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Experimental Analysis of a Small-Scale Rotor at Various Inflow Angles 时间:2018-10-09 22:25 来源:未知作者:admin 点击: 次 Abstract:The performance characteristics of a rotor that is typically used for small unmanned aircraft were analyzed in a series of wind-tunnel experiments. Wind-tunnel measurements were conducted with the rotor at various inflow angles in order to investigate the effects on the rotor performance of partially or fully edgewise flow as they are typically encountered with small multirotor vehicles. Rotor tests were also performed under static and fully axial flow conditions in order to investigate the aerodynamic performance during hover as well as vertical climb and descent. The wind-tunnel data were corrected to account for the interference of wind-tunnel walls with the rotor wake and the blockage due to the presence of the rotor test stand in the wind-tunnel test section. The results are presented in terms of thrust, power, and roll moment coefficients under different rotor rotational speeds for a T-motor 18x6.1. Additionally, the measured thrust and power coefficients of Master Airscrew Electric 11x7 are compared with available propeller data under static and axial flow conditions for verification purposes. It is shown that the rotor performance characteristics are strongly affected by the freestream advance ratio and the freestream inflow angles. For example, at inflow angles that are typical for multirotor vehicles between about 15° and 0° with respect to the rotor disc, thrust coefficients stay constant or grow with increasing advance ratio, whereas power coefficients remain relatively constant with changing advance ratio.
1. Introduction
Multirotor unmanned aircraft have demonstrated their potential for diverse applications that include military, civilian, and commercial remote sensing missions. Their ability to takeoff vertically, hover, transition quickly between forward flight and hover, and precisely maneuver within confined spaces make small multirotor vehicles well-suited for applications for which many fixed-wing unmanned aerial vehicles (UAVs) are unsuitable. Generally, small multirotor aircraft rely on multiple fixed-pitch rotors to generate lift and thrust loads that are required for their advanced flight control and navigation systems. Subsequently, these vehicles have unique abilities for undertaking missions that human-piloted aircraft often are unable to accomplish efficiently and safely [1]. The flight characteristics and dynamics of small multirotor vehicles, however, are greatly impacted by the aerodynamics of their rotors [2, 3]. Thus, in order to further improve the efficiency and utility of small multirotor vehicles, the aerodynamic characteristics of small rotors need to be thoroughly understood for the different flight conditions that these vehicles encounter and that differ significantly from traditional propeller aerodynamics with largely axial inflows.
Although stability and control modeling of small multirotor vehicles have received significant attention (e.g., [4, 5]), the rotor aerodynamics of these vehicles have only been studied to a limited degree. Most of the studies in this field have been constrained to investigations of static and axial flow conditions. Brandt and Selig [6] tested 79 propellers of different diameters ranging from 9 to 11 in (22.9 to 27.9 cm) under static and axial flow conditions. They reported thrust and power coefficients as well as propulsive efficiencies of the propellers in terms of the advance ratio for different rotational speeds. Similar experimental results were reported for various low-Reynolds-number propellers with applications to unmanned aerial vehicles in references [7–13]. Additionally, a number of studies have proposed prediction models for evaluating the performance of low-Reynolds-number rotors under static and axial flow conditions. However, these prediction models need to be tuned in order to account for low-Reynolds-number effects. The adjustments of the prediction models are commonly performed by comparing the results with those obtained from experimental measurements [14]. For instance, McCrink and Gregory [15] presented a model based on blade element momentum theory (BEMT) with several corrections for predicting the performance of low-Reynolds-number propellers. The BEMT model was validated using wind-tunnel measurements. Lee et al. [16] developed and experimentally validated a source-doublet panel method to investigate effects of different design parameters on the hovering performance of coaxial rotors.