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The NACA Vent disaster and repair

posted Jun 15, 2011, 4:48 PM by Bill Thomasson   [ updated Jun 24, 2011, 12:01 PM ]
I must have been having a really bad day when I started my rear NACA Vents.  I am installing rear vents that will be connected to an overhead console providing overhead air similar to what airliners have.  This is not part of the "Standard" RV plans, but is a modification that many builders make.  This being the first major modification that I have made, I read many other builder blogs and looked at lots of pictures before deciding to proceed.  I thought I had a plan, and I followed it, but I had not evidently thought it through enough or paid enough attention to detail, it was going soooo smoothly I was very happy as I completed my first rough cut and stood back and looked over it as I was about to start filling it to the final shape.
First I made a cardboard template of the cutout from a cerial box and the back side of the NACA vent.  I was very proud of my work in getting it to exactly match both the inside and outside of the vent's cutout.  Now an someone that has seen, or installed a NACA vent before will see not one, but 2 problems with this picture.  First the template is on backwards, the narrow part should be at the front, the wider part at the back.  I say wider part, because although the vent opening is that shape, the skin is only supposed to be cut back to the widest part of the vent.  So when you first cut it backwards, and then cut too much out this is what you get.(after a lot of yelling and screeming)
After putting the vent up behind it and seeing that the flange of the vent covered the areas that had been removed, I felt it was probably salvageable. (At this point it had still not dawned on me that I should have stopped the cut at the widest part, and was feeling very proud at how the back curve came out).  I posted on VansAirforce.com to get other builders oppinions as to if I should just fill the missing skin with epoxy, or if I should cut out little slivers of aluminum to fill in the missing pieces.  It was then that I realized that I had also cut way too much in the back of the vent..... A lot more yelling and screeming, and possibly some crying.  Luckly after a sleepless night, I had several very helpfull posts, including one person that had done the exact same cutting out of the back side (although not the messup of the reversed vent to start).  Using his repair as a guide, but extending it to cover the front messup as well this is what I did.
First I cutout a doubler out of sheet stock that is twice as thick as the side skin. and is wider than the vent.  The hole in this plate is what the original hole should have looked like.
I then cut a 1/2 circle and two slivers to match the missing pieces.
I cut them so they would hang over into the vent about 1/16" past the backing plate, then dimpled all parts, riveted the doubler onto the skin, and the back curved piece onto the doubler.  I did not rivet the small slivers on as those rivets are giong to go through the vent itself, but prosealed them onto the backing plate.
Because I wanted the rivets (into plastic) to be more structual (although just structual in the sence of holding the slivers on, I also build a thin backing plate for all the rivets that will go behind the plastic, so the rivets sit against aluminum on both sides, sandwiching the plastic vent.
I then prosealed and riveted the vent (and slivers) on.
I then mixed up some epoxy and spread a thin coat over the entire area.
 Then waited for it to dry, and then sanded it all flush. (lots of sanding... should have put a thinner coat on).
I still need to do a little more sanding and filling, but I am calling it complete for now. My lesson learned, look at the big picutre before making any cuts, especially if you are "off plans". Left side vent and repair 14 hr. Right side vent done right the first time 45 min.
Left Side
Right Side