Bulding an RV-7 in Oz

Panel Wiring

The instrument panel is the intelligence heart of the aircraft. It's instruments tell you how the aircraft is fairing. It's avionics allow you to communicate with other aircraft and ATC. It's navigational aids allow you to fly by instruments. Its switches allow you to activate or kill all the systems. There is a lot to do to connect everything up, and this has been one hell of a learning experience.

Early on, I decided that i would do all the wiring myself. It is commion for builders to get others to do their wiring, and that's fair enough. You don't want faulty connections etc giving you grief in the air. I decided to do it all myself, mostly because I actually enjoy the challence of electronics, and my engineer technical counsellor said that he thought it would be good to do it ourselves.

There is a lot to learn. Basically there are THREE major goals to this part of the project - 1. to wire up the avionics stack and connect all aerials and headset/microphone devices; 2 to wire up the engine and electrical flight instruments; 3. to wire up the connections from the battery to the electrical bus and starter, to wire up the connections from the alternator to recharge the battery and supply current in flight, and to connect the lights, strobes, pitot heat and autopilot servos in teh wings and fuselage..

Avionics stack

Audio panel - allows the pilot to select inputs to and outputs from the various radio instruments - Garmin GMA 340
Communication VHF radio - the basic communication device - King
ADF Nav Radio - it has a needle that points to a beacon or a radio station and enables intrument flight
Transponder - communicates with ATC and tells them who the blip on the radar is

Instruments (electric)

Autopilot main control
Gyro turn co-ordinator
EI Timer (for IFR)
EI Fuel level indicator
EI Electricals - Volts/Amps
EI Oil Temp and Pressure
EI Tachometer and engine timer
EI Temperatures - CHT, EGT, OAT

Battery, alternator and starter

Battery enclosure and connections
Master relay
Starter relay
Alternator output to man bus via ALT breaker and Ammeter
Master relay output to main bus

The basic problem

All of the devices listed above need wires going to them to provide

Power (+14VDC) - this comes from the master relay and the alternator via two massive 8 gauge wires to the main busses in the panels (in the case of the Alternator, through a 60A breaker and the EI Ammeter/Voltmeter
Ground - these are usually provided by screws and bolts connecting directly to the metal airframe which is common electrical ground. This is NOT used for audio lines, which are usually sheilded dual pairs. The sheilds are all grounded at a common ground near the radios
Probe sensing inputs (where applicable) - these are temperature, pressure and fuel level sensors, connected by wires to the various instruments
Lighting and dimming inputs (where applicable) - I have the Van's dimmer circuit which I hope is compatible with the EI instruments. It is driven by the NAV light power switch (along with eth NAV lights)
Aerial connections (for radios, ADF and transponder) - sheilded cable and connectors are required
Nav Lights, strobes and heated pitot - these are carried by suitable gauge wires connected to these devices

The basic circuit

Vans and various publications offer versions of the basic circuit of aircraft electrics. It is pretty simple really -

The battery is connected to the master bus by the MASTER RELAY. This relay is normall open. but is closed when the pilot closes the MASTER switch in the cockpiy. This delivers power to the STARTER RELAY and the MAIN PANEL BUS in the cockpit, which can power everything else.
The ALTERNATOR output is connected to the same MASTER BUS via a 60A circuit breaker. It therefore can charge the battery and power everything as required, once it is energised. To work it needs a FIELD connection to be energised, via a 5A circuit breaker. The same DPDT switch that makes the MASTER connection also makes the FIELD connection.
The STARTER RELAY connects the MASTER RELAY output from the battery to the starter motor. When energised (by the pilot) it enables the STARTER MOTOR to turn over the motor, hopefully to start the engine
The MAIN PANEL BUS is a couple of pieces of copper or brass which connect to the various switch/ circuit breakers in the panel.
In my aircraft these are -
- Auxiliary Fuel Pump (5A) - to pump fuel from wing tanks to engine when the engine-driven pump is not working i.e. before start, and for safety during take-offs and landings
- Radio Master (10A) - enables all the radios to be switched off at one location - makes it easy to turn on radios AFTER engine-on
- Nav lights (10A) - applies +14V to the nav lights to illuminate them at night. Also used as "dimming" indicator for night mode instrument displays
- Strobes (5A) - provide peak visibility in day and night conditions
- Landing light (10A) - applies +14V to the landing light when required
- Pitot heat (10A) - for IFR, but I will definitely NOT venture into known icing conditions
- Autopilot master - a panel switch that powers up the autopilot. If it goes berzerk, I can disable it with this switch
- Alternator FIELD breaker (5A)
- Instrument/gauges breaker (10A)
- Main Alternator breaker (60A, feeds to the BUS - all others feed from the BUS)

Wirws

I used aircraft standard TEFZEL coated wire, purchased from an aircraft supply company and Vans. The gauge (thinkness) of teh wire must macth the load carried. More current needs thicker wire to avoid heating and fire risks. Van's Instructions have a sample circuit diagram which includes wire selection guidelines.

Making crimp connections

The basic building block of the instrument panel (and most other) wiring is the AMP crimp connector. They come in various hole sizes for different wire diameters, and various attchment hole sizes where the terminal is screwed to a terminal strip other attachment.. A medium-size connector can take one medium sized wire (say 12 ga) or two small size wires (say 18ga). So if you want to join two wires together you need a larger diameter than would normally be used for that wire. The basic rule seems to be that, once strippd, the actual metal wires (either 1 or more) should just fit snugly through the smaller diameter hole and just peep out the other side (image 1), while the insulator should fit into the larger-diamater part of the hole, but not the smaller. In image3 1 below I have stripped two small diameter wires (20ga) and fed them both up into the hole of the blue plastic skin (medium size) crimp connector. Had it been only one strand Iwould have used the smaller (red plastic skin) diameter bore type of connector - you get the picture? In this case Iwanted to join two wires together in the connector. Image 2 shows the crimper squeezing and image 3 shows the result. You MUST test that there is a good electrical connection, and that pulling on either wire does not dislodge it from the connector.

Foreward terminal strip - connections to all EI instruments

Image 4 shows the forward fuse superstructore right upper rib on the left (looking from the front), with a black terminal strip bolted in place. This terminal strip carries all the common lines required to power and light the six Electronics International Instruments - mainly Power, Ground and Lighting/Dimmer. You can see small silver screws (with built in star washers) on each terminal, and a few wires with crimp connectors l located on these terminals. None of the wires are yet bundled together so it is a bit of a rats nest. Image 5 shows the other side of the sub panel. You can see wire bundles running through plastic bushes, taking the wires from the front terminal strip to the EI instruments in the left instrument panel. Ihad to leave a lot of slack in these runs to allow the panel to be removed with access to the big round connectors which join the instrument wire harness to the panel wiring. This front terminal strip connects wires from each instrument to:

Power from the Instrument/gauge breaker
Panel dimmer power supply
Ground (not on the terminal strip - a small brass machine screw attacheed to the rib itself (aircraft ground).

Image 6 shows this strip seen from the left side of the aircraft - you can see the ground connectors to the left. These are held in place with star washers and locknuts. The primer has been removed to allow a good connection. The main problem with this strip will be difficult access if I ever need to change any of the connections. I intend to test everything before the top skin goes on, so hopefully this will not be a major issue.

The fron of the subpanel provides mounting space for the Transponder height encoder (image 7) which connects to the transponder via a DB 25 miniatue connector. You can also see the wire harnesses running to each of the EI instruments, through snap-grommets in holes through the subpanel.