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We continue our study on the aerodynamic investigation of two propellers mounted partially overlapping and in tandem configurations. In the previous study, the rear propeller was placed above the plane of the front propeller (negative stagger). The current study placed the rear propeller beneath the plane of the front propeller (positive stagger). Different vertical and horizontal spacings, incidence angles with respect to the freestream, and advance ratios were tested experimentally at the University of Dayton Low-Speed Wind Tunnel. Thrust and torque were measured separately on each propeller. A significant increment in the power required of the tandem propeller system was observed when compared to the results in the previous study under negative staggered conditions. A higher loss in system efficiency at small forward flight speeds is also observed due to propeller wake interaction. The advance ratio at which this performance loss occurs is affected by the propeller spacing and incidence angle.

View Video Presentation: https://doi.org/10.2514/6.2023-2464.vid Modern VTOL configurations often seek minimal footprint simultaneously with minimal disk loading (often competing constraints), the latter to improve propulsive efficiency at low speeds. One approach is to stagger neighboring propellers in partial overlap. We consider such partial overlap of two propellers in tandem, varying the vertical and horizontal spacing, inclination of the propeller plane with respect to the freestream, and advance ratio. The rear propeller was mounted above the plane of the front propeller with a maximum of 25%-diameter overlap. Thrust and torque were measured separately on each propeller. The efficiency of the propeller pair, expressed as a ratio of thrust to power, never completely approaches that of propellers in isolation. But for vertical separation of 0.5 diameter or greater, there is a saturation, where larger separation does not result in further improvement.

View Video Presentation: https://doi.org/10.2514/6.2021-0614.vid Our propeller partial ground-effect study was extended to annulus ground-plates. Correlations between force data and flow visualization indicate that onset of strong outward radial flow on the ground plate, as normalized ground-separation distance decreases, appears together with thrust-augmentation and power-reduction in ground effect. Thrust increase in inverse to ground-effect, or ceiling effect, was found to depend just on ground plate area, regardless of plate geometry, whether the plate is an annulus or a disk. Our earlier infinite-ground correlation was modified for ground/ceiling plate geometries studied here, with an error of less than 6%.

We extend our recent electric propeller ground-effect study to consider a circular ground-plate, instead of planar-surface of assumed infinite extent. Parameter studies include propeller to plate diameter ratio, propeller diameter to ground-offset ratio, and propeller pitch to diameter ratio. As with classical ground effect, benefits of thrust-augmentation and/or power-reduction with proximity to the ground, depend on the propeller pitch to diameter ratio, with stronger ground-effect for smaller pitch to diameter ratio. Flow visualization suggests on/near the ground-plate suggests a projected dividing-streamline, external to which, the flow is largely radial. A ground-plate of half of the propeller diameter was found to have minimal distinction from that of an unimpeded free-stream, while when the plate and propeller have the same diameter, the resulting ground-effect already resembles that of the infinite-plate.