Joining the Fuselage
With the two fuselage sides completed, it was time to disassemble the fuselage jig on the workbench. Thankfully, the two fuselage sides turned out very similar, so the jig had done it’s job well. The blocks and clamps were removed and the fuselage outline was also erased so I could start with a blank slate for the workbench.
The two fuselage sides are joined by a set of cross-struts and braces on both the top and bottom sides. The rear portion of the fuselage has diagonal braces which form a torsion truss to take twisting loads imparted by the tailplane. The front portion of the fuselage has cross-struts perpendicular to the fuselage centreline to allow for pilot and passenger egress. The floor will eventually be glued to the underside of these cross-struts on the bottom side.
I was very nervous about joining the sides of the fuselage together as there is a huge potential to accidentally build in misalignment and twist the fuselage. This could permanently tilt the tailplane requiring significant shimming or ultimately, require a rebuild of the fuselage entirely.
The fuselage is constructed upside-down on the workbench as the top longerons are flat – this should aid in keeping the fuselage aligned correctly. The first step was to run a string along the workbench to establish the fuselage centreline. From this centreline, I marked the desired width of the fuselage at various stations along the fuselage. The fuselage sides need to be bent inwards so that the two sides meet at the rear. I was a little nervous about how much stress this would impart on the longeron – but i was pleasantly surprised to find out that the force required to bend the longeron (without steam bending) wasn’t much at all. I made a series of squares out of scrap pine to hold the fuselage sides vertical.
The squares were screwed into the workbench at the appropriate widths and then the fuselage sides could be inserted into the jig and bent into shape. The shape can’t be fully achieved until the two sides are tapered at the rear so that they join together at the proper angle. To cut the taper, I measured the angle between the fuselage centreline and the rear of the fuselage side truss and made a router scarfing jig for this angle. This jig consisted of two tapered blocks screwed to the workbench on either side of the fuselage side truss.
The router was attached to a flat board and was run over the area to cut the taper. Once the taper was cut on both sides, I test-fit the fuselage sides in the jig and found it to be a perfect fit – I’ll put that down to luck rather than good planning!
Now that the taper was cut, the final fuselage shape could be established. I started by cutting a few of the struts to length so I could start joining the fuselage together.
I had a limited number of clamps so it wasn’t feasible to attach all the cross-struts and braces at the same time. Hence, I proceeded to cut a few members at a time, glue them in – rinse and repeat.
The front half of the fuselage does not have diagonal braces on the floor as it relies on the shear capacity of the plywood sheets. As these aren’t attached yet, I was worried about the frame racking out of square. To mitigate this, I decided to install the rear seat back now as this would provide some resistance to racking. The seat back was cut from 1/8″ plywood. Due to the complex geometry in this area, the seat back plywood is forced to bend – particularly where it is attached to the vertical side supports. This means that the horizontal seat supports actually need to be [slightly] curved – otherwise, the plywood would have a very sharp kink that could cause a break. Once the curve was cut, the seat back and supporting elements could be glued on.
Once most of the cross members were attached, the fuselage was dimensionally stable and could be lifted off the workbench onto some saw horses. This provided much better access to all of the joints. Each of the joints could be leveled using a chisel and file and some gusset plates were attached.
At this point, the fuselage is a manageable unit that I can move around and I could attack various other parts in the order that seems to make the most sense to me at the time. The creation of the fuselage was incredibly satisfying as I finally have something that “looks like a plane” – or at least a fancy canoe?
I think my mum would prefer it to be a canoe!