Projects – Airframe & conceptual design

The Sirocco NG ultralight aircraft

The Sirocco was an old ultralight design from France, from the dawn of composite airframe construction. We designed it from scratch to yield an airframe that was so much lighter that it would fit in the new German ultralight class, all the while using a 4-stroke engine that was twice the weight of the old 2-stroke it replaced.

The Sirocco NG, as built and designed by us on it’s first few flights.

By changing the main structure over to an optimized CFRP structure and getting rid of many of the inserts, hard points and bracing wires, we reduced the airframe to a third of it’s original weight. This resulted in a total structural airframe weight of under 60 kg (132 lbs). With the engine and whole airframe parachute system mounted, empty weight was still comfortably below the 115 kg (253 lbs) limit of LTF-L.

After passing structural certification testing, the company ran out of funds and the design and production tooling was sold abroad, where it shortly thereafter started flying.

A full CAD model was built to both simplify production and run various structural and FEM analyses.
Production of the prototype
CAD model of the wing

The Warren truss wing

Conventional composite wing construction still resembles “black aluminium” wings. With ribs, main spar, drag spar, separate upper and lower skins, discrete hard points for the attachment of flaps and ailerons, many composite wings not only resemble aluminium wings, but share their drawbacks.

Conventional wing structures, wood (upper left) and composites, showing a distinct spar and cored skins.

Starting from scratch allowed us to reduce the entire wing design to two parts: a wing skin and a corrugated shear web where the control attachment points are already in place. By reducing parts count and simplifying the joining of the wing components, we reduced labor by more than half.

By optimizing the shear web and skins to avoid foam cores; structural wing weight was reduced by 38% compared to an optimized traditional composite wing.

Prototyping and testing validated both the reduction in labor and the predicted strength.

Test item to validate design of a Warren-truss wing.
Structural optimization of a conceptual Warren-truss wing design.