Some technical particulars

Geometry The glider incorporates a unique bird-like wing geometry and a bird-like method of control. The wing is made up of four wing parts: a pair of inner wings and a pair of outer wings. The inner wings are swept slightly forward and have a notable dihedral angle while the outer wings are swept back and have negative dihedral (anhedral).

This combination of sweep and dihedral angles provides very good lateral stability properties. The outer wings are slightly twisted to provide the required longitudinal stability.The layout gives very well balanced combined stability characteristics. The notorious Dutch-roll tendency, which many conventional tailless aircraft have, seems to be avoidable with this new configuration.

Layout The Exulans prototype with its wing area of 12m2 has a wing span of 12m yet weighs only 65kg. This low mass is achieved by using carbon and aramid fibers in the composite structure. This is desirable in order to make foot launching from mountain slopes or car launches possible. It has been estimated that the glide angle is at least 25:1 which, by low wing-loading standards, is exceptional. It is more than twice the glide angle of conventional hang gliders.

Flaps

Large flaps are fitted to the inner wing to enhance landing and take-off properties. The flap is designed in such a way as not to cause a net pitching moment when changing its setting. The flap effectively changes the overall wing twist and can therefore be used for spin recovery. Using the radio controlled model it was shown that full deployment of the flaps would restrain auto rotation and unstall the wing tips to recover the glider form a spin. Normally the flaps would be used to make the final landing approach angle drastically steeper while also reducing the touch-down speed.

Trimming Primary pitch control is done by the elevons (combined elevator and aileron) on the outer wing. However, for trim control in pitch the new concept of variable wing sweep is used. The outer wing is joined to the inner wing by a hinge, which allows sweep changes in the horizontal plane. By sweeping the wings forward the glider would be trimmed for slower flight without affecting the span-wise lift-distribution. The upward deflection of the elevon otherwise needed for this condition would adversely affect the lift-distribution. Therefore, by implementing the variable sweep control concept, tailless aircraft could be designed with smaller wings while meeting the same landing requirements. Besides other obvious advantages this reduces total parasite drag while improving the wing loading. At slow flight the undisturbed lift-distribution results in lower induced drag when compared to the slow flight regime achieved by elevon deflection. Therefore, the overall aircraft efficiency will improve if variable wing sweep is implemented as a means of trimming control.

Elevons The elevons are also used as ailerons for roll (and yaw) control. In a naturally coordinated turn differential deflection of the elevon would also provide the desired yawing moment in the direction of the roll. In the quest for low drag additional innovations feature in the Exulans research project, like the gap-free control surfaces.