I need your help. I am beginning to plan the autopilot for my Bede BD-4C airplane. I selected an MGL Avionics iEFIS for my glass cockpit and, as with many EFIS systems these days, the autopilot software is built-in. I need only add a pair of autopilot servos to have a fully functional 2-axis autopilot which will be fully coupled to the navigation instruments, both GPS and ILS.
Here is where your help comes in. I have never mounted autopilot servos before so read on for my questions and share your opinions.
I have a really good place to put the servos in the BD-4C: on the floor, immediately behind the rear passengers’ feet, next to the tunnel. All three controls are accessible in this spot:
- horizontal stabilator push-pull rod
- rudder cables
- aileron cable
Here is the first photo, looking toward the back of the plane. The wood at the bottom of the photo is where the rear seat passengers put their feet. The cable running left-to-right is the aileron cable. You can see the stabilator push-pull rod and the rudder cables inside the tunnel. I set both autopilot servos on the floor, next to the tunnel. Naturally, I will have to cut through the tunnel wall to do the actual installation.
In this photo, both servo arms move fore-and-aft so one could actuate the stabilator while the other could actuate the rudder. (Click on the photos to see larger versions.)
Here is a second possibility. I could rotate one servo 90º so that it could actuate the ailerons instead of the rudder.
Question #1: Given your druthers, which would you control, the rudder or the ailerons?
Next, I need to figure out the physical installation. The skin is pretty thin so I need to attach the servos to something beefier. I am thinking of reinforcing the floor with a sheet of 0.125 aluminum. I would brace it with a piece of aluminum angle, attached to the floor reinforcement and to the lower anchor for the rear seatbelts. Here is a mock-up.
Question #2: What do you think of this scheme for mounting the autopilot servos? Is it strong enough?
Please let me know your thoughts, either with a comment here or by emailing me directly.
Hi Art, do a Yahoo search of Roger Mellema’s BD-4 newsletter #27. He discusses installation of an autopilot system in his BD-4. Made sense to me. Hope this helps.
Thanks, Mike. I found the article here: http://old.bd-4.org/newsletter27.html and it contained some good info. In particular, Mellema wrote about how much better the S-TEC wing leveler worked when connected to the rudder than to the ailerons.
A 1-axis autopilot normally controls the ailerons and is known as a wing leveler. A 2-axis autopilot includes control of the elevators as well as the ailerons. If you were to connect the aileron controls to the rudder, then, if the wings weren’t level, the rudder would simply stabilize (or “coordinate”, i.e. “center the bubble”) of the turn the wings were executing. I would definitely connect a 2-axis autopilot to the ailerons. That way, the autopilot will level the wings. If you have a rudder with a trim tab, you can adjust it to balance the bubble. If not, then you may have to leave a foot on a rudder pedal, but that’s preferable to keeping your hand on the stick/yoke.
As for the mounting issue, how much maximum force would be on both control cables. Add those 2 forces together and it may give you an idea. Keep in mind, if the autopilot breaks free, you may lose the ability to move the control surfaces it is connected to. This isn’t a small decision. Better to overbuild.
As long as you don’t have any flexing in the floor when you put a load on the control rod. You can experiment with that until you get it “beefy” enough.
This is my biggest challenge: finding beefy enough components to support the servos without flexing.
#1. I’d druther connect the ailerons to the autopilot. There’s a lot more control there. Our RV-12 autopilot is linked to the aileron and elevon. I’m sure Vans Aircraft had good reason to do it that way.
#2. forces on the servos probably won’t be that great. I think your mounting plan should work ok as long as you secure the .125 al plate well.
Best of luck
Actually, the RV-12 has flaperons, not ailerons. Oops.
And they don’t call it an elevator trim tab with the elevon. It’s counterintuitive. The proper term is antiservo tab. Go figure…
Art, definitely ailerons and elevator for two axis control. With the lack of dihedral on the BD-4C you won’t “pick up” the wing like you could in the Arrow, you will just get yaw, which you (and your passengers) won’t like the feel of. Usually when you go to the third axis it is because you need a yaw damper, most jets have them, they operate full time but can also be controlled by an autopilot when needed (think autoland on a jet.)
Dave, on a full flying stabilator the anti servo tab is working with keep the surface at neutral, it also provides force or “feel”. That is the “anti” in the name, resisting movement. The PA-28 series has had them since 1960 or so. A servo tab is used to help the surface move, the DC-9 series airplanes have them. They help reduce the force needed to actuate the surface. Mr bill can provide a far better description of how they operatethan I can, I’ve never flown the -9 and only know it as an example.
Cheers