Mission-based optimal morphing parameters for rotors with combined chord and twist morphing

Background: The rotor blades with fixed geometry in today's helicopters do not give the best performance throughout the duration of any mission. However, low-speed and high-speed flights have different geometrical requirements for the shape of the most efficient rotor blades. With advancements in morphing technologies, these can be applied to change the shape of the blades between different flight regimes. Methods: Two different helicopter rotor morphing concepts - namely, the linearly variable chord extension and the torque-tube based twist - under the framework of the European project SABRE were investigated for their optimal geometric parameters using a Particle Swarm Optimization (PSO) algorithm. Since the morphing parameters were dependent on the mission profile, three different missions representing typical helicopter applications were chosen. The optimization problem was posed both as single objective (power) and as multi-objective (power, tip elastic torsion and vibratory hub load). Based on the insights drawn from these investigations, a rotor was set up including both morphing concepts in a single blade. Results: The rotor with combined chord and twist morphing was shown to give performance improvement of 6.8% over the baseline blade for a whole mission while keeping the penalty on the elastic torsion and vibration of the rotor to a minimum. The performance improvement was higher at 13% for hover and low speed flight of µ = 0.14. Conclusions: Chord and twist are both important parameters determining the efficiency of a rotor blade. Since they have non-overlapping requirements, combining the two morphing concepts into a single blade can yield higher performance than the individual ones.