Akaflieg Karlsruhe AK-X

On 12/9/2017 6:38 PM, Michael Opitz wrote:
I hope these students design and build a contest viable swept wing
sailplane.

Lets hope that, unlike the Horten gliders and their earlier SB-13 Arcus,
it doesn't have a nasty high-speed pitch oscillation, that seems to be
an unwanted feature of swept wing tailless aircraft.

I've seen flight reports about a Horten S.IV when it was being flown in
US comps that mentioned that a 'pecking' oscillation limited its
cruising speed and flight reports about the SB-13 show that it shared
this behaviour.

The DH.106 Swallow, an early British swept-wing jet, also showed the
same high speed oscillation. That killed Geoffrey de Havilland Jr. when
the pitch oscillation broke his neck during a high speed run in the 2nd
prototype and almost killed Eric 'Winkle' Brown in the 3rd - he said
that the oscillation was very violent and thought he only survived
because he was smaller than de Havilland and so didn't get his head
slammed into the canopy. When he slowed down below a critical speed the
oscillation stopped as suddenly as it had started


-- Martin | martin at Gregorie | gregorie | dot org

Martin,

That was my father who re-built and flew the Ho-IV in 1952 USA contests. He
limited himself to no more than 80 mph IAS in order to keep away from the
pitch "pecking" characteristics which made him uncomfortable. From the
late 1930's until the mid 1940's, he was one of the world's top flying wing
test pilots, so he knew exactly what he was doing when he flew the Ho-IV.

When I showed him the SB-13 designs and photos, the "pecking" was the
first thing he mused about before they even flew it. Once they flew it,
and it was established that it had the same issues, he just said that he'd
thought that they would have solved that issue by that time.

That is not to say that other designs did not have those issues, or had
not solved them earlier. My father and Heini Dittmar were the two
Lippisch primary test pilots who were with the Me 163 for basically its
entire existence. That team knew it wanted to go fast, and that slow
speed handling issues would only be amplified at higher speeds, so they
concentrated very hard on good handling characteristics on that bird. Their
results showed exceptional handling as reported by Capt Eric Brown among
many others. Dr. Lippisch and his team overcame those aerodynamic issues,
while the Hortens and the DH.106 Swallow apparently never did.

Dad was from the Lippisch camp, but through a unique set of circumstances,
he had also gained the trust of the competition, which was the Horten camp.
Thus, the Hortens let him fly a number of their creations as well. His
basic opinion was that the Hortens designed beautiful looking aircraft, but
the handling was always marginal at the very best, even in the slower speed
regimes. He never thought that the Hortens would succeed in high speed
flight as long as they didn't make vast improvements on their designs.

The Ho-IV which he flew in 1952 came from England. It had belonged to
Geoffrey de Havilland Jr. I don't know if it was the one that Robert
Kronfeld had brought to England, or if de Havilland had brought it there
from Germany after the war. Then, de Havilland sold it to another British
test pilot named Hollis Button who brought it to the USA, and promptly
lost control on his first take-off, thus crashing it. Now, Button was left
with a broken glider that nobody knew how to fix. He hunted Dad down after
he had heard that my father had the skills to fix it. Dad agreed to fix it
as long as he could fly it for the entire next (1952) year's contest
season. So, Dad repaired the Ho-IV over the 1951- 1952 winter and flew it
in several contests in 1952. Thereafter, the Ho-IV went to Mississippi
State University where Dr. Gus Raspet profiled the wings and made
performance measurements. After that, it was parked in a disassembled
state in various places, winding up in the desert SW USA. It was reported
that someone scavenged/stole the metal outer wing panels for use on some
experimental aircraft. Eventually, the Planes Of Fame Museum in Chino, CA
got it, and now have it hung on display in one of their hangars.
Unfortunately, it is very poorly restored with Styrofoam outer wing panels
in the wrong shape, as well as a rather grotesque looking replacement lower
fuselage/pod:

https://www.google.com/search?
q=planes+of+fame+Horten+IV&rlz=1C1CHWA_enUS602US60 3&tb
m=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwijoqS1lv7 XAhVjh
OAKHfxjArEQsAQIKA&biw=1280&bih=918#imgrc=_

Pictures from the Mississippi State flight performance testing, where Dad
had checked out Professor Dezso George-Falvy in order to continue the
program. These photos show how the restored glider really should look:

http://www.nurflugel.com/Nurflugel/H...s/ho_iv/Falvy_
Pics/falvy_pics.html

My own personal guess on the "hunting/pecking" problem is that it probably
resides with aero-elastic issues and the swept back high aspect ratio wing.
Given the newest carbon-kevlar+? materials, maybe the bending and torsional
stiffness of a newer construction wing can help overcome some of these
issues.... We will see if and when the SB-13 gets into the flight testing
phase...

RO


Thanks for taking time to write this up, Mike. It happens to be stuff I've
been interested in and wondered about for many years!

Bob W.

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