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Old Mar 14, 2005, 08:20 PM
plane obsessed
Cody WIlson's Avatar
Denver, CO.
Joined Nov 2003
3,846 Posts
New spar system for EPP toy's!

I actually have something positive to contribute foar a change!

Sorry no pics- but it is a pretty simple concept to "get"

Long ago in a galaxy far far away I had this idea- a super simple and light spar that consisted of two strips of carbon top and bottom-slit into the foam and secured with thin CA with the EPP between them acting as a shear web- well, come to find out Richter RC has been doing this on their Weasels forever. So I know I can't take credit for that... Then My friend Karl and I were talking about a set of Raider cores that didn't have the spar holes cut- I reminded him about the thin rods (like Richters) I used on my Slowfly wings ( which were stiffer, but still pretty wobbly). Unfortunately they would need to be ordered, and Karl (like me) wanted it NOW! Luckily our LHS has 1/4" X .007/.014 thick carbon ribbon that comes in 36, 48, or 72" lengths. He tried it out and said it worked great. So I can't take credit for that either...

Well, I just got a new Raider 'core only' and decided to try the same thing Karl did- I cut some ~3/8" deep slices into the wing on the CG out to the thickest part of the chord and then made a smooth radius and followed it out to the tips top and bottom. I was very careful to follow my lines while maintining a prefectly vertical slit. I had one 48" piece of .014 and two 48" pieces of .007. After carefully sliding the thinner pieces into place ~1/16" under the surface of the wing I cut the thicker ribbon in half and slid the 24" pieces into the center section of the slot- which by chance went about 1" past the bend on both sides where the ribbon sweeps back with the wing. I did this top and bottom, set the core in the beds to make sure I didn't leave any warps etc... and laid down a bead of thin CA to fix everything into place. It is very important top let the CA cure overnight without the use of kicker- this not only allows the CA to wick, it also prevents hard crusties (stress risers) that like to tear EPP apart under stress.

So I come home tonight and pick the thing up- wiggled the tips up and down to see if the moved- and they did, but were noticably stiffer then even my little (.060) carbon rods on my slofly's. It was time to test out the center section- Stiff is an understatement! It is almost totally rigid accross the center section! I couldn't flex it with my hands alone- just deforming the EPP temporarily where I was trying to grip. So I did what any good Neandrathal would- I pressed my forehead into the center of the wing and tried to flex it again- still very very little flex! The only thing I can compare it to is the inherent give in some Foamular or High Density blue insulation/ refrigeration foam- It feels just as stiff if not stiffer! I picked up my good ole (beaten) Raider with it's 1/4" or 5/16" carbon rod with strapping taped wing and did the same flex test- It felt like a wet noodle in comparison!

The surface area that is provided by the ribbon for it to bond to the EPP is great- only the compression forces will distort the spar- and probably snap it if under extreme stress- BUT the part that is trying to "stretch" will be held very nicely. And the inherent stiffness of CA'd EPP will help keep the ribbon on the compression side from distorting. [Edit- I just snapped one of the .007 ribbons where it compressed and the ribbon made an "s" turnn about 5" in from the tip- I can't really make a true "I" beam, but some thinned goop and bidir strapping should help to dampen the distortion (at least on the surface )]

All in all I am very impressed with this method. It won't work on anything with Dihedral or an extremely thin wing, but for a light weight super stiff spar on a combat wing I'm sold- just next time I will use .014 all around!

*Full of hot air*
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Old Mar 14, 2005, 08:55 PM
Registered User
Milwaukee G Mitchell, Wisconsin, United States
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You mean like they use on Mark Drela's SuperGee?
http://www.charlesriverrc.org/articl.../supergee2.pdf
Phil Barnes did some extensive testing here
http://www.rcgroups.com/forums/showthread.php?t=323741
I know it's not in epp but some very good info.
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Old Mar 14, 2005, 09:55 PM
plane obsessed
Cody WIlson's Avatar
Denver, CO.
Joined Nov 2003
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Yep- pretty much like that

Goes to show- there ain't nothing new under the sun.
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Old Mar 14, 2005, 10:02 PM
Fun ain't cheap!
droydx's Avatar
United States, CA, Santa Barbara
Joined Apr 2003
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It's amazing how stiff even a 1/64" thick by 1/4" strip of plywood inserted vertically into a foam wing will stiffen it up. Do this top and bottom and have a strip of filament tape and you've got a "spar structure"
Cody, You always have something postitive and good to add to the forum!
Keep 'em coming.

Droyd
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Old Mar 14, 2005, 11:21 PM
Feeling FrSky
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United States, CA, Santa Barbara
Joined Feb 2003
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Hey Cody, even if you weren't the first, you expended the brain power to come up with that stuff independently. Don't worry, eventually you will come up with something new, and we'll be stoked to hear about it!

FWIW, post pictures for those who can't quite visualize it based on your narrative. I'm sure there is something to learn here.

Steve
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Old Mar 15, 2005, 12:37 AM
Charlie
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Canada, BC, Victoria
Joined Dec 2004
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I cut most of my wings right through at the thickest part of the section and then glue together again with a strip of ply full length. Cheap, easy, stiff.
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Old Mar 15, 2005, 03:11 AM
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Germany, NDS, Ronnenberg
Joined Jan 2005
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Hy there! Time for my first post!

Last year I found this link, which describes a spar system similar to what you build: http://www.yourzagi.com/smiley.htm. The differently curved spars add a torsional stiffness to the wing which reduces flex around the lateral axis of the wing (which is necessary for sweeped wings but nearly irrelevant for planks).

After that I modified this arrangement and build my F4Y-Mini Mark II (the F4Y-Mini is a Zagi-like-1m-span wing sold in Germany from Peter Kienzle (http://www.epp-fun.de)). I put two 6x0,6mm flat CF-spars in the wing (one on top, one bottom) and curved them in a way that I called smily-face doublecross, which means that the spars are crossing each other two times on each wing half. The spars are glued in vertically with epoxy. This plane has got no crosswise taping with filament tape! Only the middle area and the LE is taped (it's a combat plane ;-) ).

The servos with the horns were completely buried in the wing (servo horns pointing down) and the rods are routed through the wing to the upper side. I also added a fixed elevon (like used by NCFM at the halfpipe: http://www.rcgroups.com/forums/showt...7&page=6&pp=20) and slots for ballast on top of the wing. This wing is aerodynamically clean and very stiff due to the spar system.
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Old Mar 15, 2005, 08:37 AM
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Cody WIlson's Avatar
Denver, CO.
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Wow! Nice work. What I did looks almost identical to your top pic- except the bend is further back from the LE. I never would have thought to do the bottom as you have. Thin carbon ribbon is cool stuff!

Thanks for the support and links fellers- and all the other idea's. Nothing I hate worse then a flexy wing! ('cept for maybie a cheating woman, but that is neither here nor thare)

Hey Charlie- you ever snapped one? Do you leave the original spar in when you do this? Do you use Epoxy?

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Old Mar 15, 2005, 08:54 AM
R U killing the kittens?
artmonster's Avatar
Chaos, UC
Joined Jun 2003
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Nice work.

I went the Barbarian rout to stiffen my "Caca Volando" Zagi.........

I just Probonded Big-ol' Carbon Arrow Shafts just behind the leading edge.....

It's now stiff as a Siberian corpse, but lethal to anything it runs into.

""If it ain't broke, hit the freakin' thing with a hammer!"" is my engineering motto.
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Old Mar 15, 2005, 08:58 AM
That Freeking Laird Guy
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United States, CA, Riverside
Joined Feb 2002
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I did something similar on the second Boomerang I built about 6 years ago. After glueing the two wing halves together I took it over to the band saw and cut the wing similar to the spar line in Robiwahn's first wing pic. I cut the wing in two though. I have a leading edge part and the trailing edge part and it curved just like Robiwahn's does. I then took Carbon fiber tow (lots of it) wetted it our with epoxy layed it on the cut and then taped the two halves back together while the epoxy/carbon tow cured. It gave me a really really stiff wing.

TFLG
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Old Mar 15, 2005, 09:15 AM
It could happen...
InTheLift's Avatar
Torrance, California
Joined Jan 2004
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Interesting thread, interesting ideas. I'm continually amazed at the level of effort some of you put into your wings...far more than my tape'um and toss'um approach.

Welcome to the boards RobiWahn.
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Old Mar 15, 2005, 10:46 AM
Charlie
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Canada, BC, Victoria
Joined Dec 2004
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Cody, no I haven't snapped one....YET. But I only really use them in bigger stuff. For anything under 60" I still use carbon fibre tubes. The curvy flat stuff looks excellent though. I have heard of people using that flat steel band that is used to tie up pallets for shipping in larger wings but I have't tried that. I have a hard time with steel in wings, kind of like concrete boats.

Heres a picture of my biggest wing with a 2mm ply spar the whole length of the wing at the thickest part of the section. The front joiner tube goes through the spar and then another ply spar comes off the main one for the back joiner tube. Flys great but a bit stately for me.

I glue everything with PU
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Old Mar 15, 2005, 05:25 PM
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AntonL's Avatar
Melbourne, Victoria, Australia
Joined Feb 2004
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Quote:
Originally Posted by RobiWahn
I put two 6x0,6mm flat CF-spars in the wing (one on top, one bottom) and curved them in a way that I called smily-face doublecross, which means that the spars are crossing each other two times on each wing half. The spars are glued in vertically with epoxy. This plane has got no crosswise taping with filament tape! Only the middle area and the LE is taped (it's a combat plane ;-) ).
That's quite an impressive EPP spar system, RobiWahn! I am interested to know where you get your CF as it seems that your lengths are longer than the max that I can get CF flat spars in (EDIT: 1.0 m). Do they, by any chance, ship overseas?

Anton
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Last edited by AntonL; Mar 15, 2005 at 05:38 PM. Reason: wrong length quoted
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Old Mar 16, 2005, 02:39 AM
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Germany, NDS, Ronnenberg
Joined Jan 2005
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Quote:
Originally Posted by AntonL
That's quite an impressive EPP spar system, RobiWahn! I am interested to know where you get your CF as it seems that your lengths are longer than the max that I can get CF flat spars in (EDIT: 1.0 m). Do they, by any chance, ship overseas?

Anton
I didn't use spars which are longer than 1m. What I forgot to mention is, that only the inner parts are 6x0.6mm. The outer wing sections are stiffened with 3x0.5mm CF-spars. Also the additional spar near the TE on the bottom side is 3x0.5mm. I put the spars in the slot with an overlap of around 30mm (approximately between the green lines in pics below) and epoxied them together.
I bought the spars at carbon-composite technologies here in germany (http://www.altec-composite.de/).
A friend of mine put a complete build report of the plane that I wrote on his homepage (http://www.krasnodembski.de/robi/) - Sorry, but its in German :-/.
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Old Mar 16, 2005, 02:48 AM
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Germany, NDS, Ronnenberg
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And thats the completed plane (a bit bigger than in my avatar ;-) ). Weighs 275g (9.7oz) w/o ballast and holds up to 120g (4.2oz) of ballast. Much faster than the original design (220g/7.8oz) I build first and a lot stiffer. Despite the higher weight the new one flys quite well in lighter winds. But it rocks a lot better in higher winds with max. ballast :-).
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