Symmetry Breaking of a Small UAV Propeller Under Oblique Inflow: A Physics-Based Blade Element/Momentum Analysis

Oblique inflow breaks the nominal rotational periodicity of a geometrically symmetric propeller by introducing an azimuth-dependent velocity component across the rotor disk. This study develops a transparent reduced order blade element/momentum framework to examine the resulting once-per-revolution aerodynamic loading of a representative two-bladed small UAV propeller. The formulation combines blade element aerodynamics, momentum-based induction, Fourier decomposition, and advancing–retreating blade diagnostics over moderate oblique inflow conditions. Particular attention is given to distinguishing quantities that arise directly from the assumed harmonic structure from quantities that provide independent aerodynamic information. A first-order analytical expansion shows that, for small in-plane velocity, the normalized first-harmonic loading scales approximately with the in-plane velocity ratio and therefore with J sin(α), consistent with the numerical model. The analysis additionally reports the absolute first-harmonic load amplitude, spanwise development of the asymmetry, cross-plane hub load components, circulation imbalance, and sensitivity to aerodynamic model assumptions. Published axial and non-axial propeller data are used to establish the range over which the reduced order trends can be interpreted. The results indicate that advance ratio and inflow angle govern the modeled cyclic asymmetry, whereas quantitative magnitudes remain sensitive to sectional aerodynamic assumptions and induced flow representation. The framework is therefore intended as a computationally inexpensive screening and interpretation tool for oblique inflow symmetry breaking rather than as a substitute for phase-resolved CFD or experimental measurements.