This experimental military two-seater helicopter, which first flew on 7th August 1944, differed radically from earlier Kellett designs and was nicknamed "Synchropter" because of its two intermeshing three-bladed rotors.
This configuration, familiarly known as "egg-beater" and used in 1938 by the German designer Flettner for his Fl.265, first appeared in the United States on this helicopter with blades which overlapped as the rotors turned in opposite directions.
The distance between the XR-8's two hubs was 1.22 metres, and power was directly transmitted to the rotors through pairs of bevel gears.
P.Lambermont "Helicopters and Autogyros of the World", 1958
Technical data for Kellett XR-8
Number of seats: 2, engine: 1 x Franklin O-405-9 rated 245hp, rotor diameter: 10.97m, overall length: 6.89m, height: 3.36m, weight fully loaded: 1347kg, empty weight: 1052kg, cruising speed: 136km/h, maximum speed: 160km/h, absolute ceiling: 2050m