According to the pilot of the tailwheel-equipped airplane, he planned to perform a touch-and-go landing, and proceeded with a “wheel landing.” During rollout, with both main wheels on the runway, “an unpredictable and unknown force” caused the airplane to depart the right side of the runway. The airplane continued toward runway signage, and the pilot applied engine power to avoid it, which resulted in the airplane’s “ground loop to the right.” The airplane’s right wing and propeller struck the ground which resulted in substantial damage to the wing and fuselage. Weather reported 14 miles south of the accident site at the time of the accident included clear skies and calm winds. The pilot reported there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.