On August 3, 1997, at 0725 central daylight time, a Larsen T-Bird II, N41873, piloted by a private pilot sustained substantial damage when it collided with a tree line during initial climb from the ultralight strip (1,500 feet by 200 feet, dry/grass) at the Whittman Regional Airport, Oshkosh, Wisconsin. Visual meteorological conditions prevailed at the time of the accident. The personal flight was operating under the provisions of 14 CFR Part 91 and was not on a flight plan. The pilot received minor injuries and the one pilot-rated passenger sustained serious injuries. The flight was originating at the time of the accident and had the planned destination of Portage Municipal Airport, Portage, Wisconsin. According to the pilot-in-command's written statement, approximately 40 feet above ground level the airplane, "Would not climb hardly enough to clear the first set of trees." The pilot reported that he, "Could not get the nose down to gain airspeed. Turned aileron left, plane went right. Used only rudder, and it turned left but went into a full stall hitting trees." The aircraft came to rest after impacting a tree line and a chain-link fence. The pilot further stated, "When we arrived at the field that morning someone was in the cabin with the door open. I believe he or someone tampered with the controls." Witnesses to the accident reported that the airplane's engine was operational and was at full power at the time of the accident. Post accident investigation revealed no anomalies with the aircraft or engine. Control continuity was confirmed for both roll and pitch axis with no anomalies found.