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Akaflieg Karlsruhe AK-X



 
 
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  #11  
Old January 31st 16, 03:07 PM posted to rec.aviation.soaring
Tango Whisky
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Posts: 402
Default Akaflieg Karlsruhe AK-X

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
  #12  
Old January 31st 16, 04:26 PM posted to rec.aviation.soaring
BobW
external usenet poster
 
Posts: 504
Default Akaflieg Karlsruhe AK-X

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.
  #13  
Old January 31st 16, 05:52 PM posted to rec.aviation.soaring
Jonathan St. Cloud
external usenet poster
 
Posts: 1,463
Default Akaflieg Karlsruhe AK-X

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

  #14  
Old January 31st 16, 06:47 PM posted to rec.aviation.soaring
Tango Whisky
external usenet poster
 
Posts: 402
Default Akaflieg Karlsruhe AK-X

Just a typo: the dummy fuselage was released at 2000 ft.
  #15  
Old January 31st 16, 08:27 PM posted to rec.aviation.soaring
[email protected]
external usenet poster
 
Posts: 60
Default Akaflieg Karlsruhe AK-X

Thanks Bert. Fascinating to read about such a special glider.
  #16  
Old February 1st 16, 12:40 AM posted to rec.aviation.soaring
Martin Gregorie[_5_]
external usenet poster
 
Posts: 1,224
Default Akaflieg Karlsruhe AK-X

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 |
  #17  
Old February 1st 16, 05:20 AM posted to rec.aviation.soaring
Michael Opitz
external usenet poster
 
Posts: 318
Default Akaflieg Karlsruhe AK-X

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


  #18  
Old February 1st 16, 05:42 AM posted to rec.aviation.soaring
[email protected]
external usenet poster
 
Posts: 172
Default Akaflieg Karlsruhe AK-X

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


  #19  
Old February 1st 16, 09:11 AM posted to rec.aviation.soaring
Tango Whisky
external usenet poster
 
Posts: 402
Default Akaflieg Karlsruhe AK-X

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.
  #20  
Old February 1st 16, 05:29 PM posted to rec.aviation.soaring
Jonathan St. Cloud
external usenet poster
 
Posts: 1,463
Default Akaflieg Karlsruhe AK-X

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.


 




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