Efficiency of tandem UAV propulsion fans with toroidal-helical connected blade tips

Abstract

Unmanned Aerial Vehicles (UAVs) are increasingly in demand for both civilian and military missions. This has become especially evident during combat operations against the Russian Federation’s occupying forces, where battlefield advantages have been achieved precisely through the use of UAVs in many cases. However, UAVs have a major drawback that hinders their widespread use: a high noise level generated by propulsion fans. Attempts to reduce fan noise structurally often decrease fan performance. The greatest source of noise is the vortex shedding at the blade tips, the strength of which, in turn, depends on the blade diameter, as vortex noise (acoustic power level, APL) is proportional to the sixth power of the cross-sectional velocity of the blade. Thus, by reducing the blade diameter and increasing the number of blades, the acoustic power level can be significantly reduced. However, reducing the diameter also decreases the fan performance. Recently, interest in the design of toroidal fans has grown. The toroidal shape helps minimize tip vortices by restricting the flow and controlling edge leakage. However, due to the nonlinear relationship between blade pitch angle and performance, setting the blade pitch in toroidal fans is a very complex process. Furthermore, the manufacturability of such designs remains a significant challenge. Therefore, the authors set themselves the goal of combining the advantages of the classical fan design with those of the toroidal configuration into a single structure and evaluating the results without sacrificing performance. A tandem fan configuration with a toroidal-spiral integrated blade tip was proposed to achieve a sharp reduction in diameter (while maintaining nearly the same performance). A comparative analysis of two configurations (the toroidal and the classical tandem fan) with a toroidal-spiral integrated blade tip was conducted using a specially designed test bench based on normalized efficiency, power, and thrust coefficients. In addition, the acoustic power level (APL) of the vortex noise was assessed. A comparison between a five-bladed tandem fan with a toroidal-spiral integrated tip and a five-bladed toroidal fan showed that the acoustic power level remained nearly identical (with the same fan diameter and rotational speed) despite significantly higher performance.