Helicopter rotor blade modal tuning using internal preloads

There are many benefits of variable speed rotors if the associated dynamics problems can be alleviated. Existing passive and active methods are unsuitable due to their mass/power requirements and effectiveness over the necessary frequency range. The concept of inducing controlled 'stress softening' to alter the natural frequencies of a rotating structure in a vacuum and in turn avoid resonance is explored in this research. This paper presents an experimental and computational demonstration of this concept in the context of a small scale rotor blade representation. The model is successfully validated away from and within regions of coupling in which veering was present, and was therefore used to assess the effectiveness of the concept on full sized rotorcraft blades. Full scale assessment demonstrated that adequate separation can be achieved without requiring excessively large forces. The aim of the research is to create a semi-active method for the alteration of the blade's resonant frequencies to avoid resonance within a range of rotor speeds.