On February 26, 1999, at 1157 hours mountain standard time, an experimental Helio Jergins H 800, N666X, sustained substantial damage during a hard landing at Prescott, Arizona. The airplane departed Prescott about 1110 on a local flight to test a newly installed engine. Helio Enterprises of Prescott operated the flight under the provisions of 14 CFR Part 91. The airline transport pilot/owner, the sole occupant, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot stated the airplane had been an uncompleted airframe when the original manufacturer went out of business. It had been purchased and completed by another individual. He installed a Walters M-601 free turbine engine on the airframe and was seeking Federal Aviation Administration (FAA) certification of this combination. A previously installed engine had fuel control problems and exhibited metal contamination in the oil. The airplane had flown approximately 5 hours with that engine. It was removed and replaced with this engine, a newer version with lower time. This was the first flight of the airplane since this engine had been installed. The pilot said he intended to stay within 50 miles of Prescott to do some basic control checks; this part of the flight was successful. He returned to Prescott and performed several landings. He stated he maintained over 70 knots in the pattern for these landings. Feeling more comfortable, he decided to make a more aggressive STOL (short takeoff and landing) approach. This would require him to fly about 60 knots and required more precise power control. He was trying to determine if this engine would not lag in response time and offer the power control required. Shortly after turning from the base leg to final, the propeller slipped into the beta (reverse pitch) range. At an altitude of 150 feet he didn't think he could lower the nose and attain the 70-knot airspeed he would need to flare normally. He elected to maintain his three-point attitude and tried to slightly increase power to get the propeller to come out of beta. Normally, beta is obtained by depressing a thumb lever that lifts a pin, allowing the control lever to be moved into the beta range. No response was noticed so he continued to slowly advance the power. The airplane touched down at approximately 1,000-feet-per-minute rate of descent. This sheared the bolt for the right main landing gear and the bolt holding the tail yoke onto its spindle. The right main gear collapsed and the right wing struck the ground. The ground strike wrinkled the right outboard wing panel, bent the outboard portion of the flap and aileron, and buckled numerous ribs. The tail yoke smashed the bottom of the rudder. The starter and fuel control units separated from the engine. The FAA accident coordinator examined of the airplane, engine, and documentation. The proper technical manuals were available and the installation appeared to comply with the manuals. He did not observe any mechanical damage to the controls or linkages that controlled the propeller. All linkages and jam nuts were secured and the linkages appeared to be properly adjusted.