On May 9, 2020, about 0645 central daylight time, an experimental Titan Tornado S airplane, N318WH, was substantially damaged when it was involved in an accident about 3 miles north of Haskell Airport (2K9), Haskell, Oklahoma. The pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight. The student pilot stated that the airplane had experienced uncommanded yaw on previously flights which he was trying to eliminate. After some research and a conversation with the airplane kit manufacturer, he installed vortex generators. The vortex generators did not eliminate the yaw, so he installed makeshift winglets on the stabilator, which were made from ½ inch plywood and aircraft speed tape. He did not ask the airplane kit manufacturer about the use of winglets on the stabilator, nor was he required to. The student pilot conducted a test flight with the makeshift winglets and the uncommanded yaw was eliminated. Since the winglets proved successful, he created new winglets out of composite material, then attached them to the stabilator with glue and rivets. He completed 3 to 4 test flights with the new winglets and remained in the traffic pattern for all of the test flights. The airplane maintenance logbooks did not contain any entry for the winglets because the pilot was still conducting test flights and was not ready to have a mechanic sign off on the installation. The student pilot stated that on the morning of the accident, he intended to complete the first winglet test flight away from the airport and at a higher altitude than the other test flights. He departed from 2K9 and climbed about 600 feet per minute to 2,000 ft where he intended to level off. He stated that the airplane flew really well with no anomalies noted at that point. As he decreased the pitch attitude, the airplane suddenly vibrated aggressively and it "felt like the tail was thumping." He decreased the engine power to slow the airplane down and descend in altitude, but the thumping and vibrating continued. He pitched the airplane down for an off-field emergency landing. Before he was able to land, the airplane rolled inverted and descended into the trees. The responding Federal Aviation Administration (FAA) inspector stated that the airplane was found in a densely wooded area with the right wing separated and significant impact damage to the entire airplane. The stabilator control horn was found fractured and disconnected from the push-pull tube. A postaccident examination of the stabilator control horn revealed that it fractured due to overstress from gross mechanical deformation. The part also exhibited impact damage from the stabilator being actuated from control stop to control stop during the flight. The airplane kit manufacturer engineer stated that winglets added to the stabilator would change the balance of the control surface and cause flutter. The stabilator is installed with a counterweight calibrated specifically for the stock stabilator. He had never seen anyone add winglets to the stabilator before and he hoped that a builder would contact him before adding winglets. He would tell a builder not to add winglets because it would alter the balance on the control surface.