Square trailing edges

[Dwayne Donnelly]

I think this has been discussed before but I can't find it, has anyone tried square trailing edges on the flaps and elevator?

[jfv]

That's what I use.  Works fine.  I like the way it looks and theoretically has some aerodynamic benefits (less drag, etc).

[Ken Culbertson]

I think this has been discussed before but I can't find it, has anyone tried square trailing edges on the flaps and elevator?

Yes and they do seem to give you a quicker response than a rounded or pointed TE.  I use a 4mm CF strip for my TE on the flaps and elevator.  It both makes if square and virtually warp proof.  Aside from the quicker response and a little added stiffness from the thicker TE, I see little difference.  I have gone to using Gurney fences on my elevators.  I just replace the 4mm with an 10mm cap, 3mm on each side exposed.

Here is a link to a rather long discussion of the subject:

https://stunthanger.com/smf/stunt-design/flap-trailing-edge-shape/msg577461/#msg577461

Ken

[Dave_Trible]

I've played with them on and off for a long while and I'm not sure there is enough aerodynamic difference to matter.   One benefit though is they make the flaps more robust and less likely to warp.   My own rule of thumb these days is if the flap or elevator is 1/4" thick or less-leave them square.  If thicker I glue on a 3/32" sq.  basswood strip to the trailing edge then taper from the mid-chord point down to the strip.  To me it looks better yet leaves some meat for stability and the strip gives me an even straight edge that also resists knicks.  I have an airplane here I designed about 1983 that has full square flaps 3/8" thick.   I remember it seemed to cause more drag and I had to increase prop pitch to keep forward speed up but it flew very well.  An article I wrote at the time showed my thinking that it did make response a little faster in theory but today I'd want to be able to prove that back to back on the same airplane.   Not sure you'd notice it.

Dave

I just saw Ken's entry;   I might add that if anything the square trailing edge would act about like the Gurney flap or elevator tabs which is to say,  be a self-neutralizing devise to force the surface back to neutral once the control pressure is let off.

[Matt Colan]

I switched to square trailing edges after Frank Williams explaining it to me. He said the square trailing edge makes the air not wrap around the trailing edge so it can make the flap act like it’s a little bigger than it is. It’s also a lot easier to build!! I build 1/4” flat flaps and it takes me 15 minutes to cut flaps as opposed to 3/8” tapered flaps would take a much longer time to shape correctly and prevent sanding in warps. I do square trailing edges on the elevators too but I also build in the elevator wedges/gurney flaps with 1/64” ply and they stick up an 1/8” above the trailing edge

[Dwayne Donnelly]

Thanks for the replies, good reading as always. 

[Martin Quartim]

I also do not taper my flaps and make it with a square T.E. I felt the same thing Frank told Matt. I just would say that the flaps becomes more efficient without creating more drag than a bigger flap.

I believe the air stream from top and bottom of the wing meet in conflicting angles and when these streams converge it creates a zone of turbulence and vortices,  with a thicker and Sq. T.E these streams will meet further away from the TE and in the case of a thinner and rounded T.E. these stream will converge right at the edge of T.E.  I am not an aeronautical engineer and I am guessing. Perhaps Frank/Howard/Brett/Paul W. can better explain what happens at the T.E.

Also I understand we are supposed to make very sharp edges to allow  airflow  depart the surface easier.

Martin

[Curare]

I do tend to keep squared trailing edges on everything even with tapered control surfaces, it's just the thickness that varies. I do it similarly to Ken with a carbon strip on the TE, it stiffens them up and adds a lot of ding resistance.

i've had a number of planes with untapered flaps and elevators, and I don't think I can tell the difference at the handle, but then again I'm pretty dumb.

[Dan McEntee]

I think this has been discussed before but I can't find it, has anyone tried square trailing edges on the flaps and elevator?

  This design concept was around back in my R/C sailplane days in the late 70's and 1980s. I think it was proven as part of the revamping and restoration of a low speed wind tunnel in upstate Illinois by the Selig-Donovan. The club I was a member of made some of the test sections that they used for evaluation. It was determined that minimum drag was achieved either two ways. One was a razor sharp trailing edge, so that the air flow from top and bottom of the wing would evenly meet coming off the trailing edge with no turbulence. This was often achieved by putting a narrow strip of 1/64" ply between the top and bottom trailing edge sheeting so the sheeting could be sanded to that razor edge and it was pretty darn tough. When carbon fiber became more common, that was used also.  The other way was with the square trailing edge, and I don't think the thickness of the trailing edge mattered, just make the trailing edge plat and as square as possible. This was the obvious choice for many as it was much less labor intensive..
   Type at you later,
    Dan McEntee

[Matt Neumann]

 I do square trailing edges on the elevators too but I also build in the elevator wedges/gurney flaps with 1/64” ply and they stick up an 1/8” above the trailing edge
[/quote]

Matt Colan, Elevator wedges/gurney flaps?  Please explain.

[Paul Smith]

I believe that the sharp TE that reduces drag is good for events where you need and want speed:  speed, racing, combat, and carrier.

But in Stunt, you actually want to fly slower, so drag is good.

The Al Rabe Bearcat used huge fuselage drag to show the airplane while keeping power high.  So a sharp TE would be a step in the wrong direction for those who want high power and limited airspeed.

[Gerald Arana]

  This design concept was around back in my R/C sailplane days in the late 70's and 1980s. I think it was proven as part of the revamping and restoration of a low speed wind tunnel in upstate Illinois by the Selig-Donovan. The club I was a member of made some of the test sections that they used for evaluation. It was determined that minimum drag was achieved either two ways. One was a razor sharp trailing edge, so that the air flow from top and bottom of the wing would evenly meet coming off the trailing edge with no turbulence. This was often achieved by putting a narrow strip of 1/64" ply between the top and bottom trailing edge sheeting so the sheeting could be sanded to that razor edge and it was pretty darn tough. When carbon fiber became more common, that was used also.  The other way was with the square trailing edge, and I don't think the thickness of the trailing edge mattered, just make the trailing edge plat and as square as possible. This was the obvious choice for many as it was much less labor intensive..
   Type at you later,
    Dan McEntee

Hi Dan, I too was part of that Selig/Donavan study. I submitted a sample for their wind tunnel tests. Fun project.

As you know, parasitic drag in gliders is a bad thing. The best (least draggy) is the sharp TE, next is the square TE and the worst (most drag) is the rounded TE.

But of course in CL drag is a good thing! Especially when going straight down! YIKES!

Cheery, Jerry

[Leonard Bourel]

Hey Dwayne My Nats and Worlds ship has all square trailing edges Works good for me and yes easy to build no taper in flaps or elevators