I am continuing to install wires in my Bede BD-4C airplane. This week’s work included figuring out an appropriate conduit through which to run wires on the right side of the cabin.
Many BD-4 builders put the wires inside the tunnel which runs through the middle of the fuselage. I decided against the tunnel for several reasons. The two biggest were a) I did not want to drill more large holes through the main landing gear box, and b) I did not want to work my wiring around the rudder cables and the stabilator push-pull rod, all of which need to move freely and not rub against any wiring.
Since there are wide sills under the door openings, I decided to run a large plastic tube under the right side door and use it as a conduit for the wires. At the back, it terminates near the autopilot servo which controls the ailerons (roll). The wiring continues rearward in the plane for the other autopilot servo (pitch), the magnetometer (electronic compass), and the VOR/glideslope antenna. Still to be run through this conduit are wires for the ELT (emergency locator transmitter) and the intercomm for the passengers in the back seat.
I installed a 1/2 inch tube that was 3/8 inches on the inside. Not big enough. Out it came and I replaced it with a 3/4 inch vinyl tube that is 5/8 inches on the inside. This is plenty big and I think it will easily accommodate the remaining wires. If that is too tight, I can always add a second conduit on the left side of the cabin.
Enough words; here are the pictures. 🙂 The tricky part was the back of the conduit, by the autopilot servo. It has to transition from underneath the door, around the square channel at the back of the cabin, avoid the bellcrank for the ailerons, run underneath the aileron control cable, and lift up above the edge of the aluminum angle so that the wire insulation will not get worn through from vibration. Here is what I ended up with. (Click the photo to see it larger.)
The vinyl tube kept collapsing as it went around the bend by the square channel. I added a bit of ridged plastic tube as a stent to keep the vinyl tube in shape.
It is a little hard to see in the photo but there is plenty of clearance between the aileron cable and the vinyl tube. I checked carefully.
You airplane buffs will notice the black nylon cable clamps. I will replace them with the usual MS21919 cushion clamps. Nylon was simply what I had on-hand for today’s work.
Here is the wide shot, showing the conduit running all the way along the right side of the cabin. It tucks neatly under the door sill. No one will be kicking it as they get into and out of the plane. I will cover the section by the bell crank and autopilot servo with interior paneling. The exit end (by the servo) is under the rear seat.
Next, I pulled some wires. It went pretty easily, except I forgot to pull a string through so I will be pulling one wire back, attached to a string, so that I can pull the rest of the wires. Here is the end near the servo. You can see the RG-58 coax for the VOR antenna, the white CAN bus cable for the autopilot servos and the magnetometer, and red and black power cables for all three devices.
Here is what it looks like from the other side.
Obviously, this is still a work in progress. The wires need to be fastened down between the servo and the conduit. I will also add some nylon spiral sheath to the wires to protect them from the edge of the aluminum angle.
Looking farther aft, you can see the rats nest of partially installed wiring, trailing backward.
In the preceding weeks, I took care of a couple of other small jobs. I installed shells on the DB-25 connectors for the wiring at the wing roots. The tefzel insulated wire is so small, and so slick, that I added a wrap of self-sealing silicon tape to the connectors. This was especially important on the right wing, where there are only a few wires in the connector.
Both wings have wires for the navigation/position lights, the strobes, and a landing light. The left wing also has wires for heater in the pitot tube.
The other small job was to put connectors on the wires for the fuel level senders, one for the gas tank in each wing. I wanted to run these through the same DB-25 connectors that carry the other wiring but that would not work. The DB-25 connectors are inside the wing spar tube. Because of the way the spar is constructed, there is no way to get the fuel sender wires inside the spar so I ended up needing another connector, just for three conductors. The aviation department at Advanced Auto Parts had a good solution: trailer hitch connectors. They are sturdy and easily replaced if damaged.
The unterminated black wire in this photo is a “local ground.” It has been bolted to the metal wing near the fuel tank. The black wire that runs into the connector will be grounded near the instrument panel.
Looking great as usual. I like the plastic tube. I’ve used that in planes as well as electric cars I’ve done. Most recently used it in my 1972 GE Elec Trac E-15 electric lawn tractor restoration.
Art,
Have you investigated the ramifications possible with running autopilot wires, antenna wires etc. in the same conduit. I’ve heard people say certain types of components should be kept isolated from others. You may need to talk to the manufacturers of the units to see if they play well together.
Gale
Gale,
There have been many discussions about interference on the aeroelectric list. As long as the wires are installed per the manufacturers’ specifications, everything should be fine. I am very aware of the issues, though. The intercom seems to be most susceptible to a “ground loop.” If my understanding is correct, you need to carefully ground the shield in exactly one location, not at both ends of the wire. Then all should be good.
I was also careful with the shielding for the strobe lights. You can see the details in my diagram here.
— Art Z.