Resonant passive energy balancing for a morphing helicopter blade

The idea of morphing a helicopter blade by using compliant structures promises augmented capabilities in terms of manoeuvrability and fuel efficiency. To achieve morphing, compliant structures work by elastically deforming to achieve the desired response, and therefore actuation must work against the inherent structural stiffness in addition to external loads. Passive Energy Balancing has previously addressed this problem for quasistatic loads, by adding negative stiffness elements in parallel with the structural stiffness, so that stiffness is reduced almost to zero and lighter actuators may be used. This work extends this idea to the case of dynamic actuation, where negative stiffness is optimally used to reduce the natural frequency of a morphing blade, so that it may resonate at the desired actuation frequency. A negative stiffness mechanism in parallel with the structural stiffness can be used to tailor the natural frequency of a morphing blade system. Furthermore, the negative stiffness mechanism introduces nonlinearity that has some benefits in stabilising the resonant response amplitude compared to a linear resonance, and is also shown to be beneficial to achieve a weight efficient mechanism. A spiral pulley negative stiffness mechanism has previously addressed this problem for quasistatic loads and is extended here to achieve linear frequency tailoring and nonlinear frequency tailoring, respectively. The equivalent stiffness of the extended spring used in the rotating system has been investigated. Resonant morphing strategies exploiting dynamic tailoring have been studied showing encouraging preliminary results.