Experimental modal analysis of a rotating tendon-loaded helicopter blade demonstrator

A novel concept which uses an internal tendon to apply a compressive force on the blades for resonance avoidance of rotorcraft, so-called ‘active tendon concept’, was put forward recently. This research enhances the active tendon concept further toward its basic experimental validation in the rotating operational conditions. To progress in this direction, a new fully dynamically instrumented demonstrator combined with the displacement-based axial load application system is developed and assessed experimentally in non-rotating and rotating conditions for the rotor speeds ranging from 0 rpm to 500 rpm and axial compressive loads ranging from 0 N to 500 N. The identified modal dynamics is used to validate the comprehensive reduced-order model which is subsequently used to support the interpretation of the rotating results where the applied experimental methods lacked in resolution of range. It is observed that the rotating system features anticipated modal dynamics in the form of centrifugal stiffening and compressive softening of the beam-dominated modes, whilst the tendon-dominated modes display rapid stiffening effects with the applied axial loads. As a result of these opposing trends, further interactional phenomena such as modal veering are observed experimentally and in model-predicted results. One consistently observed beam-dominated mode is used to extend the range of the experimental observations beyond those supported by the models toward the damping characterisation throughout the veering interactions and complex strain mode analysis. The results presented in this work confirm the anticipated modal behaviour and suggest that the rotating systems can be influenced through controllable preloads whilst in the state of rotation. Additional vibration and load control opportunities arise from the emergent and tunable interactional dynamics between the primary beam or blade system and the secondary augmented tendon preload application system.