Design and analysis of passive variable-pitch propeller for VTOL UAVs

The design requirements for the propellers of vertical take-off and landing (VTOL) UAVs are contradictory in the cruise phase and VTOL phase. To alleviate the contradiction, the concept of passive variable-pitch propeller (PVPP) using the aerodynamic moment to change the pitch was proposed by previous researchers, which can adaptively adjust the pitch as the advance ratio changes. However, this type of PVPP has not been widely used due to its complex balance module. This paper constructs a new framework to design such PVPP to simplify its balance module and increase its utility. We proposed new methods to parameterize the shape of PVPP and established mathematical models for each optimization in the design process. Subsequently, a case study verified the feasibility of the design method to simplify the trim module. By optimizing the position of the blade-pivot axis, the design results can work efficiently in multiple working conditions and can also achieve the highest efficiency at a specific advance ratio, which is of practical significance for VTOL UAVs to reduce energy consumption and increase payload in mission profiles. We conducted additional analysis on the characteristics of PVPP based on two aspects: (A) the influence of the blade airfoil on the pitching moment characteristics of PVPP; (B) the comparison between the PVPP with normal layout and the PVPP with canard layout. Overall, the methodology and general rules proposed in this paper provide a theoretical basis and technical support for designing PVPP for UTOL UAVs.