u/Arbalete_rebuilt

Two spars do not make a wing. Now we need some ribs – and quite a lot of them. The very first wing rib has just been positioned between the spars.

Unlike the wing ribs of the N-20.1, which were cut from 10 mm thick Okoumé plywood sheets, the ribs of the N-20.2 are made from a plywood web with laminated spruce caps.

The first step is to make the spruce caps. For this, a stack of 5 mm thick spruce boards is glued together with Aerodux and then cut to the required dimensions.

Meanwhile, the 3D printer is already busy in the background, producing the templates for the inner contour of the ribs. These templates are used to position and glue the prepared spruce caps onto the plywood web.

The outer contour of the rib is then machined. Once again, my friend's ingenious CNC router comes into play. It has already proved itself to be an indispensable aid on this project on several occasions.

For the first time, all the components manufactured to date have been assembled together. The fuselage, complete with the vertical stabiliser, is mounted in the assembly jig, and both wing spars have been successfully inserted into the fuselage. Reaching this important milestone was only possible thanks to the perseverance of a dedicated team.

As is often the case with projects of this nature, not everything went according to plan. It was only during assembly that we discovered the rear spar has to be installed before the front spar. Since the front spar had already been fitted, it had to be removed so that both spars could be installed again in the correct sequence.

The next phase is the manufacture of the wing ribs. These will be produced now and then installed one by one as construction progresses.

Today marked another major milestone in the project: the two wing spars were transported from the original workshop in Bleienbach to the new workshop in Nesselnbach.

For this exceptional move, a truck and low-bed trailer were used, generously made available by Hanspeter Meyer from the Giezendanner Collection. Despite the impressive size of the trailer, the two 7.6-metre-long spars occupied almost the entire loading deck – a truly impressive sight.

Once the valuable cargo had been carefully secured, Hanspeter Meyer and Eduard Hofer climbed into the cab and set off for the new workshop. Travelling at around 80 km/h on the motorway, I followed closely behind, keeping a watchful eye on the precious cargo throughout the journey.

On arrival in Nesselnbach, the two spars were carefully unloaded and placed directly onto the assembly jig, which was already waiting for them. During the previous few days, I had dismantled the jig in Bleienbach, transported it to the new workshop, reassembled it and carried out all the necessary measurements to ensure it was perfectly aligned.

The fuselage is also already in its new home. Two weeks earlier, it had been moved from my home workshop to the new facility by Max Vogelsang using a special transport.

With that, all the aircraft's major structural components are now together under one roof for the first time. The next exciting step will be to fit the wing spars into the fuselage for the very first time—a significant milestone that will allow construction of the wings to begin.

During one final, highly focused work session, the closing plywood skinning was fitted to both wing spars. At 8 mm thick, these plywood strips proved considerably less cooperative than their thinner predecessors. The centre section therefore had to be pre-bent on the original spar form while damp and then allowed to dry completely before installation.

The surfaces of all internal cavities were carefully sealed with varnish by experienced hands beforehand. The same applied to the surfaces of the plywood strips themselves. This step, too, was carried out with the precision and attention to detail that had become the hallmark of the project.

The application of the adhesive was once again entrusted to a well-practised team of experienced specialists. Drawing on the lessons learned from the previous two skinning operations, the amount of adhesive could be matched almost perfectly to the task at hand.

Looking back, the construction of the two wing spars was an exceptionally demanding undertaking. Both their geometry and structural design resulted in a series of complex manufacturing and assembly processes.

The quality of the materials and the precision required in their workmanship left no room for compromise. Thanks to the outstanding support, countless hours of work, and remarkable dedication of everyone involved, a flawless result was ultimately achieved. It was an exceptional achievement by the entire team and one that all those involved can be justifiably proud of.

At long last, work on the wing spars is moving forward again. The investigations and approvals required for the use of an epoxy adhesive were extensive and ultimately resulted in a 26-page report. Anyone interested drop me a line.

Last week, we received positive signals from Bern on our report, allowing the suspended work to resume. Naturally, we were already waiting in the wings. The individual plywood panels for the spar skins had previously been joined using a total of 26 scarf joints, creating four strips nearly eight metres long. The front and rear skins are a substantial 8 mm thick and therefore had to be pre-shaped while damp.

For the bonding process, the entire family-and-friends network was mobilised once again. By now, they have all become highly capable adhesive specialists, applying the epoxy with remarkable concentration and precision. The many helping hands were equally indispensable when it came to positioning the long plywood strips onto the spars. Eight metres of plywood develops a mind of its own surprisingly quickly.

Once the two inner skins had been bonded in place, it was time for the final inspection before closing the spar. This was carried out by CAA-approved wood aircraft construction expert Christian Nyffeler.

His verdict was pleasingly uneventful:

Flawless – the spars can be closed.

At the moment, I’m spending most of my time doing calculations, building test specimens and test rigs, and running tests — you know, the fun stuff.

The pilot harness attachment has now passed its ultimate load test (finally). It’s bonded to the fuselage structure and, just in case that wasn’t enough, also bolted on. The calculations already showed that either the bonding or the bolts alone would be perfectly capable of taking the expected loads. But obviously, that wasn’t quite convincing enough, so a physical pull test was required as well.

So, naturally, this meant building an identical attachment like the one in the aircraft, designing and manufacturing a suitable test rig complete with instrumentation — and then actually running the test. No big deal.

The required tensile force of 1659 kg was reached without any drama and then comfortably exceeded. Even when pushing the load cell all the way to its 1000 kg limit, the attachment still refused to fail. How inconvenient.

So yes, it’s now been proven — quite thoroughly — that the attachment will keep the pilot firmly in the seat, even under extreme loads. Just in case there were any doubts left.

No new blog posts don’t mean nothing’s happening behind the scenes – quite the opposite. At the moment, though, we’re facing a few challenges that are noticeably slowing down progress on the build. One of the main issues is proving a suitable epoxy adhesive for constructing the wing. But let’s take it step by step:

After assembling the upper and lower spar caps, the next step is the side sheeting of the spars with plywood. The bonding area is very large and also geometrically complex. The adhesive we’ve been using up to now, Aerodux, is no longer suitable here, as the required working times fall well outside its specifications. A modern epoxy-based adhesive seemed like the ideal alternative. At the same time, we’ve been working on the structural strength calculations for the wing, as required by the BAZL. For that, however, reliable strength values for the bonded joints are essential.

And that’s exactly where the problem lies: for epoxy resins used as adhesives in timber construction, there are no sufficiently well-supported strength values available. While there are various products that are regulator-approved, no manufacturer worldwide provides solid, verifiable data for this specific application. In the end, that means every aircraft builder has to prove the suitability of their chosen adhesive through extensive testing – and that’s exactly what we’re doing.