On February 29, 2000, at 1545 eastern standard time, a Burak XF4, experimental airplane, N9VY, collided with the ground following a loss of propeller thrust at the Brooksville Hernando County Airport in Brooksville, Florida. The personal flight was operated by the pilot under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The pilot received minor injuries and the airplane sustained substantial damage. The flight originated from Brooksville Airport, in Brooksville, Florida, at 1500. According to the pilot, 35 minutes in to the flight, while circling Hernando County Airport, Brooksville, Florida, at 3000 feet, the pilot experienced a loss of propeller thrust. The pilot stated that he heard a loud "bang" from the engine compartment and experienced loss of thrust. The pilot said that the engine tachometer went to redline, and the propeller rpm decreased. According to the pilot, changing the engine rpm did not change the propeller rpm. The pilot declared an emergency and started an emergency descent for a landing on runway 27. As the pilot maneuvered the airplane on the final approach, he cleared a stand of trees on the final approach to runway 27. While on short final to land, the airplane initially collided with the ground short of the runway surface and subsequently came to rest on a taxiway. According to the pilot, the examination of the wreckage revealed that the Rex Flex Coupling Assembly failed. The Rex Flex Coupling assembly connects the engine to the propeller drive shaft and pusher propeller. The pilot reported that three out of the six Rex Bolts (#16087) failed and one bolt was found with the head sheared off. According to the pilot, he cannot recall the torque applied to the flange assembly during installation nor did he recall the maximum required torque for this installation. The pilot stated that the Flexidyne Drive Shaft was designed for a 100 horse-power engine. The pilot/builder stated that he installed a 150 horse power engine on the airplane.