to appear sharp to the outer flow.

BTW, off topic, but if you are a test pilot at PAX, you might know my friend and former classmate, Bill Shoemaker? I think he is out on a boat right now.

 

[RobByers]

That's Kutta Enough for me

What I meant is that it is all ribs down the rudder. The initial radius of the rudder TE was about 1/8 inch when I measured it. Not sure if mine was not bent enough originally or it got spread a little bit the way it was packed in the box with stuff wedged inside it. I managed to squash it down to about 1/16 inch radius so I am in the 2-3/32 range now so I am happy now.