On May 8, 1994, at 1500 central daylight time (CDT), a Grumman TR-2, N1639R, registered to David R. Johnson of Wichita, Kansas, and piloted by a private pilot, was substantially damaged during landing on Runway 16 (2,675' X 30' dry asphalt) at the Riverside Airport, Wichita, Kansas. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight had not been operating on a flight plan. The pilot and passenger reported minor injuries. The flight departed Mountain Grove, Missouri, at 1250 CDT. The pilot stated he was checked out in the airplane by the previous owner. The previous owner had told him to fly the final approach at 90 miles per hour (mph) because the stall warning system was set high. During the final approach to runway 16, the pilot said he flew 90 mph and had extended 50 percent of the landing flaps. He said the result of this was a higher than normal final approach. To avoid a touch down zone overshoot, the pilot said he retarded the power too idle. The pilot said he over rotated during the landing flare and that his first touchdown was in a nose high attitude. The airplane porpoised and became airborne three times. The pilot stated, on his NTSB Form 6120.1/2, "I thought about (doing a) go-around but decided to keep it down and salvage the landing." After porpoising three times, the pilot said the pilot-rated passenger (passenger) told him the airplane had to go-around. He said the passenger applied full power and that the airplane turned to the left and exited the runway; becoming inverted after rolling over soft soil. During an interview with a Federal Aviation Administration Principal Operations Inspector (POI) the pilot said both his hands were on the yoke when the passenger applied power. The pilot submitted a written statement to the POI. On it he stated, "...he (the pilot rated passenger) applied full power to the engine, causing aircraft to drift left and off runway. I was still concentrating on the effects of the porpoising, and was not prepared to handle the sudden power increase." The passenger stated the pilot had not extended the landing flaps and that the final approach speed was 115 mph. According to the passenger, the pilot "...flared and ballooned into the air. As it settled back on the runway, it bounced into the air again." The passenger stated he told the pilot to go-around after the airplane porpoised and bounced into the air two more times. "The last time the airplane came off the runway (it was) approximately 30 degrees to the centerline." The passenger said he told the pilot to add power and go-around a third time. Not observing a response, the passenger said he applied full power as the airplane departed the runway's left edge.