This goes back a bit, but my son did a science fair project on combat wing airfoils. He built 4 planes. They all had the exact same layout and stab size, with the tip 75% of the root chord and the spars straight at 25% chord. The control used what we were using at the time on competition planes- an elliptical section leading edge transitioning at the spar to a straight to the trailing edge. The second plane used a pollywog Eppler airfoil(designed for extra lift on glider tails, recommended by Steve Sacco), the third had the highpoint pushed back with a pronounced curve at about 50% chord going back to the TE(similar to many F2D airfoils at the time) and the fourth was the same as the first, but with a much more streamlined shape(it still had a decent LE radius) but more streamlined than an ellipse.
Testing included level flight speed, time to do 4 figure eights, and a measurement of the width of the figure eights. The results were interesting.
All the planes had virtually identical speed. Any differences were due to slight variations in needle valve setting and wind.
The Eppler airfoil seemed to get the figure eights about 10% narrower.
The biggest surprise was in the eights. The Eppler foil was a few tenth's of a second faster. But the biggest surprise was that neither one of us could do a consistent, low level figure eight to measure with the flatback airfoil. We both ended up crashing several times because the plane would not turn consistently at the interesection. You'd start to change directions and it wouldn't do anything for a moment and then suddenly go to full control the other way. The stock wing seemed a little steadier and more pointable, with the other the Eppler and the sharp leading edge a little edgier.
Gary James did test 4 combat airfoils with X foil. They all were virtually identical results. Phil Granderson's had a small up kink in the curve at around 7deg AOA which gave it a 5% or so advantage there with a fairly sharp peak lift. Gary's pet airfoil had the the highest Cl at a couple degrees higher AOA. My stock airfoil was right in the middle of the pack, but a little lower peak lift. The Eppler 475 pooped out a couple degrees lower.
The bottom line was that in a useable range up to maybe 10 deg AOA there was only a slight difference, with PG's being just a bit better. Nobody ever flies a combat plane anywhere near the peak lift, so peak lift and stall characteristics never come into play. So now everybody in F2D is using the baseball bat leading edge, which I think Rich Brasher came up with on foam planes. The high point goes as far forward as 17-18% of chord.
Phil C.