The pilot was attempting a high speed taxi test when the airplane became airborne. A normal climb was made to approximately 1,000 feet AGL. The pilot made several left turns while in level flight and proceeded back toward the airport. During the approach, the propeller system's variable control mechanism slipped which allowed the propeller to go to the full feather position. The airplane's airspeed was between 160 to 180 mph when this occurred, according to the pilot. The airspeed then dropped to approximately 100 mph and the pilot initiated a forced landing to an open field. During the flare prior to touchdown, the pilot added power and the airplane yawed. The left wing tip struck the ground and the pilot lost control of the airplane which then impacted the terrain. The pilot stated the propeller control system probably vibrated loose during the flight. This system is controlled by the pilot from the cockpit with a vernier knob. The pilot built the airplane and had not flown it before the accident.