A comprehensive study of a coaxial, co-rotating rotor in hover
Creators: name: Johnson, Chloe Marie
name: 0000-0002-1170-9627
Publisher: name: The University of Texas at Austin
Year: 2023
Subjects: Rotorcraft
Experimental rotorcraft
Stacked rotor
Coaxial rotor
Experimental fluids
Aeroacoustics
eVTOL
Rotor noise

description: Recent innovations in electric vertical takeoff and landing technologies have spurred research of coaxial, co-rotating (stacked) rotors. A stacked rotor employs two sets of two-bladed rotors offset axially and azimuthally. This thesis focuses on the experimental measurements of a 1.108 m stacked rotor in hover at a tip Mach number of 0.42. The goal of this work is to quantify the effects of rotor geometry and operating conditions on the performance and noise in hover. A coaxial stacked rotor test stand was designed that allows for variable axial and azimuthal spacings and differential collective, or unequal collective on the upper and lower rotor blades. Axial spacings ranged from 0 to 1.75 blade chords and azimuthal spacing ranged from -90° to 90°. A method to characterize the acoustic facility was developed and implemented to quantify the effects of reflections on acoustic measurements. Stacked rotor performance measurements revealed significant variations of individual rotor thrust (67% upper rotor thrust, 50% lower rotor thrust) and power with azimuthal spacing, leading to total rotor thrust and power variations. Variations were greatest at small axial spacings and unequal collective pitch angles on the upper and lower rotor (differential collective), achieving 16.4% thrust change with 15.3% power change at zero differential collective and 76.5% thrust change with 46% power change at negative differential collectives. Moreover, large thrust variations with small azimuthal spacing changes suggest a new method for rotor thrust control. A numerical model was developed that coupled blade element momentum theory with a vortex panel method, with which it was revealed that chordwise circulation along the rotor blades is the dominant factor resulting in measured thrust variations. Rotor noise measurements revealed that at the given operating conditions, tonal and broadband noise are of similar magnitude and are both sensitive to stacked rotor parameters. Tonal noise increased at nonorthogonal azimuthal spacings due to a larger 2/rev noise component. This phenomenon was most pronounced at zero differential collective and nonzero axial spacings. Broadband noise variations were examined through tip vortex trajectory measurements extracted from flow visualization images. It was found that broadband noise around 3 kHz was greatest when the lower rotor operated below the upper rotor tip vortex. Broadband noise decreased by 1.5 dB as the distance between the rotor blade and vortex increased to 0.6 blade chords. Furthermore, if the blade operated in the vortex core or passed above it, broadband noise was reduced by 4.5 dB. The distance between the lower rotor blade and upper rotor blade tip vortex was highly dependent on azimuthal spacing and blade tip clearance. From these results, a quiet stacked rotor was obtained by optimizing tonal and broadband noise through rotor spacing and differential collective. Tonal noise variations are achieved by changing the axial and azimuthal spacing, while broadband noise can be controlled by altering the vortex trajectories with differential collective. This resulted in a mode of operation with 20% more power but a 2.5 dB reduction of total noise.