Last time, we learned about composite sandwich structures and we built a custom bulkhead for the BugE’s baggage compartment using Burt Rutan’s Moldless Composite Construction techniques. Today, we’re going to finish the bulkhead by building a support tray for a Grizzly retracting cable reel and by adding a small project box for the countdown timer (described in the last post).
The first step in building the support tray is to construct a set of walls around the cable reel using a couple of different types of structural foam. The blue foam in the photo above is the same 3/8-inch-thick PVC foam that was used to make the bulkhead. This foam, which comes in sheets of varying thickness, is good for making flat panels. The yellow foam is urethane foam. Urethane foam is preferred when the application calls for making complex shapes because it is very easy to carve and sand. In fact, the best sanding block for urethane foam is another piece of urethane foam. All of the foam pieces were cut on a band saw (although a hand-held hacksaw blade would work just as well) and glued to the the bulkhead (and each other) with 5-minute epoxy. I decided to leave about 1/8″ clearance on all sides of the cable reel to leave room for the fiberglass facings and to make installing the cable reel easier.
Once the foam was shaped to my satisfaction, the next step was to glass the interior of the foam. I did this in three separate lay-ups — the bottom of the traywas done first, followed by the left side and the right side. This was done so that gravity would assist me in keeping the lay-up flat during cure.
For each lay-up, micro-slurry was applied to the foam and dry micro was used to form fillets on the inside corners. I used three plies of bidirectional fiberglass for each of the three interior lay-ups. Fiber orientation for each ply was on the bias (at a 45-degree angle to the edges of the foam). The bottom lay-up overlapped onto the bare foam of the sides by at least 2 inches and the side lay-ups overlapped onto the bottom lay-up by the same amount. All three lay-ups lapped onto the bulkhead by at least one inch. After each lay-up was performed, peel-ply was added and the lay-up, the fiberglass and peel-ply were trimmed to within 1/4-inch of the foam and the lay-up was allowed to cure. After cure, the peel-ply was stripped and excess fiberglass was trimmed off with a hand-held hacksaw blade and a 60-grit sanding block.
After all of the interior lay-ups had been completed, the foam was carved in preparation for the exterior lay-ups. Foam was removed from all of the edges so that a glass-to-glass bond could be achieved around the perimeter of the tray. The one exception to this was is the area where the cable exited the cable reel. Here, the foam was left uncarved (see the picture above). I did this so that I could show you how to deal with exposed foam edges and we’ll look at that shortly.
I also made a pair of hardpoints out of 1/8-inch-thick aluminum so that I would have some way of attaching a retaining strap for the cable reel. You can see one of these in the picture above. The 5 lower holes (away from the camera) allow wet flox to bond the straps into the fiberglass structure and the uppermost hole (closest to the camera) is where the retaining strap will attach. All foam and micro were removed in the area under the hardpoint to promote a solid bond with the fiberglass.
As with the interior lay-ups, the three exterior lay-ups were performed one-at-a-time (bottom, left side, and right side). The foam was covered with micro-slurry and the interior corners were filleted with dry micro. On the left and right sides, the hardpoints were installed with generous amounts of wet flox prior to adding the fiberglass facings. Three piles of bidirectional fiberglass were applied on the bias on each side, wrapping around adjacent corners by at least two inches and lapping onto the bulkhead by at least one inch. Peel-ply was applied over each lay-up and the epoxy was allowed to cure overnight. After cure, excess fiberglass was trimmed off and the glass edges were trimmed flush with a 60-grit sanding block.
To finish the exposed foam around the area where the power cable emerges from the reel, I used a Dremel to remove about 1/4-inch of foam between the two fiberglass facings, all the way around the perimeter of the cut-out. Then, using a mixing stick, I troweled in dry micro to fill the “trench’, leaving the micro proud of the edges of the fiberglass. After cure, I sanded the excess dry miro flush with the edges of the fiberglass. This leaves a nice hard surface that can be primed and painted along with the rest of the bulkhead.
The final job consisted of mounting a small ABS-plastic project box that will be used to hold the countdown timer. The exterior of the box was roughed up with 60-grit sandpaper. Then, wet flox was used to adhere the box to the bulkhead. After cure, two plies of bidirectional fiberglass were applied to the outside of the box, lapping 1 inch onto the surrounding bulkhead. The glass was covered with peel-ply and allowed to cure. Red electrical tape was used as a temporary clamp to keep the fiberglass from pulling away from the edges of the box. After cure, the edges of the fiberglass were trimmed and sanded flush with the lip of the box.
Before installing the bulkhead in the BugE’s fairing, I decided to wire up the 110 VAC circuitry to make sure that everything was working properly. The photo above shows the countdown timer, to the left of the cable reel. The fuse, above and to the right of the timer, protects the 48-volt charger. I’m not thrilled about having exposed line current in this area and I am going to see if I can find a better fuse holder to do this job. The retaining strap for the cable reel is made out of 1″-square aluminum tubing, cut at a 45-degree angle and with some angle brackets bolted to each end.
The next photo shows the two chargers and the electrical sub-panel, mounted and wired…
… and the last photo shows the wiring on the electrical sub-panel.
I’m happy to say that everything worked as expected. When I plugged in the cable reel, the charger interlock relay energized. When the count-down timer was activated, the chargers came on line. Best of all, none of the precious blue smoke stored within the electronics escaped.