The powered-glider pilot reported that, during the return flight to the airport after an informal glider competition, about 10,000 to 11,000 ft over mountainous terrain, he was unable to find lift, the glider descended to 9,000 ft, and he started the sustainer engine. He added that, after the glider climbed back to the initial altitude, he shut the sustainer engine off, stowed it, and continued to the destination airport. He reported that, about 4 miles from the airport, the glider began to sink about 1,000 ft per minute. He attempted to restart the engine but was unsuccessful in the limited time available. He chose a field for an off-airport landing, the glider touched down, but the ground was slightly uneven, which resulted in the right wing dragging. The glider veered 20° to the right before coming to rest. The glider sustained substantial damage to the horizontal stabilizer and elevator. The pilot reported that there were no preaccident mechanical failures or malfunctions with the glider that would have precluded normal operation. An automated weather observation station at an elevation of 7,950 ft, located about 14 miles east-southeast of the accident location, reported wind from 270° at 10 knots, gusting to 15 knots; temperature 79°F; and dew point 9F. The calculated density altitude was 10,300 ft.