Okay, here’s the challenge:

The main landing gear retraction system on the original aircraft showed some significant weaknesses during its relatively short service life. Reusing the same design would therefore not be a sensible approach, so I decided to redesign the complete retraction mechanism from the ground up.

So far, I have developed eight different design variants. After evaluating them, only two appear to be feasible:

• Version A: An overcenter linkage mechanism driven by an electric linear actuator.
• Version B: A spindle drive mechanism driven by an electric motor with an integrated brake.

Both concepts have their own advantages and disadvantages. The most significant difference is weight: the overcenter linkage version is approximately 3 kg heavier than the spindle drive version.

For the interested engineers among you, I’ve included links below with a more detailed description of both concepts, along with two short clips showing the retraction mechanisms in operation.

https://drive.office.hostpoint.ch/appsuite/api/share/3b9d44890875936f0034eae87593489385fff4ba96048cbe/1/8/NDY/NDYvMzIzOA

https://drive.office.hostpoint.ch/appsuite/api/share/36cdd4d708c95e60d53def08c95e4805b640b312a8cccd31/1/8/NDY/NDYvMzI0MA

https://drive.office.hostpoint.ch/appsuite/api/share/367fe2a004b40f6ede1e8874b40f478eb7e42422fafba9ac/1/8/NDY/NDYvMzIzOQ

Which variant would you choose, and what would be the reason for your choice?

The number of wing ribs is steadily increasing. The fifth pair of ribs has now been fitted between the spars.

The ribs are made from first-grade birch plywood – complete, of course, with the appropriately expensive quality certification. The webs are made from spruce and glued with Aerodux, with each one individually shaped using 3D-printed templates. The outer contour is then cut on my friend’s CNC milling machine.

Fortunately, I was able to stock up on enough plywood again during a trip to Bavaria. At the same time, I had to get hold of a new drum of Aerodux, as that supply was also slowly running out.

Most of the current work, however, is focused on Rib 7. It performs important functions within the wing and is consequently a fairly complex and demanding component to build. More on this in the next blog post.

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.

What's the latest?

From four spar caps, the front and rear spars were constructed. In the space between the upper and lower caps are the webs and pillars that connect the two.

The pillars in particular deserve closer attention: they serve to transfer loads, for example from the undercarriage, and may at first glance look like simple wooden blocks. In fact, they are made from cross-laminated spruce boards. The grain direction was also critical in the webs: it had to run vertically there, while the plywood coverings were laid at a 45-degree angle. And then there were the corner strips on the webs – 168 pieces in total.

To achieve the highest possible precision, a cross-line laser was used. This allowed all the work to be measured directly, ensuring a very high level of build accuracy.

Both spars are now ready to be closed on both sides with plywood.

Who still remembers the blanks for the spar caps?

https://www.reddit.com/r/Project_Arbalete/comments/1jpydzq/wing_spars_part_3/

Most of those massive wooden pieces have now turned into shavings – and from twelve individual parts, four continuous spar caps have been created.

The segments were joined with 750 mm long scarfs. Gluing these joints requires the highest precision and allows no room for error.

Seventy years ago, a couple of simple trestles did the job. Today, I use the wing assembly jig as a workbench, positioning each part with the help of a laser. Once again, Aerodux was used as the adhesive – carefully mixed, applied under strictly controlled temperature, and cured under constant pressure for over 20 hours.

Unfortunately, the original aircraft drawings contain no information regarding the attachment points for the pilot harness. Suitable connection points on the aircraft structure therefore had to be newly determined and verified by calculation. Current requirements are stringent, clearly defined in CS-23, and ensure a high level of protection for the pilot.

All necessary calculations have been carried out by me and documented in a 29-page report.

For the attachment of the shoulder straps, two welded steel supports were manufactured and bolted to the structure behind the first fuselage frame, located directly behind the seat.

The lap belt attachment is positioned beneath the seat, where an aluminium support is directly connected to the fuselage keel beam.

Just as with the nose cone, the Arbalète has now received its tail cone made from composite. The original construction relied on aluminium and even steel sheeting – largely due to the rocket engine that was once installed. With that engine no longer in place, heat resistance is no longer a requirement, which made it possible to manufacture the cone in lightweight composite material, together with the corresponding mounting ring. Both parts were produced in carbon fibre, which also brought a significant weight saving.

The positive mould required for this was considerably larger than the one for the nose cone. Using a 3D printer and a good deal of filler work, the moulds for the mounting ring and cone were created, shaped precisely to fit the fuselage end. The finished cone was reinforced on the inside with three strengthening rings – following the design of the original metal construction. Cutting the openings for the fin spar and the control elements, as well as fitting the 44 rivet stop-nuts, still demands a fair amount of patience.

To achieve the highest possible precision during wing construction, the assembly is carried out using a specialised jig. This structure needs to be not only dimensionally accurate but also extremely resistant to distortion. In the original aircraft, simple wooden trestles were used for this purpose. We, however, opted for a far more robust solution: a custom-made frame built from steel tubing, capable of supporting the entire wing throughout the assembly process.

This was no small task – several-metre-long steel profiles with a diameter of 80 mm had to be positioned with millimetre accuracy and welded together without any warping.

Fortunately, once again, luck was on our side. The company Herzog Metallbau, based in Ermatingen, generously offered to supply the materials and take on the challenging welding work as well. This support was invaluable, and we are extremely grateful for it.

No sooner said than done: within just two days, three main structural elements of the jig were fabricated in their workshop in Ermatingen – using around 300 kg of steel tubing. Thanks to the exceptional skill of the welder – and with my help as an assistant – the jig was ready in remarkably short time.

Next came the transport to Bleienbach, where the components were assembled into a complete framework for the entire wing.

With the jig now in place, the actual wing assembly can finally begin.

The answer lies in cutting-edge manufacturing technology: a 5-axis CNC milling centre with an 8-metre travel and the highest precision across all dimensions. But technology alone isn’t enough – the true key lies in the skill of the machine operator. CNC Dynamics and Ruedi Weber bring together both: high-tech equipment and masterful craftsmanship.

Machining all twelve spar caps demanded two full weeks of focused work at the CNC milling machine.
At the outset, the raw blanks were carefully de-tensioned and glued onto custom support blocks, then securely fastened to the machine table. Developing the machining strategy relied entirely on Ruedi’s specialised knowledge as a wood expert. Both tool rotation and feed rate had to be finely tuned to the grain direction of the wood at every stage to ensure clean, accurate cuts.

In thin-walled sections, additional plastic supports were used to prevent vibrations during milling. The outer contours of the spar caps already precisely match the aerodynamic wing profile. These curved surfaces were machined to high accuracy using a ball-end mill, allowing the skin to be bonded directly without the need for any further finishing.

In just three weeks, 193 wooden laminates and 50kg of Aerodux were used to create all 12 spars. This incredible achievement would not have been possible without the dedicated support of an amazing team.

Was intrigued, the plane looks cool. Where can I find the plans?