Rudder Trim Tab

Today was a long (but fun!) day, just building one relatively small part that isn't in the basic plans. I wanted to add rudder trim capability since it seems like a pretty popular option, and should make cruise flight easier and more efficient. I've also seen a number of people retrofit their flying -10's with a rudder trim system, which tells me it's worth the hassle.

The project started with determining the proper size and location for the tab. After some researching online, I ended up with a desired tab size of 7" x 2.375" x 3.75" to be placed between the stiffeners in the same section as the center attach point:

It took a few tries to get the lines just where I wanted them, but that's why we draw with a marker instead of a saw :-)

Once the lines were sketched out, I clecoed both skins together with the outer surfaces touching in order to produce both cuts at the same time. I put a "reinforced" cut-off wheel on the Dremel tool, and went to work on cutting. A little while later, I had these this:

The two rudder skins after cutting and deburring

The area the trim tab will fit into. It is approximately 0.5" tall at the top and 0.75" tall at the bottom.

Next, I went to work on building the tab itself. I decided early on that I didn't want to reuse the scrap pieces that I had cut out from the rudder, but instead use new 0.016" sheet aluminum so that I could roll the edges around and make a piece that looks continuous. Figuring out the geometry of the whole deal took quite some time, but I ended up with a design that only will only have one line of rivets holding the whole piece together (other than the trailing edge):

The resulting part actually came out just about exactly as I had envisioned! The control horn *almost* fits completely inside the rudder, but it does bump into the skin just a little bit on its way from side to side. Still better than protruding any significant amount, which is what I first thought I'd have to deal with. I think all I'll need to do is cut a small C-shaped notch out of the opening in the rudder and that should solve the problem-- then I won't even need to use the Avery fairings I bought.

The tab seems to have more than enough travel in both directions, and with the control horn's geometry, it will require about 1" to go from stop to stop, slightly favoring the left side (for right rudder). Since the T2-7A Servo has 0.7" of travel, I think this will work out perfectly and I should be able to adjust the linkage to work as needed.

That's it for now... I'm wanting to finish the tab and servo mechanism before riveting the skins together, but it appears that aside from that, the rudder is getting close to being complete!