Aerodynamic Performance of Variable-Pitch Propellers for High-Altitude UAVs

As the air density changes dramatically from high to low altitude, the matching of high-altitude propeller and electric motor under low-altitude condition is one of the main challenges that aerodynamic design of high-altitude propeller faces. To solve this problem, variable-pitch propeller is the most commonly used and feasible solution. In this paper, the Reynolds-average Navier-Stokes solver and an efficient surrogate-based optimization toolbox are used for the aerodynamic design of a fixed-pitch propeller. Based on the optimal fixed-pitch propeller, a series of pitch angle are employed to obtain the variable-pitch propeller with maximum thrust. Then, the aerodynamic characteristics of fixed-pitch propeller and variable-pitch propeller under typical working conditions are studied and compared. The results show that under the condition of low-altitude and low wind speed, the fixed-pitch propeller reaches the rated torque of the electric motor at a lower rotational speed, resulting in a lower shaft power and thrust. By adjusting the pitch angle, the variable-pitch propeller can achieve higher rotational speed under the same rated torque constraints, thereby absorbing more shaft power and achieving larger thrust. The lower the altitude, the more obvious the advantage of the variable-pitch propeller. Compared with fixed-pitch propeller, the maximum relative increment of the maximum thrust can reach more than 80% for variable-pitch propeller. It is concluded that variable-pitch propeller can significantly improve the matching of propeller and electric motor, bringing wider flight envelope of UAVs.