The pilot reported that during a test flight the airplane encountered moderate turbulence so he decided to return to the airport for landing. While turning to the crosswind leg in the pattern, the airplane suddenly banked sharply and descended uncontrollably. The pilot was able to regain control about 500 feet above ground level, and managed to turn the airplane to the downwind leg, but he continued to have difficulty maintaining control. The pilot then decided to make an emergency off airport landing to a field. During the landing roll, the airplane collided with obstructions and nosed over. The wings and fuselage were substantially damaged. After the accident, the pilot reported that the loss of aircraft control was due to a jammed rudder for undetermined reasons.