The Piper J3 (Micro-X) needed a thicker rubber, ie. 0.085" (14 inch loop) and it did fly just for a short moment last Saturday, albeit in very big right circles and was stalling (needed additional weight upfront at the nose). I launched it too close to the wall and it banged the wall and broke the prop blade thus ended my flying session on Saturday (as the prop blade kept breaking upon impact or landing on the ground.
Subsequently, I added nose weight to balance the model further upfront. Added close to 0.8g at the nose. So AUW now (without rubber) is at 6.8g. Added a rudder right trim tab and compensate with a wing tab just under the right side, and the flight circle got much smaller, about 15 ft radius.
I had some time today after work to go to my nearby hall to fly the Piper J3 Cub. It flies like a dream now with the same 14" 0.085" loop, and had a ball of a time at the hall. I had 6 perfect flights, and I had trouble flying it low from hitting the walls, the ceiling and the ceiling lights. This hall ceiling is slanting down from the centre to both sides of the ceiling with the lowest side portions about 15 - 18ft high.
I could only put in max 1,200 winds to just keep it just under the ceiling, and I was getting consistently 50+ seconds (with one particular flight was close to a minute). I think a thinner rubber say 0.070 - 0.080" will be ideal for the low ceiling venue whereas the 0.085" will be ideal for bigger and higher site. I believe the model can go past 1.5 minute plus at our regular indoor high ceiling flying site once I crank in 1,500 - 1,800 winds (can't wait to do that during my next flying session).
Here one flight that I recorded with my phone (you can see the obstacles that the plane had to go through).