Comprehensive Analysis of Coaxial Rotor Dynamics on a Support Arm

The Rotorcraft Comprehensive Analysis System (RCAS) was used to analyze a coaxial rotor coupled with a support arm. A generic small-scale (1.35 m diameter) stiff (1^st flap > 2/Rev) coaxial rotor (D/4 spacing) was analyzed with two basic support arm geometries. This blade stiffness and rotor spacing is much larger than typically analyzed and can be representative of some eVTOL and UAM configurations. A study is first presented of an isolated coaxial rotor model for varied rotor blade flap stiffness to show the sensitivity of the fixed-frame rotor hub loads to the placement of these very stiff blade modes. Tuning the rotor blade stiffness reduced the maximum vibratory vertical hub force from 195 N to 125 N and the maximum vibratory hub moment from 175 N-m to 105 N-m. The tuned rotor was then coupled with a rectangular cross section support arm to show the effects that the support arm frequency placement has on vibratory loads. Tuning the support arm by changing the cross section to circular while keeping weight and length constant reduced the peak hub vibratory force from 650 N to 275 N and the peak arm tip vibratory moment from 325 N-m to 225 N-m. The paper characterizes rotor dynamics for small-scale stiff coaxial rotor systems on a support arm and provides insights on how to design such a system to minimize vibratory loads.