Finally an answer to the P-38 prop rotation, courtesy of the good folk at the Aircraft Engine Historical Society.
When the XP-38 was written off in the crash at Mitchell Field it had a total of 11:50 hours on the airframe, 7:02 of that being the record breaking flight from Los Angeles to New York. Naturally, in the 4:48 hours devoted to testing, little had been achieved in envelope expansion, particularly the high speed regime where tail buffeting was first observed on the follow on YP-38. The XP had a short life of two weeks, first flight being the 27th January, 1939, and crashing 11th February. The YP was a substantially redesigned and differed greatly in detail to the XP, the first YP rolling off the production line in September 1940. Detailed wind tunnel testing during the design of the YP resulted in the production of the attached graph detailing power on/off stability with various prop rotation directions. Hence “Kelly” Johnsons comment the chosen direction of prop rotation made it a better gun platform. No consideration was given to any “critical” engine aspect. Interesting too that the direction chosen for the prototype was the worst possible choice, and that rotation in the same direction enhanced stability to some degree, albeit introducing torque effects and handling issues at the stall.
