KW MkII Development - Composite Sandwich Panel Experiments

[@PhilipLardner1967]

I wonder if a Nomex core material filled with 'dry' micro balloons remove the need for the two 1mm sheets of foam either side if the Nomex. 'Dry micro' uses just enough epoxy to bind it. It's quite stiff and light. I haven't looked at the numbers / weights yet. I have a small sample of 3mm hex-cell Nomex and a bucket of micro balloons in my workshop, so I may make up a test sample to weigh and test. Keep it coming Rol - I learn every time I watch you!

[@KlingbergWingMkII]

Yep, that's a common method for "potting" the core where bolts and other fasteners will pass through. Too heavy for large areas. I experimented with it in the very early stages of my project for the LE of my D-tubes - they were bulletproof but too heavy.

[@AI.77169]

Composites have so many applications beyond aviation, so every video you share is full of fascinating possibilities!!

[@duduc998]

if you are not using prepreg, you can use a paint roller to apply some resin on the nomex just enough to wet one side of it. then stick it to the carbon part which is already soaked with minimal amount of resin. this way you can avoid using unnecessary resin within the cells.

[@KlingbergWingMkII]

You have "special powers". I ordered rubber rollers earlier this evening for just what you suggest, except using urethane glue. I'm also going to try pouring a thin layer of glue in a tray and dipping the honeycomb core in it. I'll be doing videos on the results. Foaming epoxy could also be an interesting experiment.

[@duduc998]

@KlingbergWingMkII nice! I tried dipping but for the small pitch of the cells I had and the viscosity of the resin used, it didn't work out due to resin membranes bridging the cell gaps. Maybe less viscous infusion resin would have worked though.

[@lemauz.]

I wonder if CNC routing a cavity pattern for the honeycomb would save you a layer of foam. Like foaming thehoneycomb, minus the mess... Thanks for sharing.

[@KlingbergWingMkII]

routing which material?

[@lemauz.]

@@KlingbergWingMkIIThe foam, carving thin channels for the honeycomb pattern to embed fully while keeping it low tech enough to be feasible at home.

[@KlingbergWingMkII]

Based on my recent experiments it looks like routing the foam might be essential. I have found that the honeycomb cannot cut into the foam using pressures available to a home builder. The routing would have to leave a very small layer of foam so the various pieces stay together.

[@rickboerma3392]

It seems to me that if you have foam on the outside and honeycomb in the center that the shear stiffness (or compressive stiffness) near the skin is lower, equal to a normal foam sandwich. This decreases the local buckling stress. Do you than have any advantage of the honeycomb in the middle of the panel? Let alone that you now have 2 additional bonding interfaces in the sandwich which consume resin as well. Looking forward to your findings.

[@KlingbergWingMkII]

Sorry I was not clear in the video. My goal is to get the outer skin of foam at the same level of the honeycomb so that both the foam and the honeycomb are bonded to the composite skin. In that way one could gain the advantage of high compression strength of the honeycomb and the added bonding area of the foam. Recent tests have show that it is nearly impossible to press the honeycomb into the PVC foam, so, nothing gained so far. However, my experiments continue. BTW, compressive strength and shear strength are not the same. The are generally 90 degrees out of phase to each other. It is the compressive strength of the core material that prevents local buckling when the sandwich panel is undergoing compressive and/or bending loads. I am making this clear so that my viewers do not get confused.

[@cambc9730]

You use VaRTM to produce your panels? Do you worry about degassing your resin before you start the infusion or do you do a hand layup first then pull vac?

[@KlingbergWingMkII]

Resin infusion is way, way too heavy for this type of aircraft. I spread epoxy in the fabric by hand and vacuum bag to the core. You can see this process in many of my early videos. One exception is DOW XPS styrofoam as it is too weak in compression for vacuum bagging. See the spar layup videos for a demo of that material.

[@michaeldunning7471]

@@KlingbergWingMkII Can you provide the video name? I didn't see XPS mentioned in the "Spar Cap Layup" one and I've watched all your videos, so can't remember either 😄

[@KlingbergWingMkII]

I use xps as the core foam for the spar: kzbin.info/www/bejne/a56kpZZ9l5eBi9U

[@geobum]

Thoughts on end-grain balsa?

[@KlingbergWingMkII]

The honeycomb core is 2 lbs/cu Ft, the best one would do with balsa is around 6 lbs/cu ft and it would take a lot of work to find enough blocks of that density to do just one aircraft. However, end gran balsa does have very good compressive strength and I suppose that is why it is used in some boats and windmill blade. But, too heavy for my purposes.