One of the few things that I would like to work better on my Van’s RV-8 is the aileron trim. It’s the standard spring bias system common in RVs today. Whether manual or electric, the result is that it applies a spring force to the stick to pull it left or right. This is pretty much akin to putting a thumb on a control yoke to counter a heavy wing.
The trimtab on the early RV-6. The author heard that it might once have been an STC’d kit for a Mooney.
I was pleasantly surprised to find that my wife, Louise’s, RV-6 has three-axis trim-and it is done with trimtabs on the control surfaces. You have to remember that her plane was kit No. 4, built when there was still a lot of development work being done on the model, and different things were being tried. The builder installed small trailing-edge trimtabs on the left aileron and the rudder, with servos embedded in the surfaces themselves and a pushrod to what is not much more than a piece of wide piano hinge riveted to the trailing edge of the surface. They are amazingly effective, providing crisp and quick trim to the aircraft. While they probably add a touch of drag when deflected, it is not hard to put them in trail if you want, and flying the plane out of trim is draggy anyway.
Because we are building the RV-3 as a technically advanced airplane, and because the stick grip has a "coolie hat" that just begs to drive aileron trim, I decided to copy the RV-6 trim system. Van’s actually has early drawings for the system, but I discovered that they were going to be hard to adapt to the smaller ailerons of the -3, and our RV-6 wasn’t built to those plans anyway. This was going to be a "design as you go" effort, but that is half the fun of homebuilding.
It appears that the “hardware” is going to fit inside the aileron.
The Firgelli PQ12 linear actuator. The company makes three sizes of actuators, and this is the smallest.
The first problem I dealt with was sticker shock at the price of a servo. The Ray Allen servos that are used almost exclusively in the homebuilding world are awfully precious, and the "compact" version is even more costly. Sure, I wanted aileron trim, but for $290 per servo? I talked about this with other builders on and off, and one day I received a note from one of them suggesting that I take a look at the Firgelli web site (www.firgelli.com). This company appears to have a line (three different basic sizes, with different gear ratios for each) of compact linear actuators intended for use in RC (radio control) and robotics applications. I was intrigued, and collaborated with a few engineers in the RV world to see if we could make the force and strokes work for moving small surface trimtabs. Surprisingly, the tiniest actuator appeared to have just what we would need, so I ordered a pair for our project. At $65 each, they were a real bargain compared to the alternatives.
This picture shows the simple mounting brackets and fabricated yoke for the threaded pushrod.
The actuators arrived in a little over a week, and I didn’t waste much time hooking them up to a 9-volt battery to try them out. Encouraged by what I saw, I laid one on the full-size side view of the aileron, and there was no doubt they would fit. Using some of my memories and pictures of how the trim servo was mounted in my RV-8 elevator, and having access to RV-6 and -8 drawings (plus the two airplanes), I began making bits and pieces to mount the servo. I opted for a completely self-contained "plate" design. The servo would be mounted to a plate through which the pushrod would pass to get back to the trailing edge of the surface. This would then cover a large access hole cut in the aileron, and be easy to remove and replace for maintenance and modification. The trimtab would simply be a piece of piano hinge pop-riveted to the trailing edge itself. Not having a piece of hinge with 1.5-inch flanges handy, I built a tab up using skinny hinge and aluminum scrap. The side view drawing of the aileron was invaluable for determining the shape and height of the control horn, which I fabricated and riveted in place.
It is actually easy to retrofit this into a finished aileron. It just takes getting over the fear of ruining the part.
The Hole Picture
Probably the toughest part of the job was cutting a large hole in our quickbuild aileron. If I messed it up, I was going to have to buy parts and build another one. But sort of like cutting a hole in your leading edge for landing lights, it sounds worse than it is. Using the removable plate concept made it simple, and the air nibbler made quick work of the task. The plate stiffens the area nicely, and fabricating a frame/backing plate made drilling the holes simple (I used it as a template).
The small size of the actuator meant that I had to make a few adjustments to "normal" attaching hardware. For instance, I had to build a special attaching yoke for the threaded pushrod that we used to link it to the tab. Metric hardware is included with the actuator, and the brackets had to fit that, but overall there was nothing that couldn’t be solved with a little thinking.
The actuators have built-in position feedback, and while I am going to bring that wiring out and into the aircraft, I won’t really know how it works until I get more of the avionics hooked up. As with the Ray Allen servos, the speed of the actuators can be reduced by lowering the voltage, and they are somewhat quick at 12 volts, so a speed-reduction circuit may be desirable. My next job will be to use what I have learned to build a similar tab for the rudder. I see no difficulties with this, though the dimensions will be slightly different.
With a few nutplates and rivets, this will be ready to fly.
More Applications, Perhaps
Can these actuators be used for pitch trim? It sure would be nice to use less expensive pieces, but I consider pitch trim to be an important flight control, so we’ll have to be sure that the rates and travel are adequate. I am happy with the quality of the Firgellis, and that is a good first step. I doubt that these little units will have enough oomph to handle the larger elevator tab, but the company makes more powerful units in a bit larger size. We’re working on the calculations now, and time will tell.
Creating the aileron trim was a lot of fun, and it was nice to have a model from the early -6 to work from. I built the new parts without doing any drawings-another fun way to work in the shop, just real-time prototyping, cutting and fitting, letting the material and space available be my guide. It’s a different way to design (for an engineer), more like art than science. But when it works, it is rewarding-the essence of homebuilding in a prefab kit world.
Paul Dye is an aeronautical engineer, commercial pilot and avid homebuilder with 30 years of leadership experience in aerospace operations and flight testing. He is also an EAA tech counselor and flight advisor who currently flies an RV-8, which he built, and is working on an RV-3.