Mon May 3 2010

Safety wiring pitch servo

Well, today I spent a couple hours putting safety wire on the bolts for this pitch servo. I really suck at doing safety wiring...especially in an awkward area, so it took me around a dozen tries to come up with this pathetic job. My fingers were raw and had been poked too many times to count. Between taking this picture and posting this text, I had considered redoing the safety wire yet again, but found out that the TruTrak service bulletin which required changing the arm retaining screw to one with a patch of locking compound was not fully effective. At least two VAF members have posted that their screws are backing out after complying with the service bulletin. This is a serious issue since the original incident resulted in controls lockup as described in this thread. Fortunately the pilot was able to force the controls to free up and make a safe landing.

After talking with my friend, Tom Webster, about how these brass bushings are supposed to work, I removed the pilot-side control stick to check the operation. Three things are important here.

  1. When the bolt is torqued, it should clamp the ears of the U flanges on the weldment onto the ends of the bushing so that the bushing doesn't turn.
  2. Since the bushing doesn't turn, the motion happens between the brass bushing and the steel tube welded onto the stick. This needs to be a close fit to avoid slop, but there can't be any binding.
  3. Third, this interface is supposed to be greased with "your favorite grease". I've been unable to find the statement in the instructions that indicate this, but I found several references to it on VAF.

There's no real consensus on VAF about what is the right grease to use here. Some guys say they use Aeroshell 5 or 6 since that's what's used on their Hartzell prop and why have two greases? Another guy used Aeroshell 33 since that's a lithium based grease. Several indicated they like Lubriplate grease (although they didn't indicate which type). After much searching, it appears that Lubriplate 630-AA is the correct type and I think I'm going to go with that. I've been having a heck of a time finding it locally, though.

The other thing to mention is that although the left stick pivoted freely on the bushing, the right stick did not. I decided to ream all four weldments (two sticks and two aileron bellcranks) to 0.376 inches. My friend, Jim, was nice enough to help me out. What we found, however, was that three of the four weldments were already 0.376 or larger, but that didn't correct the binding on the bushing. Measuring the bushing with a micrometer showed it to be a couple thousandths too large in diameter. So, after getting back home, I chucked that bushing in a drill and polished it down to about 0.374.