The non-certificated pilot was performing a high speed taxi test at a privately-owned, dirt and gravel-covered airport, in his tricycle-gear equipped weight-shift-control airplane. He reported that after arriving at the airport, he assembled the airplane, and then did a thorough preflight, which included checking wind conditions aloft by releasing helium balloons. He stated that the airplane had recently undergone an extensive rebuild, and he intended to do series postmaintenance taxi tests before flying the airplane. During the first high speed taxi test, as the airplane reached a takeoff speed, the pilot pushed the control bar forward, and the airplane became airborne. In an attempt to land the airplane and gain additional airspeed, the pilot pulled back on the control bar, but the airplane began to roll to the left. The pilot then applied full right control bar input, but the airplane continued to the left, and it subsequently collided with a stand of trees, sustaining substantial damage to the wings. The pilot stated that there were no preaccident mechanical anomalies with the airplane that would have precluded normal operation. In the recommendation section of the NTSB Accident/Incident Reporting Form 6120.1, the pilot stated that the accident may have been prevented if he would have been more familiar with the airplane.