On September 17, 2017, about 0820 Pacific daylight time, a Trudel GP 4 airplane, C-GTPX and a Farnsworth RV-8 airplane, N248DF, collided in midair about 1 mile southeast of the Reno Stead Airport (RTS), Reno, Nevada. The GP 4's airline transport pilot and the RV-8's airline transport pilot were not injured. The GP 4 sustained minor damage to the propeller; the RV-8 sustained substantial damage to the right wing and aileron. The GP 4 was registered to a private individual and was operated as Race 96. The RV-8 was registered to the pilot and was operated as Race 26. Both airplanes were operated by the pilots under the provisions of Title 14 Code of Federal Regulations Part 91 as an air race flight. Visual meteorological conditions prevailed, and no flight plan was filed for either flight, which originated from RTS about 5 minutes prior to the accident.The pilot of the RV-8 reported that he was positioned in the number 7 slot of a line abreast formation during the start sequence for the sport medallion race. As the flight was completing their turn toward the race course, prior to the pace plane releasing the flight, he felt an air current push his airplane down; he reduced power and maneuvered to stop the closure toward the airplane in the number 6 slot. At this time, the pilot estimated he was about 5 ft low, and 3 to 5 ft in front of the number 6 airplane and estimated he had about 15 ft of wingtip clearance. About 5 to 7 seconds later, the pilot heard a buzz and thump sound, followed by an uncommanded roll to the left. The pilot stated that he was able to level the airplane and landed uneventfully on runway 32. The pilot of the GP 4 reported that he was positioned in the number 8 slot of the formation, during the start sequence for the sport medallion race; he was located to the right of the RV-8. As the flight descended toward the race course, he saw the RV-8 drop below his wing, and he reduced power and initiated a slight right bank in an attempt to back out of the formation. The pilot stated that shortly after, his airplane collided with the RV-8. Following the collision, he pitched upward and rolled to the right to avoid the surrounding airplanes. Subsequently, the pilot landed uneventfully on runway 26. The pilot located in the number 9 slot reported that he was located on the outside of the flight, and that they were entering the start of the chute via an echelon turn to the left, when he saw that both the RV-8 and GP 4 further behind the formation flight. The pilot stated that both the RV-8 and GP 4 were advancing forward toward the flight as the flight was beginning to fly a line abreast in preparation for release to enter the race course. The pilot further stated that the GP 4 moved forward, back into position very quickly. At that time, he saw the GP 4 try to slow or correct his position to the right using ailerons. Shortly after, the propeller on the GP 4 contacted the right aileron of the RV-8.