Rotor Unsteady Free-Vortex Wake Model And Investigation On High-Fidelity Modeling Of Helicopter Flight Dynamics

The modeling of rotor unsteady aerodynamics and rotor/fuselage/empennage aerodynamic interaction have been the bottleneck in the high-fidelity mathematical modeling of helicopter flight dynamics. To solve this problem, an unsteady free-vortex wake model for transient or maneuvering flight and its impact on the methodology and accuracy of the modeling of helicopter flight dynamics are investigated in detail in this dissertation.Firstly, an unsteady free-vortex wake model for maneuvering flight is developed. Based on the lifting-surface and vortex methods, the blade aerodynamic model and the method of representating the vortex wake trailed behind the blade are presented with the trade-off between accuracy and efficiency. A new vortex roll-up model which relates the core radius, span station and circulation of the initial tip vortex with the blade bound circulation distribution is deduced for eliminating the empirical parameters in conventional rotor free-vortex models. To solve the numerical problem of unsteady free-vortex wake model, a new time-marching finite-difference algorithm(BC2D) with high efficiency is developed and its numerical stability is examined in detail for physically correct. Then, an unsteady aerodynamic model of rotors based on the unsteady free-vortex wake model and airfoil unsteady aerodynamics with dynamic stall is established to analysis the rotor transient aerodynamics in maneuver flight. To verify the model,a series of tests with model rotor on the Whirling Beam are conducted for the acquisition of the transient responses of rotor loads following the abrupt increases of collective pitch, cyclic pitch and angular rate in both hover and low-speed forward flight. The comparisons of calculated results with test data indicate that the rotor unsteady aerodynamic model developed is effective and there are significant effects of wake dynamic distortion on transient characteristics of rotor inflow and rotor loads. The inflow gradient across the rotor disk induced by the wake curving distortion is not simply proportional to the wake curvature.Furthermore, a new comprehensive model of helicopter flight dynamics is built up, which synthesizes the rigid fuselage motion with 6 degrees of freedom, the rotor unsteady aerodynamics including the flexible blade dynamics with nonlinear flap-lag-torsion coupling, unsteady free-vortex wake and unsteady airfoil aerodynamics with dynamic stall, the effect of rotor/fuselage/empennage aerodynamic interaction as well as the propulsion system dynamics. A numerical method is presented to transform the equations of motion of coupled rotor/ fuselage into first order differential equations with explicit form which can be sovled using standard numerical algorithm with high efficiency.Finally, numerical methods of helicopter trim and dynamic response are developed for the comprehensive model of helicopter flight dynamics. The free-vortex wake dynamics and the coupled rotor/ fuselage dynamics are treated with loose coupling for trim analysis but with tight coupling for response calculation. To validate the comprehensive model, the UH-60A helicopter is taken as an example to predict the trim characteristics and dynamic responses in maneuvering flight. The comparisons of calculated results with flight test data indicated that there is great effect of the free-vortex wake dyanmics and aerodynamic interaction on the fidelity of comprehensive model while the effect of the flexible blade dynamics is small for the helicopter with articulated rotor. Compared with the dynamic inflow model, the unsteady free-wake model improves the accuracy of trim results in hover and low-speed forward flight conditions as well as the dynamic responses in maneuvering flight, especially off-axis responses.