This is specific advice on installation of the Digitrak II Autopilot in a Vans RV-8.  This could also prove valuable to builders of other Vans kits or other aluminum kitplanes.

Mechanical Installation

Installing the pitch trim went according to the plans.  But the roll trim drawing isn’t correct.  It shows the bracket backwards.  This wasn’t a big deal.  You can see in the picture how everything still fits.

The drawing showed how long to make the pitch push rod.  To be sure, I connected the elevators, the F-841 push rod, and the F-635 elevator bellcrank to the servo.  Then I made sure it could travel from stop to stop without reaching the limit of travel of the servo arm.

The roll was a little harder.  The drawing showed the push rod to be only as long as the two ends.  So I was assuming that was the right length.  I didn’t finish hooking it up until the wings were mounted, and the ailerons fully installed and rigged.  Only then did I know the full travel of the WD-807 control column.  Then I clamped the torque tube bracket to the WD-807 with a side grip cleco and ensured I had full range of motion and didn’t reach the limit of the servo arm.  After I was satisfied, I drilled the hole and used a blind rivet to hold the torque tube bracket.

Wiring

I started wiring the autopilot at the bench.  I had already measured, cut, and marked the wires that needed to go into the d-sub connector.  Working in an upright position with good lighting is much easier than on my back under the instrument panel (See Helpful Hints for Soldering Terminals).  After the panel end of the “bundle” was wired up, I threaded the other ends through the bulkheads and conduit.  See Use multiple wire bundles for what I would have done differently.

Unfortunately the smaller d-sub connectors don’t fit through the holes and conduit.  This means I had to wire them inside the fuselage.

True to form, I also had to change the pins for the roll trim and add the jumper between pins 1 and 2 for the pitch trim.  According to the Digitrak II Wiring Diagram Notes 1 and 2, if you need to reverse the direction of the servos, you will have to make these changes.  Murphy’s law struck again, and I had to make both changes.

If your servo installation looks the same as mine in these pictures, you should start with main unit pin 21 to roll pin 4 and main unit pin 20 to roll pin 5.  You will also need to jump pins 1 and 2 on the main unit.

Testing

I tested the electric connections by hooking up the battery from my truck to power the unit.  Don’t hook up a trickle charger as it might ruin your electronics.  The small 3 amp power supply I had wasn’t enough to run the servos, so I used the battery.

It was easy to check the roll, I just set the unit to HDG Hold, then turned the knob.  The autopilot tried to turn the plane to intercept the new heading.

The pitch trim was a little harder because I had already mounted the autopilot unit into my instrument panel and didn’t want to pull it out.  So I put the Digitrak II into ALT Hold mode and raised and lowered the tail.  This was enough to raise the unit itself, causing it to move the elevators.  I would suggest keeping the unit out, where you can easily move it up and down to do this test.

Return from Autopilot to Electrical Systems

Return from Autopilot to Kit Plane Advice Home