With the elevator debacle under control I decided to sweep up the metal shavings and switch gears to Section 12–empennage fairings–one of the few tasks left to do. Section 12 covers the installation of composite fairings to elevator, rudder, and stabilizer tips. The molded tips come as part of the empennage kit but some must be modified by building up one side with raw fiberglass cloth. The plans are extremely basic, describing a two-step process through very clean-looking engineering drawings. Little did I know how much “character building” would be involved in saga that unfolded over the course of multiple weeks.
Some time ago I had ordered the composite practice kit from Aircraft Spruce. I ordered the version that comes with Burt Rutan’s “Moldless Composites Sandwich Homebuilt / Aircraft Construction” (a $15.50 value!), figuring it would be useful to learn from the master. For some reason this SKU was backordered (the kit with just the raw ingredients would have shipped immediately) and so it didn’t arrive until early January. When it arrived I wasn’t sure I’d made the right decision since the manual’s contents weren’t any more illustrative then what I can find online through EAA and YouTube.
Regardless, the kit came with everything I needed to get started, including West Systems resin and hardener, fiberglass cloth, flox and micro, cups, stirrers, gloves and more. The accompanying manual suggests several practice projects with which to get familiar with the materials and processes but, frankly, none seemed all that interesting. I suppose back when Rutan wrote the manual (apparently using a typewriter and ink pen) someone reading it would be mystified by this “magic” process. Today, when composite manufacturing is practically live-streamed (not only aircraft, but boats, custom cars, and more) you (or, rightly, me) feels like they understand what’s going on.
And thus I decided to jump right in.
The tasks called out in the plans are fairly straightforward and go something like this:
- Trim excess material to ensure the fairing will mate with the matching aluminum part.
- Remove any excess epoxy/gel coat from the joggle along the flange perimeter.
- Match-drill the fairings using the pre-punched holes in the aluminum skins.
- Dimple the skins and countersink the holes in the fairings.
- For fairings with an open side, enclose with three layers of fiberglass cloth and resin.
- Attach the fairings (except for the lower rudder fairing, which waits until lighting is sorted) using CS4-4 blind rivets
There are several considerations that affect the fit and finish of the fairings. Rather than provide a chronological narrative of the process (since that would result in a 70-page blog post) I will present some highlights of each stage along with lots of pictures as well as my observations, learnings and frustrations.
Mis en Place
As when making a great meal, each stage begins with prep work. First you must sand off any resin/gel coat in the “joggle” where the fiberglass parts get riveted to the aluminum structure:
I tried using a razor blade for the first few passes but this proved cumbersome and not all that effective. I found using a flat file much easier. Just hold the long edge of the file snug against the corner and work it back and forth until it’s square. It’s a bit hard to see in the image below but goal is to get a nice clean edge so there isn’t a gap when you rivet these onto the metal components.
The next step is to trim excess material from each part. I used a Dremel tool with a cutoff wheel as well as painter’s tape and a reference line to reduce splintering.
One thing to be careful of at this stage is the width of the flange. The plans specify a nominal width but I would recommend making it a bit oversize and then sanding to fit. Otherwise you risk not leaving enough material to support the rivets (see below).
Match-drilling and countersinking the fiberglass parts is pretty much the same as when working with aluminum, involving clecos, #30 and #40 drill bits, and countersink cage.
Where Are The Dressing Rooms?
One thing you might have noticed in the photos is that the fit between the rudder and rudder fairing was, er, not great. In case you missed it, here’s a close up:
Oh, by the way, see that cleco right above the counterweight attach screw? The placement of that rivet hole is ambiguous in the plans drawings (note the lack of handy-dandy dotted lines below) but the text states you need to use an odd number of CS4-4 rivets and there does appear to be a hole drawn on the forward end so I assumed a rivet was needed there. If my rudder falls off because I misinterpreted the plans then now you know why.
In retrospect I would recommend prepping the fairings while assembling the tail section components so you can check the fit and attempt any adjustments. However, because the rudder was already built when I got the kit I didn’t have that opportunity. My first thought was to attempt to improve the fit with some composite treatment. (I had never used composites before and so assumed they were effectively magic materials.) After stewed on the issue, though, I decided to try to improve the fit of the aluminum skins. Keep in mind this piece has to line up with the vertical stabilizer tip fairing and I determined just slathering on fairing compound was not going to provide the finish I wanted.
My fix involved removing the rudder counterweight, filling in the top screw hole as well as the infamous 19th rivet hole with JB Weld epoxy (using duct tape over the holes), clamping the skins tight around the fairing, and re-drilling the holes.
You can see how much the top portion of the rudder skin squeeze together (about 1/4 inch) the final photo above. You can also see a layer of JB Weld I applied between the two skins after reassembling the skins. Eventually all this will be covered with a layer of composite material.
Sometimes You Can Be Too Trim
The rudder fairing was the first piece I tackled and (naturally) I took off a bit material more than needed. To avoid having the fiberglass crack at some point in the future I borrowed a tip from another builder and patched in a thin strip of 0.020 aluminum using a strip of bi-directional glass cloth and resin.
You can see that I only added the aluminum to the forward sections where the flange was too thin for my liking. Also, the tips of the cleco clamps are wrapped in packing tape to prevent them from becoming permanent fairing accessories.
I made a similar mistake when trimming one of the h-stab fairings, cutting it to close to the end, right through a rivet hole.
In this case, though, because I needed to build up the open side with fiberglass, I assumed (correctly for once!) that enough material would get laid down to provide a reasonable anchor point.
Cutting in Three Dimensions
One of the challenges of trimming these fairings in particular is getting the upper and lower curves to align with the elevator fairings. You want to follow the contour of the elevator tip and fairing, leaving just enough space to ensure the elevator can move freely but not leave so much clearance that it doesn’t look pleasing to the eye. As with my aspects of the build, the solution is to work slowly, making small adjustments as you repeatedly attach and remove the fairing using clecos.
You can see above that before you close out the aft portion of the h-stab fairing with fiberglass it protrudes a bit outside the edge of elevator fairing. My solution to this was to make the temporary foam rib (see below) a bit taller than the profile of the h-stab fairing, thus “tucking in” the outboard curve.
R V Serious?
See what I did there? Subtle Latin reference. I crack myself up sometimes!
One of the most mysterious aspects of constructing the fairings involves enclosing the open side of the elevator and vertical stabilizer fairings with fiberglass cloth. The diagram in the plans that tries to explain this is deceptively ambiguous.
The first stab is to fashion a temporary rib out of foam and shape it to the fairing profile. I ordered some PVC Divinycell foam from Aircraft Spruce for this even though the composite practice kit came with some blue foam insulation pieces. I figured I might need it for other projects at some point in the future. At this point he plans are not very explicit as to how to go about forming the rib. Here are a couple of things that I took away from the process:
- You can approximate the profile of the fairing by firmly pressing it into the surface of the foam.
- Given the curve of the elevator counterweight, the rib should be made somewhat concave, not straight as it is shown in the plans.
- Unless you like digging out pieces of foam and epoxy from hardened fiberglass, consider wrapping the temporary rib in packing tape. Argh!
- Rather than making the rib flush with the back of the fairing as shown, recess it 1/8″ or so to provide a “well” to fill with fairing compound/micro-balloons. Argh again!
- Make sure to test fit the fairing and attach with clecos until you are happy with the profile. Yay!
As you can see from the last photo, the first layer of cloth detached from the fairings in a few places. I figured this was okay because most of the strength was going to come from the two layers applied inside the fairing. As long as I was happy with the shape (you can see this in one of the photos) I decided to move on. Also, because I did not apply packing tape there was some foam residue on the inside of the fairings but, again, since this was going to be saturated in resin in the next stage (and no one except me–and, well, you–would know about it) I felt it was okay.
The next two layers of fiberglass cloth went on pretty easily, though getting it to lay down in the tight inside curves was a but tedious. Incidentally, even though the composite kit came with calibrated pumps for the West epoxy system I found it easier to measure the resin and hardener by weight (5 to 1) since I was working with such small volumes. I used a small digital scale I had bought for the kitchen.
Super-sharp shop shears (alliteration!) and a Dremel tool with cutoff wheel made quick work of cleaning up the excess cloth and resin.
This Page Intentionally Left Blank
After reinforcing the fairings the next (and last) step in the plans is to rivet them on to the appropriate tailfeathers.
While the plans don’t dictate anything beyond this I have seen numerous builders apply composites to the joint between the two materials to make a more visually pleasing interface. In fact, the junction between the two components did look pretty amateurish. I contemplated leaving them as-is to reinforce to the FAA that this aircraft was, in fact, built by a total noob but, in the end decided to attempt to replicate my esteemed peers’ work.
In retrospect, I have to question that decision for the sake of my sanity. What became apparent quite quickly is something I’m sure builders of Long-Eze, Glasair and other composite aircraft are well acquaint with–the frustrating and fruitless hunt for smooth perfection. In a way, it is the definition on irony. I am building an aluminum aircraft, where subtle ripples in the skill and rivet lines are part and parcel, but somehow believed the tiny pieces of fiberglass at each tailfeather tip needed to exhibit the characteristics of the Hubble telescope’s main mirror. In case you are similarly afflicted I’ll briefly describe the high points here but, in truth, I am reluctant to relive this portion of the build.
I started by applying a layer of West Systems 410 filler to the individual parts in order to create a semi-smooth surface. I followed this up with a sanding and a layer of epoxy and micro-balloons. More sanding and a second layer of micro–followed by more sanding–yielded satisfactory pieces which I then riveted on to the main structures. I varied sandpaper grit from 80 to 220 and found these Durablock sanding blocks to be super useful.
Once riveted, I applied a layer of painter’s tape to protect the aluminum skin and repeated the 410-sand-micro-sand process to file and feature the gaps between the fiberglass and aluminum parts.
Once I had achieved reasonable good contours I hit the tips with a coat of SEM self-etching primer, which helped highly any ridges, valleys and pinholes. (It also offers a layer of protection to the layer of Alclad sanded off during the process.) Then it was back to spot filling with thinner and thinner coats of micro and more sanding.
In the end I was reasonably happy, if exhausted. Remember that gnarly gap between the rudder and rudder fairing? Viola!
There was also a pretty nasty gap where the v-stab met the fairing that I was able to disguise.
Now, to see how it all goes together!
Building N726AG 