On August 14, 2015, about 1110 eastern daylight time, an experimental, amateur-built Zenith CH750, N1750Z, was substantially damaged when it impacted trees and terrain shortly after takeoff at Queen City Airport (XLL), Allentown, Pennsylvania. The private pilot was seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight, which was operated under the provisions of Title 14 Code of Federal Regulations Part 91. The flight was originating at the time of the accident. The pilot reported that after takeoff from runway 25, the engine lost partial power about 100 feet above ground level. The pilot further stated that the engine would not keep him flying. He attempted to return back to runway 7, but the airplane impacted trees and a creek bed, resulting in substantial damage to the airframe. According to the pilot and Federal Aviation Administration (FAA) records, he held a private pilot certificate with a rating for airplane single-engine land. The pilot reported 328 hours of total flight experience. The wreckage was examined at the scene by a FAA inspector. The inspector reported that the airplane was submerged, nose down, in about 2 feet of water. About 10 gallons of fuel were recovered from the airplane's fuel tanks. The airplane's airworthiness certificate was issued in 2012, and since that time it had accumulated 75 total flight hours. The inspector found the number 1 spark plug missing from the cylinder head but still attached to the ignition lead. The pilot's son took several pictures of the spark plug, ignition lead and cylinder head. He verified that the threads were stripped out of the cylinder head, and the threads on the spark plug looked to be in good condition.