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Old Jul 12, 2012, 03:28 AM
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Shaper Dave's Avatar
Oahu Hawaii
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DLG Wing Spars

Hi Guys,

Wondering what type of spars your using in molded DLG wings. I have been going through different types on my test wings and have not been all that happy with the amount of flex versus weight.

Dave
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Old Jul 12, 2012, 11:55 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
USA, OH, Worthington
Joined May 2002
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I use 100psi foam or rohacell, cut to profile with a 4oz uni cap in mine. There's a few inch long carbon sock over the foam at the center joint.
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Old Jul 12, 2012, 11:57 AM
Aurora Builder
United States, MD, Lusby
Joined Nov 2003
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Dave,

Internally I'm using a 12.5mm chord-wise width strip of 100 psi foam (shaped to the top and bottom of the wing skin). Externally I use a single layer of 4.1oz uni-carbon, starts at 15mm wide at the root and tapers to 8mm wide at the wing tip. I will double check all those dimensions this evening when I am cutting more material. I get a complete set (4x pieces) of carbon wing spars out of a 2" wide, 29.5" long strip of 4.1oz uni carbon.

Have not had an issue with bending stiffness underneath a carbon d-box, if the foam shear web is properly mated to the wing skin.

Hope that helps,
Sam
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Old Jul 12, 2012, 01:36 PM
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Thanks guys,

I have been using a lighter foam spar with carbon shear webs on both sides but this has been a little too flexy.
I will go your route with a denser spar. Any one using a balsa end grain shear web in the foam spar?

Dave
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Old Jul 12, 2012, 02:38 PM
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I built several early on with end grain instead of 100psi foam. I couldn't see any difference at all.

If you have the capability to acquire / fabricate end grain balsa, it will work fine.
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Old Jul 12, 2012, 02:55 PM
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I don't think that it's necessary to use end grain balsa. I've successfully built 60" DS gliders that undergo much higher loads than 10 ounce DLG gliders using just horizontal grain balsa. The grain orientation only really matters once you encounter a shear failure. In my sample testing the vertical grain is only 4% stronger than the horizontal grain. The vertical grain samples where also heavier due to the extra adhesive that is required.
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Old Jul 12, 2012, 03:57 PM
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I never used a horizontal shear web or come across the use of one for model use. I think the term Shear Web is always used in the vertical aplication.

Thanks for the responses,

Dave
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Old Jul 12, 2012, 04:36 PM
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Quote:
Originally Posted by Shaper Dave View Post
I never used a horizontal shear web or come across the use of one for model use. I think the term Shear Web is always used in the vertical aplication.

Thanks for the responses,

Dave
I used to think so to until I read this by Mark Drela:

"a shear web is commonly made from sheet balsa, whose grain can be either vertical or horizontal. Contrary to popular belief, the grain orientation does not affect the wood's ability to withstand a shear load, which is in effect compression along one diagonal, with equal tension along the other diagonal."

and

"It's likely that the endgrain is much more resistant to peel from the hard sparcap than from the soft rib wood, so that the vertical-grain web is likely to be stronger. But this is only conjecture, and tests of spar samples would be the only way to confirm this."

After reading this I did my own batch of testing. In 4 different spar samples of varying construction the end grain web was on average only 4% stronger...but always heavier. In fact, in my first test a light weight spar with 4.1 ounce UD carbon caps and horizontal grain tested stronger than the vertical grain sample.

DLGs use very thin spar caps. The caps fail far before enough shear force is generated in web to cause the horizontal grain to fail. Increasing the web strength does nothing for you if the caps fail before the web does. The compression strength of the web (90* to the bending axis) is only vital if the shear web allows the two caps to slip past each other. If the caps don't slip then the only compression force perpendicular to the bending load is from the buckling of the spar cap.

If the web is wrapped or socked with +/-45 fibers then the fibers take the load rather than the web core. In fact some winch launched Supra models have used a 60psi foam warpped with glass as the shear web.
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Old Jul 12, 2012, 05:29 PM
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HKM HighEnd uses a horizontal grain balsa shearweb with lightweight glass wrap.. They are waaaaayy easier to make then a vertical grain web too. I've tried both in my 60" ds plane, and could not tell any difference, both in flight and post mortem
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Old Jul 12, 2012, 05:54 PM
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United States, CA, Tehachapi
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Interesting. I would have thought that there would be a much larger difference in strength between the vertical and horizontal grain balsa when used as a shear web. I've been gathering a number of different foam samples to test as shear webs. If there's any interest from others I'll happily post my results here in the composites forum. I'm currently planning on testing 15, 40, and 60 psi Dow foam, Spyderfoam, Rohacell 71 IG, 71 IGF, and 31 IGF, and Depron. I'd test Hi-load 25 but I can't find it. Are there any other foams y'all think I oughta add to the list?

Brandon
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Old Jul 12, 2012, 06:51 PM
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There's no such thing as HiLoad 25. Dow 25psi foam goes by other names, the most common is the "Square Edge".
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Old Jul 12, 2012, 07:03 PM
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Ah. Thanks for the correction.
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Old Jul 13, 2012, 04:45 PM
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You could add Dow 100psi to the list -- others have mentioned using it.
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Old Jul 13, 2012, 05:32 PM
Composites guy
North OC, Ca.
Joined Jun 2005
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Just an Fyi-
I have looked at some wet lay-up spars under high magnification of the cross-section under a microscope. The cross-section can have quite a high void content and unless you can bleed the resin content down, they are usually resin rich.
It may make sense to pre-cure them under high pressure in a tool and then add them in the lay-up.
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Old Jul 13, 2012, 05:38 PM
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United States, CA, Tehachapi
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Quote:
Originally Posted by BillO View Post
You could add Dow 100psi to the list -- others have mentioned using it.
If somebody sends me a sample I will gladly test it. It's unobtanium around here and a bit heavy for my uses (DLG cores). The only reason I'm testing the Rohacell 71 is because I have some scraps on hand that I can use.

That's good to know about the wet layup spar caps. I hadn't heard that before. I might just have to play with pre-curing my spars now. The reason I haven't done it until now is because 1) it's just another step, and 2) I can't seem to keep it from making a bump or divot where the spar goes. With molded wings the latter isn't really an issue.
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