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Old Jan 29, 2007, 02:36 AM
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Geode's Avatar
San Jose, CA
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Idea
Bumblebees - Investigation of Geodetic Structure for 34" and 60" Balsa DLG's

I've build three balsa buildup DLGs with emphasis on light wing loading for sport flying (see http://www.rcgroups.com/forums/member.php?u=30056 for detail). These DLG’s have very good flying and handling characteristics due to their light wing loading (and, of cause, SGII/Apogee based design). Lauch height is also very decent and matches 60~80% of my composite ones.

All three of them serve me well (BTW, I'm not a power launcher). But when it comes to my power launcher friends, weakness in thin balsa wings start to show up. Solid balsa LE and CF capped ribs provide ample bending strength for DLG launch. However, due to the thin airfoil (4~7%) and solid balsa based LE (grain along wing span direction), the tortional strength is no comparison to composite wing.

Here is an example of wing bending during lauch,
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Old Jan 29, 2007, 02:39 AM
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San Jose, CA
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This thread is intended to investigate the geodetic structure for 60" DLG. I decided to use the scaled tip panel of my 60" DLG as a test case and build a 34" Bug-like MDLG before I jump on the full sized DLG project.

34" Bumblebee M --> start from here
60" Bumblebee XL --> go to post #64.


Here is the highlight of the “Bumblebee MDLG”,

· Planform – elliptical wing tip from my 60” DLG with geodetic ribs
· Airfoil - AG04/08/09/10 transitional (same as composite Apogee)
· Tail feather - laminated balsa with HT08/HT23 airfoils
· Covering - Laserjet printed tissue (http://www.rcgroups.com/forums/showt...tissue+printer)
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Last edited by Geode; Apr 24, 2007 at 11:49 AM. Reason: Add Bumblebee XL & XL4 links
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Old Jan 29, 2007, 11:13 AM
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3D rendering and AG04/08/09/10 based ribs/spar.

Ready for the real work ......
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Old Jan 30, 2007, 01:47 AM
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San Jose, CA
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A Simple Experiment

The advantage of geodetic structure on tortional strength can be easily seen in the following experiment. This results encourage me to adopt the geodetic structure for full size DLG.
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Last edited by Geode; Jan 30, 2007 at 05:18 AM.
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Old Jan 30, 2007, 02:03 AM
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Glendale,Az.
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That looks like the way they make them in Germany, Nice, Doug
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Old Jan 30, 2007, 10:16 AM
trying to kick the habit
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Watching...
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Old Jan 30, 2007, 11:04 AM
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San Jose, CA
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Quote:
Originally Posted by dougmontgomery
That looks like the way they make them in Germany, Nice, Doug
Thanks for your input. I searched in RCGroups and Googled the WWW, here is what I found:

Sobox build-up accent 400 HLG - S7072 airfoil
http://webhome.idirect.com/~sobox/electric.html

Slowstick geodetic buidup wing - somewhere in the Electrical Sailplane forum (edit: found it http://www.rcgroups.com/forums/showt...light=geodetic)

Firework III Composite DLG (Disser Wing) - http://www.pcm.at/english/fireworks/index.html#cover

Aspirin Composite DLG (Disser Wing) - http://www.tud-modelltechnik.de/aspirin/preise_en.htm

Please share with us if I miss the links you have in mind. Thanks.
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Last edited by Geode; Jan 30, 2007 at 11:54 AM.
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Old Jan 30, 2007, 05:23 PM
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Grand Rapids, MI
Joined Apr 2004
567 Posts
Ridges?

Neat project - but won't you end up with ridges that don't line up with the airflow?
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Old Jan 30, 2007, 05:26 PM
working to the closest cm
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brisbane australia
Joined Nov 2002
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Stiff built up wing - carbon D box?

Hi Geode
I like you tenacity and your blog shows some great balsa models _ I can only build light when I use composites.

My latest compisite wing is an electric version of the HLG terminator - wing 1.5m - 95gm.

Despite my love of simple composite construction I often linger on this site:-
http://www.gregorie.ukfsn.org/freefl...g/cf_wing.html

I wonder if the freeflight technique may be stiffer than your beautiful geodetic structure? I think the Freeflight competition builders went down the geodetic path before they switched to carbon D-box construction? Here I have in mind your desire to build stiff hard launching dlg wings?

Watching with interest
cheers Jeff
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Last edited by jirvin_4505; Jan 30, 2007 at 05:29 PM. Reason: spelling
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Old Jan 30, 2007, 09:45 PM
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San Jose, CA
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2motheus,

True. This is just a quick-turn-around experiment to see the net improvement from geodetic structure. In the next project, there will be D-box in the LE to minimize the ridges. In this way, I can get a feeling of how much improvement from D-box and how much from geodetic structure.

Jeff,

Thanks for your link on carbon D-box. Carbon D-box certailly is the way to go and it is field proven. What I want to achieve is a low-cost or scratch-build-able 60" DLG that is easily accessable to more people. And the modern thin DLG airfoil is a must. The cost of carbon D-box or composite ones will likely scare away a lot of pilots.

With all of these insightful inputs, I believe my next 60" DLG project will combine sheeted (or solid) balsa D-box and geodetic structure for the wing. In this way, I have the best of both worlds with reasonable weight penalty.
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Old Jan 31, 2007, 02:09 AM
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Some more progress. Right wing half and the spar.
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Old Feb 01, 2007, 11:01 AM
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San Jose, CA
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Now the carbon fiber comes to help,

Spar system is similar to Taboo. Quick and easy to install, but very strong. Bending strength is overkill, I'll try 1 mm next time.

The wing in current state is already as strong as my conventional build-up wings with CF rib caps in terms of both tortional and bending strength.

Current weight - 34 grams. Not bad.

Cody, no honey there. Only CA.
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Last edited by Geode; Feb 01, 2007 at 12:01 PM. Reason: Grammer error
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Old Feb 01, 2007, 12:59 PM
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eitanro's Avatar
Tel Aviv-Yafo Ben Gurion, Israel
Joined Sep 2004
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That's a nice idea to make a Geodetic structure, but as Jeff said - free flight fliers don't use it any more because the D-box's strength is much higher than Geodetic.
in this Photo it is a self built wing, 2.37 meters, and it weights only 170 g"r.
For a DLG it is possible to make it much lighter because it doesn't have the same strength requirement as a free flight glider.
Eitan.
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Old Feb 01, 2007, 08:34 PM
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San Jose, CA
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Eitan, I totally agree with you. Composite materials and Carbon D-box are the way to go. But, I'm not abandon the geodetic structure yet.

As I replied to Jeff in post #11, D-box is great, but geodetic strucure can also improve the tortional strength. Combination of both will likely help improving the overall strength and make the balsa build-up wing one step closer to composite wing at a reasonable weight penalty (I hope). There is no conflict between D-box and geodetic structure.

Similar concepts have been adopted in multiple hollow molded DLG's, such as Aspirin and Firework, by offering disser wing or carbon D-box as an option to improve the overall wing strength. Consider this project is a balsa build-up wing equivalent of composite disser wing :-).

I saw leading edge hollow molded DLG wings re-embrace CF geodetic structure for overall strength improvement. Will the same trend happen for free flight glider too ?
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