Electric Trim Tab HOBBS: 205.0 hrs

As I get the empennage finished up and ready for storage, one of the things I havent addressed yet is firing up my servo motor to test its function and to also cut my all-thread rod to length. On top of that, a known problem is the all-thread rod not having enough clearance for the motor to run its full range of motion. I am mostly impressed with the engineering on this aircraft but this is something that is less than stellar. I noticed three things that are not designed properly with the elevator trim tab.

First, the all-thread rod interferes with the elevator skin, the clearance cutout is not long enough. Second, the plastic fitting that attaches the all-thread to the trim tab motor also interferes with the skin. And lastly, I am not getting the full up and down travel that Van's says in the plans. I dont see any way that you could get that much travel with this design. Van's calls for 35/25 degree travel up/down. I am only getting about 20/20 degrees. I have room to adjust this slightly if needed to get more down travel but I'll save this til later. For now it is good enough. This is another issue that other builders have had so I feel confident it isnt just me installing it wrong. This is not the end of the world however and maybe thats why Van's hasnt fixed it. The all-thread interference is easily fixed by elongating the slot, some guys bend the all-thread rod but I wasnt comfortable with that option and didnt see the need. The full travel in the Van's specs is not needed from what most pilots say.

Here is my 6V battery that I bought from Home Depot to test my motor. It runs a little slower at this reduced voltage but it is adequate for testing purposes. I ran the motor to the stops both directions with no problems. Just switch the wires to reverse directions. Of course it doesnt go that far when attached to the trim tab because of the constraints mentioned above. This little experiment was kind of fun and made for a pleasant evening work session, the motor moved nice and smoothly.

In this photo, you can see where the all-thread rod interferes. This view is without any modifications.

And here is a shot of my intended cut to elongate the clearance cutout. First I drilled a 1/4" hole and then trimmed with a cutoff wheel in the dremel tool.

And the finished result after filing the sharp edges and then running some scothbrite over the edges to smooth it all up nicely. Turned out very nice, looks professionally done. :)

In this view you can see it run all the way to its constraints after trimming. You can see that the all-thread rod is all but touching the skin and the black plastic attachment is also resting up against the skin. This is the limiting factor in the travel of the motor. Disappointing, but as I said, it'll have to do for the time being. The system is nice and tight. One thing you need to be concerned about with an elevator trim tab system is in-flight flutter which can be catastrophic. Its very important for the trim tab system to have NO play or loose parts, and it seems to satisfy that requirement nicely from what I can tell. The piano hinge and the motor do their job well.