XP-38:
Each engine had a General Electric B-1 turbosupercharger. To combat torque, the propellers rotated in opposite directions, a special version of the Allison engine being produced with a left-hand rotating propeller shaft. The engines had inwardly-rotating propellers.
YP-38 & production:
The propellers were outward-rotating rather than inward-rotating as on the XP-38. This improved the aircraft's stability as a gunnery platform.
Warren M. Bodie, in his book The Lockheed P-38 Lightning: The Definitive Story Of Lockheed's P-38 Fighter, states that, "Engine rotation was changed so that the propellers rotated outboard (at the top), thereby eliminating or at least reducing the downwash onto the wing centersection/fuselage juncture. There was, by then, no doubt that the disturbed airflow, trapped between the two booms, was having an adverse effect on the horizontal stabilizer.
The likelihood of an engine failure on take-off was considered less important than increasing the aircraft's capability as a fighter.
This P-38 experience is quite opposite to the case of the P-82 (twin Mustang). The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25. In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control.
This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved.
The experience did, however, prove the superiority of inward-rotating props for controllability & aerodynamic factors except in the P-38!
Last edited by GreenKnight121; 27th Nov 2009 at 00:31.