Stabs Trims & Controls II

4/21/2009

It's been 3 months since my last blog post! Although, that doesn't mean I've been lazy ;-) I have I think, been more active recently than ever before on the project. Trouble is, some tasks just end up being like, 2 steps forward then 1 back - like the elevator trim tab mechanism...

Utilizing an old salvaged fuselage was supposed to give me a leap frog across the build-timeline...this certainly doesn't seem to be panning out that way. Each time I focus upon a specific area, I seem to keep falling into the mental trap where I think "if it was flying for 25 years in a previous life, it must have worked OK". So, I end up wasting lots of time trying to replicate, then usually give up and redux. The trim tab connector rods being a perfect example: After lots of head scratching and hundreds of elevator throw tests with the trim set in different positions trying to understand a binding issue, and worse, an over-center connector rod causing the tabs to literally flap around but only in a specific configuration, I FINALLY came to the conclusion that the trim tab horns/connectors had been back wards for 25 years, and that the slight bend in one of the old connector tubes was not someone's inadvertent foot in a box during storage, but was caused by binding at full elevator travel.

How did I come to the conclusion it was back wards? Easy, I took a look at the revised 90's plans and noticed they were opposite :0| . I cut the horns off, made new ones, bringing the pivot point as close to the trim tab hinge point as possible as per plans - then voila!

While changing out the bent connector tubes, I also decided I needed some form of fine adjustment for the trim tabs, so I added rod end bearings.

For the connection to the cockpit, I decided on using the vernier control as per the old Mac Mckensie Skybolt newsletters because of the fine pitch tuning available. This involved welding on tabs at short intervals on the fuselage so as to avoid any 'snaking' movement of the cable during operation. It now works very well, with no play anywhere whatsoever. I also wanted trim stops, so I fabricated one stop at the center servo horn at the tail fashioned from a bolt, so adjustable, and the opposite stop is an aluminum tube reamed to fit the shaft at the vernier control. All that is left to do is to make the pretty placarded gauge that gets adhered to the vernier control sliding mechanism.

In my previous blog post, I listed some things to be done, here they are:

Once the horizontal stab' was set and bolted down. I did notice a few things I didn't like:

The port elevator is lower than the starboard side :0|. This I will fix. Running a line across, the difference is 1/2". Luckily, when the starboard stab is clamped at neutral, the whole of the port elevator is off. This means there's no funky twist anywhere to fix, I just need to either weld a patch plate on the port elevator horn, or make a new horn, set the port elevator to neutral, then match drill through from the starboard horn.

Done. I welded on a patch plate, filled and polished the old hole with weld, clamped everything at neutral, drilled and reamed.

The original elevator travel to the stops at the reverser is 29 deg up and 24.8 down, they should be 25 up and down. Down is ok, but I am going to cut off the stop tube at the reverser and weld on another to achieve the 25 degrees.

Done

With the elevator at neutral, the rear control stick in the cockpit is not vertical, so I will be changing out one of the control connector tubes for a new one and adjusting its length to suit.

Done. I also replaced a riveted rod end bearing with an adjustable type.

I finally got me a little lathe...SO handy for making bushings! It's only a harbor freight 7x10, but at only $350 (on sale plus a coupon), I am impressed.

I have also been busy on other stuff like, the PAX shoulder harness bar which is all tacked in place, seatbelt attach fittings and playing with a sliding canopy mockup...more to come in my next blog post.