Stability sensitivity analysis of a helicopter rotor

A sensitivity study of blade stability in forward flight for a hingeless rotor with respect to design variables is carried out using a direct analytical method. Design variables include nonstructural mass distribution (spanwise and chord wise), chordwise offset of center of gravity, and blade bending stiffnesses (flap, lag, and torsion). The formulation for blade steady response is based on a finite-element method in space and time. The vehicle trim and blade steady response are calculated iteratively as one coupled solution using a modified Newton method (coupled trim analysis). Eigenvalues corresponding to different blade modes are calculated using Floquet transition matrix theory. The formulation for the derivatives of the eigenvalues with respect to the design variables is implemented using a direct analytical approach and constitutes an integral part of the regular stability analysis. For the calculation of the stability derivatives with respect to a total of 30 design variables, there is an 85% reduction in CPU time using the direct analytical approach compared to the frequently adopted finite-difference approach. A parametric study showed that nonstructural mass and chordwise blade e.g. offset of outboard elements, and lag bending stiffness of inboard elements, have powerful influence on blade stability.