I spent about half the weekend putting what I think are the finishing touches on my electrical system schematics, at least for now, and making a shopping list of components to buy. Tomorrow I'll place the first order with B&C for wire, terminals, fuse blocks, and stuff like that. Luckily I don't have to buy the really expensive stuff (e.g. alternators) just yet.
The latest revision of the drawings is here.
I ordered and received my second aux battery, a Panasonic UP-RW1220P1. This guy is designed for UPS applications, which is basically how I'm going to use it. It's a little larger than the previous one I was messing around with, but the flatter shape of it actually gives me more options for mounting. It also has 1/4" connectors instead of the flimsier 3/16" ones on the other battery, which is a bonus.
This is the battery tray I made for it. It's all made from 1/16" aluminum angle. I actually started out intending to build it out of 1/8" angle, but it became apparent that it was going to be horrendously heavy and overbuilt that way. Since I'm building an airplane, not a Soviet tractor, I scrapped what I'd done and built this one instead. I was peeved at throwing away half a day's work, but the final version turned out pretty nice.
It's mounted beneath the floor on the passenger side of the baggage compartment, in the wedge-shaped bay just outboard of the strobe power supplies. I attached the side angles to the floor ribs with blind rivets, then shot and bucked the round-head rivets that attach the left wall of the tray since there was no way to get a squeezer down in there. First rivets I've shot in approximately forever.
Here's the rest of the pieces installed with screws. Like a ship in a bottle, I had to make it able to be disassembled so I could put it together inside the fuselage and still have good riveting access.
The hold-down bar is another piece of 1/16" angle, with one of the legs cut down to about 1/8" wide to clear the baggage floor above. The bolts are AN3-12A's.
Here's a perspective view showing exactly where in the airplane the aux battery is located. It's getting quite cozy in there under the baggage floor.
If you look at the datasheet linked above, you can see that the battery's terminals are on the side, which made the design of the tray a little more complicated than it should have been. The right-hand wall of the tray has a notch cut out to provide plenty of clearance around the positive battery terminal. The negative terminal has enough room around it to get the FastOn connector over it, and beyond that I don't really care if the ground wire touches the airframe or not.
After thinking about my chosen battery location some more, I decided that it involves too many compromises, so I'm not going to use it. I found another battery that actually has more capacity and will fit better, in a different location in the airplane. Stay tuned.
Meanwhile, I updated my electrical system design document – the latest revision is here.
One component that I've designed into the electrical system of my airplane is a small auxilliary battery, which will help keep a few critical systems online during engine start. Unfortunately there aren't a great many places where you can easily mount a second battery. I thought for a while about putting it under the baggage compartment floor or even under the seats, but in the end I decided to choose a small battery and mount it to the F-721A forward canopy deck, behind the panel on the passenger side.
I went to the local battery mega-store and picked up this 3.3 Ah VRLA battery. It is a store-brand clone of a Panasonic LC-R123R4P and is the same size, so I should be able to get replacements in the future if I need to. This is a pretty small battery (note the tape dispenser for reference) but it's the biggest one that will fit between the panel and subpanel.
I fabricated some parts out of scrap…
…and made this simple battery box. The base is 0.050" alclad, with 1/8" angles riveted to it (double-flush where necessary). It's probably way overbuilt, but I just used the materials I had on hand. I'm not sure why I primed it – force of habit and superstition, I guess.
Just like the main battery box, this one features a bar to hold down the battery. Unfortunately the battery's terminals are both on top, putting them close enough to the hold-down bar to make me worry about them shorting to it. I worked around this by riveting the bar to a strip of 1/4" nylon that is wider than the bar is, this preventing the bar from being able to contact either terminal. I'm not sure why I had nylon sheet stock in my junk bin, but I'm glad I did.
Here's the box bolted in place, with the hold-down bar temporarily taped on until I can pick up some long bolts of the correct size. Note that the bolts all come up from the bottom – this is because the fuselage skin prevents you from lifting the battery straight out of the box. To remove the battery, you actually have to undo all four bolts and slide the whole battery box out first. Hopefully this won't be too much of a hassle, as I don't anticipate needing to replace this guy more often than every few years.
Here's what it looks like with the skin clecoed in place. There is about 1/8" of clearance between the skin and the aft corner of the battery, which explains why I had to let the rear hang off the F-721 a little bit.
After I took this picture, with the skin still clecoed in place, I unbolted the battery box and removed it just to prove to myself that it can be done. It's actually not a terrible process – I've done worse maintenance on airplanes and lived to tell the tale.
I've also been working on some schematics for the airplane's electrical system. I put together a first draft, along with some notes, and sumbitted it to the Aeroelectric list for review by the learned elders. If you're interested you can cick here to see my current electrical diagrams and some text, including an explanation of how the aux battery will be used.
I clecoed the forward top skin back on temporarily in order to match drill the holes in the new F-7103C angles at the top of the panel.
Then I cut and match drilled the stiffener angle to the bottom of the panel frame. Per the advice I received from Affordable Panels, I made it short enough so that it just fits between the air vents.
I coutnersunk the stiffener angle for its nutplates. There are eight or nine nutplates that are used to attach the panel plates to the frame, and a bunch of rivets that attach the angle to the frame itself. The angle doesn't get riveted on until you're really ready to install the frame for good – the frame will be in there permanently once you attach the stiffener angle and remove the ability to flex and wiggle the frame in and out of the fuselage.
I countersunk the various rivet and screw holes in the panel frame…
…as well as the screw holes around the perimeter of the panel plates.
I clecoed all the panel frame pieces back into the fuselage, and fetched one of the cockpit interior pieces for another test fitting. Not just because it looks cool, though…
I wanted to see how well the interior fits the panel in the area of the vents. The answer is that it fits perfectly. Excellent. Also, I'm feeling like a genius for buying the interior early, since it's gone up in price significantly since I got mine.
The F-793 vent brackets tie the lower corners of the panel to the fuselage side skin. The holes in the skin are prepunched, and after lots of measuring I determined that if you simply mark a centerline on the bracket, line it up with the holes and drill, you'll have placed it with the proper amount of space to accomodate the vent and hose adapter.
Here's the first F-793 bracket. The semicircular cutout is to let it go around the body of the air vent – I made it with successive iterations of bandsaw and drum sander. I drilled the bracket for MK1000 miniature nutplates, which are the only ones that would fit within the remaining flange area.
Here's a test fit – looking good:
The second bracket was a lot easier to make, since I could simply transfer the shape of the cutout and the hole locations from the first one.
I drilled rivet holes along both sides of the radio stack area and match drilled to some 1/8"x3/4"x3/4" angle stock. The thinking here is to use these angles as a place to attach the forward end of the radio trays. I'm going to wait to rivet them to the panel until I have the radios, though, so I can actually measure the hole locations and make plans accordingly.
It was too windy outside to spray paint, but I was impatient and wanted to get a batch of stuff primed, so I did it just inside the open garage door. I got a little paint on the floor but I at least had the sense to move the car to protect it from overspray. It was also cold as heck with the door open – building is easier when it's not winter.
When the primer was dry, I riveted on about a bazillion nutplates:
If you use the stock Van's panel, it attaches to the F-7103 B&C angles via #8 screws and nutplates. With the modular panel, the screws are replaced with 5/32" rivets since the frame is designed to stay permanently attached to the airplane. These are big suckers that my squeezer didn't really like dealing with, so I back riveted them all. It's not often I get to use the red rivet gauge!
Here's another shot of the panel frame components, all finished and ready to install in the airplane.
Next, I riveted the F-721C angles to the canopy decks. There's a definitely order to this – you have to rivet the bracket to the fuselage, then rivet the nutplate to the bracket. If you do it the other way around, the nutplate would get in the way.
Last but not least, I riveted the vent brackets to the fuselage.
Construction of the panel frame is now finished, and the panel is ready for me to start playing around with avionics.