The ducted coaxial propeller (DCP) has great application value in eVTOL aircraft because of its high safety, compactness, and low noise. A numerical simulation method for the DCP is established using the sliding mesh technique. A DCP was designed and manufactured for the lift and power test to verify the numerical method. The characteristics of airflow separation inside the DCP were studied, and the influence of the vortex restrain ring (VRR) on the suppression of airflow separation and on lift augmentation of the duct is analyzed. Results show that, when the tip clearance ratio increases from 0.336% to 1.342%, both the total lift and aerodynamic efficiency decrease by about 11.3%. The influence is mainly reflected in the formation of the tip vortex, airflow separation in the straight, and diffusion sections of the duct. Tip vortex and airflow separation increases DCP energy dissipation and clogs the inner wall of the duct, reducing the effective inner diameter and airflow through the duct. Moreover, the role of the duct is weakened, and the wake is contracted, which increases the induced power loss. By adding a VRR to the diffusion section, the tip vortex and airflow separation can be effectively suppressed, which can increase the aerodynamic efficiency by 5.1%.

The ducted coaxial propeller (DCP) is highly advantageous in the design of eVTOL aircraft due to its safety, compactness, and low noise levels. To study the aerodynamic characteristics of DCP in hovering, a novel eVTOL was used, and a slip grid model was established to solve the three-dimensional unsteady N-S equation. The aerodynamic characteristics of DCP were compared to those of the free coaxial propeller (FCP) and ducted single propeller (DSP) to reveal the interaction mechanism of unsteady flow between the duct and propellers. The results indicate that the duct significantly mitigates the intensity of tip vortexes by changing the characteristics of propeller tip winding, which reduces the corresponding energy loss. Additionally, the static pressure loss is decreased by the reduced radical-induced velocity in the slipstream area. Finally, the induced power loss is reduced by the decreased axial-induced velocity and suppressed wake contraction, resulting in DCP having 39% higher aerodynamic efficiency than FCP and the duct accounting for 41.7% of the total lift. Although DCP generates 1.77 times more lift than DSP, its aerodynamic efficiency is only 91.08% of DSP.

The propulsive wing is a new concept wing of automatic propulsion with high lift coefficients and has great application value in plant protection and forest fire control. The propulsive wing wake is a reverse Bénard–von Kármán (RBvK) vortex street, which is considered a thrust-generating wake. The wake structure will change greatly at high angles of attack and lead to changes in the aerodynamic performance of the propulsive wing. To explore the optimal working range and the wake characteristics of the propulsive wing, the wake transition and aerodynamic efficiency of the propulsive wing in cruise are numerically studied. The results indicate that there are three types of structures for the propulsive wing wake. When α ≤ 20°, the wake transits from the RBvK vortex street to the critical state with the increase in cruise speed, and the Strouhal number approaches 1.9. The critical wake region decreases gradually with the increase in the angle of attack. The maximum propulsive efficiency is 0.17 at a cruise speed of 15 m/s. When α > 20°, the wake transits directly from the RBvK vortex street to the Bénard–von Kármán (BvK) vortex street and the Strouhal number approaches 0.34. The maximum propulsive efficiency appears at a cruise speed of 10 m/s, which is close to the BvK vortex street boundary. Before entering the stall state, the lift efficiency of the propulsive wing increases with the increase in cruise speed and angle of attack, up to 3–5.

The electric vertical take-off and landing (eVTOL) aircraft offers the advantages of vertical take-off and landing, environmental cleanliness, and automated control, making it a crucial component of future urban air traffic. As competition intensifies, demands for aircraft performance are escalating, including forward flight speed and payload capacity. The article presents a novel eVTOL design with propulsive wings and establishes methodologies for propulsive wing unsteady numerical simulation and wind tunnel experiments, analyzing its aerodynamic characteristics and lift enhancement mechanism. The results indicate that the cross-flow fan (CFF) provides unique airflow control capabilities, enabling the propulsive wing to achieve remarkably high lift coefficients (exceeding 7.6 in experiments) and propulsion coefficients (exceeding 7.1 in experiments) at extreme angles of attack (30°~40°) and low airspeeds. On the one hand, the CFF effectively controls boundary layer flow, delaying airflow separation at high angles of attack; on the other hand, the rotation of the CFF induces two eccentric vortices, generating vortex-induced lift and propulsion. The aerodynamic performance of the propulsive wing depends on the advance ratio and angle of attack. Typically, both lift and propulsion coefficients increase with the advance ratio, while lift and drag coefficients increase with the angle of attack. The propulsive wing shows significant advantages and prospects for eVTOL aircrafts in the low flight velocity range (0–30 m/s).

Quadcopters will possibly enter the vortex ring state (VRS), which may jeopardize flight safety of the aircraft, in descending as helicopters do. But aerodynamic characteristics of the quadcopter in the VRS are quite different with the helicopter due to the aerodynamic interference between the rotors, and they were hardly studied. In this paper, the flow fields of a quadcopter and a single rotor in vertical descending are studied through CFD simulation, and the variation laws of lift and power are analyzed. The results show that the single rotor and the quadcopter will enter the VRS like helicopters. Compared with the single rotor, the quadcopter enters the VRS at a smaller decent speed (0.65 ^v _h vs. 0.785 ^v _h). The single rotor reaches its largest lift loss at 19.6% in the middle stage of the VRS, while the rotor of the quadcopter reaches it at 10.6% in the late stage. When the quadcopter enters the late stage of the VRS, its lift variation between rotors is up to 5%. For flight safety, the maximum speed of the quadcopter in vertical descending should be limited to a safe speed ( ^V _c ^{= 0.65 v} _h). This study can provide a reference for the safety of quadcopter flight.