On May 9, 1998, at 1400 mountain daylight time, an experimental Eller BD-5B, N194EC, impacted terrain when the canopy partially separated during a test flight near Meadow Lake Airport, Falcon, Colorado. The airline transport certificated pilot, and sole occupant, received serious injuries and the aircraft sustained substantial damage. No flight plan was filed for this local area flight operating under Title 14 CFR Part 91. Visual meteorological conditions prevailed. According to the pilot, he had conducted glide and power off stall tests and was making a low pass in the pattern when the aircraft began a violent vibration and involuntarily entered a left turn. The pilot said he corrected the turn and the canopy partially separated due to vibration. Due to the drag from the canopy, the pilot had to maintain a 20 to 35 degree nose down attitude to prevent the aircraft from entering a stall. He said he raised the nose and entered a full stall condition just prior to impact with the ground. Examination of the aircraft by an FAA airworthiness inspector provided evidence of a crack in one blade of the wood propeller between 15 and 18 inches inboard from the blade tip. The blade was sent to the National Transportation Safety Board Materials Laboratory for examination. The laboratory report is attached and states that the blade contained no obvious area of mechanical damage but was cracked about one half to two thirds through the cross section of the blade arm. The laboratory examination found no evidence of weakness in the wood and no evidence of inadequate bonding between the plies of wood. The aircraft examination also provided evidence that the canopy holdback fitting was fractured. The fitting was removed and it was also sent to the Materials Laboratory for examination. The laboratory report is attached and states that the fracture faces were typical of overstress. The propeller and canopy holdback fitting were returned to the owner/builder following examination.