To investigate the aerodynamic interference caused by tanker wake on helicopter during aerial refueling, a numerical simulation method for the coupled flow field of tanker and helicopter based on the Reynolds-Averaged Navier-Stokes (RANS) equations is developed. The A400M transport aircraft and UH-60A helicopter are used as research subjects to analyze the characteristics of the isolated tanker wake and the coupled helicopter/tanker flow field. Results show that the tanker wake primarily consists of rotating tip vortex, inner propeller slipstreams, fuselage wake, and horizontal tail wake, forming a highly unsteady and strong vertical velocity field behind the tanker fuselage. Notably, lateral offset maneuvers can effectively reduce the turbulence intensity of the wake. The wake-induced upwash leads to an overall increase in the rotor thrust coefficient by approximately 2.5% to 4.4%, accompanied by pronounced fluctuations compared to baseline conditions. Meanwhile, the moments of the rotor and tail rotor increase markedly. As the rotor moves laterally toward the docking position, the unsteady disturbances intensify, resulting in more significant thrust oscillations (originally “tension oscillations”). During the standard aerial refueling procedure, the helicopter can generally avoid the strong wake interference area during the traverse phase, with minimal changes observed in both the thrust coefficient amplitude and the rotor thrust distribution. However, at the docking position the influence of the wake amplifies vortex energy within the tip vortex, causing a notable enlargement of the vortex structures. Within the rotor disk region, the tip vortex tubes become increasingly deformed and broken. Furthermore, the vortex distribution between the front and rear rotor blades shows asymmetry, with the front blade vortex slightly stronger and tending to spread downward. Under the full-configuration simulation conditions, the large rotor tilt angle limits the fuselage’s effect on the surrounding flow field. Meanwhile, the presence of tail rotor vortex increases turbulence beneath and behind the fuselage. Compared to the baseline state, the rotor thrust distribution changes significantly: thrust on the forward side of the rotor blades increases sharply, and the tip vortex deformation leads to negative thrust zones appearing near the blade tips. Wake-induced turbulence generates high pressure around the helicopter nose, reducing the fore fuselage pitching moment by about 60% and causing pronounced high-frequency oscillations. This dynamic behavior can adversely affect the helicopter pitch stability during docking maneuvers. The wake decreases torques on the receiver hose by roughly 95% and 88% in the longitudinal and lateral directions (originally “structural loads”). However, the cyclically alternating torque fluctuation may accelerate fatigue damage. Simultaneously, the wake increases the vertical torque amplitude on the receiver hose by approximately 20%, which may raise the risk of oscillatory deformation along this direction.

Aimed at the safety of helicopter flight in the highly dynamic wind field of urban low-altitude environment, a wind field simulation and reconstruction method based on unsteady Reynolds-Averaged Navier-Stokes (RANS) and Snapshot Proper Orthogonal Decomposition (Snapshot POD) is proposed in this paper. A comprehensive flight simulation platform is developed, integrating the simulated wind field, a helicopter flight dynamic model, an explicit Model-Following Control System (MFCS), and a simplified precision pilot model. Focusing on the issue of airflow disturbances from nearby obstacles in urban environments, a spatiotemporal, dual-source wind-induced threat identification model is established, which fuses turbulence threat and operational exceedance threat, and is incorporated into the construction of safety maps used for trajectory planning. The neighborhood search strategy of the A* algorithm is improved to enhance the trajectory’s environmental adaptability, forming an automatic helicopter flight simulation method for complex urban wind environments, integrating wind field prediction, trajectory planning, and flight simulation. Applied to real-world environment flight simulation, the Improved Safety Map-based A* method (I-SM-A*) can reduce the number of path nodes and approach the target faster compared with the Traditional A* method (Trad-A*). The designed trajectory can effectively isolate the wind-induced threat caused by the building airflow. This results in a reduction in pilot workload, as evidenced by decreases of 10.6% and 8.0% in the time and frequency domains, respectively. The flight simulation platform can accurately track the designed trajectory and achieve reliable automatic flight planning in complex urban windy environments.

The low-altitude environment for rotorcraft is characterized by complex wind fields resulting from the combined effects of terrain and wind conditions. This study investigates the aerodynamic interference mechanisms of three typical low-altitude wind fields on a rotor system, generated in a wind tunnel, including obstacle wake, spatial constraints, and random turbulence. Unsteady aerodynamic forces and moments acting on the rotor were measured using a six-component force balance. The influence of varying wind conditions and interference locations on rotor aerodynamic characteristics was assessed through time-frequency analysis. Furthermore, a disturbance wind field simulation method based on Detached Eddy Simulation (DES) is proposed. Numerical simulation results were then used to analyze the characteristics of the rotor-environment coupled flow field in various scenarios. The results reveal that rotor wake recirculation induced by spatial constraints decreases rotor thrust, and the reduction is exacerbated by increased confinement. The large-scale periodic vortex shedding within the obstacle wake region results in low-frequency aerodynamic perturbations. Increasing the rotor height above ground and the horizontal distance from the obstacle increases high-frequency fluctuations within the flow field, leading to a migration of the rotor thrust power spectrum peak towards higher frequencies. Random turbulence exhibits energy concentration in the high-frequency band, exhibiting a comparatively small effect on the low-frequency sensitive region associated with pilot workload. This research reveals the inherent relationship between energy distribution and rotor aerodynamic response characteristics in various scenarios, providing the theoretical basis for the design of turbulence suppression in rotorcraft.

An advanced Actuator Surface Method (ASM) coupled with Computational Fluid Dynamics (CFD) is developed and applied to the complex unsteady aerodynamic simulation of helicopter. By introducing an improved three-dimensional anisotropic Gaussian kernel, this method effectively addresses the severe aerodynamic load fluctuations commonly associated with traditional Virtual Blade Method (VBM) due to turbulent flow around blade elements. To manage the issues of regional shape and grid cell quantity variations caused by virtual blade sweeping, a universal hybrid grid generation strategy is established without body-fitted and disk interpolation grids, which enhances the computational stability at both blade elements and blade edges. Aerodynamic numerical simulations of helicopter are performed using this method, focusing on rotor/fuselage interaction dominated by rotor wake motion and fuselage blockage effects, Blade-Vortex Interaction(BVI) induced by tip vortices, and maneuvering flights involving collective pitch ramp increases. The results indicate that the advanced ASM demonstrates reliability and robustness in the simulation of complex unsteady flow fields around helicopter. Under similar computational accuracy, the advanced ASM improves computational efficiency by nearly 40 times compared to the overset-grid-based full Blade-Resolved (B-R) method, and by 6 times compared to the VBM. It shows significant advantages when applied to complex full-aircraft interaction and maneuvering flight conditions that require substantial computational resources.

Rotor-to-rotor interaction among neighboring rotors of a multirotor has great significance for aerodynamically efficient multirotor design. Current research is conducted to analyze aerodynamic performance of different octocopter configurations amid hover and forward flight. Conventional and coaxial configurations are studied and a hybrid configuration is also proposed to rectify the disadvantages associated with the earlier two. Comparison is carried out for the aforementioned configurations along with comparison of coaxial and hybrid octocopters with bigger diameter rotors in the same confined space for high thrust requirement missions. Vertical spacing of coaxial configuration is also studied. Virtual Blade Method (VBM) is considered herein due to its great computational efficiency. The results show that there are 11.89% and 14.22% loss in thrust for coaxial octocopter compared to conventional and hybrid configurations with normal size rotors and 15.61% loss compared to hybrid configuration with bigger rotors in hover, whereas coaxial square configuration performs the worst in forward flight with a lift loss of 9.1%, 14.77% and 18.8% compared to coaxial diamond, conventional and hybrid configurations with normal size rotors and 9.96% and 17.82% loss compared to coaxial diamond and hybrid configurations with bigger rotors. Combined FM shows that hybrid configuration outperforms other octocopter configurations in overall aerodynamic performance.

A robust Reynolds-Averaged Navier-Stokes (RANS) based solver is established to predict the complex unsteady aerodynamic characteristics of the Active Flap Control (AFC) rotor. The complex motion with multiple degrees of freedom of the Trailing Edge Flap (TEF) is analyzed by employing an inverse nested overset grid method. Simulation of non-rotational and rotational modes of blade motion are carried out to investigate the formation and development of TEF shedding vortex with high-frequency deflection of TEF. Moreover, the mechanism of TEF deflection interference with blade tip vortex and overall rotor aerodynamics is also explored. In non-rotational mode, two bundles of vortices form at the gap ends of TEF and the main blade and merge into a single TEF vortex. Dynamic deflection of the TEF significantly interferes with the blade tip vortex. The position of the blade tip vortex consistently changes, and its frequency is directly related to the frequency of TEF deflection. In rotational mode, the tip vortex forms a helical structure. The end vortices at the gap sides co-swirl and subsequently merge into the concentrated beam of tip vortices, causing fluctuations in the vorticity and axial position of the tip vortex under the rotor. This research concludes with the investigation on suppression of Blade Vortex Interaction (BVI), showing an increase in miss distance and reduction in the vorticity of tip vortex through TEF phase control at a particular control frequency. Through this mechanism, a designed TEF deflection law increases the miss distance by 34.7% and reduces vorticity by 11.9% at the target position, demonstrating the effectiveness of AFC in mitigating BVI.

Accurate airspeed measurement is crucial for helicopter flight safety. This article establishes a numerical simulation method for the rotor/fuselage aerodynamic interference flow field based on the unsteady momentum source theory and investigates the influence of rotor wake vortex interference on helicopter airspeed measurement. Using a reference helicopter as an example, we analyze the variation characteristics of airspeed values within the entire speed range for different rotor thrusts and airspeed probe locations. Due to factors such as rotor blade motion and fuselage blockage, there are differences in the static pressure phase and peak values at different airspeed probe locations. The phase difference is influenced by the impact of rotor blades, while the peak value difference is affected by the rotor disc load and fuselage blockage. The study also identifies and explains the “airspeed hysteresis” phenomenon caused by rotor tip vortices passing through the airspeed probe area within specific speed ranges. The variation of airspeed in the airspeed probe area can be divided into three regions: the downwash-dominated region, the hysteresis region, and the incoming-flow-dominated region. For conventional configuration helicopters, the hysteresis region occurs only at medium to low speeds. Further analysis of parameter effects suggests that the closer the airspeed probe is to the front of the fuselage and the smaller the rotor thrust, the further forward the speed range of the hysteresis region. When the airspeed probe is positioned near the nose, no significant hysteresis appears. Finally, the article provides recommendations for the placement of airspeed probes from the perspective of aerodynamical interference.

Numerical studies were performed to investigate the mechanism and potential of several active rotors for reducing low-frequency in-plane thickness noise generated by rotating blades. A numerical method coupling the blade element theory, prescribed wake model and Fowcs Williams-Hawkings (FW-H) equation was established for rotor noise prediction. It is indicated that the excitation force on the blade tip can generate anti-noise that to partly cancel the in-plane thickness noise with an appropriate actuation law. Results from the phase, frequency and amplitude sweeps show that the excitation force direction and actuation law are the crucial factors affecting the noise reduction, which determine the noise reduction area in the elevation and azimuth directions, respectively. The active trailing-flap rotor can generate the in-plane excitation force, but because of large lift-drag ratio the anti-noise is mainly from the vertical lift, which is caused by flap deflection similar to a variable camber airfoil. For the harmonic control rotor and active twist rotor, the excitation force is also attributed to the vertical blade lift. The vertical force can reduce the noise near the rotor plane, it will also cause the noise increase in most other areas. Finally, two new active rotors were proposed to generate the in-plane chordwise and spanwise excitation force. With the modified actuation law, the noise in most areas around the rotor was reduced, which improved the acoustic characteristics of rotor significantly.