Thinking about a Third Autopilot Servo

[jaredyates]

Has anyone else thought of installing a third autopilot servo? The rudder would be more challenging than the aileron and elevator, because the rudder doesn't use a closed loop system. My goals would be to come up with a method that makes it virtually impossible to foul the pilot-driven rudder cables (obviously), and that it be possible to install the system in a covered fuselage, because, well, I don't have an uncovered fuselage.
This thread shows a version of rudder servo on the RV:

http://www.vansairforce.com/community/showthread.php?t=119555

It looks like the goal is to create a little closed loop somewhere in the system, and connect the servo to that. Here is another idea with a bellcrank mounted to the airframe, but it looks much more cantankerous:

http://www.flion.com/flionwiki/index.php5?title=Yaw_servo_installation

Both of these installations look to me like they put a lot of side displacement on the rudder cable, and being that the RV-10 has a little black plastic grommet at the bulkhead and not even a fairlead, this seems problematic. Wouldn't it be great if a bridle system was able to apply the force in a more parallel and coaxial way to the original rudder cable?

Here's what I imagine for the rigging: visualize a capstan servo with a bridle wire that is oriented span-wise across the fuselage. That bridle wire would then turn aft 90 degrees at a pulley (probably a few more degrees than 90) and join the rudder cable with a bolt-on plastic block like in the Garmin VAF link above. The pulley would need to be vertically as close as possible to the rudder cable, to keep from displacing the rudder cable vertically. I'd create a bolt-on bracket that would hold the pulley onto one of the mostly vertical fuselage tubes, and position it laterally so that the aft-pointing cable exits the pulley on the same vertical plane as the rudder cable. This would eliminate left-right deflection of the rudder cable.
Station F, right behind the cabin, is the easiest to access, but bottom of the mostly-vertical t10 tubes is cluttered with cabin sides, and it wouldn't be easy to bolt on a pulley bracket. Moving back to station G, there is better access to the mostly vertical t7 tubes, and it is close enough to still be accessible from the cabin. The rudder cables don't have a fairlead at station G, so there may be concern with rubbing if they bounce up and down and collide with a pulley bracket. Also, as the rudder cables move aft in the fuselage, they also move up vertically a little. This would give more mounting area on the t7 tube for a pulley bracket.

The standards tell us that a fairlead shouldn't exceed 3 degrees of cable deflection, so that leads me to pulleys. I haven't been able to find the standards for pulleys, other than here:

http://www.aircraftspruce.com/catalog/appages/phenolicpulleys.php

Perhaps someone knows of a better source? I've tried using the specs from the ACS page to narrow down the best pulley to use. Since the bridle cable is 1/16 inch, that rules out all of the MS20220 series. The MS20219 is allowed for 1/16, but the -1 is not allowed for flight controls. So that leaves the -2 and -4. The -3 has a huge diameter bore, so we can rule out that one.
Regarding the MS24566 series, it says AN210-1A, AN210-3A, AN210-1B and AN210-3B shall not be installed on frequently used aircraft controls to bend the cable more than 15 degrees from a straight line. So in that series only the -2 pulley would be acceptable. For some reason the -2 pulley is larger than the -1 and -3, thanks to the guy who came up with that.
I'm not sure to what degree these standards apply in this type of application, especially with not having their source. And we are bending the secondary cable, not the flight control cable. Doesn't a rudder autopilot count as secondary? I'm not sure. It seems that rudder trim would be secondary.

Please share any thoughts that you might have on this topic!

[BTAZ]

Not sure if there is room, but I toyed with the thought of controlling the tension in the front pedal return springs.

Basically putting a small pulley in the vicinity of the existing tab on the firewall tubing that the return spring currently hooks in to.

Then a length of 1/16 cable from the front rudder pedal through the new pulley and up the firewall.

Then another pair of pulleys to turn the cables 90 degrees so they can be interconnected with a pair of springs.

The center of the two springs would join on the end of a lever that could be turned left/right to bias the tension one direction or the other. In your case, this "spring bias" would be controlled by a servo.

Another place you might get some ideas is to look around for a Stinson 108-2 or -3. They have rudder trim from the factory and is required when upgrading from the Franklin to a 180 hp on up Lycoming/Continental so there might even be documentation on the Univair site.

[Rollie]

I guess if I were going to try this, I would consider mounting the servo as far aft as possible and running a pushrod parallel to the cable to one rudder horn. Simple, positive control, and it doesn't interfere with the cables. So on one side you would have the cable and a pushrod exiting the fabric, maybe spaced a couple inches vertically so they never interfere with each other. Might not be possible with the fuselage covered. Not sure how far you can get back into the four place tail. In the Patrol that would be only as far as your arms can reach from the baggage area.

[JimParker256]

How about using an anti-servo tab on the rudder itself, and one of the small RC Allen servos mounted inside the rudder? I've seen guys do it that way on RVs and other metal planes, but I'm sure it could be done on our planes as well. Not sure how that would interface with the autopilot itself, but you could definitely have an electrical switch to control the servo, and thus the trim.

Any of these seem like a lot of additional weight and complexity, versus a fixed-position trim tab set to neutralize yaw at your normal cruise speed... That's probably what I'll do, to keep things as light and simple as possible. If it's not there, it can't break or require maintenance, and the weight of "nothing" is tough to beat!

[Battson]

I would look at running a second set of cables forward from the rudder pedal horns, around pulleys, and upwards parallel to the firewall where the cables can meet and connect the servo to the cable behind the instrument panel. Include one spring in series with the cable run for tension, remove the springs at the pedals.

That approach has several advantages:
- you keep the weight of the servo forward
- no need to run wiring through the closed fuselage
- lots of structural parts to mount things onto with clamps or by bolting onto the firewall directly - no welding required
- its a relatively easier area to perform the install, because you can remove the boot cowl if necessary
- its easy access for maintenance in future

I have been toying with the same idea for an electric rudder trim install.

[jaredyates]

I was thinking less about a trim type of servo and more like an autopilot servo.

[Bdflies]

So, Jared, why are you considering the 3rd axis? Is adverse yaw enough to bother the back seat folks? Or does the 'hawk lack yaw stability to the point where turbulence makes it wag?
Just curious.

Bill

[Battson]

I would look at running a second set of cables forward from the rudder pedal horns, around pulleys, and upwards parallel to the firewall where the cables can meet and connect the servo to the cable behind the instrument panel. Include one spring in series with the cable run for tension, remove the springs at the pedals.

That approach has several advantages:
- you keep the weight of the servo forward
- no need to run wiring through the closed fuselage
- lots of structural parts to mount things onto with clamps or by bolting onto the firewall directly - no welding required
- its a relatively easier area to perform the install, because you can remove the boot cowl if necessary
- its easy access for maintenance in future

I have been toying with the same idea for an electric rudder trim install.

This concept could be applied using a larger autopilot servo, or a trim servo. The autopilot servo is probably the easier application.

[jaredyates]

I like the concept of closing the loop at the front, but am having a hard time visualizing where there would be room for all of that stuff.

[jaredyates]

So, Jared, why are you considering the 3rd axis? Is adverse yaw enough to bother the back seat folks? Or does the 'hawk lack yaw stability to the point where turbulence makes it wag?
Just curious.

Bill

The adverse yaw makes the autopilot unable to maneuver the airplane effectively with roll alone.

[Battson]

I like the concept of closing the loop at the front, but am having a hard time visualizing where there would be room for all of that stuff.

I think one would need to use smaller gauge cable, so the pulleys could be small enough to fit in the little space available.
Seeing as it's not a direct control linkage, just the autopilot, I don't see any risk of using a smaller cable. If it breaks, then you just have to fly the plane yourself. But the chances of that happening would be small, the normal control cable gauge is definitely oversize to create a factor of safety.

[Battson]

Or one could use a mechanical arm linked to the rudder pedal to convert the horizontal motion to vertical, and so remove the need for a pulley in that location.

[jim.mclaughlin924]

Would a light bungee cord interconnect between the aileron and rudder work? Kind of like the systems used on certified planes.

[flyboy]

I had the need for varying degrees of left rudder in my Patrol with different power settings. I also had a trim servo from another project laying around so it was attached to a tube on the left side ahead of the flap handle. I ran a 1/16 cable from the servo back to the rear rudder pedal and attached it with a spring. It took a little trial and error on the size of the spring but it works great.

[Bdflies]

Great info! I printed those pics, for future reference. I'm seeing a need for rudder trim, but haven't really gotten around to thinking about it, yet. I'm grounded, waiting for a vernitherm.
Am I seeing a flap handle extension?

Bill

[jaredyates]

Fantastic Dennis, how is it working out?

[flyboy]

The rudder trim works very well. It does increase the right rudder pressure ( now stretching two springs) so I need to release the tension before landing. Bdflies you are very observant. Yes we extended the flap handle 3 in. It made that 3rd notch a little easier, especially for Donna. The 4th is doable for me but not for Donna.

[Bdflies]

Aha! I'd heard you were using the 4th notch. I was about to reactive an old gym membership.. Either that, or slow below 60 before trying again, but I'm thinking that the 4th notch must feel like air-brakes. A 3" extension sounds better to me! Thanks for the pic Dennis.

Bill