fs24 – Phönix

Fiber Reinforced Composites

The fs24 Phönix was the very first aircraft in the world to be made from fiber reinforced composites. At the time, the limits of the then widespread wooden construction were made clear by the slow emergence of laminar profiles. These new generations of profiles demanded a better surface quality than that of typical wooden wings. Only the use of negative molds for the construction of the fiberglass sandwhich wings allowed for the required surface.

Originally, a monocoque design was developed for the fs24 to attain a lower wing loading than contemporary sailplanes. In this design, balsa stiffened with a paper-glue layer was the main support material. Due to financial reasons however, the project took longer than expected, even leading to a halt for some time due to external problems. Progress in the monocoque and aerodynamic design let to a concept that was deemed more worth it. This among other things led to support from the state of Baden-Württemberg.

For the new monocoque design, balsa was still retained as the supporting material, but fiberglass reinforced polyester resin was used as a stiffening material. Strength tests proved the worth of this design: Both materials are stretched equally due to their similar breaking strains. A special polyester had to be used, which hardens completely during polymerization in the air without being too sticky. This polyester could also directly be dyed, allowing the final lacquer to be kept relatively thin.

Since at the time, rate of climb while thermaling at low/medium speeds was seen as more important than max speed while flying straight ahead, emphasis was given on achieving a low wing loading. Since a low wing loading reduces the glide ratio at higher speeds though, an attempt was made to improve the aerodynamics as much as possible to compensate for this. To this end, the fuselage form, profile design, fuselage angle of incidence, wing loading, aspect ratio, and other details that have an effect were optimized for the aircraft’s median cruising speed. A new calculation method was also developed to create the profile. The fs24’s glide angle can be influenced via flaps on the undersides of the wings which can flip out up to 90°.

All in all, the fs24 has pleasant flight characteristics and good control surface effectiveness. The aircraft does not require to be trimmed when deploying the flaps and can perform steep approaches with them. Directly after the first flight, some time in December, it was apparent that the aircraft could still thermal while other sailplanes couldn’t even fly anymore.

The fs24 prototype currently resides in the German Museum in Munich.

Specifications