If you have been following along, you know that I have spent the last couple of weeks tossing ideas around, and getting lots of suggestions, for how to mount autopilot servos in my Bede BD-4C airplane. This is the fourth in my series of blog posts on the subject and, I am happy to say, I have actually drilled aluminum and bolted stuff in place. If you are just joining the party, you can jump back to the other three posts:
- Planning a BD-4C Autopilot Servo Installation
- Thinking about Mounting Autopilot Servos in the BD-4C
- Autopilot Rudder Servo First Test
Among all the people who generously offered me their advice, one has some significant aeronautical design experience. Another person owns a BD-4 that has been flying for about 40 years with autopilot servos installed and was kind enough to help get some photos to me. Based largely on their advice, I decided:
- not to put electric motors inside the enclosed landing gear box, and
- to put the servos on the stabilator and ailerons (not on the rudder)
I had been worrying quite a bit about how to rigidly mount the servo motors. They weigh about two pounds each and the BD-4C is surprisingly short on hefty components to which to attach stuff. The skin is metal but very thin and flexible. The aluminum angles that comprise the bulk of the fuselage are 1″ x 1″ x .063″ and twist significantly if I bolt a servo directly to them.
From the plane that has been airborne for 40 years, I learned that it is OK if the servo mounting is not perfectly rigid. Specifically, it is OK if the back of the motor is not also attached to something. That opened up several good locations and greatly simplified the mounting.
I put the aileron (roll) servo under the back seat, on the copilot’s side of the plane. The aileron cable is well exposed and I was able to attach the servo support to the large aluminum angle that is the lower, right edge of the fuselage and to a diagonal piece that runs across the bottom of the fuselage. This photo was taking looking toward the back of the plane. (Click on any photo to see a larger version.)
Here is a photo looking forward. Without the servo in place, you can see how I fabricated the mount.
I decided to mount the stabilator (pitch) servo farther back on the floor of the fuselage. By moving it behind the baggage area, I gain access to a bellcrank in the middle of the stabilator push-pull rod. I can attach the servo actuator to this bellcrank without drilling any holes and risking weakening this “somewhat critical” component of my airplane.
Here is the long shot. You can see that I needed only a single 10 inch piece of angle and it mounts securely between an angle running across the bottom of the fuselage and a second angle running diagonally. The bellcrank is the black piece at the upper left side of the photo.
Here is a close-up looking aft, with the servo set in position.
The next jobs are to fabricate the connecting rods for the two servos, riveting the rod-ends into pieces of 6061 aluminum tubing and then to run wiring from the front of the cockpit back to the servos.
I was trying to remember who had an auto-pilot in their BD-4. Couldn’t remember.
Hi Art. Are you installing inspection panel by the autopilot incase you need to change the motor?
It will be awkward but I ought to be able to service the pitch servo from the baggage area. The roll servo is easy to reach, underneath the back seat.
It is Eric’s plane.
Art, your work is looking really nice! I think you’ll be very happy with controlling roll vs yaw. If the mounts are rigid in the direction the servo exerts force (which it looks like they are) you should be good, you’ll know when you start flight testing!
BTW, your last picture caption looks like it should read “Elevator (Pitch) autopilot…”
I fixed the caption. Thank you for pointing that out, Jim.