Experimental study of the impact of the number of blades on the profile drag of UAV helicopter rotors in hover

Abstract

Due to the motion of tip and root vortices of helicopter rotor blades and the inevitable blade-vortex interactions in multi-blade rotors, the profile drag of all individual blades is increased, when compared to the absence of any blade-vortex interaction. Different factors like the speed of rotation of the rotor and the number of blades greatly impact the magnitude of the profile drag increase. This effect is additionally worsened with the low- Reynolds operational conditions of small-sized UAV helicopter rotors. In addition, there is an initial upward motion of the tip vortex and thus there are three scenarios for the blade-vortex encounter, depending on whether the vortex emitted by the previous blade is still above, on or below the plane of rotation of the blades. All those uncertainties motivated the authors to carry- out this present experimental study. The results are shown in a convenient form. In hover, the profile drag increase is shown to be greater with increased number of blades and higher rotational speeds as there is less time for the preceding vortex to clear the path of the following blade.