High-Advance-Ratio Coaxial Rotorcraft Aeromechanics

Current vertical-takeoff-and-landing aircraft designs are generally a compromise between efficient hover performance and forward flight performance. With both high-speed capability and range being important future requirements, the aforementioned compromise is even more of a challenge. Fast edgewise flight of the helicopter is mainly limited by the asymmetric aerodynamics at the rotor disk and associated retreating blade stall.

Rigid rotors are one key technology enabler for coaxial rotor systems that are capable of high forward flight speed. The suppressed (or at least significantly reduced) flapping motion of the rotor blades allows for smaller rotor spacing and so for significant hub drag reduction.

Recently, experiments have been performed by the University of Texas (UT) at Austin, USA, with such a high-advance-ratio coaxial model rotor system in hover and also in a wind tunnel for advance ratios up to 0.5. In order to further the understanding of the interactional aerodynamics, structural dynamics, and flight dynamics of rotorcraft with coaxial rotor systems, particularly at high advance ratios, the Institute for Helicopter Technology shares a collaborative effort with the UT and performs comprehensive analysis using the computational modeling capabilities with the aeromechanics rotorcraft code CAMRAD II together with the experimental data set measured by UT.

The current research intends to provide more insight and to better understand the unsteady forces/loads and their aerodynamic and dynamic sources, such as aerodynamic interactions between the upper and lower rotors, blade deflections and their effects on blade clearance, in order to produce a real (validated) predictive capability for these challenging rotor systems, and ultimately, to enable technology and performance improvements.