Honeycomb Panel Floor Boards

[Archer39J]

Managed to get my hands on some sheets of 1/4" 2-ply fiberglass nomex core honeycomb panels, 3lb core. I plan to use this for the floor boards and possibly the aft bulkhead. Compared to .032" 2024 I calculated the weight for the sandwich panels at 1/2lb heavier over a 4x8 sheet, so fairly close in weight. I also plan to leave them unfinished, with thin carpet over them, and I'm thinking about neoprene edge trim to close out the open core along the periphery (via McMaster). Between the frames the panels rest on I'm considering neoprene again, but I'm concerned about vibration and wearing through the fiberglass, I just don't know yet.

My reasons for doing this are:

-Sound absorption, the sandwich panel will provide greater noise protection than the aluminum sheet without having to coat it with something heavy.

and

-Stiffness, I've seen some excellent examples of beading the AL floor boards. While this looks very nice it's something I'd rather not have to do. For actual flooring you usually see 9lb core, but the 3lb seems quite stiff enough for this application. No high-heel punch through to worry about in my BH

I'm debating whether I want to pot thru-inserts or just blow clearance holes through the panel to attach them. I think I'll need some blind inserts to attach things so if I'm doing that I might as well pot the others too. Anyone have experience using these in GA, or for my application specifically? I'd love to hear your thoughts about this, I only have experience in commercial interior monuments but these seem like a fine alternative.

[JimParker256]

Based on my experience maintaining a Grumman that was built with Nomex-core honeycomb panels, you want to keep water away from them if you're in freeze/thaw country. Here in TX we don't worry about it much, but any part where the panel is vertical and water even "might" get in is worth sealing carefully along all the edges. If water gets into those panels (doesn't take much at all) then freezes (expands), the system can begin the delamination process. I would also definitely use some kind of "thru-inserts" as you called them, because the skins (and honeycomb inserts) can be easily torn. (I'd probably seal the heck out of that hole, as well, since you could be getting in with muddy/snowy boots, allowing water in.)

I hope to find a reasonably inexpensive honeycomb panel source and use it to line the entire baggage compartment. I might even use that corrugated plastic stuff they use to build model airplanes – assuming I can determine the burn characteristics are OK. I'm not planning to line the rest of the cabin, but my plans for the plane include using it for Pilots N Paws rescue flights, and I'd hate for some pup to claw through the Oratex covering...

[Archer39J]

Good call on the water intrusion, that's definitely something I was thinking about in closing out open core. I think edge pot would be too brittle in this application so capturing the edge with trim and some sealant is what I'll do, maybe RTV, maybe structural epoxy, we'll see.

For the inserts one ditches out the core some distance from the hole before potting it in so there's basically a slug of epoxy around it, should seal it out nicely.

Thanks Jim!

[svyolo]

If the mounting holes are near the edge, just dig out the core and inject some thickened epoxy. Farther away from the edge you can drill through one skin, take a Brad nail or piece of steel wire and put a 90 degree bend in it. Put that in a drill and put it in the hole and tear out the core. Inject (syringe) thickend epoxy. When it hardens, drill through. Doing 20 holes only takes 2x the time as doing 1.

Thickened epoxy is good to seal the edge. A syringe helps, but a small ziplock bag with a tiny hole cut in one corner (bakers bag style) works great.

[Archer39J]

Hmm, that certainly would be easier than messing with NAS inserts. This would only work for through inserts though, anything I wanted to attach would still need a blind insert.

[svyolo]

Yeah the epoxy "plug" is the proper way to mount hardware in cored fiberglass boats. You don't want to get the core wet, and the core can't hold the compressive loads of the bolts. The manufacturers used to use plywood at hardware bases but these always failed after 10-20 years. They didn't know fiberglass boats were going to last forever.

I am going to use some kind of sandwich panels for the floors and interior panels in the baggage area. The ones I have seen so far are a bit expensive. Flat ones are easy to make, and I think I can make them just over 1/2 the weight of .03 aluminum sheet.

I am going to use the thinnest kevlar cloth for the skins, and foam core, probably 1/4 inch if it will fit.

[Archer39J]

Ah I see, yeah I'm not used to marine applications. I'm fortunate to have acquired my panels as scrap, otherwise it wouldn't be worth it to me.

[Archer39J]

Some photos of the install so far. These panels are incredibly rigid and have almost no flex so I'm having to trim them more than the aluminum floor boards, I'm thinking about how they'll be removed when the interior is covered and trying not to cross those planes. They make a nice 'thud' when screwed down and seem quite an improvement to the aluminum sheet, only flight will truly tell. The screws and washers are meant to be temporary, I've yet to pot the through inserts but I'm tempted to leave them as is and use nylon washers to make a good seal, that would keep water out. I'm not thrilled about potting inserts and having them not line up with my holes, I'll try the ones under the seats first and see how they turn out. I'm going to close out the gaps and seams with aluminum fairings and I also plan to make rudder cable guards. I have some nylon u-channel to go around the edges, but I'll probably just use AL tape on the hidden areas. I was able to get all my floor panels out of a single 4'x8' sheet.

[lsa140]

Thanks for sharing this, I've been weighing different options and it is great to see what other builders are doing.

[Archer39J]

So I'm wondering what others would recommend. I can use either flush or countersunk through inserts. I was thinking flush as I could then seal it up with a nylon washer and prevent any liquids from running down into the fastener, clip-nut, and tab assembly. But obviously having the fastener heads protrude on the floor is not ideal. Is my concern about sealing up the inserts a valid one? What do people with aluminum floors do? I don't really recall anyone dimpling them.

[Bcone1381]

I am using traditional AN526 machine screws which are Truss-head, non-structural, phillis head. The heads will protrude above the floor and I have no concern about that. But I am using AL floor boards too. All that dimpling....the floor, the steel tab...not ideal for an AL floor. If I spend too much time refining my project it won't get done. I love building and a number of refinements will happen after its flying. But for now it's BUILD ON!

[Archer39J]

Yeah it's funny how several of my changes have fallen by the wayside in favor of just getting the build done.

[PaulSA]

If you are using machine screws, you have presumably used nutplates on the steel floor tabs. Dimpling not required. With the floor in place, simply countersink using a #40 tool which fits the #6 nutplate screw hole. Assuming you are using 032 AL, you will cut through the AL a little into the steel but then finally fit with a tinnerman washer.

And if it's anything like my RV-10, all these little things that you where going to come back to later and "fix once it's flying" simply won't happen or will take years to get around to!

[Chris In Milwaukee]

Coming back to this topic, I asked my composites instructor while at SportAir Workshops this weekend about this. He said that he would use a Forsner bit to cut through the top layer and bore down to, but not through, the bottom layer. Cut out a circle just a smidge smaller than the hole from a common high-impact plastic cutting board or UHMW plastic. Drill and countersink the plastic for your flush fastener (flat head screw, etc). Use a structural epoxy (Aeropoxy or comparable) to glue the disk into the hole you bored. Mix up some dry micro balloons and fill in the gap between the disk and the hole. Then cover it with one layer of light cloth as if it were a patch. That’s about as strong as it gets, right there.

[svyolo]

PaulSA;
That is my favorite way to build. Get it flying. Don't add 7 years with side projects. If it is important to you, you will do it quickly after it is built. If it is less important, maybe it will take you a few years, or maybe it is not that important, and you never do it. But get it flying, as until then, it is a dream that you doh't even know if it fits your requirements. because you don't even know your requirements.

I know my requirements, and they are minimal.

[Archer39J]

Indeed, that's why I'm sticking with the push-pull controls per the design. I'm also going to start out with the stock wingtips, figure I can swap them out later if I really feel like it but getting in the air is the main priority. Right now my only big side project is the skylight.

[JCD23]

How do you fit a 1/4” floor board with the rudder cables. Don’t they interfere?

[svyolo]

I will check tomorrow but I think 1/4 inch is OK.

[Archer39J]

How do you fit a 1/4” floor board with the rudder cables. Don’t they interfere?

I'm spacing the rudder pedals and the control stick assembly up 1/4" to compensate. Which means new brake cylinder arms, and modifying the flap handle quadrant some. Every little change....

[JCD23]

Wouldn't be easier to just re-route the Rudder Cables under the aircraft at least in the cockpit area?

[Archer39J]

Not sure, probably not in a finished frame with fairleads already welded.