On August 1, 2014, about 1415 central daylight time, a Mosquito XE amateur-built helicopter, Canadian-registered CF-HEZ was substantially damaged while maneuvering near Wittman Regional Airport (KOSH), Oshkosh, Wisconsin. The private pilot was not injured. Visual meteorological conditions prevailed for the flight. The helicopter was owned and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 without a flight plan. The local flight departed approximately 1410. Just after takeoff, one tail rotor blade separated from the tail rotor and gearbox assembly. The helicopter started to rotate and impacted the ground in a level attitude, spreading both skids out horizontally. Both skids and the tail boom were substantially damaged. According to the Federal Aviation Administration inspector who examined the helicopter, the tail rotor blade had separated cleanly from the assembly. Neither the collar nor the pins used to secure the blade were located. The inspector reported that the tail rotor assembly was a unique design being tested and was not used in any other application. The pilot reported that he had been testing this tail rotor design for four years. He stated that a tension/torsion strap and two steel pins were used to retain the blade. The pilot stated that the strap was not completely restricted from movement and over time movement and wear resulted in the failure of the retaining assembly. The pilot reported that there were 82 hours on the tail rotor assembly and it had been 6 hours since it was last inspected.