Paul,
Thanks for the heads up on the P-39. Do you have any references to the problem(s) and cure?
Thanks,
Jim Rudholm

 From the book "V's for Victory".

Paul Lamar
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A New Generation of Fighter Aircraft

Overall, it had taken several months for Allison to both literally invent and develop the new type of hydraulic vibration damper sufficiently to complete the contract required engine qualification tests prior to authorizing the first flight. Using the first model of the vibration damper, work went ahead in preparation for the first flight. Active development of the hydraulic damper for the XP-39 continued until July 31, 1939 when the 35 hour qualification test was successfully completed.

After identifying the 1/2-order vibration from the long extension shaft, Allison at first argued that the Army should proceed with flight tests of the airplane and simply minimize operation in the high speed range where the vibration became objectionable. Wright Field objected, noting that should a single cylinder short-out during high power operation that 1112 of the power of the engine would be contained in the vibration, a condition they maintained as unsafe. This was confirmed by Allison on their workhorse engine when they demonstrated a violent shaking when the event was simulated. As a result Allison designed new extension shafts that were 30 percent stiffer by increasing them from 2-1/2 inch diameter and 5/32 inch
(0.156 inch) wall to 2-5/8 inch diameter and 0.200 inch wall. This, in combination with the further improved hydraulic damper, raised the system natural frequency and better dampened torsional vibrations so that there were no serious resonant speeds for torsional vibration encountered from 600 to 3100 rpm.127 The new extension shafts were not available prior to the airplane being sent to the NACA for wind tunnel testing, further suggesting that the airplane was never allowed to attempt a full power performance demonstration of its capability before being rebuilt as the XP-39B.

Finally, on April 6, 1939, the XP-39 flew for twenty minutes with Bell test pilot James Taylor, USNR, at the controls. The engine incorporated the hydraulic vibration damper, but not the stiffened extension shaft, so care was taken to minimize operation of the engine in the high vibration ranges.
John Kline, the Allison man assigned to the XP-39 relates that, "We had the Goo Damn'st Party that night at the Biltmore that you ever saw!"128

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I have a picture of my mother getting out of a P-39 in Iran in WWII at a
lend lease Russian/American base-I think it was called Sarchesma or some
such, and she was in the 'Persian Gulf Command' unit. She went up in it with
a Russian pilot, and did so, she told me, because it had a door and she
could get in 'daintily', although she consented after learning this plane
had a door at a dance the night before with the Ruskies (they drank a lot,
she said, and danced well......). She was an RN charge nurse at the base
hospital there near the border.
Here she is (on right) with her working buddy.


Marc Wiese


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