Carbon Skin/Nomex Core sandwich panels are one of the world’s best strength to weigh materials. And it’s affordable. However,
- the edges of sandwich panels can be easily damaged.
- the panel itself is not good at handling point loads.
- the skin can peal from the Nomex with enough force.
- the edges are also not aesthetic,
But all of these issues can be fixed (see below photo).
There are several approaches to edging Nomex Sandwich panels:
o Edge, by bonding on a flat extruded carbon or plastic bar or C-channel
o Edge with caulking for lightest weight or epoxy adhesive for highest strength
o Use solid balsa core that doesn’t need edging
o Get over the sex appeal. go functional, don’t edge
But sometimes you have to edge for functional reasons:
o Drill a hole that must supports a load, like a bar going through it, so the hole has to be solid
o The area around a hole or edge has to support a side load
… can use a washer if the load is light enough to be spread over the washer
… if the load is high, then edging is required
o The edge is facing an impact zone or wear zone
o The edge needs to be softened (carbon on edge can be sharp)
o The skin is subject to peal forces.
In this example, a Vibration Dampened Electronics Platform (VDEP) is being built to sit within the frame of the ship (motor masts, spars, rotors, and prop cooled ESCs). The VDEP needs to hold the electronics, battery, and camera; approximately ½ a kilogram. That load goes through four holes in the Carbon/Nomex panel.
Two carbon extruded rods (axles) connect the VDEP (think car body) to the frame (think car frame and motor) by going through via four Lantern Style dampers laid sideways (think shocks or struts) that are part of the frame and through four holes in the VDEP. Thus the holes need reinforcement because they carry a load that is too high for Carbon/Nomex material. See photo below.
To reinforce the holes and make the edge aesthetic and bomb proof in crashes, the edges of the VDEP were filled with epoxy adhesive (Scotch-Weld EC 2216). Note that there are many different types of fillers. And many different types of processes. This is the Ditch Route process.
Step 1: Ditch Route the shapes into the Carbon/Nomex Sandwich Panel. But first drill locating holes so the piece can be put back under the 3-axis router again for a final route and be sure to not turn the router off or save the zero point. Route through the top skin and the Nomex, but not the bottom skin. Leave the bottom skin to make it easier to fill the “ditch” with the filler of choice.
This photo shows the ditch for an inner cut-out, and outside profile, and a hole.
This photo shows light shining where the ditch is coming through the skin on the far side, which in this case is only a single ply of carbon.
Step 2: Cut the Nomex flags. The flags will interfere with the filler penetrating the cells along the edge of the cut. This goes a lot faster than it looks when using the right tool.
Step 3: Fill the ditch with the filler of choice. Focus on the side of the ditch that matters. Cure the filler (I used an oven at 200F so the 2216 would cure in 30 minutes).
Step 4: Route the filler and panel all the way through. Use the locating holes in the panel to set the workpiece back under the router. Set z to go all the way through the panel.
And that’s one way to build a light weight and extremely durable 250 FPV flyer.
Replies
I'm curious about gps, you have good signal there?
This is the stuff they use in larger helicopters. Great Advice Mr. Frantz! And it's a beautiful lil guy up there!
like a mother looking at it's baby ... only an engineer can look at that quad and say it's beautiful :-)
A balsa core wouldn't need edging, but it would need sealing to avoid moisture getting into the balsa.
Another option would be to use foam core material such as Rohacell.
Using an adhesive for edging is probably the easiest. It might save you some weight if you make the first cut with a 0,5 mm offset. Then the final cut would also remove some of the (excess?) adhesive.
good idea on the cut ... especially if you don't trust the repeatability of the CNC. on mine, the excess is all cut away as it repeats the trace fairly close.
stay away from foam core. for the reasons you pointed out i love the weight of the stuff and it's edge but it has no peal strength and doesn't show skin ply failure until it is too late.
your idea on balsa is great. don't worry about edge seal. it will absorb moisture through the skin and edge and then it evaporates if you leave it exposed to open air. for several years we made the mistake on aircraft sealing the edges of floor panels. all it did was seal the moisture in. under aircraft carpets, its a dank and wet place. balsa core is also the lighter solution when the edge to area is larger.
Impressive specs and writeup.
I love the look and feel of CF plated balsa core! Even to the touch, it so warm and smooth....Yes I know=> geek out.
But that is purely an esthetical viewpoint.
Nice work !
What is the weight of the frame ?
The frame is a test piece to determine if a VDEP is needed for high quality FPV (I'm hoping not so the frame can be made much lighter and simpler). But i can answer what is the weight of sealing the edge of panels.
The choices are (weights for one side piece approx 230 mm x 40 mm (using epoxy edging; there are lighter alternatives):
- Nomex core with no edging 12 g
- Nomex core with edging 17 g
- Balsa core 22 g
- Normal plastic laminate 60ish+ g (the difference is huge)
So if a 1/8" thick work piece has an area to edge length of less than 10ish, it might be lighter to use Balsa core.
Another metric. Jim and I are building indestructible 250 FPV racers that weigh with battery less than 350 grams. Since F = ma or a = F/m, you can imaging the increase in acceleration when composites are used to build the racer and you decrease mass (m) by a factor of 2. These babies really rip. Now ... if i could just learn to control one :-)
That's a very neat way of sealing the edges. I particularly like the way you've connected the VDEP to the main frame; it'll be interesting to see the effect on vibration.
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