Trim tab

At the end of my flying season in 2014, I found a crack on my trim tab during a preflight inspection. I stop-drilled it as a temporary measure, but promised myself to make a new one. Here is what my old trim tab looked like:

The trailing edge extension has been added at some point by one of the previous owners, but I'm not 100% sure why. The most probable cause is that it didn't have the full +7.5/-42 degrees travel recommended in the assembly manual so it was decided to add more surface.

Below is a better shot at the crack. The end of the control horn and the beginning of the trailing edge extension are almost on top of each other. This is certainly the main cause of the crack developing.

Without knowing the full story behind the trailing edge extension, I decided to err on the conservative side and make a one-piece type trim tab with the same area as the old one:

The brown block behing the control horn is phenolic. I read somewhere that some Pitts Model 12 are made that way and I thought it was a good idea to distribute the load more evenly along the chord of the tab. I also increased the depth of the control horn for the same reasons.

I went to my friend Paul's shop to build the parts. Big thanks to him for his great help! He machined the phenolic block to a perfect fit! The trailing edge bend was not straightforward to do. What we ended up doing is tack weld a rod on the edge of a flat plate and use this as a dye to press the metal sheet between two blocks of steel. I was very happy with the result.

Then I went to see my other friend Scott to use his press drill to make all the required holes. This is probably the most sheet metal work I'm ever going to do on this airplane:

Then  I cleaned all the parts and used Alumiprep/Alodyne to create a corrosion protection coat:

Finished the assembly using Avex rivets and AN525 8-32 screws for the control horn:

Next step will be the paint and installation on the airplane!

Finalizing the tail surfaces installation

Once all the tail surfaces were in place, I made initial measurements with a smart level as well as a conventional level to know how far I was from the desired 2 degrees of incidence on the stab as well as zero washout as suggested by Doug Sowder's article on Pitts rigging. I was quite happy to find that without any tension on the tail wires, I was very close to the target incidence, with a little washout on both sides.

I used a tool that my friend Paul Goyette and I made for tensioning the wires. It is similar to the one that Steen sells, but made out of nylon and with a cylindrical shape rather than cubical. I think this home made tool is even less prone to damaging the wires than aluminium and would also cause less damage should I mistakenly drop it on fabric. I have two of them, one for the tail and one for the wings.

I slowly increased the tension by adding 1/2 turn on all wires, measuring angles and repeating until getting in the middle of the target range of 250 to 325 lbf tension. As I approached the final tension I realized I was inducing anhedral to the stab by doing 1/2 turns everywhere at each iteration. This is probably due to the top wires being longer and therefore stretching more that the bottom wires. When I realized that, I did one round of tensioning the top wires only, which brought me back to zero dihedral.

I used the Holloway Engineering tool along with an electronic torque wrench to measure the tension. 

Thanks to my girlfriend Isabelle who helped during the final stage of the tail installation!

Elevator surfaces

Today I installed the elevator surfaces on the h-stab. First I cleaned up the gap seals and the lubricated them using lithium grease. I glued the seals outboard of the strap hinges to the stab so that they don't slide off in flight. I used one bead of Permatex weatherstrip adhesive for that. The two bolts on the pictures are the AN4-17A going through the strap hinges.

I installed two new KP4 bearings to the root of the elevator surfaces and then fitted the surface in place:

The lithium grease greatly reduced the friction when moving the elevator. Before that, it used to make the sound of an old door cracking. Now it's quiet and smooth.

Next time I plan to install the rudder surface and tail wires!