Performance and Vibratory Hub Loads of a Mach-Scale Coaxial Rotor in Hover

A Mach-scale, rigid, counter-rotating coaxial rotor system, 80 inches in diameter and operating at tips speeds up to 625 feet per second, was built and hover tested in three configurations: two-bladed single rotor, four-bladed single rotor and two-bladed counter-rotating coaxial rotor. The blades were untwisted with a VR-12 airfoil profile and a constant chord of 3 inches with a 0.15 inch trailing-edge tab. The rotors were tested up to a maximum blade loading (CT/μ) of approximately 0.095. Dynamic hub load data, as well as lower rotor pushrod loads were recorded with synchronous averaging implemented using a main rotor shaft 1/rev trigger. Mean loads were used to analyze rotor performance, as well as to validate an in-house, free vortex wake model (FVM). The performance of the coaxial configuration was found to be indistinguishable from that of a four-bladed single rotor of equal solidity and planform while individual upper and lower rotor performance was degraded compared to a two-bladed single rotor. The FVM was shown to accurately capture both total system loads, as well as trends in upper and lower rotor thrust sharing. Preliminary results for vibratory thrust loads indicated larger coaxial rotor vibratory loads at several n/rev harmonics compared to an isolated single rotor. The azimuthal variation of lower rotor thrust, compared with that of an isolated single rotor, revealed evidence of aerodynamic interaction with passing upper rotor blades.