A Review of Coaxial Compound Helicopters: Aerodynamics and Flight Dynamics

The coaxial compound helicopter has enhanced the maximum flight speed, while retaining the hover and low-speed maneuvering capabilities of conventional helicopters. As a novel configuration, its coaxial rigid rotors and tail-mounted propeller introduce unique aerodynamic and flight dynamic characteristics. This paper reviews both the aerodynamic and flight dynamic characteristics of coaxial compound helicopters. This review begins with an introduction to the unique features of coaxial compound helicopters, such as lift offset and control redundancy. In the aerodynamic section, various numerical simulation methods used in aerodynamic research are summarized, containing Computational Fluid Dynamics trimming methods. The aerodynamic characteristics of coaxial compound helicopters are reviewed in terms of aerodynamic interference, aerodynamic loads, and aerodynamic noise. Aerodynamic interference between the rotors is a major cause of unsteady aerodynamic loads, which in turn lead to aerodynamic noise. Subsequently, this paper introduces the models involved in flight dynamics modeling, detailing the rotor aerodynamic model, inflow model, blade motion model, and aerodynamic interference model. Based on this, the trim, stability, controllability, and flight control design of coaxial compound helicopters are reviewed. The differences in trim results between coaxial compound helicopters and conventional helicopters, as well as the control coupling effects of coaxial compound helicopters, are addressed. Finally, this paper summarizes the aerodynamic and flight dynamic characteristics of coaxial compound helicopters and provides some suggestions for future research.