Aeroelastic stability analysis of time-periodic MBS-models with application to a helicopter in forward flight

Abstract

In this thesis, a trim algorithm is implemented in the Python library STARS, which simultaneously determines the trim solution and the time-periodic steady state. This algorithm is called parallel trim in this thesis. The solver settings of this algorithm are optimised concerning the quality of the solution and the computation time first. Then the algorithm is used to determine trim solutions in the forward flight of a helicopter rotor. Finding the trim solution is considered independently of determining the time-periodic equilibrium states for optimising the solver settings. The trim solutions are optimised using a 4-bladed MBS model of a helicopter rotor, the 7AD rotor. It could be confirmed that the determination of the trim gradient has a critical influence on the convergence behaviour of a trim algorithm. The tolerances for time-integration in the MBS software Simpack have the most significant impact on the quality of the steady state solution. By comparing an aeroelastic stability analysis of a simplified trim, which can be performed without a trim algorithm, and a 5-DOF trim with the parallel trim algorithm, it could be confirmed that the rotor trim influences the stability behaviour of a helicopter rotor in forward flight. A realistic trim for the rotor could be found even with simple assumptions, such as a parabolic aerodynamic fuselage drag and a constant gravitational force.