Novel empirical models for estimating aerodynamic coefficients of small UAV propellers

Abstract

Designers of small UAVs are often faced with a hurdle in the propeller selection stage in preliminary design due to the lack of simple yet accurate models to estimate small propellers’ performance (thrust coefficient, power coefficient, and efficiency variation with advance ratio). It might even seem impossible to have accurate propeller performance models as the performance depends on the propeller geometry, and small propellers have complex geometries that are not readily available. Nonetheless, by analyzing the performance data of over 170 propellers (diameters ranging from 2 to 18 inches) from different manufacturers, we show that: (a) the thrust and power coefficient curves can be approximated as second and third-order polynomials in advance ratio, respectively, and (b) the coefficients of these polynomials depend predominantly on the pitch ratio. Leveraging this observation, we develop novel empirical relations that determine the coefficients of the polynomial performance curves as functions of the propeller pitch ratio alone. The efficacy of the proposed performance estimation models is demonstrated by accurately predicting the performance curves of several propellers that were not used to construct the empirical relations. Further, using the developed empirical relations, we propose a method to select a suitable propeller that provides high efficiency for a given set of preliminary UAV design parameters like the required thrust and operating velocity.