Modeling Performance of the Joby S4 Propeller in Descent Conditions

Abstract:
Newly designed eVTOL aircraft utilize propellers that operate with a large range of propeller rotation rates. Traditional nomenclature uses nondimensionalization based on the blade tip speed, and input reduction based on a similarity assumption under constant advance ratios. In this study, we explore the validity of this similarity assumption in the context of hover and descent scenarios for a variable pitch eVTOL propeller with rotation rates ranging from 54%-100% of the maximum value. In hover, the relative Reynolds number and Mach number effects are found to be relatively minor. As the axial descent ratio increases, prior to the onset of vortex ring state, the similarity assumption breaks down, and the mean thrust coefficient varies up to ±10% under different rotation rates. A similar breakdown is observed for descent conditions with higher edgewise flow. A detailed exploration shows that the effect is primarily due to relative Mach number effects, which alters the tip vortex wake interaction at the disk, and to a lesser extent changes the loading along the blade. Since the breakdown in the similarity assumption is isolated to a small region of the propeller operating envelope, a surrogate model is developed to preserve the reduced input dimensionality while accounting for the relative tip speed effects in the affected region of the envelope. This is achieved by first generating the surrogate over the full domain, assuming validity of the similarity assumption. A separate high-RPM surrogate correction layer is then built to capture the worst-case tip speed effects in the descent region of the envelope.

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