Friday, May 2, 2014

Spar Pin Safety Reed Switches Adjusted

After letting the epoxy used to enclose the magnets in the F-1217C spar pin stoppers cure overnight the first item in the agenda was to make the necessary adjustments to allow the reed switches to close from the magnetic field present when the fuselage spar pin is in the proper position.

This process requires a little trial and error along with patience. Initially when the right fuselage spar pin was in place and the F-1217C spar pin stoppers were properly seated in the locking hole on the arm rest, the reed switch did not energize. Van’s instructions say to trim away 1/32" from the spar pin’s tube handle that the F-1217C pin stoppers are inserted into. This requires removing the screw, pin stopper and spring from the assembly, grinding down the tube, deburring and reassembling the parts to try again. The instructions say to keep doing this but do not remove more than 1/8”. The reed switch for the right fuselage pin began working after the fourth cut so just a little shy of 1/8" of material was removed in total.
Checking the reed switch circuit with the right spar pin locked in place to see if the reed switch contacts closed from the magnetic field around the spar pin stopper. After the fourth trim all was well.

One may ask why trimming the steel tubing the spar pin stoppers slide into has any effect on the reed switches. Frankly, I could think of at least two better ways of doing this but in a nut shell the steel tubing acts as a shield because the magnetic lines of force will pass through the steel tube easily so they remain concentrated within the steel and don’t spread out with enough concentration to energize the reed switches. Trimming the tubing shorter at or below the level of the reed switch increases the amount of magnetic field that will be present in the vicinity of the reed switches.
The spar pin assembly on the left of the photo is the right spar pin after final trimming. Note the length of the tubing the spar pin stopper slides into is almost 1/8" shorter than the yet to be trimmed spar pin on the right.

The builder is striving for a fine balance here…. the reed switches should ONLY energize when the spar pin stopper is locked in place. You don’t want the reed switch to become energized anytime the spar pin stopper is in the vicinity of the reed switch, only when the magnet in the pin stopper is fully up and inside the locking hole on the arm rest. If the builder gets too aggressive with trimming the tubing, which could easily happen if the tubing is not trimmed in small increments, it will create a much larger magnetic field around the spar pin than desirable. This will likely allow the reed switch to become prematurely energized just by having the spar pin in the general vicinity of the reed switch … not good because we only want the reed switch to energize when the spar pin stopper is locked in the hole on the arm rest and at no other time.

The same process was utilized on the fuselage spar pin assembly for the left side. Overall it required less trimming and the reed switch was working great with just a little shy of 1/16" trimmed off the tubing. Once both reed switches were working flawlessly, the rest of the wiring was completed. Both the reed switches were wired in series and connected to a wire in the WD-00046 wiring harness that according to the drawing went to pin 5 of the connector at the instrument panel. An ohm meter was used to take a reading between ground and pin 5 of the connector and there was continuity. Removing a spar pin broke the continuity. Time to move on.
With both fuselage spar pins in place and locked in position, both reed switches were energized and there was continuity to pin 5 of the WH-00046 connector at the instrument panel … a good thing!

Work continued doing the mundane things such as making wiring labels and crimping connectors where necessary. All and all, good results for the day.