Graupner Cirrus
A flying buddy of mine, Jack Hiner, had a Cumulus back in the 70's. He flew it extensively and did well with it. As I recall it had a tendency to tip stall because of the sweepback and no washout. Jack cured it by slitting the bottom skin in a triangle that began at the trailing edge about 5 inches from the tip and ran in a straight line to a point on the tip which was about an inch and a half from the trailing edge. He bent the trailing edge up at the tip about 1/8 inch and glued a thin strip of balsa in the gap of the slit with yellow carpenter's glue. This added a bit of washout and decambered the tip airfoil a bit.