On July 31, 2017, at 0835 central daylight time, an experimental amateur-built Chapman T-Bird II, N204BC, collided with the terrain shortly after takeoff from the Washington County Airport (8WC), Potosi, Missouri. The private rated pilot was seriously injured and the airplane was substantially damaged. The airplane was registered to and operated by a private individual as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual flight rules conditions existed near the accident site at the time of the accident, and a flight plan had not been filed. The local flight was departing at the time of the accident.Two witnesses reported seeing the airplane in a "nose dive" shortly after takeoff. One of these witnesses stated the airplane was having "difficulties" and heard the engine prior to the descent. A third witness was a friend of the owner and identified himself as "part owner" of the airplane. He reported the takeoff and initial climb looked normal. The airplane then entered what looked like a right crosswind turn, which was the normal procedure after takeoff on runway 02. He stated the wing "dropped" in the turn and the airplane continued a descending turn until it disappeared behind the terrain. This witness had flown the airplane in the past and stated that the airplane tended to "drop a wing" in turns which required a quick recovery. He also stated the airplane was a little "squirrelly" because it reacted quickly to control inputs. The accident pilot, who was not the owner, had not flown this airplane before the accident flight. The part owner stated they looked over the airplane and discussed the handling characteristics of the airplane, including the need to keep the airspeed above 50 mph and the engine above 6,000 rpm. The part owner stated that due to the pilot's size and the small cockpit, he had a difficult time getting in the airplane, so he adjusted the rudder pedals forward. He stated the control yoke was contacting the pilot's leg when he turned it to the left. The part owner stated that he made several modifications to the airplane at the registered owner's request. The modifications included changing the landing gear from a conventional to tricycle gear, installing different brakes, installing a different shaped fuel tank, changing and raising the seats, modifying the steering assembly and most recently changing the flaperons to a flap and aileron design. The airplane had been flown three times between the time the flight control design change was completed and the accident flight. The registered owner flew it twice and the part owner flew it once when he inadvertently became airborne during high speed taxi tests. The part owner stated they did not conduct any flight tests or aerodynamic performance calculations after the modification. He did not hold, nor was he required to have a repairman certificate for this make airplane. The last condition inspection recorded in the aircraft logbook was dated February 15, 2017. There were no entries in the logbook stating the airplane met the flight test requirements following a major modification as outlined in the Experimental Operating Limitations for the airplane. The Operating Limitations for the airplane state, in part, "…after incorporating a major change as described in 21.93, the owner is required to re-establish compliance… ." "…the owner shall make a detailed log book entry describing the change prior to the test flight… ." A review of the aircraft logbook did not reveal any such entries. Additionally, the part owner stated that he didn't feel the changes to the airplane were a major modification, because they made the airplane better than the original design.