After adding an electric fuel pump and associated plumbing to alleviate fuel flow issues in my Rose Parrakeet, I had installed a battery and temporary wiring to power the pump while I worked out a permanent solution. True to its 1930’s roots, the Parrakeet has no electrical system at all: no starter, alternator, battery, or radios (I use a handheld), nothing. I was reluctant to use an electric pump with the complexity of the associated wiring and batteries, etc., but for various reasons it made the most sense.
I wanted a backup for the pump’s power supply, however, though it should never be needed. The pump draws 1.6A so the 5Ah lithium-iron phosphate (LiFePO4) battery I picked would run it for over 3 hours, which is more than enough as the pump is only used for a few minutes during takeoff and climb. Periodically charging the battery on the ground would be sufficient … provided I always remembered. Adding a second battery wouldn’t add much weight (it only weighs 1 1/2 pounds) and would provide comforting redundancy.
But adding a battery and wiring leads one down a slippery slope of finding other things to power. Besides the handheld radio, I use my phone while flying for navigation and flight track recording, and sometimes a larger tablet if I’m going cross-country. These devices have sufficient battery for most local flights, of course, but may not be enough for an extended trip, and having a backup is always good.
I decided to build a system using two batteries, with a switch and wiring so either could power the pump. At the same time, the battery not powering the pump could be used for auxiliary power for the electronics. But it would be useful to have voltage monitoring as a measure of charge state, and of course circuit protection (fuses or breakers). This is starting to get complicated!
The phone and tablet use USB power cables, while the Icom handheld radio requires a power adapter providing 11 VDC (not 12 V, curiously). Icom sells a cigarette lighter power adapter for its radios, of course, though I didn’t yet have one, having always used the “wall wart” charger at home to keep the battery charged. For the other devices, I could use a panel-mount USB power port—there are even TSO’d versions for standard category aircraft—but my experience is that USB power supplies tend to fail after a few years, and the fast charge standards keep changing. Installing a simple cigarette lighter receptacle instead would give me the option to easily swap out the USB adapter, and even get a replacement at any convenience store if one failed while away from home. So, at this point I was looking at a system with two cigarette lighter receptacles: one for USB devices and one for the radio adapter.
Finding a location for all this was tricky. The Parrakeet is not a large airplane and space in the cockpit is tight. There is a small baggage space in the turtledeck, but its capacity is limited and I wanted to keep that available for other things. I also wanted easy access for charging or servicing. I had mounted the temporary battery on the cockpit floor, just forward of the control stick. There was just enough room there for two batteries and the associated components.
At the same time, I was considering the appearance. I didn’t want to spoil the plane’s 1930’s antique vibe with modern electronics (at least the radio and phone/tablet are removable), but I recognized that some compromise was necessary. As the cockpit floor is plywood, a wooden enclosure would not look out of place, and wood is usually my preferred material for projects, where feasible. Besides, I had a sheet of eighth-inch plywood left over from a friend’s Minimax project. The dark mahogany plywood wasn’t a color match with the lighter cockpit floor, but being a typical cheap pilot, I used what I had.
Once I started laying out the electronics and batteries, I realized I had a small amount of extra space. The Parrakeet is short on storage space, and there’s no place easily accessible in flight for small items. Adding a small compartment or glovebox on top of the batteries would give me a place for those items. The design started to coalesce with the two batteries in the bottom of the box, a storage space above with a hinged lid, and an angled panel facing the pilot for switches, power outlet, etc. The angled panel also provides the needed clearance for the control stick in the full forward position.
Electrically it’s straightforward, but there is a bit going on. The two batteries feed through fuses to a DPDT master switch with crossover wiring so battery Number 1 powers the pump while battery Number 2 feeds the aux outlets, or the switch can be flipped so Number 2 powers the pump and Number 1 feeds the aux devices, with a center “off” position. Separate switches control the pump and the aux circuit. I found some nifty little digital voltmeters I could use for each battery, and indicator lights showing the circuits are powered.
Partly due to space constraints, I decided to use only a single cigarette lighter socket (for the USB devices), and three 5.5 mm power receptacles on the side of the box. One is for aux power, where I will plug in a pigtail cigarette lighter socket for the radio adapter. That one is controlled by the switch, which also controls the cigarette lighter socket. The other two are for charging the batteries, wired upstream of the selector switch. Charging is done in the center “off” position so the voltmeters and indicator lamp aren’t connected while charging.
The box design went through several iterations in the CAD system before I actually cut any wood. I wanted the entire box to be easily removable from the plane for charging or servicing. I made a few cardboard mockups to see how they would fit in the space and how the box would need to be manipulated for removal. Once I was satisfied with the design, it was time to start making sawdust.
Since the box is nonstructural, seeing no significant loads, I chose to use Titebond glue for its ease of use. I wouldn’t use it for primary aircraft structure (though some have), but for lightly stressed items like this it’s fine. Besides the leftover plywood, I used a radial arm saw to rip some strips for corner reinforcement out of a Douglas fir 2×4 with unusually straight grain. Purchased with a bunch of other 2x4s for a home improvement project, the wood was too good to waste on house framing, so I had put it aside for a project such as this. For the two blocks that slide over the mounting pins and the hinge mounting reinforcement, a harder wood was appropriate, so I used ash milled by friends from insect-killed trees cut near our cabin. The removable glovebox floor, which also serves as the battery holddown, is quarter-inch plywood.
A false bottom with half-inch space under the battery compartment floor accommodates the removable attachment to the floor, along with room for the Molex plug connecting to the pump and the pump switch mounted near the throttle. Except for fuses and the 5.5 mm power connectors on the sides of the box, all the electric components were to be mounted on the removable faceplate, and the glovebox lid would be attached with a piece of aluminum piano hinge and a push-button latch. I considered making the glovebox lockable, but as it’s hard to reach except when sitting in the seat, and non-obvious, I decided a locking lid wasn’t necessary.
Assembly of the box presented no special challenges, though it required an assortment of small clamps and several evenings of gluing up subassemblies. I tested the slide-on mounting on a scrap sheet of plywood before finishing the box, in case any adjustments were needed (they weren’t). Then it was a matter of locating the electrical parts and drilling holes before applying several coats of polyurethane varnish.
For the control labels, I wanted something in keeping with the vintage look, though in reality the airplane builders of the 1930’s probably wouldn’t have bothered with labels. I tried using the industrial laser at work to mark the wood directly, but there was a fine line between getting a crisp mark and setting the wood on fire, and I couldn’t get consistent results. Also, the wood smoke wouldn’t be appreciated in the toolroom! I learned that marking on wood requires a different kind of laser that we didn’t have. I got better results using the laser to mark on thin brass strips, which were then attached to the box using 3M’s VHB (Very High Bond) adhesive tape.
The wiring presented no special challenges, though like many builders I found myself wishing that I’d made the box just a teeny bit bigger. One of the side rails had to be relieved to clear the terminal block wiring.
I originally planned to mount fuses for the two batteries either on the panel or on the box side, and I ordered the fuseholders, but the tight space made that impractical, so I used inline fuseholders inside the battery compartment instead. They won’t be replaceable in flight, but when a fuse blows there’s generally a problem that’s best sorted on the ground, and nothing here is safety-of-flight critical.
One last thing before going to the hangar was to weigh the completed box, which came out at 5.86 pounds including both batteries. I also checked the balance point of the box so I could determine the moment arm for an updated weight and balance.
Installing it in the plane was made easier by removing the fuselage side panel, though that’s a tedious exercise in itself, involving lots of screws and the partial removal of adjacent panels. Even though I had tested the slide-on mount on a piece of scrap plywood, it still took some shimming under the anchor blocks to get it to work smoothly. I also had to replace a brake hose with a shorter one to clear the box, but this was anticipated, and it was old enough to warrant replacement anyway.
To remove the box, the captive thumbscrew at the bottom of the panel is first loosened, allowing the box to slide back about an inch to clear the pins. Lifting and tipping it over gives access to the Molex connector on the underside; once unplugged the box can be removed from the plane. Installation is the reverse.
Flying the plane for a while with the fuel pump switch next to the temporary battery on the floor had made it clear that it would have to be moved to a more ergonomic location. The original plan was to move the radio push-to-talk (PTT) switch to the control stick from its “temporary” location near the throttle and use that hole for the pump switch, but I found I liked the PTT where it was, so the pump switch went next to it. At the same time, I concluded that a push-button switch would work well for the pump as I could press it in the same motion as moving the throttle forward for takeoff. Unlike a toggle or rocker switch, push-buttons don’t give a visual indication of their position, but both the light on the panel and the pressure gauge directly under the switch show that the pump is operating, so I was comfortable with this.
All that was left was to run the wires down to the connector on the floor. The very last bit of wiring was to install the connector … and I discovered that the wrong connector had been shipped to me, in a bag clearly marked with the correct part number I ordered. Grrrr, more delay. But I was able to test everything with the pins inserted individually into the mating connector (everything worked), and finally everything else could be buttoned up. When the correct connector arrived a few days later, it only took a couple of minutes to install.
It always seems to take a year or two to get a new used plane to where I want it and “make it mine.” There are still some minor things to do, but with the fuel and electrical systems done and functional, the plane was finally ready for general flying.
