Yaw stability in gliding birds

A new concept for describing the yaw stability in gliding birds is presented. This concept introduces dynamic stiffness in yaw as an appropriate indication of stability. Other than the conventional metric of static yaw stability given by the gradient of the aerodynamic yawing moment with respect to the sideslip angle, the dynamic stiffness does not only provide a qualitative indication of stability but also a precise quantitative measure of the restoring action in the yaw axis. With the use of scaling relations, it is shown that the dynamic stiffness of birds is sufficiently high though their static yaw stability may be very small. The underlying mechanism is that the yaw moment of inertia is more reduced with a decrease in size than the restoring aerodynamic moment. Reference is made to the yaw stability in aircraft and related flying qualities requirements. Thus, numerical values are derived which can be used as a standard of comparison providing a rating basis for the dynamic yaw stiffness in small flying objects, like birds. Furthermore, it is shown that the wings of birds produce yawing moments due to sideslip so large that a sufficiently high level of dynamic yaw stiffness can be achieved. From the results derived in this paper, it may be concluded that birds - unlike aircraft - need no vertical tail for yaw stability.