Aeromechanics of Coaxial Rotor Helicopters using the Viscous Vortex Particle Method

Abstract

Coaxial rotor helicopters are a candidate for the next generation of rotorcraft due to their ability to achieve high speeds without compromising hover performance. Coaxial rotors are designed to offload the retreating side of the rotor in high speed flight to delay the effects of reverse flow and blade stall which limit the speed of conventional single main rotor helicopters. The proximity of the two rotors induces periodic blade passage effect loads and unsteady rotor wake interactions absent in single rotor configurations. Coaxial rotors employ stiff composite hingeless blades to prevent the possibility of blade strike. At high speeds, the coaxial rotor operates at reduced RPM to avoid the drag penalty on the advancing blade tip. This combination of rotor lift distribution, periodic blade passage effect, unsteady rotor wake interaction, combined with stiff hingeless blades and reduced rotor RPM implies that a coaxial rotor system requires a specialized aeromechanical analysis.

The goal of this dissertation is to develop a comprehensive aeromechanical analysis capable of modeling the aeroelasticity of stiff hingeless counter-rotating blades and the complex rotor-wake interactions present in a coaxial rotor system. The rotor wake is modeled with the Viscous Vortex Particle method, a grid free approach for calculating vortex interactions over long distances. The spanwise blade loading in attached flow is obtained from a computational fluid dynamics based rational function approximation unsteady aerodynamic model. The ONERA dynamic stall model is extended to capture three dimensional effects due to flow separation. The combination of the viscous vortex particle method with reduced order models for spanwise loading captures the unsteady coaxial rotor loads with computational efficiency. Trim procedures are developed to determine control inputs for a coaxial rotor to maintain equilibrium in hover and forward flight. In forward flight, two different trim conditions are considered: trim with propulsor off, and trim at level attitude. The two trim conditions have a significant impact on the vibratory hub loads, rotor inflow distribution and the aeroelastic stability. A unique aspect of the coaxial rotor is that its stability in both hover and forward flight are governed by equations with periodic coefficients. Therefore, a periodic aeroelastic stability analysis based on Floquet theory is applied. A new graphical method is developed to identify coupling between the blade modes of the two rotors.

The aeromechanical formulation is applied to a rotor resembling the Sikorsky X2TD coaxial helicopter. In hover, the rotor experiences 8/rev blade passage loads due to oscillations in the blade bound circulation induced inflow. Increasing the collective pitch increases the coupling between the flap and lag modes of the blade. The aerodynamic interactions lead to an inter-rotor coupling of the first flap modes. In forward flight, the effects of trim condition, advance ratio, lift offset, and separated wake on the hub loads, inflow distribution and aeroelastic stability are examined. The results indicate that the aeroelastic stability of the lag mode is reduced in forward flight at a level attitude compared to hover.

This study provides an improved physical understanding of the aeroelastic interactions in coaxial rotors. The work presented in this dissertation has the potential to facilitate design and development of future high-speed coaxial rotorcraft.