The drivetrain system of tiltrotor aircraft is a complicated multibody system. Traditionally, rotorcraft drivetrain systems are modeled by the finite element method using an equivalent mathematical model with all the elements spinning at the same rotational velocity and structural properties scaled according to gear ratios. Such a process can be complex and computationally expensive, especially for large-scale problems. This paper proposes the dynamic analysis of a tiltrotor drivetrain, coupled with flexible blades’ lagwise motion, using a novel multibody system modeling and analysis method based on the transfer matrix method. The proposed method eliminates the need for equivalent processing of the drivetrain system components and does not require the derivation of the overall governing equations based on the Hamilton principle. Instead, they are directly formulated according to the system’s topology graph. Virtual branch and geometric elements are introduced to decouple any topological structure of the drivetrain system into multiple independent chain systems, further reducing the modeling complexity.

Blending the agility of aerial drones with the covert capabilities of underwater submersibles, the aerial-aquatic rotorcraft has garnered substantial interest due to their unparalleled capacity to traverse both air and water. Nevertheless, a critical hurdle for these vehicles lies in mitigating the adverse effects of repeatedly transitioning between these environments, particularly during water-surface takeoffs. Currently, research on the interference caused by rotors approaching water surfaces remains limited. This paper introduces a novel adaptive rotor aerodynamic model based on continuous finite vortex theory to predict rotor thrust within gas–liquid flow field. Initially, the model’s sensitivity to system parameters was analyzed to optimize its predictive capabilities. Subsequently, a comprehensive ground/water experimental setup was designed to investigate the intricate aerodynamic interactions between the rotor flow field and water. By varying rotor sizes, the characteristics of the rotor flow field and water surface were examined at different rotor-water surface distances. The performance of different modeling methods was analyzed based on the rotor experimental data of a diameter of 0.38 m, and the prediction results were quantified using the percentage of the mean-square error. The results show that the average error of the finite vortex rotor model is the smallest. Finally, a novel transition boundary is proposed to divide the rotor flow field of the gas–liquid mixture into two stages. The thrust loss zone is defined to delineate the safe operating range of the aircraft, providing a basis for the design of aerial-aquatic rotorcraft.

Aerial-aquatic rotorcraft, capable of navigating in both underwater and aerial environments, has gradually become a focus of research in recent years. The physical properties of water and air differ significantly, thus the rotor experiences a complex two-phase flow environment during the cross-medium process. Using the lattice Boltzmann method (LBM), this study focuses on the aerodynamic characteristics and flow mechanisms of a rotor as it gradually approaches the water surface from the air. Combined with corresponding CFD results, it is shown that at a height of approximately 0.8 times the rotor radius above the water surface, the rotor enters a flow state similar to ground effect. As the rotor continues to approach the water surface, its downwash significantly interacts with the water, inducing the formation of a distinct liquid crown structure accompanied by strong vortex ring effects. This leads to a decrease in air pressure beneath the rotor, causing a continuous decline in rotor lift during this period. The lift characteristics exhibit a turning point, where the lift decreases instead of increasing, indicating significant gas-liquid two-phase flow interference. After the liquid crown structure breaks up, the rotor lift stabilizes. At H/R = 0.2, the rotor lift increases by approximately 26% compared to its performance in the air. At H/R > 0.4, the growth rate in rotor lift gradually slows, decreasing at a rate of 2% to 5%, until it eventually converges to the lift level observed in the aerial condition. This paper preliminarily reveals the initial patterns of the thrust characteristics of rotors under near-water surface conditions and their influencing mechanisms, providing a reference basis for subsequent theoretical research and practical applications.

Aerial-aquatic rotorcraft have garnered extensive attention due to their ability to operate in the air and underwater. However, their widespread application still faces challenges, the most prominent being the stable transition across water surfaces. Neglecting the near-water-surface effects during the transition process may lead to distortion in its system dynamics model, increase the difficulty of controller design, and even result in failure during the transitions. Most previous studies have relied on the ground effect theory to simulate the transition across water surfaces, but these efforts have shown limited accuracy in predicting rotor lift due to the significant differences between ground and water. In the present paper, we investigated the rotor's aerodynamics in proximity to the water surface using the potential flow theory, based on which a rotor aerodynamic model incorporating the near-water-surface interference is proposed and optimized by Laplace's law. Experiments conducted at various heights above the water surface and with different throttle settings have validated the superior predictive capability of the proposed model. The results indicate that the lift increases as the rotor approaches the water surface, but the increment is less significant than when it approaches the ground. Additionally, the lift increment induced by the near-water-surface effects diminishes at higher rotation speeds, manifesting as lift loss ratio ranging from approximately 5% to 44%. Finally, the aerodynamic model of the rotor and the correction method adopted in this study significantly improve the prediction accuracy of rotor lift under both near-ground and near-water-surface conditions. For small-and medium-sized rotors, the averaged prediction error is reduced by approximately 60%−80%.

Aiming to elucidate the complex aerodynamic interactions between rotor and wing during hovering, forward flight, and transition modes of tiltrotor aircraft, an enhanced hybrid vortex particle method is developed. This method leverages Neumann boundary conditions and Hess equivalence principle for efficient computational analysis. By incorporating a single panel-multi vortex particles conversion and adaptive vortex particle quantity control, the adaptive adjustment of vortex particle quantity is achieved, further optimizing the computational efficiency. Validation against wind tunnel data demonstrates precision and efficiency of the method compared to traditional viscoelastic vortex particle methods. Subsequent numerical simulations and flow field analysis of the rotor/wing model unveil intricate aerodynamic interactions. In hovering mode, while the wing's blocked downwash slightly enhances rotor lift, the dominant negative lift due to significant wing download adversely impacts overall load-carrying capacity. Increasing collective pitch mitigates this lift loss. During transition, the rotor wake initially induces substantial lift loss on the wing, followed by a notable increase and negligible influence in later phases. The wings alter structure of the rotor wake, but the effect on rotor performance is minimal. In forward flight, rotor/wing aerodynamic interactions is weak, albeit with a reduced lift-to-drag ratio. These findings provide valuable insights into the aerodynamic complexities of tiltrotor aircraft, contributing to the development of aeroelastic stability analysis, high-fidelity flight dynamics model development and performance optimization.

High-speed helicopters use technologies such as advancing blade, reduced rotor rotation speed, and auxiliary propeller to achieve high-speed flight. The coupled system formed by coaxial rigid dual-rotor, variable speed drive train system, and high-power output tail propeller brings new challenges to traditional helicopter torsional vibration analysis. Firstly, a new modeling strategy based on the transfer matrix method is innovatively proposed to address the problem of complex multi-mode coupling torsional vibration system of high-speed helicopters. Compared to the modeling strategy of conventional finite element method, the present method does not require the equivalent processing of the drive train system, nor does it require the derivation of the overall governing equations based on Hamilton’s principle. System governing equations can be directly obtained according to the topology structure of the system. In addition, a virtual geared branch element is innovatively introduced to decouple the topology of the drive train system into multiple independent chain systems, further significantly reducing the difficulty of modeling. Finally, the coupled torsional vibration dynamics of high-speed helicopters under different working conditions is studied based on the proposed method.