Tail effects on yaw stability in birds.

Bird tails, which are an aerodynamic surface in the horizontal plane, are treated with regard to their effects on yaw stability. Reference is made to wings of very small aspect ratio similar to the values of bird tails in order to identify features which are significant for the aerodynamic yawing moment characteristics due to sideslip. It is shown that there are yawing moments of considerable magnitude for this aspect ratio region. Furthermore, the lift coefficient, which also exerts an influence, is included in the treatment of yaw stability. To show more concretely the addressed effects for birds, the yawing moment characteristics of the wing-tail combination of a pigeon, which is considered as a representative example, are treated in detail. For this purpose, a sophisticated aerodynamic method capable to deal with the mutual flow interactions between the tail and the wing is used to compute results of high precision. The yawing moment characteristics of the pigeon wing-tail combination with respect to the sideslip angle and the lift coefficient are determined, with emphasis placed on the contribution of the tail. It is shown that there is a significant contribution of the tail to yaw stability. The findings of this paper on the contribution of the tail to the yawing moment characteristics are supported by an evaluation of existing experimental data. Furthermore, the physical mechanisms are considered which are the reasons for the stabilizing role of the tail. These effects concern the contribution of the drag acting at the tail to the yawing moment. In addition, it is shown that extended legs and feet, when exposed to the airflow, can contribute to yaw stability.