A series of parachute flights were planned from Weston-on-the-Green. The first flight was uneventful. Thirteen parachutists boarded for the second flight, and the aircraft was cleared to take off from a dry surface of grass runway 01 (take off run available 3,194 feet). Approximately 80-83 kt (scheduled rotation speed 71 kt) the pilot pulled back on the control column, causing the aircraft to feel nose heavy and the nose did not rise. The pilot aborted the take-off, commenced braking, and set the condition levers to the full reverse position. As the aircraft decelerated, it turned to the right to avoid trees and bushes ahead, striking a small earth mound while traveling at approximately 15-20 kt. The accident resulted in the aircraft coming to a halt.A series of parachute flights were planned from Weston-on-the-Green. The first flight was uneventful. Thirteen parachutists boarded for the second flight, and the aircraft was cleared to take off from a dry surface of grass runway 01 (take off run available 3,194 feet). Approximately 80-83 kt (scheduled rotation speed 71 kt) the pilot pulled back on the control column, causing the aircraft to feel nose heavy and the nose did not rise. The pilot aborted the take-off, commenced braking, and set the condition levers to the full reverse position. As the aircraft decelerated, it turned to the right to avoid trees and bushes ahead, striking a small earth mound while traveling at approximately 15-20 kt. The accident resulted in the aircraft coming to a halt.