Trim tab

I took a little time for shop clean up tonight, now that the engine is not hanging on the hoist anymore. After that I worked on reinstalling my new trim tab. As you can see on this post, I had to make a new one since the old tab was cracked due to a bad modification which led into fatigue.

Before installing the new tab on the airplane, I applied transparent polyeurethane erosion tape on the bottom since it is highly exposed to prop blast. Here is how the bottom of my old tab looked like when I removed it:

One other thing I realized when I removed the old tab was that the attachment holes on the piano hinge were badly ovalized:

Again, this demonstrated to me how little details can be very important. The old screws were standard AN526 SS screws with almost zero shank length. The S-1S parts catalog calls for screws with a small shank so that no shear load is applied on the thread. I'm pretty sure that if the correct fastener was used, my old aluminium piano hinge wouldn't be as bad. I decided to replace the crews with AN 525 washer screws. You can see the subtle difference between the two here:

Here is a shot of the final installation compared with the old tab.

Again, big thanks to my friend Paul who patiently helped me building the new tab. Thanks also to my friend Joe who helped me with the paint (I hate painting).

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!