Aerodynamic performance of plant protection UAV rotor at different altitudes

The performance of plant protection drones can depend mainly on the atmospheric environment in plateau areas. In this study, a plant protection unmanned aerial vehicle (UAV) rotor test bench was designed with adjustable rotor speed and real-time monitoring of engine speed, rotor lift, and output torque. The rotor speed was adjusted via the engine throttle, where the throttle line was pulled by the servo crank. The output speed of the engine was achieved to adjust the rudder angle, according to the PWM signal duty cycle. A set of data acquisition software was developed for the rotor test bench, in order to monitor engine speed, rotor lift, and torque parameters, and then display them in real time. The overall structure of the test bench consisted of the rotor, transmission, power, data acquisition, and servo control system, together with the platform. A sensor, control, and data acquisition were built with a data acquisition card as the core, and then the infrared remote control was added to increase the safety of the test. The rotor test bench was equipped with a DLE430 dual-cylinder inline two-stroke engine, with a rotor radius of 1.51 m, an airfoil of NACA 8-H-12, and a blade number of 2. This design fully met the technical indicators of the rotor system in the test state, such as the strength, stiffness, vibration, and accuracy. The blade element momentum was adopted to explain the aerodynamic characteristics of blades. The computational fluid dynamics (CFD) simulation was used to complete the solution. The rotor aerodynamic performance was numerically simulated at the speeds of 800, 1 000, and 1 200 r/min, respectively, within the altitude of 0, 1, 2, 3, and 4 km, respectively. The second-order upwind scheme was used in the numerical simulation, indicating a more accurate performance than the first-order upwind scheme. A systematic investigation was made to explore the effects of blade angle and rotor speed on rotor lift, test bench torque, and power using quadratic rotation orthogonal experiments and response surface method (RSM). The rotor performance tests were conducted, where the lift was taken as an indicator. The viewing performance tests of spread rotor test benches were also carried out, where the torque and power were as indicators. The quadratic regression equations were established for the lift, torque, and power. The relationship was determined between the rotor lift, test bench output torque, as well as the power and blade angle. The rotor speed shared a significant correlation and a good fitting level. The experimental results show that the rotor power decreased significantly with the increase of altitude, whereas, the descent rate increased. The power increased with the increase of speed at the same altitude. Furthermore, the power at an altitude of 2 km decreased by about 26% at a rotor speed of 1 000 r/min, compared with an altitude of 0 km. The optimized rotor speed was 1 116 r/min, the blade angle was 10.44°, the maximum lift was 356.28 N, the torque was 227.35 N·m, the power was 26.54 kW, and the efficiency of the rotor test bench was 85.92% at an altitude of 4 km. Compared with the experiment at an altitude of 134 meters, the lift of the rotor at an altitude of 1.941 km decreased by 22.38%, which was consistent with the decrease of 20.22% in numerical simulation. The driving torque of the rotor decreased by about 24.21%, and the engine power difference was about 3.99%. There was a reasonable range in the error ratio between the experimental and simulation. In addition, the variation trend of the experimental results was consistent with the numerical simulation, indicating a relatively small error. The main reason for the error was the frictional resistance of the power device composed of the rotor and engine in the experimental device during sliding. The findings can provide a strong reference for the high-load plant protection UAV at high altitudes.