Shortly after takeoff near the departure end of the runway, the pilot inadvertently stalled the airplane which impacted the ground nose and left wing low. The airplane then cartwheeled and came to rest upright. According to the pilot's wife who was a witness, the engine sounded normal. She further stated that this was the first takeoff attempt from the grass airstrip. Examination of the cockpit revealed that the landing gear selector handle was in the down position. Examination of the airframe and engine by an FAA airworthiness inspector revealed no evidence of preimpact failure or malfunction. In the opinion of a pilot who had performed one takeoff in this airplane from this airstrip, the climb performance was marginal due to the large propeller blade angle of the homemade propeller. He stated that the engine performance was similar to what he would expect with a cruise propeller installed. He advised the pilot that operations were marginal from the accident airstrip.