P=39's and flat spins.

"Gordon" wrote in message
...
Tumblin' P-39s

I've read the factory findings about the in depth testing done to
determine
whether this condition actually existed and if so, how to correct it. No
amount of adjustment resulted in the factory test pilots to duplicate the
manuever described by so many combat pilots. Frustrated, the factory
asked the
air corps to provide one or more combat pilots who said they had
encountered
'tumble' to come to the factory and assist in recreating the situation
necessary to cause it. After several test flights with various data
points
matching what the combat pilots reported, no one was ever able to force a
P-39
to tumble. I read this in a 'lessons learned'-type report generated after
the
extensive testing had been completed.

I think the "tumble" stories were a direct result of what uninitiated and
green pilots encountered when they caused the Cobra to depart while coupled
into accelerated stall and an extreme rearward cg due to the gun cans
problem. An accelerated departure in any fighter can be "interesting".
With the cg aft far enough, the airplane can exhibit some wild antics as it
goes through post stall and develops increasing yaw rates. With roll
introduced as this is happening, I wouldn't be at all surprised with a very
high disorientation factor in a young inexperienced pilot in this airplane.
It would be quite easy for an inexperienced pilot to interpret these wild
antics as a "tumble"!!!
I don't see any way the airplane would have swapped ends though. To do this
requires a tremendous impetus force and a very high energy state, which
would be inconsistent with the aerodynamics found in an ever decreasing turn
radius caused by a decreasing TAS through the arc of the turn as angle of
attack increased up to and past CLmax causing a departure. The resulting
stall from this attitude would most likely have been fairly violent in the
Aircobra with an aft cg, and depending on where the ball was as the stall
commenced, either a high wing roll off or a low wing roll off into the
departure would have been a possibility in the P39, which had positive
static and dynamic stability when in the cg envelope, but with the cg aft
due to ammo expenditure, that roll off could easily have scared the living
crap out of an amateur! In the disorientation through the departure, through
the incipient post stall stage and throwing in a fully developing spin mode
going flat with a coincident extremely high yaw rate associated with this
scenario; a young pilot could easily think the airplane had "tumbled"
Actually, many pilots don't know this, but to get a prop airplane to tumble,
you have to meet specific criteria. Gyroscopic precession from the prop disk
is critical to getting an airplane to tumble, which means power!!! This
means the airplane has to be carefully set up before a tumble potential can
exist. For example, to tumble a Pitts, you set up a 45 degree up line and
slow the airplane below Va with FULL POWER. Then you nail full right rudder
with full left aileron and forward stick all at once!! This causes a 3 axis
inertia coupling that vectors on the gyroscopic precession from the prop 90
degrees ahead of the disk axis as the airplane is being maneuvered in
positive pitch. With all three axis coupled, the damn airplane develops a
mind of it's own and all you do is hang on for dear life as the energy
dissipates through whatever the airplane has decided to do without
consulting you :-))
Now, THAT'S a tumble!!! :-))
Personally, I can see no way that an aircraft configured like the P39 could
be set up to tumble at departure...violent yes...but I seriously doubt end
over end....even with an aft cg. Bell would have picked up on this right
away and they didn't. Hell, this is hard enough to set up in a Pitts, much
less the Aircobra.
My guess is a violent departure, lived through and retold by a VERY
"excited" novice :-)
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/CFI
Retired