Hi All, this is my take observations and comments on earlier Kitfox (1-3) with one mass bass balance and flat bottomed flap/aileron!
Here goes in point form.

1. I have a about 100 hours on a slick Kitfox Mk3 that I that painstakingly built over a few years. I'm not shy is saying its well put together and a lot of care and thought went into its construction. The plane is AWESOME to fly I love it!
2. I'm well aware it has a larger than normal engine in my R2800 radial. I'm ok with that and respectful of useful load and VNE
3. On my very first test flight during my first turns the ailerons felt very hard, I knew this was not right. I looked down at the flap trim handle and noticed it had pulled itself all the way forward causing the flap/aileron to go reflexed (negative AOA).
4. As soon as I put the flaps back neutral the plane again rolled properly and with the correct aileron feel, problem solved. But when letting the handle go the flaps would, due to aerodynamic loads, slowly pull back to reflexed again. I completed the test flight holding the flap handle in place manually.
5. On landing I added more tension to the flap handle friction washer. Next flight, same thing.
6. I decided to make a positive locking and staged flap mechanism, next flight problem solved. No more flap handle wandering.
7. Fast forward 100hrs to this week and I had a good fright! During perfectly calm conditions, dead calm, after a touch and go I was on down wind, I was doing only 70 kts, then a sudden THUMP and the plane started shaking (***!!!!) I thought engine / prop issue so pulled the power, still the plane was shaking. Not the engine or prop.
8. I looked out the side window and I could see that the aileron was fluttering. Ok why, its never done this in the past? Scanning the controls I saw that the flap handle had jumped out of its notch and had gone fully reflexed (negative AOA).
9. I pulled the flap handle back to the correct notch and the flutter instantly stopped. This all only lasted seconds say 10 max.
10. I landed without issue and after inspecting the plane, I found it was fine.
11. I have checked all the lash in the controls and it was as I built it, solid.
12. I made a new flap locking notch plate that is near fool proof and cant jump out.
13. NOTE: I have a separate elevator trim, I do not use the flaps for pitch trim.

Conclusions / Opinions:
14. I do not think that sloppy linkages should allow flutter to occur in a well designed air frame. This is an aerodynamic issue.
15. Later Kitfox have more that one mass balance along the length of each aileron. I believe this is related.
16. Later Kitfox have a much thicker flap/aileron airfoil and resultant heavier torque tube down the length of the aileron. I believe this is related.
17. Flying an early kitfox (1-3) with the flaps reflexed and inducing a "stick slap" (sudden movement) can induce an aileron flutter.
18. This is a design issue that can be managed but not eliminated.
19. Earlier Kitfox have flap/aileron that are thinner in section, compared to the later models. With one large mass balance at the outer length. I speculate in conditions already mentioned above that the aileron can twist and get out of phase with the one larger mass balance and the flutter begins.
20. I speculate that later model Kitfox (4-7) rectified this, likely, design issue by making the flap/ailerons stiffer in construction, with a larger torque tube, and thicker airfoil, having more than that one mass balance and a revised (better) control system.

See my plane here: https://youtu.be/2lrEZkFqktc