ECLAIRE: A modular aircraft for study and teaching

Composite monocoques and conventional frame structures may be very different, but they share one characteristic: the load-bearing structure and outer skin are closely coupled. Changing the contour therefore means changing the structure and manufacturing the fuselage again.

For a monocoque, that also includes the mould, which can be the most labour-intensive part of the process. For an aircraft intended to be reconfigured, this is the crucial limitation.

A platform whose fuselage can only be changed by making a new mould is not a platform. It is a finished aircraft.

The load-bearing fuselage structure is a skeleton made from three-millimetre birch plywood. At its centre is a square hollow section that carries the internal loads. Two frames for the wing attachment and the landing-gear mounts connect to it.

The outer form comes from an FDM-printed PETG shell. It has no function within the primary structure. It carries its own weight and the aerodynamic loads on its surface, transferring them into the skeleton.

The two are joined through three fixed interfaces whose geometry must remain unchanged. Away from these interfaces, individual shell segments can be adapted and reprinted as long as their maximum permitted mass is not exceeded. Within these limits, the load-bearing structure remains unchanged.

For the structural design, the CS-VLA certification specification from crewed aviation served as a reference. It defines specific load cases with numerical values that can be checked independently. On a teaching platform, traceability is part of the product.

Reduced to 14 load cases, the calculation covers the operating envelope: maximum load factors in flight, gust loads, landing impact and ground handling. A safety factor of 1.5 was included in the calculation.

The load cases are evaluated with a purpose-built analytical design tool. Its parametric structure allows the same design method to be repeated for other missions in a comparable size class.

All analytical checks meet the defined criteria, even though the cross-section was designed not only for strength but also to accommodate payload and avionics.

Two requirements determine the design: the shell must be replaceable segment by segment and must not take on a role in the primary structure. Otherwise, the separation between carrying loads and defining form would be lost.

PETG was selected from four filaments. It provides sufficient temperature and UV resistance, impact toughness and low warping. The parts are printed without support structures, which places the segments vertically on the print bed and makes the printer's build height the governing limit.

The shell is therefore divided into eight segments. Their arrangement follows the available build height, print orientation and position of the interfaces.

Thin free-standing edges include printed stabilising ribs that are snapped off after printing. They are designed directly in CAD rather than generated by the slicer.

The skeleton parts are cut with a laser cutter. Apart from the walls of the square hollow section, they all come from a single standard sheet of plywood measuring 600 by 600 millimetres.

When the sheet comes off the laser and the contours appear for the first time, there is suddenly something tangible in front of you. For me, that is one of the best moments in any project.

The real challenge was not cutting the parts but assembling them. The load-bearing layers of the frames are deliberately left intact, so they have neither tabs nor slots. As a result, the parts do not position themselves during assembly.

Printed assembly jigs were developed to hold every part in place while the adhesive cures.

Material costs for the complete fuselage airframe, including raw materials and adhesive, were below €100. It also required no master pattern, mould, autoclave, vacuum equipment or dust-free workspace. For student project teams, this infrastructure is often the real barrier — not the material and not the ability.