Framing an Aileron Trim Tab

First real day of work – 5.5 hours.  ($5.25 spent on drill bits)

These are the ailerons as I got them.  As you can see from the one on the left, it is missing the nose ribs.  I started looking everywhere for them, and finally found them mounted inside the other aileron, along with 2 extra tail ribs on the outboard section.  I really mulled over why this happened and what to do about it until I remembered that KW had told me he was going to add aileron trim (just like me!)  He was beefing up the aileron to accept a servo and be strong enough to separate a trim tab.

While I was figuring out what became of the nose ribs, I looked up the cost to buy new ones from Vans.  In stock, $13 each.  Wow!  I’m used to Cessna prices – and I like this!  I am also missing the aileron counterbalance weight for the left aileron.  It cost $10 from Lowes – its a piece of 1/2 inch galvanized water pipe.

Anyhow, the aileron trim tab KW began is on the outboars side of the right aileron.  I was thinking inboard, but since he began on the outboard, I’ll go that route.  I want to frame up a trim tab not unlike the rudder trim tab as per the plans.

Rudder Trim tab as per Vans Plans

Existing framing.  The two inside nose ribs and tail ribs are extras added by KW.  He already drilled the skins to match the ribs, so I was committed.  Luckily, I already had a length of MS20257 hinge – the same hinge Vans calls out in the plans for the elevator trim.

Frame with hinge.

Fabricating a trim tab spar of 0.032 2024 T3.  Luckily, I had some scrap to do the job.

Folded end of spar clamped on inboard rib.  Outboard rib trimmed, folded, and temporarily clamped in place.  The spar is clecoed to the inboard rib, but the outboard can’t be drilled until the skins are fitted up.

Trimming up the middle rib.  This will also be drilled after the skin is clecoed back on.

Spar clamped in place with skin installed and clecoed.

Cutting off the skin aft of the spar.

Trim tab cutout.  Now I can drill and cleco the joints between the ribs and spar, as the final position of the ribs is set by the skin clecos.

Laying out rivet pattern for spar/hinge/skin joint.  Rivet fans are a great tool.  You can see stools in the background.  I am working in the SIU Aviation Technologies Sheet Metal lab, where I teach aviation sheet metal fabrication and repair.  This is how I just “happen” to have so much good stuff and access to so many nice tools.

Hinge clecoed in position.  Trimmed portions of ribs clecoed back onto the cut out portion which becomes the trim tab skin.

Using one of the rib ends to make a pattern for a new rib.  Don’t try a job like this without a caliper.  I’m old fashioned enough to like my dial version over the more modern (and less reliable) digital versions.

Rib form block cut out of scrap pine and radiused.

Drawing the rib blank.  I used 0.030 aluminum instead of the 0.025 of the other ribs.  That will add a couple grams.  We’ll have to check the balance before we fly!

Hand forming the flanges with a rubber hammer.  This will be followed up with a more precise sheet metal hammer.

My new rib alongside the outboard rib and the pine bending block.

Clamping the trim tab to a straight edge before drilling the new rib to the skin.

Not shown – forming the trim tab spar.

Trimming and folding up the rib to fit the spar.  I’ve cut the triangle forming block to length, and I’ll hammer the flange down.  Notice the relief holes in the corners.  This was done on the 2 outside ribs.  The center rib will not be riveted in to the spar, forming a stiffener instead of a rib.  The full aileron had stiffeners only in the center, not ribs.  My trim tab is fine with a stiffener instead of a rib.

Trim tab set in position.

Drilling the skin/spar/hinge joint.

Tim tab clecoed in place, shown slightly raised.  Raising the trim tab will push the aileron down, pushing the wing up – so the trim tab moves the same direction as the wing travels.

Bottom clecos removed to fit in the bending brake.  Lower skin bent as per elevator instructions.

Completed end view of formed trim tab.  Compare to end view of elevator tab below.  I need to get the servo before I finish the tab.  I have decided to use the Actuonix digital servos because they are light, low cost, and I can hook them to an autopilot as well as a simple trim system.  They are specifically intended to be controlled by Arduino, and that will probably be the brains on my trim computer and my autopilot system.

I’m very happy with the trim tab – it easily moves up and down through an appropriate range – about 15 degrees down, and as far up as I want.  I can control the precise stops when I add the servo.

Reminder of the elevator trim for comparison.

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