On July 29, 2015, about 1400 eastern daylight time, an experimental amateur-built Mosquito helicopter, N285XE, was substantially damaged during a forced landing near Winchendon, Massachusetts. The commercial pilot/owner/builder was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which originated from Jaffrey Airport (AFN), Jaffrey, New Hampshire. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.The pilot completed an uneventful 40-minute flight in the airport traffic pattern at AFN, then landed and refueled the helicopter with 93 octane automotive fuel. He subsequently departed about 1300 for a flight in the local area. About 1 hour into the flight, and while flying at an altitude of 2,500 feet msl, the pilot felt the helicopter suddenly "kick from loss of torque." He then observed that the engine rpm had dropped to about 30%, and he was unable to increase engine power using the throttle control. The pilot responded by performing an autorotation to an open field. During the landing, the helicopter touched down "hard," and the main rotor system struck the tailboom, resulting in substantial damage to the airframe and tail rotor drive system. About two weeks after the accident, the pilot conducted a test run of the engine on the airframe. Review of video from the test run showed the engine started without hesitation and ran at idle rpm with no obvious deficiencies noted. During a subsequent engine test run, the pilot accidently bumped an ignition system wire harness and the engine partially lost power. After examining the wire harness, he found that a "bad crimp" had resulted in a loss of electrical power to the coil pack of one cylinder, which resulted in a loss of spark for that cylinder, and the engine running on only one cylinder thereafter. According to Federal Aviation Administration (FAA) airworthiness records, the single-seat helicopter was issued experimental amateur-built aircraft operating limitations on April 20, 2015. The helicopter was equipped with an Innovator Technologies Inntec 800 two-stroke engine, rated at 85 horsepower, equipped with a Rotor Flight Dynamics Dragon Wings controllable pitch rotor. At the time of the accident, the airframe and engine had accumulated 25 total flight hours. According to FAA records, the pilot held a commercial pilot certificate with ratings for rotorcraft-helicopter. In addition, he held a repairman experimental aircraft builder certificate for a Mosquito helicopter. His most recent FAA second-class medical certificate was issued on March 11, 2011. The pilot reported 183 hours of total time, of which 25 hours were in the same make and model as the accident helicopter. His most recent flight review was dated January 3, 2015.