Abstract:
A model-scale, coaxial, counter-rotating rotor system with single-bladed rotors was tested in hover and compared to a comprehensive model developed in CAMRAD II. Measurements included vibratory hub and pitch link loads, as well as three-dimensional lower rotor blade deformations extracted using digital image correlation. The model flap dynamics were validated using a rotating frame modal extraction technique based on a modified Ibrahim Time Domain method. The CAMRAD model successfully predicted unsteady loads and deformations for the isolated lower rotor operated with cyclic pitch in hover. To investigate transient loads, measurements were taken in the coaxial configuration at a blade loading coefficient of 0.10 and for an isolated lower rotor at equivalent blade loading. The CAMRAD model accurately predicted the unsteady interaction thrust, as well as the blade flapping which was found to increase after upper-lower rotor blade passage. The CAMRAD model revealed aerodynamic forcing in the coaxial configuration consistent with vortex interaction of the upper and lower rotors, while higher harmonics observed in the experimental data were attributed to impulsive loading due to blade thickness effects.