Thursday, March 17, 2016

Com Antenna Short Wire Issue Resolved – Tail Cone Drilled For Nutplates

Following a suggestion from friend Mike, decided to try using a 90 degree BNC connector first as opposed to tearing out the entire center tunnel to gain access to the com antenna wire. Mike made the valid point lots of airplanes have 90 degree adapters (including his) so it should work just fine and will alleviate the need to move the com coax forward.  The 90 degree connector will add approximately .2db of signal loss to the antenna which in the grand scheme of things is not all that much. Fortunately, one of the local FBO’s had a spare connector for me to purchase.
This 90 degree BNC connector will eliminate the need for the elusive “cable stretcher” or tearing out the center tunnel to gain access to the com coax cable so it can be moved forward.
The 90 degree BNC connector installed on the back of the Garmin GTR 200 radio’s frame. This should only add approximately a little less than a quarter db of insertion loss.

Back to work the tail cone fairing … with the tail cone fairing still secured in position with Clecos, the #40 holes in the fairing were match drilled to a final size of #27 into the tail cone’s skin. All twelve mounting holes are drilled to #27.
Match drilling the tail cone’s fairing into the tail cone using a #27 drill bit.

While the fairing was removed for deburring, a little more filing and sanding was done to tweak the stabilator’s upper and lower skin … now there should be a 1/8" gap between the tail cone’s fairing and the upper and lower stabilator skins.

The tail cone’s fairing is secured onto the tail cone using 12 screws that screw into nutplates. The nutplates offer a little bit of a challenge because there really is not much room for drilling the tail cone’s skins for the nutplates from the inside out. Van’s suggests bending the nutplates to conform with the curve of the tail cone's skin then drill using a long #40 drill bit … but I found that would place the drill bit at quite a bit of an angle for most of the holes.
A nutplate secured with a screw and a pile of washers ready to be drilled from the inside out.

I opted to get a little creative and used the 90 degree pneumatic drill. For the most part it worked well there was still a small angle but not as much as using the long #40 drill bit.
Using the pneumatic angle drill outfitted with a #40 drill bit to drill the nutplate rivet holes into the tail cone’s skin.

There were a couple of holes where the angle drill was hitting  stuff which created either a bad angle or total interference ... so for those locations, elected to drill one of the holes from the inside and use the drilled hole to Cleco a spare nutplate curved to conform with the outside of the tail cone’s skin so the second hole could be drilled from the outside. This worked out well. Because one hole was drilled from the inside and the other from the outside it should make the angular difference minimal even though it is on a curve ... especially after the holes are dimpled.
This nutplate location had one hole drilled from the inside … then a spare nutplate was curved to match the contour of the tail cone’s skin and secured on the outside of the skin with a Cleco so the second rivet hole could be drilled from the outside.

After all the rivet holes for the nutplates are drilled, the nutplates are to be marked so they can be placed back to the proper locations after removal for deburring. The builder also needs to identify two nutplates that will receive a little more attention … in that, a tap is to be run half way through them prior to being riveted in position.
The two nutplates my fingers are pointing towards require a tap to be run half way through them prior to being riveted. I’m guessing this is done because these two nutplates are fairly close to the stabilator’s bottom skin and this will make screwing the screws into the nutplate a little easier.