On the aerodynamics of an accelerating and decelerating propeller in hover at low Reynolds numbers

Abstract

Small unmanned aerial vehicles commonly regulate propeller thrust through rapid changes in rotational speed, yet it remains unclear how the inflow, the wake, and the near-surface velocity respond to these changes at low Reynolds numbers. A commercial two-bladed propeller was tested in hover with custom blade holders at three additional pitch offsets relative to the original blade geometry, $Θ=\,$0, 5, and $10^\circ$, and accelerated and decelerated between 3000 and 5000 RPM with open-loop step responses. The resulting nondimensional acceleration and deceleration numbers, below 0.011 and 0.007, suggest negligible acceleration effects. Phase-locked particle image velocimetry (PIV) at 40, 60, and $80\,\%$ rotor radius and in the meridian plane was combined with load measurements. The thrust coefficient responds quasi-steadily at $Θ= 10^\circ$, but shows hysteresis at the higher pitch offsets, with lower values during acceleration and higher values during deceleration than under steady conditions. This trend opposes expectations from dynamic inflow theory. The effect is largest at $Θ= 5^\circ$, with deviations from the steady values of up to 8\,% during acceleration and $5\,\%$ during deceleration, compared with $4\,\%$ and $3\,\%$ at $Θ= 10^\circ$. PIV reveals that the rotor-disk inflow, blade deformation, and sectional circulation remain largely quasi-steady, whereas velocity fluctuations, blade-wake vorticity, and tip-vortex circulation vary strongly, with the largest deviations at low rotational speeds rather than at peak acceleration or deceleration.