The pilot reported that while climbing after takeoff during a personal flight with 20o flaps and an airspeed 60 kts, he noticed that the climb seemed very slow. As the airplane approached a tree line, the pilot, thinking the airplane would not clear the trees, reduced the flap setting to 10o to increase airspeed. When the pilot realized that the airplane would not clear the trees, he initiated a 180o turn, with the intent to land on another runway at the departure airport. During the turn, the airplane entered an aerodynamic stall and subsequently impacted terrain. The airplane sustained substantial damage to the forward fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot reported that the airplane flight manual stated that the climb speed should be 62 knots with a 20o flap setting.