Boot Cowling

Building the skin between firewall, panel, windshield, and landing gear.

The boot cowl, the aluminum skin aft of the firewall, encloses the space where the pilot's legs extend. It also houses most of the avionics. Essentially, it's a skin section linking two bulkheads. Complexity arises from its interfacing with the exhaust tunnel, gear legs, windshield, instrument panel, and various struts.

The key to fabricating the boot cowl lies in understanding the skin's connection points on the pilot's side, where no continuous bulkhead exists. Instead, the skin attaches to side formers, the instrument panel, and a partial bottom bulkhead, with the windshield resting atop this lofted shape and sealing against the top skin.

An important consideration is ensuring easy access for maintenance. The top skin, positioned above the panel and under the windshield, presents an ideal location for a service opening. This skin wraps around the fuselage tubes, so the required panel split can double as a service access point for the avionics. In the XHawk, I've also made the side skin removable, with the split situated below the permanently installed windshield. An additional split at the bottom allows removal without detaching the gear legs.

Firewall

The first order of business involves mounting the firewall, adjusting the flange, and trimming the flaps to the exhaust tunnel. On the fuselage, I drilled and reamed the holes for the engine mount bolts, a straightforward task requiring elbow grease. The kit's firewall comes with pilot holes for the four engine mount bolts, which must be checked and drilled to size—using a sharp drill bit and some oil aids in handling the titanium. I sandwiched the firewall between the fuselage and the engine mount, adding a washer between them to compensate for protruding welds.

Instrument Panel

For the instrument panel, I needed additional vertical space to fit my chosen PFD (Garmin G3X GSU460), as detailed in my Avionics and Electrical Overview article. This necessitated fabricating a custom panel, slightly altering the inclination of the top boot cowl skin, but without causing any significant issues. This customization also allowed me to angle the panel forward, improving ergonomics and my viewing angle.

The increased vertical size changes the inclination of the top boot cowl skin slightly but not enough to cause any problems down the line.

This customization also allowed me to tilt the panel forward to improve the ergonomics for my viewing angle.

The side details of the panel are quite interesting. For one, this shows the area where fuel lines and wires can be run to the wing. Secondly, this is also the gap where the skin stays unsupported. Stock panel looks very similar to this.

For more details see finalized panel.

Exhaust Tunnel

Fitting the bottom exhaust tunnel proved to bit a bit of a puzzle, but worked out well after some trimming and bending to align with the rest of the cowl. The transition from the firewall to the tunnel initially presents as a challenging compound curve, and some have straightened it with an angle bracket. In my experience, joining these pieces is simpler than expected; manually bending the firewall flap aft and progressively drilling and Clecoing achieves good results.

After bending, all tunnel angles match the adjacent parts. I also flattened the aft corners of the tunnel in a vise, making sure to relieve-cut and polish any overstressed areas.

This is the transition from firewall to tunnel. Here you can see how it tends to curve down slightly. No need to force this straight.

Bottom Skin

The skin aft of the tunnel, wrapping around the gear, features elongated cutouts angled forward to accommodate shock strut movement (bearhawk.tips templates here). These gaps are closed with a Neoprene membrane and are open at the back for easy disassembly.

Side Skins

For the most part, I start by creating cardboard templates from the kit-supplied metal sheets, adjusting as needed, then transferring the result back to metal sheet.

Given the complexity of the shape some trial and error, and gradual approximation is common. I often extend the material again with tape and re-trim the shape many times. This technique works surprisingly well.

Overlapping the pieces, similar to fish scales, prevents scooping air or precipitation.

For complex parts, I start with rough-cutting using snips, followed by hand files and electric sanders. A battery-powered finger belt sander and a Dremel are invaluable tools here.

Here is a useful trick to split a panel without losing material. Obviously a proper shear could do this but sometimes you only have a ruler and a box cutter.

The top and rounded corner pieces, though technically cuttable from a single piece, were formed separately due to kit material limitations.

Eventually, the top piece is permanently installed and sealed against the windshield. I also sealed the skin to the T-angles (latter with Sikaflex 221). This is also where the riveting gun saw its first use. Make sure to get some practice with this tool first. It is far less forgiving than the rivet squeezer.

To simplify side skin removal, I avoided wrapping it around the gear legs, opting for a removable bottom piece instead.

Top Skin

Similarly to the side skins I used a lot of cardboard templating to figure out the final shape. The service opening is dimensioned as big as possible within the internal area of the windshield.

The nutplates on the instrument panel are size #8 and installed wet to reduce galvanic corrosion between the carbon fiber and the cadmium-plated fasteners.

Structural Supports

I added four T-angles for structural support to the boot cowl. These work well because they align symmetrically with panel splits.

The side T-angles are riveted to the top skin pieces. The bottom skin pieces, with the objective to be removable, are attached with screws and nutplates.

Cowl Transition

Interestingly, the vertically enlarged instrument panel seamlessly integrates with the engine cowling, creating a smooth transition. The complete engine cowling fabrication is detailed in a separate article.