Building, testing and operating an experimental aircraft

The Dreaded Trim Tab

02 Nov 2010

“Fear” and “loathing” seem to be the two best words to describe builders’ reactions to confronting the elevator trim tab. Even the Van’s Aircraft Accessories Catalog offers a special replacement trim tab kit, presumably for the plethora of builders who screw it up the first time. So, needless to say, we had our concerns before starting this little project.

Trim Tab Layout Trim Tab Layout

At Oshkosh this year, Dad compiled a thorough body of research on the subject, taking somewhere around seven million photographs of RV trim tabs (of wildly varying build quality), in addition to cornering and interrogating anyone who looked like they may have built an RV trim tab in the past.

Trailing Edge Alignment Trailing Edge Alignment

On other builders’ advice, we utilized some wooden straight-edges (pictured above) to align the trailing and inboard edges of the elevator with those of the trim tab.

Mind the Gap Mind the Gap

Once we had the tab where we wanted it, we then endeavored to find the perfect position for the hinge, considering the measurements called out in the plans, as well as trying to center the hinge line perfectly in the gap between the leading edge of the tab and the elevator’s trim spar, so that its appearance would be aesthetically pleasing.

Hinge Drilling Hinge Drilling

The instructions call for you to position and drill the hinge to the trim tab first, and then use that fixture to match-drill the other half of the hinge to the elevator trim spar.

Hinge Layout Hinge Layout

Once the aft half of the hinge was clecoed in place, we put everything back into the wooden straight edges, and did our best to keep everything aligned the way we had it initially. We also used a thin wooden spacer (pictured above) to ensure we maintained the minimum required distance between the outboard edge of the trim tab and the corresponding edge of the elevator, so there is no rubbing between the two when the tab is moved up and down.

Forward Hinge Drilling Forward Hinge Drilling

Finally, after hours of tweaking, clamping, adjusting, eyeing, measuring, and hand-wringing, we decided at some point we’d just have to Man up and drill the first hole.

Riveting the Trim Tab Hinge Riveting the Trim Tab Hinge

After drilling, we deburred, prepped and primed the trim tab components and began final assembly, once again using the trusty bench-mounted Cleaveland squeezer for accurate results.

Trim Tab Complete Trim Tab Complete

Trim Tab Clearance Trim Tab Clearance

It may not win any awards at Oshkosh, but we’re pretty happy with how everything turned out, especially considering it was our first and only trim tab!

Installing the Servo and Cover Plate

Following the advice of Brad Oliver in his RV-7 blog, we strayed from the prescribed measurements a bit when positioning the Ray Allen trim servo on its mounting brackets.

Servo Cover Plate Servo Cover Plate

It takes just a little bit of tweaking to get the actuator arm of the servo to be precisely centered in the cutout, and if you follow Van’s instructions, it’ll be off a little to one side.

Installing the Trim Servo Installing the Trim Servo

Before final assembly, we did some test-fitting of the servo and cover plate into the elevator, and found that we needed to shave off the protruding corners of the nut plates a bit in order to get the servo to slide in and out easily.

Re-Priming Re-Priming

After everything fit just right, we deburred, prepped and (re-)primed the parts in question. Ooh, and we finally found a decent local supplier for our SEM self-etching primer, so no more waiting around for shipping!

Two Countersunk Holes Two Countersunk Holes

If you look closely, you’ll notice that the bottom four holes which attach the servo brackets to the cover plate are dimpled, while the top two are not. Again following Brad’s lead, the top two are countersunk slightly, which creates just enough clearance for the servo to sit on its brackets without hitting the shop heads of those rivets.

Servo Clearance Servo Clearance

If you’ve done some research, you’ll know that other folks have made shims and other such fixes to get the servo to clear. We found this solution to be simple and elegant.

Servo Installed Servo Installed

Finally, we had the trim tab servo installed and fitting properly (never mind the cotter pin, which is installed wrong!). As you can see in the video at the top of the page, we did a little tweaking to get the actuator arm the right length so that the trim tab moved roughly 25°–35° each way.

Note: The trim tab tolerances are located in the Flight Test section of the preview plans.

We also had to bend the actuator arm slightly inward to achieve the clearance and reach we wanted. I’m sure we’ll have to do more of this when we get to final assembly and first flight, but for now, it’s close enough for government work!

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