On August 23, 1997, at 1045 eastern daylight time, a homebuilt Revolution Mini 500, a helicopter, N7240E, was substantially damaged when it collided with the ground during a forced landing near the Harry Clever Field, New Philadelphia, Ohio. The certificated private pilot was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The local, personal flight that originated at New Philadelphia, was conducted under 14 CFR Part 91. According to the pilot, he had completed a pre-takeoff check, which included checking both ignition systems, and engine split test for correct operation of the sprague clutch. He established the helicopter at a hover for about 2 minutes, checking the exhaust gas temperature gage for correct temperatures. All checks were satisfactorily completed and then he departed. The pilot reported that shortly after takeoff into the wind, at an altitude of about 50 feet and airspeed of about 35-40 mph, he noticed the main rotor slowing down. He immediately applied power with no results. Shortly thereafter, the engine lost power. He said he entered autorotation, but he did not have enough rotor speed to perform a successful autorotation, and the helicopter touched down hard. The helicopter was moved to an overhaul facility in Newcomertown, Ohio, and was examined by a Federal Aviation Administration (FAA) Inspector. The Inspector stated that the examination revealed that more than 75 percent of the electrodes of the rear cylinder spark plugs (magneto side) were burned. According to the Rotax Operator's manual, it stated in part: "If both plugs have 'white' electrodes with 'melt' droplets, first suspect lean mixture." According to the FAA Inspector, examination of the exhaust manifold revealed a half inch crack within a welded seam, mid section of the exhaust manifold in the center section. According to the owner, the helicopter had a total of 56 hours since new, and the spark plugs were installed about 30 hours prior. The spark plug installed in the forward cylinder was of the same type, and its electrode indicated "normal" wear. The owner also said that the exhaust manifold was delivered from the factory with the weld. According to the Rotax Operator's Manual, it stated in part: "This is not a certificated aircraft engine. It has not received any safety or durability testing, and conforms to no aircraft standards. It is for use in experimental, uncertificated aircraft and vehicles only in which an engine failure will not compromise safety. User assumes all risk of use, and acknowledges by his use that he knows this engine is subject to sudden stoppage."