Analysis of Spin and Recovery Characteristics of Multirole Fighter Aircraft.

Abstract

The purpose of the present research is to investigate the spin and recovery characteristics of a high performance fighter configuration that exhibits yawing moment asymmetry at high angles of attack. High fidelity aerodynamic model, in a look-up-tables form, is developed using experimental data from static, coning, and oscillatory coning rotary balance wind tunnel tests. As a first step, all the possible steady spin modes, along with their sensitivity to control settings, are predicted from the aircraft flight dynamic model. Influence of high alpha yawing moment asymmetry on the spin recovery characteristics is inferred from the spin equilibria plots. Results from dynamical systems theory are applied to determine local stability characteristics of aircraft around steady spin states. The complete set of dynamic modes of aircraft in spin is evaluated and mode content in each of the motion variable is determined using modal decomposition. Investigation of dynamic characteristics of predicted spin modes is performed using six degrees-of-freedom time history simulations which showed that both, right and left flat spins are oscillatory and divergent. Standard spin recovery piloting strategies are investigated by performing numerical simulations of flat spins in open-loop configuration. Numerical simulations show that for fighter configuration with high-alpha yawing moment asymmetry, same spin recovery strategies when applied to left and right flat spins produce contradicting results. The proposed spin recovery strategies effectively reduced the recovery time for left flat spins. However, aircraft's inherent tendency to yaw towards right due to high-alpha yawing moment asymmetry renders proposed spin recovery strategies ineffective in accelerating recovery of right flat spins. Performance of a gain scheduled flight control law in automatic spin recovery is also evaluated by performing numerical simulations of left and right flat spins in closed-loop configuration. The control law also reduces the recovery time of the left flat spins but is ineffective in aiding the recovery of the right flat spins. Simulations in closed loop configuration show that the large amplitude oscillations in angle of attack and sideslip, observed in open-loop configuration, are significantly damped by the flight control law.