The proof of concept of the seat’s spar worked pretty well allowing to start with the design and production of the final part. The central idea is to make a carbon composite with birch plywood, AIREX® and aluminum core. The aluminum parts are fittings to connect the seat’s spar to the two wing’s spar joiners (steel tubing framework). These critical pieces transfer the load produced by the pilot’s and structure’s weight to the wings (roughly 8 kN or 1800 lb). Machining them was not easy, as a couple of different steps were needed—including reclamping.
A couple of takes were necessary until we had the quality we wanted. Any negligence during programming or machining ends up in a worthless piece. Here’s how the first piece evolved from left to right:
The alloy is AW 7075, which is very strong and easy to machine (6 mm solid carbide tools at 24’000 rpm, 1’000 mm/min, 1 mm cutting depth and 25% overlapping). We started with a 10 mm plate by leveling an making the pockets on one side. Two holes were made for repositioning after turning. To get a high accuracy, the holes are finished with a reamer and a dowel pin is used to reposition the piece. Having turned and reposition the plate, the other side is equally machined and the final contour is milled. The spar joiners and aluminum fittings will be joined by a fitting screw (M5 Ø6 mm). Hence, the hole will be finished later with a reamer during assembly.
So much on the fittings. As mentioned above, the spar will be a carbon composite with a mixed core. The spar caps will be made of 8×0.8 mm pultruded unidirectional carbon profiles (DPP®, R&G Faserverbundwerkstoffe GmbH), which are laminated to sufficient thickness. The core is mainly made of 8 mm birch plywood of aircraft quality. To reduce weight, holes were milled and filled with machined AIREX® discs. The whole package will be wrapped with ±45° carbon fabrics. We’ll use LG 735 Aero epoxy resin from GRM Systems s.r.o.. A little bit of Elan-tech® AS90/AW93 will be used to join the spar caps to the aluminum fittings. To prevent contact corrosion, a thin layer of glass will be wrapped inbetween carbon and aluminium.
Lamination of the spar caps comes next…
By the way, we got the steel tubing for the spar joiners. 14 meters of very strong 1.7734.5 / 15CDV6 tubes:
The material 1.7734.5 is substantially stronger than the usual 1.7214 / SAE4130. It has a yield strength of 790 MPa against 480-520 MPa of SAE4130.