Sat Apr 24 2010

Autopilot pitch servo

Today I did some more investigation into what to do for mounting the pitch servo. here you can see that the outer face of the servo arm is roughly 3/4 inch from the mounting face of the servo.

...but the outer face of the bellcrank is 1 3/32 away from the mounting face. Well that explains why folks are saying that the short servo pushrod will be at an angle. That's roughly 3/8 inch difference.

What on earth am I doing with these old (scrap) Z-brackets?

Well, after thinking about it some more, I decided to abandon the bracket supplied by TruTrak and fabricate my own. These Z brackets will be riveted to the side of the center rib. I hacked one flange off of each Z bracket so it's a 5/8 x 1 1/8 x 1/16 inch thick angle.

Here I've got the new bracket pretty much finished. It's made of four pieces and matches the mounting holes of the original bracket, but since the Z brackets are 1 1/8 inch high and the mounting angles are 3/4 inch high, this mounting creates a 3/8 inch offset...exactly what I need.

After polishing the ends of the supplied pushrod, I temporarily mounted the rod end bearings. These will need to be removed when I prime the rod.

OK, here's the custom bracket as installed. The pushrod is nearly parallel with the centerline of the fuselage and I'm able to use the large OD washer for retention of the rod end should the bearing ever fail. There's a couple things I still am not completely happy with about this mounting.

The first is that there's a gap between the two plates that make up the bellcrank where the pushrod is bolted. This should be easy to correct by simply putting a washer in here. The second issue (which I don't have a picture for) is that if the servo arm pulls back far enough, the jam nut on the rod end bearing contacts the arm. I'm not sure if it's really a problem, though, since I don't know the allowable limits of travel yet. If it is indeed a problem, I'll probably need to stack a couple washers between the bearing and the arm just like was done on bearing which attaches to the bellcrank.

One other think is that I added a stiffener angle to the back side of the center rib. I thought this would spread out the load on the web of the rib, so I don't get any cracks at the bottom Z bracket rivets.

Plumbing pieces?!?! Is this something to do with the relief tube?

Nope. It's a tool for installing rod end bearings. Please ignore my sloppy PVC adhesive. The purple primer just ran everywhere.

Here's how it works. Not sure who came up with this idea originally, but it does a good job and was cheap to make.