Planing the plane
The empennage of the Pietenpol is substantially complete now with the completion of the vertical stabiliser. Hence, I now need to start work on the fuselage. Annoyingly, the Pietenpol timber kit I purchased second-hand was missing the longerons – though the seller kindly refunded me for these. Therefore, I needed to source some timber for the longerons. As I’m in Australia, I can’t simply order some new stock from Aircraft Spruce because the shipping would be many times the cost of the timber itself. I eventually found a timber supplier with some long pieces of Sitka Spruce and set about purchasing several lengths.
Upon arrival, the timber looked good though a few of the pieces were too dense for my use – apparently, the density can vary quite a bit. However, I determined that one of the pieces was lighter than the others and may be suitable. After a few calculations, I determined that the longerons would end up about 1 kg heavier (around 250 g each) than if I used equivalent timber from Aircraft Spruce. I think this difference just comes down to natural density variations within the species and the dedicated aircraft timber shops must screen their timber carefully for density. Fortunately, timber strength is typically proportional to density and hence, my timber is likely stronger than what I would get from Aircraft Spruce. I am happy with this compromise and decided to start fashioning the longerons. The Sitka Spruce pieces can be seen below.
I would love to have a full timber workshop to do the timber milling in, however, I find a special pleasure in making things work with what I have. Hence, I set about milling the timber with a circular saw and my pop’s Triton table saw.
The large Sitka pieces were first cut with the circular saw into approx. 1.25″ thick boards that could then be run through the table saw to cut into square sections. These saws left a rough surface finish on the timber – hence, I left the pieces oversize.
With the longeron stock cut to the approximate shape, the rough surfaces left by the saws could be planed down. This was done using the hand plane due to some bad experiences with my electric thickness planer – it likes to eat timber….
For this task, I used my pop’s Stanley hand plane which I had refurbished previously. The hand plane leaves a fantastically smooth surface finish which, unlike with sanding, doesn’t contain a lot of crushed wood fibres and sanding dust. Theoretically, this should make the epoxy bond to the longerons stronger – definitely something we want …
With the longerons milled to size, I wanted to check the flexibility of the timber as we have to bend the bottom longeron quite a bit to form the fuselage profile. For this, I lay the longerons on the table shown above and clamped it in the approximate profile of the fuselage bottom chords. The timber bent well and didn’t seem to be overstressed at all.
The only problem left to solve is to eliminate a couple of timber defects in the longerons. It is almost impossible to source suitably graded lengths of Sitka Spruce that are the requisite 173″ long without encountering some timber defects (at least in Australia). There are some minor hard knots and pitch pockets in a couple of my longerons (typically only one defect per longeron). These defects are very localised and can be removed from the longerons by cutting a small section out and jointing the timber back together. The method of jointing the timber is known as a scarf joint and is covered in the FAA’s AC 43.13-1B shown below.
The scarf joint is made by cutting a long taper on two pieces and then glueing both tapers together. This gets reinforced by additional plywood reinforcing plates. The FAA’s AC 43.13-1B requires a scarf joint taper of 1:15 (min) – this equates to a taper 15″ long for a 1″ thick longeron. Many sources point to anything more than 1:12 as being stronger than the original solid timber section. To get started, I created a jig for cutting the shallow taper using my router.
The longeron gets clamped in the jig after being cut to a rough taper with the jigsaw. The router runs along the sides of the jig and cuts the taper on the longeron. After a couple of tests, I developed a good technique for clamping both ends of the longeron and this created very consistent scarf joint cuts. The surfaces of the scarf joints were very clean and I’m very confident the joint will have sufficient strength. One of the practice scarf joints was tested to failure and showed no failure of the glued scarf joint.
The scarf cut was made on both pieces of the longeron and it was glued together with T-88 and left to cure for three days. The resulting glued longeron will be used for the upper chord of the side fuselage truss as this member is not permanently bent – as bending of scarf joints is not ideal. The reinforcement plates will be added when the fuselage gusset plates are added as the gussets can double for the reinforcement plates in some locations.
Before undertaking the scarf joint, I was unsure whether I would be confident enough in my workmanship to have faith in the scarf joint in such a critical location. Thankfully though, I was so pleased with the results that I have no hesitation with the scarf joint in the longeron.