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.

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

On 14/12/2017 07:50, 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

Same reason as the dihedral on 707/DC-8, etc tailplanes I understand

- 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

Spar OVER the pilot's knees?? Sounds like emergency exits might be
interesting - and not very rapid.

- 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/

--
GC

Yep, as I said in a post above, the oscillation was damped with an aft, but then the stall characteristics turned to nasty. Most of this nastiness was cause by spanwise flow during stall which caused the whole wing to stall very rapidly. We then installed the boundary layer fences (iirc two per wing), and things improved.

Le jeudi 14 décembre 2017 06:40:58 UTC+1, Tango Whisky a écritÂ*:
Yep, as I said in a post above, the oscillation was damped with an aft, but then the stall characteristics turned to nasty. Most of this nastiness was cause by spanwise flow during stall which caused the whole wing to stall very rapidly. We then installed the boundary layer fences (iirc two per wing), and things improved.

That should read aft cg.

There actually was another nasty effect (which probably will happen with the AK-X, too):
During the ground run of the aerotow, the SB13 would lift of before the tug (as usual with sailplanes). It would then eventually fly through the vortex which the tug leaves on the runway after rotation and pitch into the ground. This happened on the (inofficial) first flight, and the relation between vortex and pitch down was discovered on the video - there has been a thin layer of snow on the runway which made the vortex visible.
Same would happen if during tow the SB13 reached a somewhat low position and came to contact the downwash of the tug's wing and its vortex - the glider would pitch down and could only be stabilized in a typical low tow position.

So the counter measure was to aerotow with a long rope (80-100 m), brief the tug pilot to do a gentle rotation/lift-off, and stay in low tow position troughout the tow.