This Righter 4-0-34 engine, one of nine total built, was intended to power radio-controlled gunnery target drones and post-war, self-launching sailplanes. It is a four cylinder, two-cycle, simultaneously-firing (in opposing cylinder pairs) engine of 34 cubic inch displacement (554 cc) that made 17 horsepower at 3,800 rpm. Ignition was of the battery-points-dual coil type. The 4-0-34A with top-mounted, unthrottled carburetor was test flown in 1944 in the one-off, experimental Radioplane RP-9 aerial gunnery target drone which achieved 133 mph in flight. The 4-0-34A, with a low-mounted, throttled carburetor and pull-starter, was also used in the post-war, 1946 Nelson-Bowlus Bumble Bee, a two-place, retractable landing gear, self-launching sailplane. Video of the Bumble Bee can be found here:    • F 3520 The Nelson Bumble Bee/Dragonfl...  
At only 17 horsepower, the Righter 4-0-34 provided minimal flight performance in the Bumble Bee and was replaced with Nelson-built engines of 25 and 28 horsepower in the follow-on, limited-production Nelson Dragonfly aircraft. There are six known surviving Righter 4-0-34 engines, however as of July 2022, this is the only running example. Based on the geometry of the nose thrust bearing retainer, the rear case machined from aluminum billet, and the absence of a data plate or any evidence that a data plate was ever attached, this engine may have been an early experimental or development model. The internal condition of the engine indicates it had been refurbished before being sold as government surplus in 1945.