Study of aerodynamic interactions in counter-rotating coaxial rotors

Abstract

Counter-rotating coaxial rotors (CCR) are ubiquitous in emerging next-generation configurations, demonstrating enhanced performance metrics such as increased forward-flight speed, flight range, and payload capabilities compared to single-rotor configurations. At the same time, multiple rotors in the CCR also result in high aerodynamic interactions that govern their performance. Therefore, it is necessary to better understand the nature and influence of these aerodynamic interactions. This dissertation seeks to address this knowledge gap by undertaking experimental investigations into the impact of select aerodynamic interactions on the performance of counter-rotating coaxial rotors. The primary focus is on studying the effects of rotor-ground and blade-on-blade interactional effects, specifically in hovering flight in close proximity to the ground.

The rotor-ground interactions significantly alter the hover performance of a rotor operating in ground effect (IGE). In the case of the CCR, the individual rotors interact with each other and the ground plane, leading to a complex aerodynamic environment. Through a rigorous experimental investigation, this dissertation shows that the CCR behaves as a single rotor when operating in IGE despite these interactional effects. However, the individual rotors in CCR show very different performance behavior compared to a single rotor, contingent upon rotor spacing and proximity to the ground plane. Similarly, the blade-on-blade interaction in CCR occurs during the crossover between the upper and lower rotor blades. These periodic events introduce transient excursions in airloads, resulting in vibratory loads and potential alterations to the vehicle’s acoustic signature. This dissertation studies the fundamental physics of these interactional effects, employing a canonical representation of the problem, i.e., a crossover between two translating airfoils. The investigation identifies key non-dimensional parameters influencing the problem and observes a significant analogy between blade crossover and gust interaction problems. In particular, applying the Kussner model, commonly used to predict the response of airfoil lift to gusts, demonstrates promising predictive capabilities, especially in the initial stages of blade crossover.