I've seen some posts about flap interface such as ending a flap someplace along the wing. A few years ago me and some of my aerobatic pilot engineer friends embarked on a bunch of testing of ailerons. As part of that testing I made a bunch of videos including these. I'm sure I can put my mitts on all the videos I did but here are a few. You can watch in slow motion and see several elements of interest to CL aircraft. Rolls in both directions are basically the same as watching a flap from top and bottom in deflection. As part of these videos, we did some turbulators in various places to see what the impact was. The turbulators were put in various positions in terms of chord from near the leading edge to on the front edge of the flap (aileron). The ailerons on this airplane basically just plane flaps very similar to what the average SL stunt model uses albeit with a round nose and hinges which reduces any inherent gap. 

These videos are "wind up rolls" meaning the rolls accelerates as they progress. It shows the effect of increasing flap deflection.





Snap rolls are performed by pulling hard in pitch until the airplane stalls then kicking hard on the rudder which causes the airplane to pivot. It's quite interesting to see the rapid onset of separation.




This video is just the aileron tufted. The turbulator is just aft of where the wing cut ends and is on the nose of the aileron. I think it is a .060" string trimmer line.




In case you might be interested...

Mark,
Very interesting videos.  I never tufted my Extra but could feel a slight oscillation at high speed and max deflection through the stick.  It was obvious enough that I could back of slightly on the stick, maybe 1/2” and it would disappear with no visual reduction in roll rate.   I considered reducing total stick travel but the oscillation was not present a slower speeds where I really needed maximum rate.

My experience with the Laser is similar which is part of what precipitated this investigation. Well, being a flight test engineer, it's a natural progression. I was trying to see if I could make it better will a simple string turbulator. I tried a bunch of different locations and diameters and none of them really helped. I didn't try vortex generators which would cause more drag in other places just to fix an end point condition which I didn't consider valuable enough.

My airplane is the Stevens variant type laser with the 23012 airfoil wing. This airfoil has a "sharp" stall characteristic and my airplane would occasionally "tell you about it" by a small dip in the wing when pulled too hard. Linda Morresey asked me about it once as she saw it at a couple competitions. Yeah downgrade but more importantly I had to explain it was a pilot issue being too aggressive during the pull. The day she asked me about it we had low ceilings and I was slow on my entry in to the box which my turn and pull resulted in a visible dip. The airplane has very small deviation in the LE radius and the tufts show it. Watch and you can see part way out there are  few that indicate separation prior to the rest.

The take away for the forum is to have a careful look in places to see. Turbulence an separation begins in the low to mid 20 degree deflection which means an onset of drag with decreased increase in lift. The other place to look is at the two tufts either side of the intersection of the wing and aileron. I've seen some discussions over this regarding full span flaps or not. There's definitely leakage around this intersection enough to make a decision? For me, full span.

Chris Rud took some movies of tufts on a stunt plane wing with VGs.

A trick from the flow-visualization guys: use fluorescent tufts and ultraviolet light. I think they did this in a dark wind tunnel, but it's dark outside half the time.  Heck, it's dark right now. 

The ailerons on this airplane basically just plane flaps very similar to what the average SL stunt model uses

I think it shows more elevator on tail than flaps on wing, wing has positive AoA with flaps down, while tail has negative AoA with elevator down - just like wing during roll.  That is why Airfoils designed for tails like Wortmann FX71 does not work on C/L wings (proofed). (sorry Mark, I had to see this thread before I wrote you that PM  )