Investigation into aeroacoustic characteristics of evtol rotors
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Authors
Smith, Brendan, John
Issue Date
2024-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Mechanical engineering
Alternative Title
Abstract
Advancements in technologies including batteries and electric drive trains has led to the developmentof a variety of concept electric vertical takeoff and landing (eVTOL) vehicles. In this
thesis, the aeroacoustics of different eVTOL platforms is considered under different conditions,
highlighting the importance of phasing, aerodynamic interactions, and rotor solidity.
A group of classical multicopters is first considered, with a quadcopter, hexacopter, and
octocopter operating at the same conditions compared against an equivalent single rotor vehicle.
These platforms are large enough to be “manned-scale”, meaning that rotors are large
enough to be pitch controlled and thus phase locking between rotors is possible. Initially the
multicopters are considered in hoverwith just two special phasing cases, “orthogonal” and “tipto-
tip”, which are opposite phasing cases in which adjacent rotors are perfectly in or out of
phase. The acoustics are compared to examine how phasing conditions compare on each multicopter
platform, with the orthogonal phasing showing much greater acoustic benefits. The
multicopters are then examined at different cruise speeds using the orthogonal phasing, with
alternative disk loadings also considered. It is found that configurations and disk loadings with
lower advance ratio maintain acoustic signatures fromphasing patterns at higher cruise speeds,
and that locations of advancing blades is a large indicator of high noise accumulation. The
quadcopter is then examined again, this time with additional, higher solidity rotors in addition
to the baseline considered in hover and cruise. Relative phasing is set to 216 possible starting
phases by varying the starting position of three of the rotors against a static rotor. Noise across
all possible phasings is compared, finding that depending on the relative phasing between rotors
noise varies quite significantly. Additionally, the orthogonal phasing considered previously
is shown to be acoustically one of the best phasing configurations for 2-bladed rotors, while the
tip-to-tip is one of the worst acoustically.
Aerodynamic interactions, which can occur due to rotor proximity to other rotors, the
ground, or wings, significantly alters the noise produced. A rotor pair in ground effect is examined,
with two ground spacings that show weak and strong aerodynamic interactions. The
interactions from the ground cause a large increase in the high frequency content of the noise,
and when the interactions are stronger this effect is amplified and the noise signal is significantly
altered. A line rotor pair is then examined in cruise, with the wake from the front rotor
interfering with the rear rotor thrust. Despite the differences in load between a rear rotor in isoxx
lation and experiencing interactions, the acoustics show almost no difference whether or not
the interactions are present. A propeller-wing assembly in axial cruise is examined, with the
interactional effects examined for both the prop and wing independently. The propeller noise
shows large increases in front and behind when a wing is present due to an increase in peak
loading, despite an overall loss in thrust. The wing noise is found to have large high frequency
content, which is more strongly predicted when higher fidelity aerodynamic simulations are
used for noise predictions.
Wind turbines share the same broadband noise models for acoustic predictions that are
used in rotorcraft. Using a methodology that combines rotorcraft and wind turbine acoustic
prediction methods, the acoustics from a single-rotor and quad-rotor wind turbine are compared.
It is found that for observers closer to the rotor plane, the quad-rotor is louder due to
sources being perpetually closer to the observer compared to the single-rotor turbine.
The rotors being considered for eVTOL concepts are typically much higher solidity than
traditional rotors, allowing for a lower tip speed which results in lower noise produced. A baseline
rotor is established, and then the solidity is increased through both an increase in chord
and number of blades. By comparing the acoustics and performance metrics of the rotors, it
is found that increasing solidity through number of blades is best for acoustics, and at higher
solidity values acoustic benefits diminish while performance penalties do not.
Description
December 2024
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY