AK8 1/3.5 scalish 5 meter scratch build - RC Groups
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Dec 31, 2009, 10:00 AM
dare to thermal
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AK8 1/3.5 scalish 5 meter scratch build


Based on an very old fuselage from an DG-300 in october 2007 i´ve started to build a semi-scale AK8 with my flying buddy (yes, we build two planes!).
The AK8 is an design from the Akaflieg Karlsruhe http://www.akaflieg.uni-karlsruhe.de...ebersicht.html , based on an DG-600 fuselage. The original AK8 is a standard class plane (15m, ailerons only).
I like the lines of the AK8 with the straight trailing edge, but i want flaps...
I´ve stretched the wings a bit and will use six wing flaps: flap1, flap2, aileron.
So it´s more a AK8-18m or an AK8-open
The wings were build from foam and sheeted with veneer (koto), but with full carbon cloth under the veneer and with a build-up spar for 25g. Wing joiner is a 20mm full carbon rod with 6deg dihedral.

Today was the first time we assembled one of the planes!
I hope this will push our motivation to finish the planes!

Bernd
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Dec 31, 2009, 10:11 AM
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pilotfrank's Avatar
Good looking bird! Do you have pictures of the build? Would like to see some of the carbon spar and wing joiner, in particular.
How many hours did you put into this project so far?

Frank
Dec 31, 2009, 11:06 AM
dare to thermal

Wing spar


We have made an easy plug to pre-fabricate the spar caps. The caps were made of 39 rovings at the wing root. We have used a little tool to get those hundreds of meters of rovings saturated.
http://www.cb-roter-baron.de/downloads.html (Tränkvorrichtung) - http://www.cb-roter-baron.de/Traenkvorrichtung.JPG
The shear web was made with glas socks over foam and glued together with the spar caps inside the spar channel of the wing. Finally we wraped the spar with kevlar.
A lot of work!

How many hours? Hundreds.... but a lot of time we wasted with bad quality of foam we ordered, the approach to build wing joiners by ourselves, new technology (spar building) and so on... Everything what could go wrong, went wrong!

Bernd
Dec 31, 2009, 11:32 AM
TEAM HORIZON
Lbuff1's Avatar
"everything that could go wrong, went wrong"

Oh..... I see you met Mr Murphy.

The plane looks great, I'm sure it will be worth the effort when you're in the air.

LEN
Dec 31, 2009, 12:17 PM
yyz
yyz
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Hinreißend!


What a beautiful glider. The planform is amazing. I have a few questions:

1) Why did you wait so long to share this build?

2) Which airfoil or airfoils are you using on the wing?

3) Did you build any washout in the wingtips?

About things going wrong, there is a saying in the US that "It's not how far you fall but how high you bounce". Not sure if there is a German equivalent but it looks like you and your partner have bounced very well.

Looking forward to following this build,

Mike

ps: I love the little machine for saturating the carbon tow


Quote:
Originally Posted by Lbuff1
"everything that could go wrong, went wrong"

Oh..... I see you met Mr Murphy.

The plane looks great, I'm sure it will be worth the effort when you're in the air.

LEN
Dec 31, 2009, 12:42 PM
Deniable plausibility
Shedofdread's Avatar
Very elegant looking sailplane. Looking forward to seeing flying photos.

Happy New Year!

S
Dec 31, 2009, 01:19 PM
dare to thermal

airfoils


"1) Why did you wait so long to share this build?"
I didn´t thought it took sooooo long time. My buddy sometimes remind me: "we will need ten building sessions...." . But we met Mr. Murphy

"2) Which airfoil or airfoils are you using on the wing?"
Its an A-7026 strak as discussed in the german rc-network forum
A-7026 2.5/14
A-7026 2.5/11.4
A-7026-(3)
A-7026-(5)
A-7026-(5)

"3) Did you build any washout in the wingtips?"
no. May be i should ?

thx for the saying. There is no similar in german. I like it!

Happy new year!

Bernd
Last edited by Bernd Brunner; Jan 02, 2010 at 10:06 AM.
Jan 02, 2010, 10:30 AM
dare to thermal

Rudder


The rudder is removable. Hinged as usual with a tube and a carbon rod.
The rudderhorn is devided in two parts: one part hinged at the fuselage,the counterpart glued into the rudder. Push-Pull cables will be attached to the part at the fuselage.
btw: weight of the rudder is 22gr

Bernd
Last edited by Bernd Brunner; Jan 03, 2010 at 11:33 AM.
Jan 02, 2010, 11:51 AM
yyz
yyz
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Wing planform drawings


You really don't get a feel for how nice the wing planform of this glider is until you look at the PDF file you created and posted.

The trend toward broader wing tips is really interesting. The eta, EB29, AK8 and other gliders are probably the shape of things to come.

Keep up the good work. It's really looking nice,

Mike
Jan 02, 2010, 01:14 PM
dare to thermal
I do remember, that similar wingtips and this planform was developed by Wil Schuemann in 1983(!!). and published in [“A New Wing Planform and
Improved Low-Speed Performance,” Soaring, Vol. 47, No. 2, Feb. 1983, pp. 16-26.]
27 years ago!

I´m really looking forward to the flights.
Ther is an interesting point, i forgot to mention. With this planform the CG is moving backwards to 140mm (root chord is 260mm). If my calculations are correct, additional nose lead will be not neccessary! But we will see..

Bernd
Jan 02, 2010, 01:29 PM
yyz
yyz
Registered User
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Yes, absolutely. I remember that period as well. Someone, maybe Schuemann himself, modified an ASW-12 to have that plan form if my memory serves me.

The new trend I was referring to is for the tips to be broader so that the crescent form leading edge doesn't end up in a pointed wing tip. This gives the nice backward sweep of the tips, like your AK8. There was some discussion about this in one of the eta design discussions but I can't remember exactly what the issue was they were trying to address. Maybe keeping the Reynolds numbers higher at the tips (less drag)?

Regardless of the reasoning, aesthetically it looks incredible!

Mike


Quote:
Originally Posted by Bernd Brunner
I do remember, that similar wingtips and this planform was developed by Wil Schuemann in 1983(!!). and published in [“A New Wing Planform and
Improved Low-Speed Performance,” Soaring, Vol. 47, No. 2, Feb. 1983, pp. 16-26.]
27 years ago!

I´m really looking forward to the flights.
Ther is an interesting point, i forgot to mention. With this planform the CG is moving backwards to 140mm (root chord is 260mm). If my calculations are correct, additional nose lead will be not neccessary! But we will see..

Bernd
Jan 06, 2010, 05:12 PM
dare to thermal

Elevator vs. Mr. Murphy


A few days ago i have worked on the elevator. Cause the hingline of the left and right half is not straight, i decided to put two servos hs125 in. I cut the servoholes and put some carboncloth in. But then i thought it would be a good idea to put the hole thing on the radiator to accelerate the curing of the epoxy. Next moring: big surprise... the elevator look like a banana - skewed.
I was able to remove the warpage with some more heat and weight. But we will see what happens on a hot day in summer.

Bernd
Jan 06, 2010, 05:27 PM
dare to thermal

Root rips


Today i glued the root rips into the wings.
For alignment, i was using a straight wing-rod and two additional rods (instead of the two pins) with distance tubes. Hope it will be straight.

Bernd
Feb 07, 2010, 03:12 PM
dare to thermal

Wing saddle


the last weeks i´ve worked on the new wing saddle. The old fuselage i´m using was made with a differnt airfoil and a smaller chord. I´ve attached a new ply rib and reshaped the wing saddle.

The whole process was far more complicated than planned... the root rip and the new rip for the fuselage were made and drilled together for perfect fit a year ago. But now the root rip was bend a bit. Cause of this the alignment pins didn´t fit - the fit was too tight. So, what to do?

I´ve expanded the holes in the fuselage, and glued in short brass tubes. Now, the wings fit perfect to the fuselage.

Bernd


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