Numerical simulation of the coupled flow field in helicopter/tanker interaction

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.