Matt's RV-7 Project

This is what I woke up to today:

METAR KLWC 211352Z AUTO 30010KT 10SM CLR M16/M23 A3048 RMK AO2 SLP340 T11611228

For those of you who can't read a metar, it was 3°F (-16°C) while I was drinking my coffee this morning. With the winds at ten knots, the windchill was thirteen below zero. Ouch! In the garage, the heaters tried but failed to get the mercury up to the good side of freezing:

I spent most of the weekend inside, trying to find ways to advance the progress of the airplane project without actually having to touch cold things. I put in some CAD hours cleaning up various aspects of the electrical system and panel design, and I also soldered together the second revision of my annunciator controller board and wrote some test software for it. This time I used an Arduino Nano for the brains, which made it super easy to get the board checked out and the drivers written. Here's a photo of the finished product… the blue goo is RTV, used to keep the electrolytic capacitors from shaking loose:

I mounted it to the center subpanel rib with PCB standoffs. After that, the cold chased me back inside.

Also, my cats hate christmas:

My airplane has two B&C voltage regulators, one for the primary alternator and one for the secondary. Each regulator has a flange with quarter-inch mounting holes, which is way overkill for any bolt you'd conceivably use to mount these things. Luckily I found these 10-32 clip nuts, which coincidentally have a bushing in them that perfectly fits into a 1/4" hole.

Regulators mounted up. I put them on the top of the avionics shelf, on the passenger side. The bolts come up from the bottom, so I can remove a regulator while laying down under the panel (assuming I ever have to). I got the ground terminals, case ground lugs, and field wires hooked up before I got tired and cold.

Here's a shot of the fusible links that I added for the alternator field power supplies. Power to each alternator field comes from its respective bus via a fusible link, goes through a 5 amp circuit breaker to the bus master switch, then runs over to the regulator and out to the alternator.

I also spent the last few evenings working on a redesigned annunciator controller. It will be ten times more awesome than the previous design, thanks to some cool shift registers that I found.

PCB layout has the same effect on my brain that Tetris does. Should I find that alarming?

I decided to install an alternate static air valve in my airplane. I figure that for the amount of effort involved, it's cheap insurance against loss of air data due to a drop of rain getting into the static system – see here for a real-world example of why you might want such a thing. Lots of RV builders use a simple fuel drain fitting under the panel as their alt static valve, but (of course) I wanted to use something fancier.

I bought this miniature pneumatic toggle valve for twelve bucks, which is a smoking deal in the aviation world. It works like a SPST switch for air, and the form factor is almost identical to a standard 15/32" electrical switch.

Since the valve wasn't fitted with any kind of anti-rotation mechanism, I used my Dremel to cut a groove in the threads for a keyway washer.

I mounted the valve in the extreme outboard corner of the instrument panel on the pilot's side. It was a little tricky to locate this hole accurately, since the panel frame makes things pretty tight in this area.

To protect against accidental activation of the alt static valve, I put one of those cool spring-loaded switch guards on it. It's fastened to the panel by the nut on the switch body, and I also drilled and tapped holes for a couple of 4-40 screws to improve its resistance to rotation.

These switch guards are so cool. I wish everything had these. A coffee machine, a vacuum cleaner, your iPod – you name it, it becomes more awesome when the switch that turns it on looks like it came out of a T-38 cockpit.

I'll finish plumbing the alt static valve once I get all the rest of the instruments installed. It will be minimally complex – just one extra static air line to run to the valve. Under normal circumstances, the valve is closed and that extra line does nothing. When the switch is flipped, the static system is opened to the cabin. Very simple.