On December 17, 2001, about 1250 Pacific standard time, a Vernon Sonerai II, N6399D, lost directional control on the landing rollout on runway 26, and came to rest inverted in an adjacent field at the Camarillo Airport, Camarillo, California. The experimental airplane was operated by the owner under the provisions of 14 CFR Part 91, and sustained substantial damage. The private pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the local area flight and no flight plan had been filed. The airplane departed Camarillo about 1000. The Safety Board investigator interviewed the pilot. The pilot stated that the purpose of the flight was to practice maneuvers, as well as to fulfill the experimental certificate operating limitations initial 40-hour requirement. He indicated that he had accrued 5 hours of flight time in the airplane, which was also the total time on the airplane. After departing Camarillo airspace, the pilot flew towards Santa Paula, about 10 miles away, to practice maneuvers. He flew about 1.5 hours before returning to Camarillo for landing. He noted a slight crosswind on landing, but nothing unusual. In the pilot's written statement to the Safety Board, he stated that after touchdown the airplane became directionally unstable with increasing amplitude until the airplane was facing perpendicular to the runway. The airplane then departed the runway, and traveled perpendicular to the furrows present in the infield of the airport. Of note in the pilot's written statement was the directional instability that he encountered during takeoffs in the 7 hours that were placed on the airplane. He made several modifications, mostly rigging changes to the tail wheel, to improve the takeoff characteristics. While in contact with the Sonerai technical representative, the owner stated that there were plans to convert the conventional landing gear airplane to a tricycle landing gear. The proposed modification would solve the instability problem, "the resulting mass induced control signals are stabilizing instead of destabilizing."