From the start, I realized that the instructions in the build manual for the cowling installation made no sense to me. I was supposed to fit the cowling before installing the engine, fitting the bottom cowl first, independent to the top cowl. Most of the group wisdom contained in the Van's Air Force forums advised to install the engine first, which I did, so the cowling could be installed relative to the engine. To me, the most visible indication of a good-fitting cowl is the way the top cowl fits relative to the spinner. Any lack of concentricity is immediately obvious. At this point, I was still following the
I first trial fitted both top and bottom with duct tape to get a general feel for what had to happen, then proceeded with the top cowl first. I leveled the top cowl as described in the manual, using a long spirit level and plumb bobs hanging from the front corners of the top cowl. Amazingly, the floor in my hangar turned out to be level, making this part easier. I checked that both main tires were pumped to the same pressure and that the wing tips were the same distance off the floor.
At this point I decided to check the procedure for cowl installation for the newer 912iS (fuel injected) airplanes and discovered that this new procedure closely mirrors what I had just done: engine first, top cowl next, then bottom cowl. I wish I had read this before starting work on this section! It would have spared me the unease and frustration I was feeling for making such a large deviation from the manual, only to find that I was unknowingly following the new procedure.
After leveling the top cowl, the next task was to set the gap between the front face of the cowl and the rear bulkhead on the spinner. I decided to set the gap at 1/8th inch, which allowed me to once again use the handy 1/8th inch spacers provided free by Lowe's in the form of paint stir sticks. Thin double-sided tape worked great here.
If I had wanted a slightly larger gap (the manual recommends 3/16th inch) I'd have used stir sticks from Home Depot which are a bit thicker.
I love the way the small gap looks. At Oshkosh 2022 I checked the gaps and general fit of the cowls on all the -12s in attendance (and other RV models, as well) and found mostly larger gaps. Supposedly, larger gaps make for easier cowling removal, but I haven't found it to be a problem (yet).
The small piece of aluminum angle clecoed to the top of the cowl forces the vertical alignment of the cowling and the spinner, although a hangar neighbor who built an RV-7 says the cowling will sag a bit after a few hours of operation screwing up that perfect alignment.
The Sharpie drawing just in front of the pilot seat is where I'll make the cutout for the NACA duct used to cool the voltage regulator.
Next, the aft edge of the top cowl is block sanded to match the forward edge of the aluminum skin covering the instrument bay. All the #40 holes for rivets attaching the cowl to the soon-to-be-installed piano hinges are marked and drilled. One errant #40 hole is shown. I could've staunched the bleeding with a cleco in the finger tip, I suppose.
The cowling is held in place with duct tape throughout this procedure. As with anything dealing with fiberglass on this airplane, it's an iterative process: trial fit (with tape), mark, remove, sand, trial fit, etc. All of the marking is done with Vis-a-Vis Wet Erase. The marks stay on well until rubbed with a wet finger tip (or paper towel if you grew up somewhere other than South Carolina).
The aft part of the piano hinge to which the finished cowl attaches was installed much earlier in the build (in my case probably seven or eight years ago). The forward half of the piano hinge, to which the fiberglass is to be riveted, was fabricated (fluted to match the curvature) and installed with its pin.
At this point I fabricated all the piano hinges and pins. There are two on top (one shown in the pic), two on each side which attach the aft edges of the bottom cowl to the airframe, and one on each side which attaches the top cowl to the bottom cowl.
With the aft half of each hinge installed, I proceeded to trial fit the bottom cowl. Lots of on, mark, off, sand, fit, repeat.
It all went fairly well once I switched to the 912iS instructions. My only screw-up so far in this section was sanding off too much fiberglass from the interface between the upper and lower cowling (shown with the hinge clecoed at left) on one side. I'm trying to decide how to recover from that and I'll describe it in part II of the cowling writeup.
Off topic
More Colorado wildlife: A few elk showed up behind my house recently.
They better be glad I no longer hunt!
