Villa

Hi superflea
I like the analysis you offered. Thanks. I have always wondered why the J3-Cub reacted so much more different than other models. For you and others interested in this subject I would like to offer something that I discovered on the three J3-Cubs I have had.
I learned to fly R/C in 1972 in PA. One old timer in the club loved the Sig J3-Cub, which was a kit, 71” wing span (WS) and usually had about a .45 engine. I saw him crash, usually on landing, maybe a dozen times, totaling 3-4 planes. There were a few other Sig J3-Cubs in the club and all seemed to eventually crash on landing; I stayed away from them since they seemed more than I could handle.
In 1980 I lost my Sig Cougar due to a failed solder connection and decided to get a Sig Cub. An old timer in my new club in NC did not fly anymore but loved to build the Sig 71” WS J3-Cub. He sold them at cost. I soon was crashing that Sig Cub. I don’t recall any useful advice and we did not have RCUniverse then. I kept rebuilding it and landing it slow and crashing. It had a violent snap roll. I was determined to conquer it but it conquered me. I gave up.
About a year later I decided to build my own Sig J3-Cub. While building the wing I noticed that the wing had almost no structural integrity. It was limp as a wet noodle. The wing struts is what held it together, but MOST important, determined the washout/wash in of the wing. I had my answer now.
My theory is that after repeated landings of the Sig J3-Cub, the top of the “weak” cabin and the light wing structure would crush a little, which would allow the struts to change the washout/wash in of the wing tips. Unless I have got this backward, it is desirable to have washout (the leading edge {LE} lower than the trailing edge {TE}) at the wing ends. The struts were forcing the wing tips into wash in and the plane was a disaster waiting to happen since the wingtips would stall first and there would be NO WARNING. All of my other planes would warn me of a pending stall by rocking the wings.
On the Sig J3-Cub I was building I made the strut wing attachments adjustable and I checked the wing tip wash in EVERY time I assembled the plane for flight. Not one time did I ever have an uncommanded snap roll on the Sig J3-Cub that I built.
I have noticed that now there are many much better J3-Cubs on the market than the Sig Cub I described. Most have a cantilever wing and at least some of the wing struts are not functional. I believe that my theory may still apply if uncommanded snap rolls are involved. Presently I fly a 71” WS J3-Cub I designed and built from Coroplast, using the Sig layout drawings I had as a guide. My wing is fully cantilevered, my struts are adjustable, and I have never had an uncommanded snap roll with it on landing. I completely forgot everything I had learned about taking off a J3-Cub and as a result had 3-4 snap rolls on takeoff. Plywood models would have been totaled. My CoroCub, as I named it, sustained little damage. I keep an eye on the washout and adjust the flying struts as needed.