On October 9, 2018, at 1904 eastern daylight time, an unregistered experimental amateur-built Supercat airplane collided with terrain after takeoff from Three Rivers Municipal Dr Haines Airport, Three Rivers, Michigan. The private pilot sustained serious injuries, and the airplane sustained substantial damage to the forward fuselage. The airplane was operated by the pilot/builder under the provisions of Title 14 Code of Federal Regulations Part 91 as a local personal flight. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight was originating at the time of the accident.The pilot reported that he recently completed construction of the airplane, and although he had previously performed multiple high-speed taxi tests and "crow hops", this was to be the first flight. He stated that the takeoff runway was a 1,000-ft-long grass strip, adjacent and parallel to Runway 9. After warming up the engine, he taxied the airplane into position, and initiated the takeoff roll. The airplane lifted off after traveling about 1/4 of the runway length, and he stated that during the initial climb he failed to maintain the correct pitch attitude and adequate airspeed, and the airplane experienced an aerodynamic stall. The airplane struck the ground nose-down just beyond the end of the runway. The airplane was equipped with a Rotax 447 series two-stroke engine. The pilot indicated that he planned to register the airplane with the Federal Aviation Administration once he had established that it could be flown. He reported that there were no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.