Akaflieg Karlsruhe AK-X

On Mon, 11 Dec 2017 00:55:27 +0000, Michael Opitz wrote:

It looks like someone may have tried to make the tips "all flying"
control surfaces, as they appear to be skewed off axis somehow.
Also, IIRC, those are elevons (not ailerons) that are missing. IIRC,
the Ho-IV used a system whereby an outboard spoiler was separately
deployed on one wing (or the other) for roll and yaw control. (in
addition to the elevons)

Yes, I was aware that this is a complex set of three control surfaces per
wing, but I'm a little confused about their function, but I think that,
listing from root to tip they were elevator,aileron,drag rudder. Is this
a fair description.

Dad found the praying mantis position ok, except for when he was at the
end of a long flying day, and his beard stubble started to become
irritated by the chin rest.

:-)

To be quite honest, I was 1-1/2 years old when Dad flew the 1952
nationals. I was left at home with friends. I never got to look at a
Ho IV cockpit with Dad in my adult life, so I can't answer your
questions. I'm sure that the fellow(s) who precisely rebuilt a Ho IV a
few years ago in Germany can answer those questions though.

Fair enough. I didn't realise you were so young at the time.

http://www.nurflugel.com/Nurflugel/H...s/ho_iv/ho_iv_
Restoration/body_ho_iv_restoration.html

Do you know if this replica was completed and flown? All I know about it
is in that article about the ongoing building project. About all I know
about it it that it is not the glider in NASM, which is the second IVb
prototype.

Dad made one or two 500 Km flights in the Ho IV during the 1952
nationals. That finished his Diamond badge. (#1 Germany, #10
International)...He was still a German citizen at the time, and did not
pick up his USA citizenship until a few years later, although that did
not stop the SSA/USA from also claiming his badge,
awarding him USA #6....

What a nice way to get Diamond distance!

BTW, have you see this article about the Ho S.IVb:

https://scalesoaring.co.uk/VINTAGE/D...n/Horten%20IV/
Horten_IVb.html

Lots of photos, good plan showing the three control surfaces per wing and
a useful write-up about construction, flying characteristics and where
the airframes went.

A bigger plan is downloadable from he

https://scalesoaring.co.uk/VINTAGE/D...n/Horten%20IV/
Horten_IV_model.html


Last but not least, there's a great write-up on the restoration of the
example in the Deutsches Museum, Munich:

http://www.twitt.org/HoIVrest.htm

It has decent cockpit photos and says exactly how the control yoke
worked: it slid back and forth on a central tube for pitch and rocked
from side to side for roll control.

FWIW, the example in NASM seems to be the restored S.IVb 2nd prototype,
though its described as a VI both by NASM and elsewhere. It seems as
though these are interchangeable names for the same design.

The best comprehensive list of Horten designs I've found so far is on

http://www.nurflugel.com under

http://www.nurflugel.com/Nurflugel/Horten_Nurflugels/
horten_nurflugels.html



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

Great stories Mike.

There is no greater gentleman in soaring (or aviation) than Rudy Opitz. I'm proud to have had the opportunity to know and fly with him during his life..

There's also Albion Bowers' work at NASA Armstrong on the Prandtl wing which has a lift distribution that appears to eliminate adverse yaw, and therefore (some of) the need for vertical stabilizers. Obviously Prandtl's ideas go way back to the early days of flight along with the above-described flying wing concepts. Not sure about the implications for crosswind landings and other practical concerns but it appears to work pretty well.

https://www.nasa.gov/centers/armstro...-106-AFRC.html

Andy Blackburn

On Mon, 11 Dec 2017 14:03:36 +0000 (UTC), Kiwi User
wrote:


Yes, I was aware that this is a complex set of three control surfaces per
wing, but I'm a little confused about their function, but I think that,
listing from root to tip they were elevator,aileron,drag rudder. Is this
a fair description.

Close.

One needs to know that the AK-X works in a completely different way
than any other flying wing ever designed. Despite its similar
appearance, it is not even similar to the SB-13 aerodynamics-wise.

1. The AK-X is a flapped wing. At low speeds, all (!!) control
surfaces move downwards, at high speeds all move upwards. Just like an
ordinary glider. The rudders are in the winglets.

2. Pitch control is done by the inner flaps which work the same way as
a canard. Pitch up: Control deflection down, and vice versa. Perfect
solution concerning lift-distribution.

3. Compare the wing sweep of the AK-X to other flying wings: It is
much greater. This shows good promise to get rid of the pitch axis
oscillations experienced by other flying wings and the CG sensitivity
that has plagued all flying wing designs so far.
However, it needs an extremely stiff wing , which has just become
possible in the last few years after the latest progress in carbon
fibre stiffness.
(Fun fact: The wing is so stiff that the structural test did not
result in the wing spar breaking but in a torsional fracture of the
wing shell!)



Comparison to previous flying wing designs:

All previous flying wings had one huge basic fault:
In order to pitch up (or to fly slow), you had to deflect the controls
up, therefore reducing airfoil camber and thus lift coefficient -
basically exactly the opposite of what you'd like to have
aerodynamically.

The wing of the AK-X works exactly like that of any flapped glider:
Low-speed flight: All flaps deflected "down"
High-speed flight: All flaps deflected "up"


The idea behind this aredoynamic design is, frankly spoken, a touch of
genius. It's the first ever flying wing design ever that in theory
will be able to compete with a conventional design in all areas of the
flight envelope up to very high speeds.

Plus, there are a couple of other benefits:
The wing uses conventional airfoils whose aerodynamic qualities can be
predicted well today. The flapped wing creates the same lift
coefficient as the wing of a conventional design, allowing high aspect
ratio and wing loading.
Behind the cockpit there's a 40 liter water tank (directly at the
center of gravity) and no other structural parts - pretty simple to
replace this tank with an angine and some serious battery capacity.


To me, the only remaining question is the influence of the wing sweep
on spanwise flow - but as I heard the guys are pretty optimistic so
far (they've got a 1:2 model flying with very good results).




Cheers
Andreas

Thank you so much Andreas. This project and Mu-31 are projects that seem like they have potential to develop new technology.


On Monday, December 11, 2017 at 12:41:31 PM UTC-8, Andreas Maurer wrote:
On Mon, 11 Dec 2017 14:03:36 +0000 (UTC), Kiwi User
wrote:


Yes, I was aware that this is a complex set of three control surfaces per
wing, but I'm a little confused about their function, but I think that,
listing from root to tip they were elevator,aileron,drag rudder. Is this
a fair description.

Close.

One needs to know that the AK-X works in a completely different way
than any other flying wing ever designed. Despite its similar
appearance, it is not even similar to the SB-13 aerodynamics-wise.

1. The AK-X is a flapped wing. At low speeds, all (!!) control
surfaces move downwards, at high speeds all move upwards. Just like an
ordinary glider. The rudders are in the winglets.

2. Pitch control is done by the inner flaps which work the same way as
a canard. Pitch up: Control deflection down, and vice versa. Perfect
solution concerning lift-distribution.

3. Compare the wing sweep of the AK-X to other flying wings: It is
much greater. This shows good promise to get rid of the pitch axis
oscillations experienced by other flying wings and the CG sensitivity
that has plagued all flying wing designs so far.
However, it needs an extremely stiff wing , which has just become
possible in the last few years after the latest progress in carbon
fibre stiffness.
(Fun fact: The wing is so stiff that the structural test did not
result in the wing spar breaking but in a torsional fracture of the
wing shell!)



Comparison to previous flying wing designs:

All previous flying wings had one huge basic fault:
In order to pitch up (or to fly slow), you had to deflect the controls
up, therefore reducing airfoil camber and thus lift coefficient -
basically exactly the opposite of what you'd like to have
aerodynamically.

The wing of the AK-X works exactly like that of any flapped glider:
Low-speed flight: All flaps deflected "down"
High-speed flight: All flaps deflected "up"


The idea behind this aredoynamic design is, frankly spoken, a touch of
genius. It's the first ever flying wing design ever that in theory
will be able to compete with a conventional design in all areas of the
flight envelope up to very high speeds.

Plus, there are a couple of other benefits:
The wing uses conventional airfoils whose aerodynamic qualities can be
predicted well today. The flapped wing creates the same lift
coefficient as the wing of a conventional design, allowing high aspect
ratio and wing loading.
Behind the cockpit there's a 40 liter water tank (directly at the
center of gravity) and no other structural parts - pretty simple to
replace this tank with an angine and some serious battery capacity.


To me, the only remaining question is the influence of the wing sweep
on spanwise flow - but as I heard the guys are pretty optimistic so
far (they've got a 1:2 model flying with very good results).




Cheers
Andreas

On Mon, 11 Dec 2017 15:48:44 -0800 (PST), "Jonathan St. Cloud"
wrote:

This project and Mu-31 are projects that seem like they have potential to develop new technology.

Absolutely.

Since you mentioned the Mu-31: Also an extremly interesting project,
much more consequent wing-root design than the JS-3, and since it's
otherwise identical to the ASW-27 it should be easy to compare the
benefits of the new design.

On Mon, 11 Dec 2017 21:41:25 +0100, Andreas Maurer wrote:

To me, the only remaining question is the influence of the wing sweep on
spanwise flow - but as I heard the guys are pretty optimistic so far
(they've got a 1:2 model flying with very good results).

If a description I read many years ago of what makes a Hoerner tip work
and why its beneficial is true, then the spanwise flow shouldn't be a
problem.

I know that a lot of tip shapes were described as Hoerner tips, but the
one I'm talking has:

- a minimum LE sweep of 10 degrees on the outermost wing panel

- a straight edge to the tip raked outward toward the TE at at least
30 degrees and should meet the TE at an acute angle,
i.e. not rounded off

- the upper surface curves down to meet the lower surface at an acute
angle

The idea was that the LE sweep promoted spanwise flow toward the tip,
which was encouraged to oppose the tip vortex rotation as it slid over
the convex tip profile. The pointed at the end of the TE anchors the tip
vortex while the roll-down of top surface flowing spanwise out along the
panel and down over the tip shape will tend to move the tip vortex
outward.

I used this tip design for many years on competition free flight F1A
gliders. It worked for me. It was notable that, while models with
conventionally rounded tips needed a lot of tip washout to prevent tip
stalling, my design worked best with unwarped [flat] tip panels.
Directional stability was good too. Minimal fin area is beneficial to F1A
performance and thermal centering, the optimum being just big enough to
kill dutch rolling tendencies. On my design the fin had to be reduced to
a surprisingly small size before the first signs of dutch roll appeared.
Benefits of solid balsa fins: you keep chopping bits off until the dutch
roll appears and then stick the last bit back on.


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

On Tue, 12 Dec 2017 01:52:24 +0000 (UTC), Kiwi User
wrote:

Hi Martin,

I'm rather worried about spanwise flow originating at the wing root
(similar to the SB-13) - but let's wait and see. The guys (and gals)
know their stuff.

The idea was that the LE sweep promoted spanwise flow toward the tip,
which was encouraged to oppose the tip vortex rotation as it slid over
the convex tip profile. The pointed at the end of the TE anchors the tip
vortex while the roll-down of top surface flowing spanwise out along the
panel and down over the tip shape will tend to move the tip vortex
outward.

I used this tip design for many years on competition free flight F1A
gliders. It worked for me. It was notable that, while models with
conventionally rounded tips needed a lot of tip washout to prevent tip
stalling, my design worked best with unwarped [flat] tip panels.
Directional stability was good too. Minimal fin area is beneficial to F1A
performance and thermal centering, the optimum being just big enough to
kill dutch rolling tendencies. On my design the fin had to be reduced to
a surprisingly small size before the first signs of dutch roll appeared.
Benefits of solid balsa fins: you keep chopping bits off until the dutch
roll appears and then stick the last bit back on.

On Wed, 13 Dec 2017 01:27:16 +0100, Andreas Maurer wrote:

Hi Andras,


I'm rather worried about spanwise flow originating at the wing root
(similar to the SB-13) - but let's wait and see. The guys (and gals)
know their stuff.

I notice that initial drawings put the wing at the bottom of the pilot's
pod, but in the 1:2 model its just below the canopy rim. Was this for
wing clearance or aerodynamics?

I'm a little surprised, too, at the quite minimal root fairings. Is this
what you were referring to when you mentioned spanwise flow at the root?

It would be interesting to see flow visualisation round them. Though, as
you say, the guys and gals know their stuff, so maybe cleaning up the
wing roots is being left for full size detailed design. After all, the
1:4 model had nothing except a couple of sensor probes at its root, so
just adding the pod was quite a big step aerodynamically.


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

On Wed, 13 Dec 2017 11:46:30 +0000 (UTC), Kiwi User
wrote:

On Wed, 13 Dec 2017 01:27:16 +0100, Andreas Maurer wrote:

Hi Andras,


I'm rather worried about spanwise flow originating at the wing root
(similar to the SB-13) - but let's wait and see. The guys (and gals)
know their stuff.

I notice that initial drawings put the wing at the bottom of the pilot's
pod, but in the 1:2 model its just below the canopy rim. Was this for
wing clearance or aerodynamics?

Several causes:
- mainly wing tip clearance: The wing tips are far behind the landing
gear and dihedral is only 2 degrees, they come down when the nose goes
up, creating ground clearance problems

- with the wing out of the way lots of space for a really strong nose
gear (one of the famous weak points of the SB-13)

- the wing spar is now over the knees of the pilot, leaving plenty of
easily accessible space for the controls, mixer and Haenle-type stick
between wing spar and instrument panel

- lots of space for the pilot (I'm 6'7" and fitedt comfortably in the
prototype fuselage on the Aro aviation fair)

In their own (German) words:
https://akaflieg-karlsruhe.de/ak-x/aerodynamik/



I'm a little surprised, too, at the quite minimal root fairings. Is this
what you were referring to when you mentioned spanwise flow at the root?

Well, aerodynamically speaking there is no need for wing root fairings
as long as the complete wing root is in an area of pressure rise.
On a conventional glider the wing is in the area of pressure loss
(aka: where the fuselage gets thinner), resulting in the need for a
wing fairing.

I'm not the designer of course, but I think you can be sure that we'll
get to know all the details in the future.

What I'm referring to can be seen on the SB-13:
http://www.airport-data.com/aircraft...00724103.html#

Look at the little red wing fences - afaik they had to be intruduced
to tame handling (stall?) characteristics.

(Bert: Dein Auftritt! Habe das SB-Buch gerade nicht vor mir, wo der
genaue Grund beschrieben wurde).


It would be interesting to see flow visualisation round them. Though, as
you say, the guys and gals know their stuff, so maybe cleaning up the
wing roots is being left for full size detailed design. After all, the
1:4 model had nothing except a couple of sensor probes at its root, so
just adding the pod was quite a big step aerodynamically.

Definitely.

BTW:
This is the project page of the AK-X 1/2 model:
https://akaflieg-karlsruhe.de/tag/12-modell/
On the spin onboard video you can nicely see how the inner flaps work
as elevator.

On Wed, 13 Dec 2017 21:50:57 +0100, Andreas Maurer wrote:

Several causes:
- mainly wing tip clearance: The wing tips are far behind the landing
gear and dihedral is only 2 degrees, they come down when the nose goes
up, creating ground clearance problems

I didn't think of that.

- with the wing out of the way lots of space for a really strong nose
gear (one of the famous weak points of the SB-13)

OK

- the wing spar is now over the knees of the pilot, leaving plenty of
easily accessible space for the controls, mixer and Haenle-type stick
between wing spar and instrument panel

Nice.

What I'm referring to can be seen on the SB-13:
http://www.airport-data.com/aircraft...00724103.html#

Look at the little red wing fences - afaik they had to be intruduced to
tame handling (stall?) characteristics.

OK, understood.

BTW:
This is the project page of the AK-X 1/2 model:
https://akaflieg-karlsruhe.de/tag/12-modell/
On the spin onboard video you can nicely see how the inner flaps work as
elevator.

Yes I found that this morning. Must be new: I've visited that page before
but don't recall seeing it then.

Fascinating to watch those inner flaps thinking they're on a canard!



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