At 23:40 31 January 2016, Martin Gregorie wrote:
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



--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

My father (P. Rudolf Opitz) was a German flying wing test pilot
specialist during the 1930's and 1940's. He got to fly a lot of what
was out there at the time. The Hortens had the same high aspect
ratio swept back wings, and they also had the same annoying and
dangerous pecking longitudinal stability issues that the SB 13
had. When Dad heard that They were building the SB 13, he
wondered if they had solved the pecking problem. When we didn't
hear much more after the initial flight tests, Dad rightfully
concluded that they had not solved the old Horten nemesis. Dad
always said that the Hortens designed beautiful looking aircraft, but
they were a real handful to fly. The pecking essentially limited them
to slow speeds only. Dad could not comprehend how the Hortens
thought they were going to go fast, when they couldn't sort out the
handling even at slow speeds. Dad did campaign a Ho IV in the
USA for the 1952 contest season. He finished 7th at the US
Nationals in Grand Prairie, TX, and won a couple of regionals. Like
the SB 13, take-off's were an issue. Dad solved the problem by
using a 300' (~92 meter) long tow rope, and a 300 Hp Stearman
tow plane (as compared to the regular 220 Hp). See this YouTube
clip at the 1:21 point for film of a take-off:
https://www.youtube.com/watch?v=0M84AKSyGZk
Notice, it is normal, and he is able to go right into a high tow
position. The pecking became uncontrollable at speeds above 85
mph, so Dad limited himself to a top speed of 85 mph. That, along
with the high powered tow plane and 300' rope allowed him to
safely campaign the glider for a full contest season. Afterwards, he
was also able to check out Dr. Raspet's pilot (Falvy) at Missisippi
State University for follow-on performance flight testing.

Before he passed away in 2010, Dad saw there was a beautiful Ho
IV restoration/(new build?) going on in Germany. The builders
never contacted us for advice, but he wanted them to know that if
they were thinking of flying it, they should use a powerful tow
plane, a 300' rope, and not to dare go faster than 85 mph... I have
never heard if they tried to fly that Ho IV after its completion or
not... It would be a shame if they broke it after all of that hard
work..

Mike Opitz - USA