The sport pilot reported that days prior to the accident flight, he adjusted the propeller and re-installed it without testing its performance. On the accident flight, he departed the runway and remained in ground effect until he reached 50 mph, then began a climb. As he flew the gyroplane to the south, the surrounding terrain elevation decreased and the pilot thought he was climbing. The engine was operating about 6,300 rpm and the airspeed was 40 mph; the engine develops maximum horsepower at 6,750 rpm and the best climb speed is 45 mph. The pilot initiated a left turn to land in a field or back at the airport, but the wind pushed the gyroplane and the pilot lost control. The gyroplane impacted the ground and sustained substantial damage to the tube frame, vertical tail, and rotor blades. The pilot reported no preimpact mechanical malfunctions or failures that would have precluded normal operation. He reported that the propeller was not adjusted properly in order to provide maximum engine rpm and subsequently maximum climb performance. Due to the slow airspeed, it is likely that the gyroplane experienced an aerodynamic stall.