Resonance Avoidance of Variable Speed Rotors using Variable Stiffness Blades

Abstract

This paper presents and evaluates a new concept for resonance avoidance of variable speed rotors. This concept is based on using a Variable Stiffness (VS) composite spar, which mechanical properties can be varied in flight. This adaptive spar is made of a mixture of composite laminates and variable stiffness layers. The variable stiffness layers are made of shape memory alloy fibers and their module of elasticity can be changed or controlled by the addition of heat. To model the structural dynamics of the blade, a one dimensional geometrically exact beam formulation has been coupled with a two dimensional cross-sectional analysis tool. The rotating frequencies of the blade are obtained by finding the eigenvalues of the linearized equations about the nonlinear steady-state condition. First the accuracy of the developed model is verified against available results for a scaled composite blade, and a very good agreement is observed. Next, the effect of adding vari-able stiffness layers on the blade frequencies is examined. It is found that by changing the temperature of the blade by only 10 degrees, it is possible to modify the resonance frequencies. This is obtained by either remov-ing the resonance crossing from the operational range or moving the crossing toward higher harmonics. Fi-nally, the proposed concept is applied to a full-scale blade and the same observations are obtained.