What can be learned from birds for achieving directional stability without a fin

Flight mechanics characteristics concerning static and dynamic directional stability in birds are subject of this treatment. It is shown how directional stability is influenced by the elements of the aerodynamic configuration (wing, body, horizontal tail) where issues of static stability in terms of a restoring moment capability are dealt with. A modern and efficient aerodynamic method for modeling the fluid flow around complex geometries and for computing forces and moments with high precision was used to obtain results on the stability characteristics. Concerning a quantitative as-sessment of directional stability characteristics, rotary dynamics with respect to the yaw axis are considered. For this purpose, the frequency of the dutch roll is regarded as an appropriate measure of dynamic directional stability. With ref-erence to aircraft experience, requirements on adequate restoring characteristics in the yaw axis are used. In an expanded treatment of dynamic directional stability, coupling effects between the yaw and roll axes are also considered. It turns out that these can yield a significant contribution to dynamic directional stability. Based on the derived results, it is shown that aerial vehicles the mass of which is comparable with that of birds can have adequate dynamic directional sta-bility characteristics in terms of an appropriate dutch roll frequency level without needing a fin.