On September 13, 1997, at 1340 central daylight time, a Reilly Thorp T-18 experimental airplane, N16LE, registered to the pilot (builder of the airplane), impacted a shallow lake following a loss of control during approach for landing at Wagoner Airstrip, Wagoner, Oklahoma. The private pilot was fatally injured, and the pilot-rated passenger sustained serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the Title 14 CFR Part 91 demonstration flight. The flight, which originated from Wagoner Airstrip at 1300, was intended for the owner/pilot to demonstrate the aircraft to the passenger who was a potential buyer. The pilot-rated passenger reported in an interview, that he had contacted the owner approximately 1 week prior to the accident to inquire about potentially purchasing the airplane. He stated that he wanted to see whether the airplane would be suited for operating out of his private grass airstrip. He further stated that he had heard that the Thorp T-18 had relatively high stall speed characteristics and high landing speeds which may not be compatible with his 2,000 foot runway. An agreement was made for the owner to demonstrate a flight to see how the airplane performed during short field operations. Additionally, he stated that he does not have any recollection of the event. His resultant injuries required extensive hospitalization for months after the accident. After about 40 minutes of local flight, the airplane entered into a left downwind for runway 17 at Wagoner Airstrip. Several witnesses observed the airplane in a left traffic pattern on the base leg for runway 17. One of the witnesses, who a pilot, stated that he observed the airplane "overshoot" the runway on base leg, saw the left wing go "down", and then saw the airplane "stall." The other witness observed the airplane make a "very steep" left turn toward the runway, heard the engine "power up" during the turn, and then saw the airplane's left wing "drop dramatically." Subsequently, the airplane descended uncontrolled into a lake, located southeast of the approach end of runway 17. During recovery the right wing was found partially separated from the fuselage. The instrument panel buckled downward, and the underside of the fuselage was found extensively damaged. Examination of the aircraft by an FAA airworthiness inspector did not reveal any mechanical or structural anomalies. The airframe, engine and pilot logs were not recovered. A stall strip was not found installed on the aircraft. Several interviews conducted with Thorp T-18 owners revealed that the stall speed of the airplane with flaps extended is approximately 65-68 knots (level wings). No flap stall speed is about 69-71 knots, and landing speed (touchdown) is about 80 knots. All three owners who were interviewed reported that in their experience, the airplane "breaks very fast" during practice approach turn stall maneuvers. The NTSB did not travel to the scene of the accident. Information received by this office immediately after the accident suggested that the pilot-rated passenger was identified as the "pilot of the airplane" by emergency rescue personnel when the two occupants were being extracted from the wreckage. With that information, a decision was made to request toxicology tests (which were "negative") for the "presumed pilot." Therefore, toxicology tests and an autopsy were not performed on the fatally injured 81 year old pilot, who was at the time, "presumed" to be the passenger. Further investigation after the accident revealed that the fatally injured occupant (who was the owner of the airplane) was extracted from the left seat. Toxicology tests and an autopsy was not feasible at the time this information was learned. NTSB Form 6120.1/2 is not attached to this report. The pilot/owner/operator is deceased and the pilot-rated passenger has no information about the aircraft's history.