Installing a complete electrical system in a Stearman carries with it several challenges .
First there is the generator or alternator and the battery installation. Last issue, we talked about one solution to the battery requirement.
After deciding on that configuration there is the next chore of locating and hooking up all of the components that are required to complete the system. This list includes the following:
- High current solenoids for the Master switch, the Starter activation and the engage solenoid.
- Voltage regulator for the generator or alternator
- Current shunt for the ammeter that is back on the avionics/control panel in the rear cockpit.
- Starting coil
- Terminal strips for wiring
One would like all of these components to have a neat looking installation, be accessible for later trouble shooting, have a minimum amount of exposed wiring and so on. Kind of like Boeing would have done it.
There is an additional requirement of not having any hot wire or terminal forward of the firewall when the master switch is off. Also there should be an accessible hot terminal somewhere to attach a battery charger after the master switch is left on overnight.
These components can be attached to the aft side of the firewall directly, or on an separate plate or support. The inter component wiring can then be installed and laced up as neatly as possible. Adding a protective cover for all of this isn’t straight forward.
The wiring to the generator , the starter, the engage solenoid, the engine ground, the pre-oiler pump, and to the magneto from the starting coil, must be routed through the firewall in some safe manner and on to the accessory. This wiring forward of the firewall, over time, tends to accumulate oil and grime on the insulation and ends up looking worse and worse as time goes by.
One solution to all of this is to install an electrical equipment box just behind the firewall. This box should be just large enough to hold all of the components listed above, and include a cover easily attached and removed with small Dzus fasteners.
The next approach is to utilize Aircraft grade braided flexible conduit for all of the inter component wiring.
This conduit is available from AirFlex, who advertises in the Flying Wire.
They have it, not only in several diameters, but in aluminum, stainless steel and brass braid.
This conduit can be customized with the farrells and collars necessary to attach the ends to bulkhead feed throughs, and to the back of Cannon type connectors.
From the EEB (electrical equipment box) it is a fairly simple matter to attach feed through elbows that go through the back of the box and the firewall. Then the flex conduit will attach to the elbow and lead to the accessory. Often there is a cover for the terminals on the accessory component, and the conduit can be connected directly to a feed through on that cover.
If there is no cover, then the end of the conduit can be fastened to a ground point close to the terminals, and a short length of wire run from the end of the conduit to the terminal.
A short length of conduit can be routed from the bottom of the EEB and down to a feed through on the battery case.
So far so good. No exposed components or wiring.
On the right side of the EEB, there will be a two pin electrical connector for a two conductor cable that will lead up the cabane strut to the wing navigation lights. Exposed, but unobtrusive cable.
Now on the right side of the EEB, there will be a 12 to 16 pin electrical connector that provides the wiring to the avionics panel in the rear cockpit. This connector has to have a couple of larger pins for the main power to the console, and as many other pins as your design requires.
A flex conduit with 1/2 inch ID will hold all of the wiring required. On the other end of the conduit will be a similar connector to attach to the avionics panel. To follow good convention, the plug on the forward end of the main conduit should have male pins, and the other end should have female. Then the bulkhead connectors on the EEB and the avionics panel will be selected to match those plugs. The reason for this is to prevent having any “hot” pins when one end or other of the cable is unplugged. That way, anything brushing across the male pins will not cause a short-out.
Now, with the capability of unplugging the main control and power cable to the avionics panel, it is very desirable to provide electrical connectors to the avionics panel for the other functions. These include:
- Wiring to PPT switches in both cockpits.
- Wiring to the phone jacks in the front cockpit
- Wiring to the Hobbs meter actuation switch. (Oil pressure or pitot tube pressure.)
- Wiring to the ELT for remote testing and control
- Wiring to the rudder navigation light.
- Power wiring to a NAV device mounted separately form the avionics panel.
Since most of this wiring is small two conductor, a high quality black rubber jacketed cable can be used. The cable leading to the phone jacks, it should be shielded.
With the altitude encoder in the avionics panel, there has to be a flex line from the “static” system to a feed through on the avionics panel. Then a tube is run from the inside of that feed through to the encoder. With this done, the complete avionics panel can be removed and replaced for trouble shooting or modification in ten to fifteen minutes.
With this configuration, there are four separate conduits and tubes leading from the firewall feed through back to the cockpits, if you have mechanical tachs, and three if you have electrical tachs.
A neat way to route and attach these to the fuselage tubes is with micarta clamps. With two strips of micarta clamped together, drill a hole for each conduit or tube right through the interface between the two strips. The holes should be of a size to snuggly hold the tube. Then at right angles to these hole drill a 3/16 inch hole between two of the tube holes. This hole will accommodate a #10 screw through the micarta clamp and through a tube clamp positioned on the fuselage frame. You can use as many of these as you think necessary.