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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My Dad related to me a number of times about P-39 Airacobra's in Edmonton.
Many were sent to the Russians, who liked them for air support. They sent
women pilots to Edmonton where they received a batch of new P-39's to fly
back. They used Blatchford Field as it was known then, now the Edmonton
Municipal. The city had a referendum and will be closing this airport soon.
The Indy Car races were run on the runways and taxi strips. Dad said once in a
while a P-39 would "wind up" and come apart in the air. Something to do with
the drive shaft he said. He just died this spring at 80.

Dale Davies`


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