On May 16, 2001, at 1939 central daylight time, a Kirchner Pietenpol Aircamper home built airplane, N25157, was substantially damaged during a forced landing following a loss of engine power near Tulsa, Oklahoma. The airplane was owned and operated by the pilot. The private pilot, sole occupant, was not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The flight originated from the Sand Ridge Airport near Collinsville, Oklahoma, at 1910, and was destined for the Richard Lloyd Jones Jr. Airport, Tulsa, Oklahoma. While en route, the pilot observed a rise in engine oil temperature. The pilot elected to make a precautionary landing at the Harvey Young Airport; however, engine rpm decreased to where altitude could not be maintained. The pilot initiated a forced landing to a bean field about one mile northeast of the airport. During the landing, the airplane touched down "hard," nosed over, and came to rest inverted. Examination of the airplane by FAA inspectors, who responded to the accident site, revealed that the main landing gear had collapsed and was partially separated. The right wing spar outboard of the lift strut was fractured, and the vertical stabilizer was damaged. An examination of the Corvair automotive engine, by the owner, revealed a stuck oil cooler thermostat valve. The thermostat valve was found stuck in the position that would prevent oil from flowing through the oil cooler. Additionally, pieces of a broken piston ring were found in the engine's oil sump. Attempts to obtain a completed Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2, from the pilot/owner were unsuccessful.