The pilot reported that, while enroute, the air was "unstable and choppy", and the airplane did not climb as expected. He added that the airplane "managed to climb to 9,500 [ft mean sea level] and maintain the altitude", but then had a "sudden and violent drop in altitude followed by a down draft". Subsequently, the airplane descended to 500 ft above the ground, the airplane was between two hills and still "struggling to climb". He observed what appeared to be a dirt plateau followed by a cliff, and decided to land on the plateau to "avoid possibly falling off the cliff". During the landing, the airplane nosed over. The airplane sustained substantial damage to the fuselage, empennage and right wing. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.