Improve on the Gottingen 398? - RC Groups
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Feb 03, 2004, 03:19 PM
Obstinate spurious drone
jeff edmondson's Avatar

Improve on the Gottingen 398?

I am building a 1:6 scale Klemm L.20 (spans 85"). It was a popular German lightplane from the early 1920's. The wood/fabric airframe was great for aero clubs. It performs basically as a motor- glider with moderate loops and rolls. German WW1 ace Ernst Udet gave many aerobatic show in them between the wars.

The full size craft used a Gottingen 398 section. FYI it is like a bloated Clark Y- almost flat bottom, 13.75% thickness, 4.41% camber. This provided both high lift (power was below 50 HP) and allowed a very tall primary spar. This was early in monoplane design and wood spars were not very capable in thin foil sections.

For scale appearance reasons I want to preserve a reasonably thick section like a Clark Y. The plane will fly in as close to scale fashion as possible (no prop hangs, or sustained inverted flight). That means slow and smooth aerobatics and some thermal
gliding too.

It will be built up wood construction, as the 1:1 was, so holding 2 thou on the section is the limit- no tooling with composites here.

Should I stick with the Goe 398 section. Are there other similar, but better sections I should consider.

FYI specs: max AUW 4.0 lbs., 12x8 MAS wood prop, Astro 020 5T @ 12V w/ 4.4:1 G.B., Kokam 2000's 3S2P, Phoenix 35 (BEC to drive 4 micro servos).

Any comments welcome.
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Feb 03, 2004, 03:42 PM
Ascended Master
Sparky Paul's Avatar
Why not stick with the 398?
You want the look, and it obviously isn't something you'd select for a trans-oceanic airplane, so looking for "better" wouldn't accomplish much.
Feb 08, 2004, 01:15 PM
Visitor from Reality
If you really want 'scale', stick with the Go 398 - otherwise you're going to lose a lot of the appearance around where the fuselage and wing meet. As you are going for a scale flight envelope, probably not a lot to be lost in the flying by doing so.

Once you change the root airfoil, you might as well toss all the scale stuff out the window and design a 'sports' wing to the same planform as your prototype.

Be interested in how you get along with this model - seems a collosal amount of airframe behind not a lot of motor! Light weight and low power suggests a calm weather only flier - something I can relate to, as I hate going out flying when its windy

In terms of span, you're looking at the same span as my 1/5th Cub, which weighs twice as much as your target and has 16 3000mA cells!

Good luck with your project - we need these more than more Cubs


Feb 08, 2004, 01:28 PM
Obstinate spurious drone
jeff edmondson's Avatar

Keep the Goe398

Paul & Dereck thanks for your replys.

I am going to stick with the scale section. I does provide a great deal of lift.

Comparing MotoCalc simulations between the Clark Y and Goe398 does give a slight edge to the Goe for a scale like flight envelope.

Yes she is quite a span for an 020. Knock on silicon, the MotoCalc estimations give a very desirable flight performance, with several prop options in the sweet spot(12x6 to 13x8). The lithiums make the big jump towards giving the motor less weight burden.

I've got the airframe plans done up. However there is an issue of Flugzeug Classic (German air enthusiasts) that just came out with a story about a restoration of a L.20 in Austria. There are extremely few of these craft around. So I have to wait 3 more weeks to get the issue before I "buy off" on my plans.
Feb 08, 2004, 10:10 PM
Visitor from Reality
Hi Jeff
The last model I saw with an AF 020 was a 52" sort-off Cub from a modern laser-cut kit. He was using the 3.3:1 offset 'box - are you using the inline Astro 'box?

One comment the builder made was that the motor / kit came recommended for 10 cells, which he tried and promptly went back to 8!

This is obviously a punchy little motor and has started me idly thinking about an old favourite - the English Electric "Wren", from the Old Warden / Shuttleworth Collection. If you think your prototype is a powered glider, the Wren demo'd by taking off, flying mostly the length of a 3000' runway, landing, turning around by means of ground handlers, and flying back again at some 10'. On calm days!

LiPos sound a must here. Still, you've probably got enough nose length so you won't be relying on battery weight for a correct CG.


Feb 09, 2004, 10:51 AM
Obstinate spurious drone
jeff edmondson's Avatar
She is a little pointed in the nose, so the planetary GB is the way to go. The nose is still a little short (not radial engine short) so the LiPo's will be forming a box around the motor, just to get them as far fwd as possible.

The 020 has got a lot of "punch", 200watts with a 12 or 13 inch prop. I coud not find a Hacker that could deliver this power for the same motor weight, they were almost double the weight.

Here is a photo of a German model Klemm L.20...

Feb 09, 2004, 01:18 PM
Visitor from Reality
I think I recall seeing that one in EFI - huge great thing, mostly fresh air with a paint job!

Not buying any more BL until the ones I have are up and flying for their living - but want to try the new Astros. I've abused an AF035G on ten cells for around 6 or 7 years and it still runs like it did when new - that is what I call economical. If his BLs are that good, I'd be happy.

you could be the first to match IMAA legal with parkflier power!

Jan 27, 2016, 01:50 PM
Registered User
i'm using the god 398 airfoil for a 21 lb plane, twin engine, and it flies just fine with small motors, 2x power 15's from elite on 3S.
it loiters with only 150 watts of power, so for endurance, this is very efficient airfoil . the plane can fly for over 4 hrs on a charge.
the stall characteristics are great, and landing speed very low. if you put it in a dive, it's like putting air brakes on, just from the increased drag profile from the airfoil.
i love this airfoil.
that's my 2 cents. you can see my plane here:
Jan 27, 2016, 06:14 PM
Entropy is happening!
Jim.Thompson's Avatar

30% Klemm.

I'm using the E205 airfoil @ 15% thick.
Build thread of my 30% scale, (or semi-scale) model here:

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