Time-Dependent Coefficient Reduced-Order Model for Unsteady Aerodynamics of Proprotors

This work presents a methodology for the identification of a periodic-coefficient Reduced-Order Model (ROM) for the prediction of perturbation aerodynamic loads on tiltrotor propellers in cruise flight. Although the result is a periodic-coefficient model, the process requires only frequency-domain aerodynamic solutions. Assuming the unperturbed proprotor in axial flow, first the matrix that collects the aerodynamic transfer functions between blade perturbative boundary conditions and generalized aerodynamic forces is derived. Then, its rational-matrix approximation, followed by the combination with the equations describing the wing/pylon/proprotor kinetic coupling, yields the aerodynamic ROM. This ROM is expressed in terms of a set of linear equations that relate the time evolution of the aerodynamic loads acting on the proprotor blades to wing/pylon and deformable-blade degrees of freedom. Numerical results concerning a three-bladed proprotor connected to a bending and twisting wing will show that the unsteady aerodynamic loads predicted by the proposed ROM are in excellent agreement with those obtained through direct time-marching aerodynamic solutions.