ROTORCRAFT TRIM ANALYSIS USING A GENERAL-PURPOSE MULTIBODY CODE

Rotorcraft dynamics represents a major analytical challenge to aeronautical industry and research centres. Complexities arising from large rigid motions, body elasticity, aerodynamic loads and control systems have to be taken into account in order to ensure the accuracy of a comprehensive analysis. Architected for the nonlinearities associated with large motion in three-dimensional space, the ADAMS general-purpose multibody code allows to automatically formulate and integrate the equations of motion for a wide range of mechanisms, including rotary wing systems (once provided with an aerodynamic force field description). However, the ADAMS simulation system lacks the capability to calculate periodic motions, as required in the helicopter trim analysis and stability evaluation. The prediction of the trimmed periodic motions of the rotor system implies the numerical solution of differential-algebraic boundary value problem. In this work we present a new approach to perform this task inside the ADAMS numerical environment. Thia approach is based on the perturbation of the minimal set of Ordinary Differential Equations (ODEs), being equivalent to the original system of Differential Algebraic Equations (DAEs) which defines the rotorcraft equation of motion. The transformation of DAEs to ODEs is based on the linearization of the local constraint manifold defined by the algebraic constraint equations, as suggested by Maggi in his work [1-3]. The proposed method is quite general and can be used to drive the ADAMS integration scheme within the periodic motion analysis of mechanical systems. The algotithm is adopted to simulate the wind tunnel trim test of a ECD BO105 mach-scaled model (EU HeliNOVI project [4]). Comparisons between numerical and experimental results are provided.