Simulation and Mathematical Modelling of Aircraft Flying Qualities with Varying Tail Dihedral

Abstract

Aircraft handling qualities depend on the empennage geometry. The size and position of tails along with the arrangement govern the stability and controllability of the aircraft. However, the empennage also effects the aerodynamic efficiency of the aircraft by producing additional drag force. By increasing the tail size, one can achieve better handling qualities, however, this can negatively impact the aerodynamic efficiency by substantially increasing the drag force. A comparison of different tail setups in terms of their contribution to stability and controllability along with their impact on aerodynamic efficiency is done. The analysis was performed using a Vortex Lattice Method. It was observed that the V-Tails provide best stability and controllability characteristics with the lowest wetted area. V-tail is a tail geometry setup that provides stability and controllability about longitudinal and directional axes simultaneously. In addition, the setup has less wetted area and interference, thus producing less drag as compared to conventional tails. The dihedral angle of a V-tail determines its contribution to both longitudinal and lateral-directional dynamics. However, there is no well-defined empirical method to compute the most suitable dihedral angle for a V-tail in order to meet the required flying qualities. This work presents a method to select the most appropriate dihedral angle of a V-tail to fulfill the requirements of aircraft flying qualities. Numerical calculations were used to generate a complete flight dynamics model with different tail dihedral angles. Subsequently, damping ratios for longitudinal and lateral-directional modes were extracted from these models. Using a curve fitting technique a polynomial was generated for longitudinal and lateral-directional damping ratios against tail dihedral angle. It was observed that by increasing the tail dihedral the longitudinal damping ratio was reduced. In addition, the lateral-directional damping ratio increased with the increase in tail dihedral angle. The lower bound of the tail dihedral angle was obtained using the lateral-directional damping limit in accordance to the flying qualities. Similarly, the upper bound of the tail dihedral angle was obtained using the longitudinal damping limit. The tail dihedral angle in between these bounds was found to be optimal for adequate longitudinal and lateral-directional flying qualities. In addition, it was observed that the mathematical model was not valid for a different flight dynamics model. This is due to the change in aerodynamic behavior of the aircraft.