Nonlinear Helicopter Rigid-Elastic Coupled Modeling with Its Applications on Aeroservoelasticity Analysis

This paper represents a new mathematical model for helicopters with flexible fuselages. The theory connects the two commonly separated research areas of flight, dynamics and structural dynamics, by considering all necessary effects of a flexible fuselage on helicopter flight dynamic characteristics. They are rigid-elastic couplings of the flexible fuselage between six rigid-body degrees of freedom and elastic deformations, along with inertial and kinematic couplings between subcomponents and the fuselage. This is achieved by using the floating frame theory and deriving the acceleration coupling matrices of subcomponents analytically. In addition, the obtained formulation is explicitly decoupled such that all unknown absolute accelerations could be obtained in a single step without iterations, which could improve computational efficiency greatly. This model is suited for aeroservoelastic analysis, and is particularly suited for heavy lift helicopters (HLH) because they usually have slower turning rotors and more flexible fuselages due to their larger scale, leading to closer frequency ranges between rigid body motions and high-frequency rotor and structural modes. The model is validated before being applied to a numerical example of a sample HLH with a feedback loop for stability augmentation. The results show that the feedback loop connects actuators, fuselage deformation, and rigid body motion and induces divergent oscillation, which is the aeroservoelastic instability that cannot be predicted without the additional sensor measurements feedback caused by dynamic deformation. Since the 10-second step response could be calculated in three seconds, the presented helicopter mathematical model is expected to be used in real-time simulations or even further in pilot-in-the-loop simulations, ultimately helping with the design of modern helicopters with nonnegligible fuselage flexibility.

Automated summary

This paper presents a new mathematical model for helicopters with flexible fuselages, which connects the research areas of flight dynamics and structural dynamics. The model is suited for aeroservoelastic analysis and is particularly suited for heavy lift helicopters.