On Sun, 31 Jan 2016 06:07:14 -0800, Tango Whisky wrote:

Many thanks for that description.

I'd wondered how, when 'pecking' had a largely unknown onset condition
and no way known of exiting it, and in addition stalls led to spins that
reversed during recovery, the airframes and pilot(s) survived the years
long test series.

Yours is the first account I've seen to say that this was due to
parachute recovery. Now I can see how all this was possible, so thanks
for that.


I've been a member of Akaflieg Braunschweig during the construction
period of the SB 13. I did write the final eport, and did present the
flight testing on the SSA convention in 1988.

The idea came up because there was the possibility to develop laminar
airfoils with decent pitch stability. So it was guessed that without the
tail boom, there should be 10% increase in performance. At that time,
the SB 12 was the firt standard class glider exceeding 40:1.

During tests with a 1/3 scale model, there was a flutter coming up which
was the result of pitch oscillation coupled to a bending oscillation of
the wing. The solution was to employ - the first time ever in aviation -
high-modulus carbon fibers (instead of high-strength carbon fibers) in
the spar caps. This pushed the bending frequency of the wings well
beyond the flight enveloppe.

Main pain during the construction of the wing structure was the fact
that the wing connection was classic, but the wing was swept back 15
degrees, so that the spar caps would experience torsion. To evacuate the
torsion into the skin of the wings, we had 45 degree fabric layed up
over each layer of rovings. 8 hours of lay-up with a team of 8 for each
spar cap...

The incident during first fligh showed a problem with a swept back wing:
When the glider hit the stationary take-off vortex of the tug on the
runway, the inboard section of the wing stalled and the nose pitched
down. That was mitigated later by a 80 m rope, and by a very gentle
lift-off of the tug. However, whenever the SB 13 hit the propwash behind
the tug, it pitched down into the low tow position- no way to come up
again. So low-tow was standard procedure.

Flight tests showed a strong pitch oscillation for forward CoG
positions, with a frequency of about 1 Herz which are impossible for the
pilot to counter. Moving the CoG aft improved that, but the spin
behaviour was a real bitch. The reason is that the inboard wing stalls
first, and due to the sweep-back, the detached flow rapidly moves
outboard.
Solution to this was putting 2 boundary layer fences on the leading edge
of each wing.

The nose wheels was a very tiny structure (no place to put serious
steel), so any incident directly led to the workshop.

With no tail boom, the SB 13 was prone to receive a spring-operated
recovery system. It was extensively tested with a dummy fuselage and
telemetry, releasing it at various configurations at 200 ft from
underneath a helicopter. It worked pretty well, with 3 canopies of 1200
sqft each.

That recovery system had fixed lifetime of 15 years, so when it was
over, it was decided to stop the flights with the SB 13, and to give it
to a museum (Deutsches Museum in Unterschleissheim, I think).

Will be interesting to see how the AK-X will work around the pitfalls...

Bert Ventus cM TW



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