Fascinating and thank you!

On Sunday, January 31, 2016 at 6:07:16 AM UTC-8, Tango Whisky wrote:
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