Akaflieg Karlsruhe AK-X

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.

At 20:41 11 December 2017, Andreas Maurer wrote:

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


Andreas,

Thank you very much for the insights. If my father were still alive
today, he would be wanting to follow the progress very closely. The
aerodynamic genius of using the wing sweep to emulate canard
characteristics as opposed to using the sweep for high speed flight
is extremely intriguing. Being able to use current modern airfoils
while reducing drag considerably, should result in very noticeable
performance increases. Also, the advent of newer construction
materials and methods which enable the builders to achieve the
stiffness that is required to overcome other previous wing's
aeroelastic issues cannot be overstated.

Please post any progress updates here whenever you may hear of
them in the future....

RO

On Tue, 12 Dec 2017 03:30:19 +0000, Michael Opitz
wrote:


Thank you very much for the insights. If my father were still alive
today, he would be wanting to follow the progress very closely. The
aerodynamic genius of using the wing sweep to emulate canard
characteristics as opposed to using the sweep for high speed flight
is extremely intriguing. Being able to use current modern airfoils
while reducing drag considerably, should result in very noticeable
performance increases. Also, the advent of newer construction
materials and methods which enable the builders to achieve the
stiffness that is required to overcome other previous wing's
aeroelastic issues cannot be overstated.

Please post any progress updates here whenever you may hear of
them in the future....

I promise.
I happen to be at the Akaflieg Karlsruhe workshop two times per year
so with a little luck I can provide you with some updates if the
Akaflieg guys allow that.


But since we are talking:
Has your father ever talked about (or even flown) the Horten VI (the
24m glider)? I was always amazed that such a thing could be built in
the pre-carbon fibre aera.

There are a couple of reports about the IV, but I couldn't find any
halfways detailed source about the VI.

And, second question:
Do you know if the Horton guys aver considered winglets instead of the
drag ailerons?

Cheers
Andreas

At 14:03 11 December 2017, Kiwi User wrote:

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



As far as I know, the replica has not flown.

Thanks for the scalesoaring links, I had not seen them before.

RO

The New Zealand Wheihe

The New Zealand Weihe was restored and I beleive flown by Billy Walker some years ago and was hanging in the Terminal building at Queenstown until a few years ago. Not sure where it is now, it looked great. Apparently not what the council wanted in the terminal.
Tom Claffey
....