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The behaviour of the state variables in post stall flight region of high-performance aircraft is presented in this paper. To demonstrate the state variables of aircraft in post-stall dynamics, the non-linear state equations of aircraft are solved. The numerical investigation has shown that a sufficiently negative lift slope can lead to limit cycle oscillation motion. The behaviour of limit cycle is demonstrated with the action attributed to relay action caused by decrement of lift coefficient jump at stall. The time-history of state variables and the effects of decrement of lift coefficient on limit-cycle amplitudes are achieved. A numerical model constructed for a high-performance aircraft F-18 is solved to illustrate the results. The effects of significant factors such as mass, thrust on limit cycle behaviour in post stall region are considered in this analysis. The couple of effects including the mass and velocity on limit-cycle amplitude of attack angle and pitch angle are obtained in this region. The results indicate that chaos behaviour of attack angle exists in the case of a 20 per cent decrease in mass and a 5-10 per cent increase in velocity. Multi-point attractor is predicted with the change in the values of attack angle. A good agreement between the numerical results and the published work is obtained for limit cycle oscillation existence at different values of lift coefficient decrement.

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Based on power spectral technique and Lyapunov approach, methodology to determine the vertical gust load on aircraft encountering atmospheric turbulence under different flight conditions is presented in this paper. Modified longitudinal short period aircraft equations of motion to reflect gust inputs are solved. Family of five linear dynamics models of increasing gust excitation complexity are developed to describe the normal load factor throughout an aircraft due to vertical gust. These models (except Model 2) give a rapid estimation of normal load factor in case complete data are not readily available. Numerical model constructed for a Boeing 747 jet transport is solved to illustrate the results. These results show that Model 5 exhibits higher frequency contents when compared with other models under different flight conditions. The normal load factor of aircraft is estimated at different probabilities of not exceeding the corresponding load factor value based on statistical technique. The Models 1, 3, 4 and 5 predict the load factor with maximum 5% error when compared with Model 2 which considered all gust penetration effects. Finally, the results show a good agreement with the published work in load factor determination, at different probabilities of not exceeding this value when encountering a turbulent vertical gust.