Aircraft Wiring for Smart People

I'm not sure what number is higher: the number of hours I've spent working on my RV-7A project (more than 1,700) or the number of seminars I've been to and number of times I've slogged through AeroElectric Connection, hoping that something will click and I'll be able to go ahead and wire up my airplane.

It's simple, I know. Most people around Oshkosh tells me it's simple. The usual conversations go something like this:

"Just take a hyrdrostatic compost nuclear whack and attach it to a B4598 connector, making sure to Wizard of Oz it to the Dufus-o-meter. Run another wire back to the platypus Mozart spectrograph, flip the switch, uttering 'boogota, boogota, boogota' and voila. Simple."

So I was a little hesitant to take in the Wiring for Smart People (Monday 10 a.m.) forum with Greg Richter, Blue Mountain Avionics, but then I figured, 'what the heck, maybe my whole problem so far has been that I'm much too smart. That's my story and I'm sticking to it.

"I adore the 'Dummies' books, but I figured anyone who can build an airplane, and stick with it to the point of wiring up systems is not dummy," Richter says.

"Hello, Greg. My name is Bob.

Greg talks about a hundred miles an hour, faster than my ability to comprehend, but fortunately he handed out a document -- Aircraft Wiring for Smart People- A bare-knuckles how-to guide -- that certainly turns the universe upside down a bit because it seems to fly in the face of a lot of conventional wisdom I've heard from other sources. (It's clear, for example, that Bob Nuckols and Richter don't see eye to eye)

On the other hand it's written in English -- plain English -- and captures a step-by-step approach in 36 pages. The forum was mostly a recitation of the document, so I'm going to paraphrase the document.

I'm guessing some of the people who read this site regularly aren't going to be happy about what's dispensed here. But that's the breaks; send a note with your response and I'll be happy to post it here. I'm merely passing along the contents of a particular forum.

"There are a thousand ways to wire anything and 800 of them work great," according to Richter. "If we build satellites and missiles so that they have to work, we should build airplanes the same way.

Greg than outlined the salient points of his book, which I've made note of here:

12 or 24 volts - He recommends a 24 volt system "If you have a battery with enough grunt to start your engine, it'll run your avionics for long er than you can remain aloft," he advises to people who want to install a 12-volt system.

Alternators: Q: How many alternators can I put on an airplane.
A: One is plenty." A second alternator is a great idea, but I recommend you put it on a second engine." If you have a good alternator and regulator (he likes B&C stuff) the most likely reason for failure is bad wiring or overload.

Secondary alternators are a neat idea, but with a single engine, it doesn't really buy you that much more time aloft, and buys you none at all when the engine quits.

Firewall connectors -- Drill a hole run the wire through it and "RTV it." The connectors that are so expensive, are aluminum. "An aluminum connector on a firewall is not going to last longer than drilling a hole and not playing with it.

Manual/auto - "My thought is simple: No manual override, cross feeds or other Apollo 13-wanna-be switches in the electrical system. If you're at the point where you're flipping emergency switches, and you're having that kind of a day, it's probably better to just land."

Batteries - How many batteries. if you go with 24 volts: One. Provide multiple busses, not multiple power sources. "Just like your house, car and computer, multiple busses and breakers solve the problem, not multiple power sources. how many people do you know who have more than one Power Company feeding their house?

Power & Ground - Whatever electric current goes into something, has to come back out and return to the battery. Anything you power up needs two wires to make it go -- one for power and one for return.

Bad grounds cause more problems than just about anything else. Richter's solution is to "ditch the whole, outdated, tragically useless concept." Two wires.

Sizing wires & breakers: Breakers are 10A for everything wired with a #18, which is almost everything. If you have a load that draws more than 8 amps, you need to put it on a separate breaker. Breakers should be loaded to less than 80% of their capacity.

A common practice in homebuilding is to size the breaker to match the size of the load. this sort of makes sense, but when you think about it, it really doesn't. The breaker is protecting the wiring, not the device being powered. If the wiring shorts out, we want the breaker to pop instead of melting the wire.

Antennas: For VHF, Nav, Com, Localizer etc : use a commercial whip antenna and make sure that the bottom of the antenna is connected to the skin of the aircraft. No paint, oil or any other yuck. This needs to be a solid electrical connection.

Transponder antennae put out a lot of watts and you don't want to sit near them. "I see people putting them under the seats, which is an amazingly bad choice. Microwave antennae you want to put as far away from people as you can."

You should be able to see 12 satellites when you're flying. "Most of the time I see 8 or 9," Richter said. "You have to put the antenna where it has an amazing vew of the sky." You want these things to be as high on the plane as you can. They're great on the cowling on the front, or the empennage on the back. Keep it outside the wing platform.

Magnetometer -- If you attach it with AN hardware, it won't work. Iron messes things up and stainless steel is still steel with an iron content.

GPS - Common troubles are putting them partially under a metal glaresheidl, beneath carbon fiber, or in the cockpit of a high-wing design. GPS can "see" through fiberglass, but not carbon, and certainly not aluminum. Got it next to anothre antenna? That's a chuck of sky you'll miss too!

Making the audio quiet - Follow this rule: All circuits are wired with a power lead and a return of the same size. This means that microphone, headphone, CD and everything else gets connected with two leads and that the return leads are all connected to this audio "ground bar." it doesn't matter where or what ground is, as long as all the wires to to that one place. The PS Engineering crew has a drawing in their installation guide that's worth looking at. (Somewhat unrelated but interested discussion here.)

Shielding -- Make sure you connect the shield at the ground buss end and cut it off flush at the other end. If you want it quiet, give it it's own lead and shield it. Shields are connected and the source end, and cut off flush at the load end.

Keep these signals on their own separate cables:

* Each headset
* Each microphone and push-to-talk (PTT)
* Each audio source (CD, cell phone etc.)

Engine wiring - Wiring in the engine compartment is simple with an Engine Pod from BMA (Installation guide here).

TOOLS

Solder and flux : - Good solder comes as a spool of soft wire and is 63% lead and 37% tin called 63/37 solder.

Snakeskin - This knitted cable covering makes the difference between an OK and an exceptional looking installation. Put your spun leads inside the snakeskin andyou're on your way to a truly gorgeous and easily repaired airplane.

Digital volt ohmmeter - You don't need anything super-cuper here, but you do need something that can measure DC Volts, Ohms and has a continuity checker that beeps. Anything else is gravy for this kind of work. Make sure the thing is digital.

Automatic wire stripper -- An automatic wire stripper saves time, money and frustration and is cheap, cheap, cheap. Nicked wires caused by bad manual stripping ae about half of the broken connections, I think, I see.

Flush cutting dykes - these are the normal "wire cutters" that everyone uses, with a twist -- they make a flush cut with no burr. Do not use them for cutting safety wire. It'll nick them badly and they won't make a clean cut anymore.

Coax stripper for BNC connectors -- These nifty little gizmos strip the outer jacket, copper braid, and inner insulation on coaxial cable in one go, and are available from seveal makers in varying price ranges. You won't be doing too many BNC connectors, but for what it costs, it's cheap insurance that your connectors will all work right the first time.

Daniels crimping tool -- This is the one to use for all the D sub connectors you see on avionics. If you are farming this out, you don't need it. These tools make a perfect crimp every time and are pretty expensive. It's worth borrowing one for as little as you'll use it, but use it you must, as there is no substitute.

Crimp connectors are traditionally is what's used on light aircraft but not on satellites because they have to work, which is why I recommend soldering everything. Crimp connectors exist not because they're better, but because it's easy to train people to use it in a low-production environment. Connectors will stat oxidizing and in about 10 years you'll have something to do.

Soldering iron - You'll need one in the 35-watt range.

Desoldering pump -- You'll need one of these for things you solder together and discover you didn't mean to. A good one costs about $20.

Solderless splices and terminals -- The automotive-style splices are horrible. I've spent more hours chasing bad butt splices and under-crimped ring terminals than I'd care to count. Soldering takes a little more time, but once it's done, it's done well and completely and that's the last you'll ever mess with it.

CONNECTORS

Power connectors -- The ubiquitous "molex connector" shows up all over the place.

Battery and alternator connectors -- Big power leads like battery cables should have big lugs sweated on with a torch. Dunk the lead in flux, heat it up and tin the exposed copper. Put some flux in the lug, getit hot and stick the tinned lead in. Add solder until the lug is full. The connection will be low resistance, won't corrode and will look good.

Coax connectors -- BNC connectors are about all you'll see in light airplanes. anything else, follow the instructions or get a pro to help you. There's nothing magic about SMC, TNC or the rest, just no sense in beating yourself up to put on ONE oddball connector ONE time.

TECHNIQUE
Learn how to solder, And the best guide to learning that is here. The only thing I'll add is that solder and shrink tube come as a pair. If you solder a lead, you have to put shring tube over the connection to keep it from oxidizing, and most importantly to strain relieve it.

If you want to read the entire Aircraft Wiring for Smart People book from Blue Mountain Avionics, simply go here.

From the archives:
Vertical Power is ready to fly
Beginning an electrical system design for your RV airplane

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