The pilot had delayed his intended departure for approximately eight hours, following overnight rain, to allow the sun to dry the grass runway surface. The runway was then walked to assess its condition. The takeoff was downhill, commencing at the beginning of the strip orientated in a 170° direction. A tailwind of approximately 5 kt was reported. The initial part of the takeoff run appeared normal to the pilot, but as the aircraft became light, it veered left and, despite full right rudder and brake, it left the runway onto the adjacent field. It then began to decelerate and, even with full power, the nosewheel contacted the ground. The pilot closed the throttle and mixture, but despite full back pressure on the control column, the aircraft came to a rapid stop and became inverted. The pilot turned off the ignition, fuel and electrical power, released his harness and vacated the aircraft by the left door.
The pilot considered that a small patch of soft earth had caused the initial swing and, as the aircraft was nearly airborne, the brake had little effect.