A dynamic grid technology based on unstructured grid was used to simulate the aerodynamic characteristics of multi-rotor aircraft in the descent state

and experimental verification was performed. This method was used to simulate the flowfield of the aircraft in the vertical descent state and the 30° oblique descent state. The aerodynamic characteristics of the multi-rotor aircraft and the law of the flowfield in the slipstream region were obtained in this state

and the variations of force and power were also obtained. The study found that the multi-rotor aircraft would enter the vortex ring state in both vertical descent and 30° oblique descent. When the vertical descent speed dropped to 4 m/s

the multi-rotor aircraft was already in the vortex ring state

the loss of the thrust reached 15%

the rotor power increased first and then decreased with the increase of vertical descent speed

the thrust and power of different rotor of the aircraft were not consistent. When the 30° oblique descent speed reached 4-6 m/s

the multi-rotor aircraft was in vortex ring state. This conclusion can provide a reference for the safe flight of multi-rotor drones.