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Jun 01, 2019, 06:31 PM
Scott
Pylonracr's Avatar
Terrible idea. This has been beaten to death many times. There is no advantage to using excessive resin other than to add ballast. Sand the cores lightly and bond the skin to the core using the absolute minimum amount of resin possible. Remember, this is a stressed skin composite, the foam is not a structural component, its only purpose it to keep the balsa in its shape.

Scott
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Jun 01, 2019, 07:06 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Pylonracr
Terrible idea. ............
Scott

That is my conclusion about it too Scott.
However, Glenn builds high performance, high wing loading slopers like the Rodent. (Do you know that model? Has to be seen flying to be believed!).
Even so, if and when I am adding more weight purposely, I add extra reo as well as extra resin.
Jun 02, 2019, 05:15 AM
by ZIPPER
ZIPPER's Avatar
Quote:
Originally Posted by Pylonracr
Terrible idea. This has been beaten to death many times. There is no advantage to using excessive resin other than to add ballast. Sand the cores lightly and bond the skin to the core using the absolute minimum amount of resin possible. Remember, this is a stressed skin composite, the foam is not a structural component, its only purpose it to keep the balsa in its shape.

Scott
I fly heavy slope planes and build for strength, weight is a small byproduct. The extra weight that's added is less that one coat of paint,
Ive done testing on skins with and without the ribbing and the ribbed skins are torsional stiffer from the 45deg cuts and stiffer along the span from the long cuts.
It works for me and Ill keep doing it.
I wouldn't do in a DLG as every oz counts.
Jun 02, 2019, 07:17 AM
Registered User
Thread OP
I wouldn't do it on my gliders, but it was interesting to hear the idea.
Jun 02, 2019, 06:03 PM
Scott
Pylonracr's Avatar
Somebody is going to have to enlighten me here. I will admit that it has been a while since I graduated from college, but there was one thing that every professor beat into our heads. Never, Ever, leave any resin in any composite matrix that is not absorbed into fabric. Not only is excess resin simply dead weight, it actually lowers the strength of the finished component. I have seen the test results on this and analyzed the data, but I am not going to post it here unless requested. There is a Troll lurking on this forum, and every time I post any factual data that is the least bit scientific, he wants to argue the facts. I do not want to play 5 year old games again......

The proper way to add torsional strength would be to laminate carbon tow at + and - 45 between the core and the skin. This would yield the optimum strength to weight without compromising the underlying composite matrix. I can not fathom how filling grooves in the foam could add any measurable strength, it is simply unsupported resin. While being initially stiff, it has no tensile strength and will fail long before the balsa skin will.

Scott
Jun 02, 2019, 06:25 PM
Slow builder
_AL_'s Avatar
Increased bonded surface area.

Al
Jun 02, 2019, 09:37 PM
Scott
Pylonracr's Avatar
Not possible, there is no structural bond through unsupported resin.

Scott
Jun 05, 2019, 08:19 AM
Aurora Builder
Quote:
Originally Posted by Pylonracr
Not possible, there is no structural bond through unsupported resin.

Scott
What is lost in a classroom environment is some amount of resin is needed to bond the skin to the core. Otherwise you will have peeling. In pre-preg layups their is an additional layer of ~0.005" resin that gets added between the core and the first layer of the carbon/epoxy matrix to ensure that there is enough resin to make a good bond. In bagged or hand layups you just ensure that there is enough resin during wetout. Another trick we do sometimes on thin cores is essentially drill the core in spots to make epoxy rivets, which greatly reduce the chance of a delamination occuring in areas that get handled or otherwise abused. For flight loads this isn't needed, but again, the real world is very different.

To emphasize this point, this is how high performance multihulls are built: https://sailpuffin.com/2019/05/18/chasing-performance/

Yes, keeping the resin down is important, that is simply a rule of mixtures calculation That is why there are only two sources for small quantity high modulus carbon fabric in the U.S, as otherwise the material only comes in pre-preg; trouble is for model building, most aren't using an oven and have access to a freezer.

The approach taken on a DLG structure is slightly different. The loads are very high during launch but the flight loads aren't bad and stiffness is critical as is weight. It is literally the highest performing composite structure on the planet outside of America's Cup 50' catamaran foils (which take 6 months to build and are solid high modulus pre-preg carbon). No one in the real composites world knows of the type of structure we are building here because it simply doesn't exist! The trick is to use foam with minimum void size to reduce the epoxy required to bond the skin to the core then compress the crap out of the core during layup to get a very solid bond with absolute minimum resin content. This isn't possible in many applications, but full scale composite aircraft blades are build the same way.

I'm also not visualizing your 45/45 technique. What is done in high load areas (under spars for example) is to slice the foam and embed carbon into the skin to reduce the buckling potential of the spar. Better is to carry this carbon top to bottom to form a shear web.

Finally, every application is different and slicing the core or prickling it with one of these: http://www.monokote.com/accys/topr2190.html to increase peel strength is a plus!
Jun 05, 2019, 09:40 AM
Scott
Pylonracr's Avatar
Sam, all very valid points. I think you and I are basically saying the same thing. My comment RE: unsupported resin was in reference to digging troughs in the foam core and filling them with resin under the skin in an attempt to gain strength. The comment made was that if these troughs were cut at an angle it would somehow increase the twisting strength of the wing. I pointed out that the proper way to add strength on the bias under the skin is to laminate tow on the bias under the skin.

On race wings I typically cut a true spar groove in the foam core and put a 1/8 x 1/4 balsa spar in with 12K tow laminated top and bottom.

Scott
Jun 05, 2019, 12:20 PM
Aurora Builder
Ah yes Scott, we are in agreement there. Carbon >> foam for improving stiffness or strength in any direction.

Some more good notes from the builders on resin uptake in foam cores: http://forums.sailinganarchy.com/ind...omment-6608385

I followed their silica priming procedure but with an even thicker mix when building some carbon panels out of klegicell. Weight was less of a concern than proper bond and waterproofing the core (even though klegicell is a waterproof foam).
Jun 10, 2019, 07:36 AM
I hate propellors
emufingers's Avatar
I pinched my wife's fabric tracing wheel from her sewing kit. Has a serrated wheel on the end. You can quickly puncture the seal from the hot wire with it, paying extra attention to any area where really good adhesion is require, say around the spar edges. Not only does ot increase adhesion but harden the foam surface and there are less finger dents, so the centre of the winf panels where they are handled during fitting. Adds very little weight ,
Jun 10, 2019, 07:57 AM
I hate propellors
emufingers's Avatar
I have been looking for something to replace 1.7 oz kevlar. It s getting hard to get nd is best to cut even with Phil Barns scissor mods. I notice 2 oz innegra is available from surf suppliers. It is much cheaper and can be cut with a hot knife. torsional strength is between glass and kevlar colour is white. The are even cloths with a diamond disser type pattern of carbon woven in, but in a heavier weight. Any comment on these?

https://www.sanded.com.au/collection...ard-fibreglass
Jun 10, 2019, 11:58 AM
Registered User
Wow, nice selection of fabrics at Sanded.com
Jun 10, 2019, 04:36 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by emufingers
I have been looking for something to replace 1.7 oz kevlar. It s getting hard to get nd is best to cut even with Phil Barns scissor mods. I notice 2 oz innegra is available from surf suppliers. It is much cheaper and can be cut with a hot knife. torsional strength is between glass and kevlar colour is white. The are even cloths with a diamond disser type pattern of carbon woven in, but in a heavier weight. Any comment on these?

https://www.sanded.com.au/collection...ard-fibreglass
Andrew (AvB) of Queensland Dynamic Soaring fame has used innegra and he very much likes it. Send him a PM or read his threads (Scratcho maybe?).
I only use kevlar for hinges; In my opinion, it has limited application for general layup reinforcement due to its poor compression strength.
The fabrics displayed on that Aus link are all open weave surfboard types. At least, they look like it.
Dick Garrard of Ironbark Composite still supplies fine weave aircraft grade fabrics and his price is the lowest I have found. His service style is a bit unusual, but he gets the orders out quick and reliably. Just make sure you clarify what you want and what you don't want!..................
Jun 18, 2019, 08:32 AM
Aurora Builder
The 2.4oz Innegra I tested a while back was very open weave and the fibers were essentially rolled so almost twice the material thickness of Kevlar. You're looking for more like a 1oz innegra, which is a great material but really tough to find. Those guys at Sanded probably know where to get some though, very neat materials on their site.


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