Coordinated turning stall and spins

Posted this to the discussion on spinning Blaniks from a coordinated turning stall.

November 9, 2003
Turning Stalls and Insipient Spins

As promised, apropos to this discussion on spin entry from coordinated
turning stalls, I took a tow this morning to 5000 feet agl and
performed a series of coordinated and cross control turning stalls.

The aircraft used was a Ventus 2bx, delivered this year. I have
approximately 75 hours in this aircraft and about 525 hours total in
the model. I flew the glider at approximately 70% of the aft cg limit.
Wing loading was 7.8 lbs per square foot. All stalls were entered in
the first positive flap position.

My intention was as follows: to perform a series of turning stalls,
both coordinated and cross controlled, to determine the departure and
post departure characteristics of a modern fiberglass sailplane.
Stalls were entered gently and in a shallow bank (lower wingtip on
horizon). Whether coordinated or cross controlled, I fixed the
controls in the pre-departure position for three full seconds after
departure (that is, no attempt was made to recover immediately after
the stall break).

Once off tow I completed two clearing turns, then stalled the glider
wings level twice to establish attitude. I then entered a coordinated
shallow left turn and gently eased back on the stick. The stall broke
cleanly. The glider initially yawed about 30 degrees to the left,
dropped its nose through the horizon, then began to increase its bank
angle and gain speed. G forces accumulated and I recovered from the
spiral dive at about 80 knots and roughly 70 degrees of bank. (As
noted above, the elevator was held firmly aft and aileron and rudder
neutral until recovery was initiated.

I repeated the same maneuver to the right. The stall break was less
clean (more mushy). Development of the ensuing spiral dive was slower,
but airspeed and bank angle both accumulated until I released the
controls and initiated a recovery.

I repeated this sequence with like results.

I then entered a shallow bank turning stall (left) while skidding
slightly. As the low wing began to drop, I applied about ½ stick
travel to the right, ostensibly to raise the dropping wing. Entry into
the spin was immediate and dramatic. The glider yawed approximately
ninety degrees while dropping it nose to about 60 degrees below the
horizon. I left the controls in this position for a count of three
(one one thousand, two one thousand…) The glider completed
approximately 1.25 rotations before I initiated a recovery (stick
forward, ailerons neutral, opposite rudder, pull up from dive).

I repeated this process to the right. However, this time, I gently
accelerated the stall (achieving a slightly higher nose attitude
before departure). Once again, I skidded the turn (10 to 20 degrees),
and tried to pick up the low wing as it stalled, this time with full
deflection of the aileron. The ensuing spin entry was even more
dramatic. I was unable to measure rotation rate (even roughly) because
the glider's nose went immediately past vertical. As I lost the
horizon I became disoriented, until I looked out at the wingtip and
found the horizon again. I nonetheless fixed the controls for a count
of three. There was no noticeable g build up until I initiated a spin
recovery. Max speed during the dive was just above 120 knots, about 20
knots more than I typically see for a recovery from a fully developed
spin.

It should be noted that my glider has a flap redline of 80 knots. In
all cases, if airspeed exceeded 80 knots, I moved the flap handle to
the first negative position.

My interpretation: while the glider exhibited a yawing motion during
the coordinated turning stall, it did not auto rotate, nor did it show
any such propensity. Some pilots may find the dropping wing, yaw
motion, and reduced g force of a coordinated turning stall
disquieting, but when compared in sequence to an actual autorotation
leading to a fully developed spin, the prior is patently docile.
Height loss after an immediate recovery from a coordinated turning
stall using a release of back pressure and coordinated ailerons and
rudder could be measured in 10s of feet. The spin, however, from entry
to the bottom of the dive recovery was measured in hundreds. Loss of
height for the first spin, from entry, through development, to the
bottom of the recovery dive was 475 feet. The second: 750 feet.

Conclusions: draw your own.