Chapter 22 - Section 3C

Auto Pilot Servos


My auto-pilot is part of the GRT Hx system. It does not require an extra electronic instrumentation unit - saving Instrument Panel space. The only hardware are the two servos that are driven and integrated to the Hx system. When the GRT system arrived, the servos (pitch & roll) were packaged in individual bags, together with a mounting bracket, rod ends, extension rods, all accessory bolts and buts, and a wire harness connecting the servos to the EFIS, and documentation.


Pitch Servo

The most convenience location for my pitch servo is on the bridge (I built earlier) above the nose wheel well, right under the canard bell crank. You may have to look closer for the beige color bridge (where the red/black servo) is being clamped down. The aluminum mounting bracket is supplied by GRT with the servos.









Optimizing Pitch Servo Mounting Location and Length of Extension  Arm

GRT documentation provided clear guidance to prevent over-rotation of the servo arm and mounting location for the auto-pilot to perform its duty without obstruction. Most of us know, at this point of our building process, there is limited space for doing all that - imagine placing the canard right over the servo (as shown in the above picture). I have to find a more convenient setting to figure all these out. It turned out to be pretty simple...


I just tape down a couple of cardboard boxes and boards on my work bench to simulate the height between the canard bell crank to the bridge. Then I sat the canard on top of the card board boxes. I decided to raised the pitch servo about 1" above the bridge surface to allow other wires to route around and under it. Again, if you look close, you can see the 3/4" U-extrusion where the servo mounting bracket is bolted to. By adhering to GRT's mounting guidance/instructions, the length of the extension arm can be determined. In my case, I have to shorten the extension arm supplied by GRT to 3.75" between rod end centers. 


Here's my final mounting position and extension arm length for my pitch servo:

- Height above support bridge (support U extrusion) = 3/4"

- Rod length between rod end centers = 3 3/4"

- Horizontal distance between bell crank center to Servo crank center = 1 3/8"

- Angle between Servo Arm & Connecting rod end to bell crank = 90 degrees

- Distance between forward edge of support bridge to front of U extrusion = 2 3/26"

- Distance between bell crank Center line to front of U extrusion = 2 3/16"


Just for my own record & documentation, an alternative Pitch Servo location can be:

- Height above support bridge = 0"

- Rod length between rod end centers = 4 9/16"

- Horizontal distance between bell crank center to Servo crank center = 1 3/8"

- Angle between Servo Arm & Connecting rod end to bell crank = 90 degrees


Since there may be forces between the servo (pushing the bell crank) and the mounting (bridge) surface, I decided to beef up the bridge a bit. In addition, the side rails (walls) will be used for tie down and hiding the wire bundles running along the bridge.










Roll Servo

The roll servo is a bit simpler that the pitch servo, as far as meeting the GRT mounting criteria. The difficult part was deciding were to mount it. I had three possible locations:

- below and fore of the control stick grip, attaching to the L-shaped bracket (originally made for the manual roll trim);

- behind the back of the pilot seat;

- by the hell hole.


I decided to mount it on top of the main gear bulk head (bulk head with 2 large holes over the main gear) because of ample space to mount the servo. Unfortunately, the right mounting bracket leg (supplied by GRT) hangs over the big holes, preventing a solid footing. Therefore, I have to come up with a different mounting scheme...


As show (left) I made a flat aluminum plate with 4 mounting holes and a large center hole to accommodate the servo. In addition, I added 4 tab bond holes at the lower part of the mounting plate. The 4 tab bonds were floxed and glassed to the aft face of the main gear bulkhead. To support the servo, I added a 1/4" thick Phenolic block and bolt it to the mounting plate as well (note the two lower holes).        









Here's another picture on the mounting location of the Servo.











Connecting the Roll Servo Arm

I made a rod end mounting arm, sandwiched by 4 layers of glass on both sides, and a conforming footing to the torque tube (as shown). After lining up with the servo arm, I secure the rod end bracket to the torque tube with 2 pipe clamps. This attachment method was used by the GRT servo mounting instructions as well.


Resultant rod length between rod end centers = 7". Center rod (without rod ends) = 5.5"








Realigning control torque tubes

After both auto-pilot servos, roll trim and pitch trim are mounted, I re-adjusted the torque tube lengths so that the the ailerons (left and right) are moving in sync with each other as performed in Chapter 19 Section 11.  The following is my new data: 


  Stop Pin at: Left Digital Level Reading Clearance Between Left Stick Grip to Fuselage Right Digital Level Reading Clearance Between Right Stick Grip to Fuselage
Neutral Pilot 12.1 >2.75" 12.2 >2.75
Neutral Co-pilot 12.2 >2.75 12.3 >2.75
Max Left Bank Pilot 32.2 .25" 6.0 .25"
Max Right Bank Co-pilot 6.0 .25" 32.1 .25"