P-38 Prop Rotation
 

Any history guru able to answer the question as to why the rotation of the props were reversed from the prototype to subsequent aircraft? On the prototype the top of the props rotated towards the fuselage, subsequent aircraft the top of the props rotated away from the fuselage. A reference would be handy if available. Thanks in advance.

The P38 (Craig Wall)

SD

Thanks SD, I had seen that in fact, "it made a better gun platform", but still begs the question "why?".

There was a discussion on this topic on the key publishing forum a while ago, I can't remember the outcome but you could search for it. I seem to remember the general consensus being that it was not ideal as it left both engines as being 'critical' in regards to engine out performance and airflow over the rudders - or something like that. I'm not an aerodynamicist so wouldn't have a clue myself. Paul

From the fact that they experimented with every possible combination of rotation, it would appear that no-one actually knew, or knows, why this should be the case. It was felt, on purely empirical evidence, rather than theoretical, to be so.

SD

There's a discussion about propeller rotation of twin-engined aircraft on the Test Pilots (top of the list at the moment) area of this forum which I hope will help.

Aerials

Brian,I`ve looked through my books and can`t find anything definitive,except I read ,perhaps in an American mag. that the rotation was changed,(swop engines across) because of vibration,or slipstream interference on the tail,which may have been because of the twin-booms.I don`t think it was fully cleared until the `J`and`L` models.Various other mods were made,including `spur` gearing,raising of the thrust-line,,intakes reshaped,change of props to Curtiss Electric,raising tailplane incidence,and electric airbrakes,and `beard` radiators..reference for that is `Famous fighters of WW2`,by William Green.

Thanks all, I've managed to source a magazine from an authoritative organisation which I hope will put the question to rest. Will report upon receipt.

I had heard that it was due to vibration when the props rotated inwards. No proof or anything, sorry.

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.
http://i101.photobucket.com/albums/m...m227/M0001.jpg

Would be interesting to see where the -322-61 Lightning 1(RAF) featured on the graph ,as both props rotated CW(from the cockpit).