Flap & Elevator trailing edges.

Hi all you stunt guru's; Does anybody have an opinion on rounded or left flat. Seems I read some where about leaving them flat or square instead of rounded, made them more efective. Maybe less turbulance? Thanks All;

>Hi all you stunt guru's; Does anybody have an opinion on
>rounded or left flat. Seems I read some where about leaving
>them flat or square instead of rounded, made them more
>efective. Maybe less turbulance? Thanks All;

Hi Jim

after I get the hinges installed I sand a V shape into the flap leading edge
see the picture of how I do mine

Full scale aerobatic aircraft have discovered that a sharp, flat, square trailing edge has some advantage. This seems a bit counter-intuitive, but some of the competition folks claim it provides an edge, particularly in the ailerons, where a little bit of a 'detent' at center in control pressure is a good thing. Like, say, when you are doing a 4 point vertical roll.

That said, the variation in Reynolds Number between full scale and stunt scale is enough to make one wonder if such wisdom is transferable.

I can say that on the Super Chief I have on the building table I am building it with the control surfaces exactly as they came out of the kit. Flat all the way from the leading edge V (for hinge movement) back to the trailing edge, where the thickness of the surface is flat. Sand every square surface (including the trailing edges) smooth and then stiffen with PolyCrylic and finish.

This is more a matter of construction philosophy for this particular ship, but I suspect from all I have read that squared off control surfaces will be perfectly fine compared to finely crafted tapered ones.

My two sense...

Cy.

>This is more a matter of construction philosophy for this
>particular ship, but I suspect from all I have read that
>squared off control surfaces will be perfectly fine compared
>to finely crafted tapered ones.

One thing is for sure, rounding the back edge of a control surface is the worst. In r/c a rounded trailing edge can induce flutter. But from what I have been reading a flat square edge is supposed to be the best, better than a taper.

>Full scale aerobatic aircraft have discovered that a sharp,
>flat, square trailing edge has some advantage. This seems a
>bit counter-intuitive, but some of the competition folks
>claim it provides an edge, particularly in the ailerons,
>where a little bit of a 'detent' at center in control
>pressure is a good thing. Like, say, when you are doing a 4
>point vertical roll.
>
>That said, the variation in Reynolds Number between full
>scale and stunt scale is enough to make one wonder if such
>wisdom is transferable.
>
>I can say that on the Super Chief I have on the building
>table I am building it with the control surfaces exactly as
>they came out of the kit. Flat all the way from the leading
>edge V (for hinge movement) back to the trailing edge, where
>the thickness of the surface is flat. Sand every square
>surface (including the trailing edges) smooth and then
>stiffen with PolyCrylic and finish.
>
>This is more a matter of construction philosophy for this
>particular ship, but I suspect from all I have read that
>squared off control surfaces will be perfectly fine compared
>to finely crafted tapered ones.
>
>My two sense...
>
>Cy.

Ron B.
F4Fguy

Cy:

What you're saying is true,to a point.In theory if you can create a sharp edge at the end of a body the flow can be "fooled" into continuing to flow as if there were no edge.I can't recall the exact reference,but,it was Dr.Kamm in the mid thirties,in Germany.

My guess is though,that at our reynolds numbers,you can't get a sharp enough edge on the corner to get the "full scale" effect.At our RNs Coanda effect will cause the flow to turn the corner,then separate,causing turbulence in the hingeline,and a "burble" over the flap.The same would be the case at the extreme trailing edge of the flap,upset flow,causing turbulence and inefficient flaps.I believe a truly sharp flap trailing edge would be a better solution for an RN of our magnitude.Unfortunately,A truly sharp edge is equally hard to obtain at the trailing edge.

Just my random thoughts on the subject.

Thanks for the response Guys; The flaps and elevator will be tapered down to about 3/32" at the trailing edge. Just don't know if I will round trailing edges, or leave them flat on the ends.

I built a 40-size flapless profile with a 1/4 thick square trailing edge on the wing and a 1/8 thick square trailing edge on the elevator. I don't have it completely trimmed out, but it looks like it is going to fly to suit me eventually. I can always glue on somae tapered trailing edge stock, if needs be.

Jim,
This might be in the area that a straight and rigid airframe is more important. For what it is worth, I flew planes with both rounded and squared off TE and I didn't seem to think one was better than the other. My guess a few more gallons of practice might make a bigger difference.

Since I have seen you fly, and very well I might add, I believe you are right. I, at my level in the stunt wars probably could not tell the differance anyway. I think I will do what looks best, as this generally works for me anyway. Sometimes I get carried away with details. Sometimes a good thing, and sometimes a waste of time. One thing for sure, if you put it out on this board you will get help to see which it is. Thanks again to all you guys who keep me on the straight and lite, (narrow).