Aileron Trim Servo

It’s been a little bit since my last logbook entry, but have some great news to share. I recently made an exciting career change — I accepted an offer to work for a startup company called Merlin Labs and I’m helping create an autonomous flight system for retrofitting into existing aircraft! I’ll share more about this in a separate dedicated post.

Back to my plane, one of the mods I wanted to incorporate is an aileron trim system. Years of flying a C172 that likes to roll to the right has demonstrated to me that my plane should have a means to trim the aileron. The Sling 4 does have electric elevator trim as standard, but unfortunately no option for the ailerons. Van’s Aircraft sells a small mod kit for the RV-10 that installs inside the wing, and attaches to the aileron push rod via a pair of springs. A linear servo (made by Ray Allen, same as what Sling provides for the elevator servo) adjusts the center point bias on a pair of springs hooked onto the pushrod, which is what accomplishes the trim. The wing and control linkage design is quite similar between the RV-10 and Sling 4, so I decided I would find a way to make it work.

I figured I’d start with an almost-complete picture to better visualize the modifications. I chose to locate the servo assembly next to wing rib #7 as this rib has a large lightening hole in just the right spot for clearance of the springs. Like on the RV-10, I’ll mount the servo to an inspection cover, but in this particular location I needed to create a larger hole since there’s only an existing small aperture in this location.

Trim servo install about 75% complete
Location for the trim servo
Snapshot of the Vans install instructions showing how the springs attach to the aileron pushrod

First task was to update the wiring harness. I’m using one of my Aerobaud control modules out in the wing to power all the lights and now this trim servo too. I recently discovered the type of tape widely used in the automotive world called Tesa tape. I’m still using a braided nylon jacket to protect the wires since they’re mostly inaccessible once the wing is closed, and then I use this tape at the ends. I used a new software package called RapidHarness to design and build this cable, it’s a really nice tool and I definitely recommend it. They have a free community edition.

Sample diagram made in Rapid Harness
Tesa tape used to cover the y-junction
Roll of Tesa tape
Mostly-finished right wing cable assembly, just needs pins and connectors

To figure out the best placement of the servo, I temporarily installed the aileron push rod, so that I could determine where to cut the hole in the skin. With the assembly jigs in place you can’t install the push rod, so I cut a small access hole into the jig on the wing root side.

I had to cut an access hole in the assembly jig to allow for inserting the aileron pushrod

I couldn’t find much guidance on how to place the servo, so I decided that the servo spring arm should be spaced 1.5x the radius of the spring away from the push rod, to preclude binding or rubbing of the spring and servo arm. The push rod itself doesn’t move linearly, but instead follows a bit of an elliptical path due to how it attaches to a bell crank, so I needed to determine what path the pushrod would follow once installed.

I measured the joystick throw for the ailerons to be 51mm from stop to stop (the stops are in the floor of the fuselage). Inside the wing, there is something of a secondary stop; the bellcrank for the aileron swings from stop to stop for the pushrod to slide 110mm – again, not quite linearly but more of an elliptical path. Obviously the stops in the fuselage will prevent the bellcrank from hitting its stops. This information was important to know because it determines where to place the servo assembly to achieve a spacing of 1.5x the spring radius, or about 14mm. Until I assemble the wings to the plane, I won’t know the exact position of the pushrod for ailerons neutral, so I’ll have to wait to install the springs later, but for now I have the information I needed to place the servo and cut the hole.

Finding the location for the trim servo

The next photos show the process of cutting the hole in the skin with a nibbler, followed by smoothing with a Dremel.

Hole cut pattern
Nibble nibble
Hole cut and cleaned with a Dremel sanding disc

I cut an access cover out of 0.020” thick 6061 aluminum, which is a little thicker than the kit plates which are 0.016”. The bracket for the servo is made up of two z-sections riveted together, and when attached to this plate will act as stiffeners.

Match drilling the access plate to the new hole

To restore strength to the skin and to reinforce the nut plates the access cover attaches to, I made a doubler from a junk piece of wing walk skin, which is 0.032” thick. The doubler is a little oversized so as to pick up a line of rivets shared with wing rib 7. Attached to this doubler are MS21047 nut plates.

Recycling a malformed wing step skin for a doubler
In process photo drilling holes for the nutplates
Test fitting the skin doubler
Test fitting the various parts

I’m off for a few days to visit family in NC, but when I return I’ll finish the installation by attaching the servo assembly to the cover. I will bond the doubler to the skin with leftover fuel tank sealant, and then pin out the electrical connectors.

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