On June 16, 1999, about 1640 eastern daylight time, a homebuilt Myers 145, N42240, was substantially damage after a partial loss of power, and a forced landing to a grassy field near the Armstrong Airport, Wapakoneta, Ohio. The certificated private pilot received serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to the pilot, he left his home about 1500, and traveled to the airport were the airplane was kept outside, and tied down. Before preflighting, he added 5 gallons of fuel, then did a fuel check. The check was "normal", and the pilot identified no anomalies during the preflight. The pilot boarded the airplane, and started the engine on the first attempt. He then taxied short of runway 8, and completed the run-up checks. During the run-up checks, engine rpm did not drop when individual magnetos were selected. In addition, the pilot scanned both engine and flight instruments, and noticed no discrepancies. The pilot announced his intentions on UNICOM, taxied onto the runway, advanced the throttle, and departed using 10 degree of flaps. He estimated that the airplane became airborne approximately 1/4 of the way down the 5,002 foot long runway. The pilot remained in left closed traffic, and turned crosswind, downwind, base, and final. While on final and 800 feet agl, the pilot noticed a partial loss in power. He moved the throttle, but engine rpm did not change. With insufficient power to make the runway, the pilot executed a forced landing to what he thought was a level grassy field. The airplane touched down "smoothly", and started to rollout. Before the airplane came to a stop, it encountered a creek bed approximately 5 feet deep, and 30 feet wide. The airplane entered the creek bed, and "immediately" came to a stop. The pilot secured the ignition system, removed the key, and disconnected the battery. He then egressed the creek bed via the airplane's wing, and was assisted by witnesses. In addition, the pilot stated that both the airframe, and engine had about 50 hours of operation. He added that on a previous flight, the engine experience a high oil temperature. He changed the oil from 50 weight to 30 weight, and installed a clean set of spark plugs. The purpose of the accident flight was to evaluate the effects of these changes. A review of field notes taken by the Federal Aviation Administration Inspector that was on scene, revealed no pre-impact failures for either the engine or the airframe. The following excerpts are from the Advisory Circular 20-27D, Certification and Operation of Amateur-Built Aircraft: "The amateur-built program was designed to permit person(s) to build an aircraft solely for educational or recreational purposes. The FAA has always permitted amateur builders freedom to select their own designs. The FAA does not formally approve these designs since it is not practicable to develop design standards for the multitude of unique design configurations generated by kit manufacturers and amateur builders.... Since 1983, FAA inspections of amateur-built aircraft have been limited to ensuring the use of acceptable workmanship methods, techniques, practices, and issuing operating limitations necessary to protect persons and property not involved in this activity."