Akaflieg Karlsruhe AK-X

[Jonathan St. Cloud]

Has anyone seen the Akaflieg Karlsruhe AK-X photos. They have flown a scale model and it looks very interesting. Wonder how it will compare to "traditional designs"? If it proformed as well or better than moe traditional design I would purchase one.

[Glidingstuff[_2_]]

From the website

"The time has come, after years of advanced development, testing, evaluation of concepts, construction of models, discarding and regenerating ideas the AK-X project is rising to a new level: We build the man-carrying flying wing of 15m glider racing class!"

https://akaflieg-karlsruhe.de/2016/01/fraesarbeiten-bei-dg-flugzeugbau/

DG have started making the full size molds for them on their CNC machine.

Looks interesting,

Paul

[Craig Funston]

On Wednesday, January 27, 2016 at 2:46:10 PM UTC-8, Glidingstuff wrote:
> From the website
>
> "The time has come, after years of advanced development, testing, evaluation of concepts, construction of models, discarding and regenerating ideas the AK-X project is rising to a new level: We build the man-carrying flying wing of 15m glider racing class!"
>
> https://akaflieg-karlsruhe.de/2016/01/fraesarbeiten-bei-dg-flugzeugbau/
>
> DG have started making the full size molds for them on their CNC machine.
>
> Looks interesting,
>
> Paul

Interesting project. I remember seeing some interesting flutter modes on the similar SB-13.

[Jonathan St. Cloud]

Any idea what happened to the SB-13? How did it fly, what was it's performance...?

On Wednesday, January 27, 2016 at 4:50:40 PM UTC-8, Craig Funston wrote:

> Interesting project. I remember seeing some interesting flutter modes on the similar SB-13.

[Dan Daly[_2_]]

On Friday, January 29, 2016 at 10:43:28 AM UTC-5, Jonathan St. Cloud wrote:
> Any idea what happened to the SB-13? How did it fly, what was it's performance...?
>
> On Wednesday, January 27, 2016 at 4:50:40 PM UTC-8, Craig Funston wrote:
>
> > Interesting project. I remember seeing some interesting flutter modes on the similar SB-13.

https://en.wikipedia.org/wiki/Akaflieg_Braunschweig_SB-13_Arcus

[Jonathan St. Cloud]

I did see this, plus the short write up at Deutsches Museum Flugwerft web site. Was looking for someone with personal knowledge of this program or a technical write up of the program.

On Friday, January 29, 2016 at 7:56:26 AM UTC-8, Dan Daly wrote:
> On Friday, January 29, 2016 at 10:43:28 AM UTC-5, Jonathan St. Cloud wrote:
> > Any idea what happened to the SB-13? How did it fly, what was it's performance...?
> >
> > On Wednesday, January 27, 2016 at 4:50:40 PM UTC-8, Craig Funston wrote:
> >
> > > Interesting project. I remember seeing some interesting flutter modes on the similar SB-13.
>
> https://en.wikipedia.org/wiki/Akaflieg_Braunschweig_SB-13_Arcus

[Martin Gregorie[_5_]]

On Fri, 29 Jan 2016 11:34:02 -0800, Jonathan St. Cloud wrote:

> I did see this, plus the short write up at Deutsches Museum Flugwerft
> web site. Was looking for someone with personal knowledge of this
> program or a technical write up of the program.
>
> On Friday, January 29, 2016 at 7:56:26 AM UTC-8, Dan Daly wrote:
>> On Friday, January 29, 2016 at 10:43:28 AM UTC-5, Jonathan St. Cloud
>> wrote:
>> > Any idea what happened to the SB-13? How did it fly, what was it's
>> > performance...?
>> >
>> > On Wednesday, January 27, 2016 at 4:50:40 PM UTC-8, Craig Funston
>> > wrote:
>> >
>> > > Interesting project. I remember seeing some interesting flutter
>> > > modes on the similar SB-13.
>>
>> https://en.wikipedia.org/wiki/Akaflieg_Braunschweig_SB-13_Arcus

Its also covered pretty well, with 3-view drawings, in Martin Simon's
book "Sailplanes 1965-2000".


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

[Paul T[_4_]]

If you have FSX or FS2004 you can also fly the SB13. Download here:
http://simviation.com/1/browse-Gliders+and+Ultralites-130-5
Seem to remember reading the SB13 was fairly tricky to fly hence it's
early retirement, and the performance was somewhat disappointing.

[Martin Gregorie[_5_]]

On Fri, 29 Jan 2016 23:39:37 +0000, Paul T wrote:

> If you have FSX or FS2004 you can also fly the SB13. Download here:
> http://simviation.com/1/browse-Gliders+and+Ultralites-130-5 Seem to
> remember reading the SB13 was fairly tricky to fly hence it's early
> retirement, and the performance was somewhat disappointing.

Yes, indeed. The students spend 6 years working on a 1/3 scale RC model
and building it, followed by another 2 years making modifications and
retesting the aircraft, but they found that it remained tricky to fly
(stalls with a rear CG usually resulted in a spin and stalls with a
forward CG resulting in the glider 'pecking'. Unless spin recoveries were
very precise the glider often flicked into the opposite spin instead of
recovering. Its behavior on tow was diabolical, it was very tricky to fly
and, as its performance was no better than contemporary Standard Class
gliders they gave it up as a bad job and put it in a museum.

'pecking' was a large amplitude, rapid pitching cycle whose causes and
mitigation were never understood.

FWIW this is a rapid summary of the two page description in "Sailplanes
1965-2000", which also includes an excellent colour photo and a full
page, dimensioned 3-view drawing of the SB-13.


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

[Andreas Maurer]

On Sat, 30 Jan 2016 02:06:21 -0000 (UTC), Martin Gregorie
> wrote:

>On Fri, 29 Jan 2016 23:39:37 +0000, Paul T wrote:
>
>> If you have FSX or FS2004 you can also fly the SB13. Download here:
>> http://simviation.com/1/browse-Gliders+and+Ultralites-130-5 Seem to
>> remember reading the SB13 was fairly tricky to fly hence it's early
>> retirement, and the performance was somewhat disappointing.
>
>Yes, indeed. The students spend 6 years working on a 1/3 scale RC model
>and building it, followed by another 2 years making modifications and
>retesting the aircraft, but they found that it remained tricky to fly
>(stalls with a rear CG usually resulted in a spin and stalls with a
>forward CG resulting in the glider 'pecking'. Unless spin recoveries were
>very precise the glider often flicked into the opposite spin instead of
>recovering. Its behavior on tow was diabolical, it was very tricky to fly
>and, as its performance was no better than contemporary Standard Class
>gliders they gave it up as a bad job and put it in a museum.
>
>'pecking' was a large amplitude, rapid pitching cycle whose causes and
>mitigation were never understood.
>
>FWIW this is a rapid summary of the two page description in "Sailplanes
>1965-2000", which also includes an excellent colour photo and a full
>page, dimensioned 3-view drawing of the SB-13.


.... and here are a couple of videos that illustrate some of the major
problems:

First flight:
https://www.youtube.com/watch?v=GY6FJZF-hS4


First (and only) winch launch attempt:
https://www.youtube.com/watch?v=WqlYORF0-oQ

Note the nose gear, and learn some German curses.


Cheers
Andreas

[Tango Whisky]

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

[BobW]

On 1/31/2016 7:07 AM, 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
>

And the above is one of those diamonds years of mining RAS sometimes
yields...thanks much, Bert!!!

Bob W.

[Jonathan St. Cloud]

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

[Tango Whisky]

Just a typo: the dummy fuselage was released at 2000 ft.

[[email protected]]

Thanks Bert. Fascinating to read about such a special glider.

[Martin Gregorie[_5_]]

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 |

[Michael Opitz]

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

[[email protected]]

Mike,
Thanks for the great history lesson.

Great information all around in this thread.

-Tom (The high school kid who helped you rig the ASW-15 when you flew it in Tucson '74-'75)

On Sunday, January 31, 2016 at 8:30:10 PM UTC-8, Michael Opitz wrote:
> 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

[Tango Whisky]

Le lundi 1 février 2016 00:43:41 UTC+1, Martin Gregorie a écrit*:
> 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 |

Don't get me misunderstood - the recovery system was extensively tested on the dummy fuselage, and it's final version was mounted into the SB 13. However, this final recovery systems had never to be deployed.
I maybe forgot to mention that the tests showed the descent rate under canopy to be around 5 m/s. Now, for the pilot that would still be a nasty impact, but by all means survivable. For the wings, this impact speed would have been fatal because the high-modulus fibers of the spar caps were known not to stand shock loads. So, if the recovery systems would have been deployed one single time, the SB 13 would have gone to the trash bin.

The pitch oscillation was triggered by any small excitation, but it was stable, i.e. the amplitude did not increase. It was just described as unpleasant.

[Jonathan St. Cloud]

Thanks so much for the information. I did see a youtube video of a spin that stopped in one direction and started in the next.

On Monday, February 1, 2016 at 12:11:12 AM UTC-8, Tango Whisky wrote:
> Le lundi 1 février 2016 00:43:41 UTC+1, Martin Gregorie a écrit*:
> > 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 |
>
> Don't get me misunderstood - the recovery system was extensively tested on the dummy fuselage, and it's final version was mounted into the SB 13. However, this final recovery systems had never to be deployed.
> I maybe forgot to mention that the tests showed the descent rate under canopy to be around 5 m/s. Now, for the pilot that would still be a nasty impact, but by all means survivable. For the wings, this impact speed would have been fatal because the high-modulus fibers of the spar caps were known not to stand shock loads. So, if the recovery systems would have been deployed one single time, the SB 13 would have gone to the trash bin.
>
> The pitch oscillation was triggered by any small excitation, but it was stable, i.e. the amplitude did not increase. It was just described as unpleasant.
> The spin behaviour was discovered during a demonstration flight at about 1500-2000 ft, and this was the only time that the recovery systems came close to being deployed. Subsequent tests where then done at 7'000-10'000 ft, with the results being impressive, but no longer frigthening. Finally, the boundary layer fences did the trick.

[Craig Funston]

On Monday, February 1, 2016 at 8:29:39 AM UTC-8, Jonathan St. Cloud wrote:
> Thanks so much for the information. I did see a youtube video of a spin that stopped in one direction and started in the next.
>
> On Monday, February 1, 2016 at 12:11:12 AM UTC-8, Tango Whisky wrote:
> > Le lundi 1 février 2016 00:43:41 UTC+1, Martin Gregorie a écrit*:
> > > 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 |
> >
> > Don't get me misunderstood - the recovery system was extensively tested on the dummy fuselage, and it's final version was mounted into the SB 13. However, this final recovery systems had never to be deployed.
> > I maybe forgot to mention that the tests showed the descent rate under canopy to be around 5 m/s. Now, for the pilot that would still be a nasty impact, but by all means survivable. For the wings, this impact speed would have been fatal because the high-modulus fibers of the spar caps were known not to stand shock loads. So, if the recovery systems would have been deployed one single time, the SB 13 would have gone to the trash bin.
> >
> > The pitch oscillation was triggered by any small excitation, but it was stable, i.e. the amplitude did not increase. It was just described as unpleasant.
> > The spin behaviour was discovered during a demonstration flight at about 1500-2000 ft, and this was the only time that the recovery systems came close to being deployed. Subsequent tests where then done at 7'000-10'000 ft, with the results being impressive, but no longer frigthening. Finally, the boundary layer fences did the trick.

There's a very nice technical article about the AK-X in the current RC Soaring Digest http://www.rcsoaringdigest.com/. Page 27

Translated from German by Google translate it was.