I had that happen with one of the old gliders I used to build. A friend and I built a set one weekend and then late one night took them out for fun. It was in England and after 1/2 hour or so the skins on the wings started to get floppy... even with a good layer of dope on them. Took back inside and hit them with a blow dryer and headed back out! Lost that one, when I was flying in some rolling hills. Caught an updraft and it just kept on going, watched it continue to climb till was out of sight!