Wednesday, May 18, 2011

Reed the directions, dummy!

I continued work on the reed switches today. As a quick review, these switches are part of a too-clever-by-half over-engineered solution to the perceived problem of a pilot trying to fly an RV-12 with no wing pins in place to hold the wings on. In other words, this is an overly complex attempt to prove Darwin wrong. Which is not to say that some form of automatic verification isn't needed, mind you, but because of the persnickety nature of the system as designed, Van's was forced to build in an override switch on the panel in case a failure of this temperamental system stranded someone somewhere. And if there is an easy-to-operate override switch, those already fated to a Darwinian end will eventually use it.

Me? I'm way too smart for that.

It is what it is, though, and I have to get these things installed and working. The first order of business was to gather up the mounting hardware. Oh goody, Adel clamps!! Also note that one of the wires on one of the switches has been modified with the addition of a hoop terminal. This hoop will be mounted onto the Adel clamp screw that holds the right-side reed switch in place. That will provide an electrical ground to that switch. The other wire will be attached to a wire that passes through the wiring duct and is itself attached to a wire on the left-side reed switch. The other wire on the left-side reed switch will attach to a wire that goes all the way back up to the avionics bay where it will feed into a computer. Presumably the computer will send a small voltage back down to the switches and monitor the voltage - when it goes to ground, both of the switches are closed (thereby taking the voltage to ground) and therefore (in theory) the wing pins are in place.

Or something like that, anyway. Like I said: overly complex.

Starting on the right side, I squeezed the switch into its new home in the tight embrace of the Adel clamp.

The clamp gets mounted under the passenger arm rest, and getting that clamp in there was a real bugger. There was no room for fancy helpers like safety wire or vise grips; it was just a bare-handed fight to the finish. Once I finally got the clamp secured, I tested the circuit by putting the wing ping plunger right up next to the little hole that allows the reed switch "see" through to the other side of the arm rest. The needled on the multimeter being way over to the right like that means that there is no resistance on the switch, which in turn means that the switch is closed. In other words, it works.

Putting the pin in place didn't work at first, however. It took a lot of wiggling and monkeying around with the switch to finally get it to work.

If I thought getting the Adel clamp in place was an onerous chore, it's only because I hadn't yet tried passing the wires through the two tiny little bushings in the frame and didn't have a good basis of comparison. It was like threading a needle with string before then trying to blindly thread yet another needle three inches away. It didn't take long to see that it wasn't going to work. I went and got a length of safety wire and tried passing it through the bushings, the idea being that I would tape the wires to it and pull them back through the bushings. I couldn't get that to work either, and eventually resorted to using an inspection mirror to get a look behind the frame to aid in getting the safety wire correctly aligned with the second bushing. Success!

Having completed the right side, I went back to the work bench to get the left side switch. You may remember that the left side switch is the one without the hoop terminal on one of the wires.


And I'm the guy that's too smart to ever forget to put in a wing pin!!

Hello, Mr. Darwin.

So, long story short shortened, I removed the incorrect placement of the first switch, replaced it with the correct switch, then installed the first switch on the left side of the plane where it was supposed to be in the first place.

I then tested the switches again.

Neither would work.

Checking the book, it says that I may have to file off some of the wing pin tube that the plunger resides in. In fact, I am allowed to remove up to 1/8". This will expose more of the plunger to the tiny hole that the face of the switch "looks" through.

Having researched this in advance, I knew that this was a likelihood. I also know that there are other approaches. Some have repositioned the switches to make them align in parallel with the plungers, while others have increased the diameter of the "view" holes to improve the magnetic view for the switches.

I'll try the grinding first, but if it doesn't work I will probably enlarge the view holes a little bit.

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