By designing the outer wings’ detach point more than 2m away from the fuselage, the bending moment is reduced to roughly 40% of the maximum, making the coupling of the wings much easier and lighter in weight.” He adds that the spar of the inner wing has a pocket at the end, into which the stub spar of the outer wing is inserted. To determine where the wing segments detach/assemble, Wehren explains that it depended on the bending moment in the spar: “The greatest bending moment in a spar is in the center of the wingspan. This way, the aircraft is balanced when the pilot is lifting it and also balanced in flight, with no aerodynamic trim control needed during foot launch.” And to compensate, the outer wings are swept back to move the aerodynamic center back to the pilot. So, I moved the wing forward in front of the pilot.

Wehren points out that an aircraft’s planform should be designed with the center of gravity and the aerodynamic center of the aircraft in the same location: “In a light, foot-launch aircraft like the xxtherm2, the main wing spar would cross the fuselage at the place where the pilot sits. The wings, each 4.8m long, are built in three parts, to enable breakdown and transport, yet deliver performance while facilitating foot-launch. The group wanted the aircraft parts to fit within a 4.8m/16-ft-long container, which could be placed on a small trailer and weigh, together with the container, less than 75 kg. The design also had to balance the demands of aerodynamics with portability, given that one must be able to disassemble and pack a hang glider in a container for car transport to competitions. And at the end, we asked the most important question: How is this supposed to be manufactured?” The xxtherm1 was designed the classic way: first some crazy ideas, then more detailed concepts, studies and design calculations. Says Wehren: “In 2006 we had only the intention to build a foot-launchable glider for our own flying, without any plan to produce it. The all-carbon composite xxtherm2, intended for such long-distance competitions, falls into the third category. Pilots in all three types have set impressive soaring and distance records over the years. The Fédération Aéronautique Internationale (FAI, Lausanne, Switzerland) defines three classes of competition hang gliders: flexible wing hang gliders, where the pilot is suspended, typically in a sling or bag, and controls a delta-shaped wing with simple weight shifts rigid wing hang gliders that require spoilers and ailerons for flight control and Class 2, Subclass O-2 rigid wing hang gliders with flight controls and a fairing or small cockpit.

Hang glider flight