The pilot stated that, before departing in his tailwheel-equipped airplane, a U.S. Army CH-47 heavy-lift helicopter departed and proceeded upwind. When the helicopter was established on the downwind leg of the traffic pattern, the pilot initiated his takeoff. At 40 ft above ground level in the initial climb, the airplane “hit the helicopter’s rotor wash” which rolled the airplane “about 135 degrees to its right.” The pilot was able to level the wings before the airplane touched down in low brush to the right of the runway and came to rest near the airport boundary fence. The airplane sustained substantial damage to the main landing gear support structure and the pilot deplaned uninjured. The pilot reported there were no mechanical malfunctions or anomalies with the airplane that would have precluded normal operation.