AHRS mount – SUCK!

6 hours.

This is going to span the last three days, so bear with me. My AHRS mount from Van’s showed up last week, but stayed in its box until Monday, because I had to go up north for family birthdays. Monday I spent a good chunk of time on the AHRS mount, which is basically a rib that attaches to the floor and a platform that attaches to that rib and also to the rib running between the baggage bulkhead and the one aft of that. The idea is that the platform lays flat relative to the longeron and gives you a safe, out-of-the-way place to stick your Attitude, Heading, and Reference System sensor. It’s also a decent place to put in the remote compass. It’s far enough away from any big steel that it ‘should’ give you a nice accurate reading. Van’s designed this part as a retrofit. It works on the 6, 7, and 9 models, and I believe the 10 as well. To install it, I have to crawl down the tailcone, drill a crapload of holes, make everything level, then drill in place. The process is this: Attach the platform to the rib, then snug the platform up to the bulkhead and center rib. Then drill the rib to the floor. After that’s all clecoed in, measure for level. I have a tailwheel model, this is difficult, so I measured the down angle of the longerons and the angle of the main spar and made the platform match that. The rest angle of the aircraft with my fat ass in it is 11.6 degrees. The spar was surprisingly level. I got the whole mess drilled in tonight.

Other projects involved some wiring. This is the bad part about multitasking. Blog entries become… Mushy. Monday morning I decided to finish wiring up the E-Bus, alternator field, and ignition/start switches. I got the E-Bus done, and it functions as designed and diagrammed on Bob Nuckolls’s Z11 drawing. I also installed the 5A pullable circuit breaker and started working on the ignition and starter switches. This is kind of a weird configuration, but it works, and even kind of makes sense. It’s hard to explain, because I’m tired, but basically these enable the magnetos in the up position. Center position is off, and spring-loaded down position on the left switch is start. The idea is you enable the left mag with the left switch, then engage the starter with the right switch, switching the right switch to the up/on position after the engine catches. Elegant, really. I got this all wired up, but didn’t hear the clunk of the starter contactor. What I got for my trouble instead was a blown fuse. I rechecked my diagram, rechecked my switches, I got out the multimeter and checked for voltage potential and continuity in various switch positions, then replaced the fuse, thinking maybe it just blew sometime earlier maybe I’d left the master on and grounded a wire or something. Blew that one. Changed to a simpler test: the starter circuit engaged by two known on/off tabs on the switch. Poof. By this time It was time to go to work, so off I went. This morning, I had a hunch in the shower: check the surge protection diode on the starter contactor. Sure enough, that was the problem. I had wired the diode with the band towards ground instead of towards the S terminal on the contactor, which meant that every time that circuit was completed, it would short to ground and fry the fuse. Good to learn this stuff in the initial stages of electrical work before letting the magic smoke out of a very expensive part. The suck of sucks was getting the diode wire off the firewall. I had to take off the current limiter, the battery cables, and the copper bus bar between main and starter contactors before I could get it loose. After I installed its replacement, with the band facing the right way, everything worked fine. Go figure.

Tonight it was back to the AHRS mount, down in the Jeffries tube, which I already described. It’s about to get sucker still: I have to dimple all the rib attach holes in the bottom skin with a pop-rivet dimpler, which means I’ll need a helper, or I’m going to have to climb in and out 24 times just to do that. I’ll also need a helper for bucking those rivets.

Comments (1)

  1. 1:04 am, April 18, 2011Sara Courte  / Reply

    Sorry. I still don’t understand it, but it is very impressve.

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