The aviation industry requires continuous reductions in greenhouse gas and noise emissions, which is resulting in a shift towards hybrid and fully electric aircraft. The propeller is an attractive form of propulsion system for electric aircraft as it can achieve high efficiencies and has excellent scalability potential. However, a major concern is the high community and cabin noise levels. It is possible to mitigate the propeller noise levels by reducing the blade loading and shifting the loading inboard, though this will often adversely impact the aerodynamic efficiency. Another method of reducing noise is by introducing blade sweep. This can result in destructive interference of sound waves. The downside is that sweep may impose significant structural deformations when the blade is loaded. These structural deformations may alter both the aerodynamic and acoustic performance. The objective of this thesis is to identify how these structural deformations will impact the aerodynamic and acoustic performance of low-noise, swept propeller blades. A parametric study was performed where the blade sweep was parametrised. The aerodynamic, acoustic and structural performances
of these propeller configurations were then analysed to identify the relations between these three disciplines.
A multidisciplinary propeller framework was used where aerodynamic, acoustic and structural tools could be coupled. For the aerodynamic analyses, a Vortex Lattice tool was developed, which can perform inviscid, incompressible simulations of any propeller geometry. The acoustics were calculated using Hanson’s Helicoidal Surface Theory, which models the blade sweep and dihedral as phase lag effects to model the interference between
sound waves. The structural tool used the Euler-Bernoulli beam theory for bending deformations and Saint-Venant’s torsion theory for twisting deflections. These three tools were coupled such that the final deformed shape of a loaded propeller could be obtained and evaluated. Each tool was validated individually using high-fidelity and experimental data, confirming that accurate results may be obtained within the tool’s applicable limitations.
The elastic blades were compared to their rigid counterparts to analyse how much the blade deformed, and how these deformations impacted the aerodynamic and acoustic performance. It was found that bending deformations are caused by the centrifugal forces, opposing any bending moments caused by the aerodynamic forces. The torsional deformations, mainly caused by the aerodynamic forces, result in a wash-out. Both the
bending deformations and wash-out of the blade reduce the local angle of attack, reducing the aerodynamic loads acting on the propeller blades. Since these structural deformations are concentrated near the blade tip, the local loading distributions are only impacted near the tip. This effectively shifts the loading inboard. The loss in performance can be significant if substantial sweep is applied, with losses of up to 20% of the thrust and torque. The efficiency, on the other hand, does not significantly change due to elasticity. The acoustic performance was mainly altered due to the changes in aerodynamic performance, with any changes in blade geometry having negligible effects. The overall sound pressure levels reduce by up to 2.5 dB, correlating with the reduction in thrust and torque. By eliminating the noise’s dependency on the overall thrust level, a clear correlation between the shifting of the local loading and the noise emissions was also found. These inboard shifting loads reduced the thrust-specific sound pressure by up to 1 dB.
It is possible to reduce the effect of elasticity by letting the structural forces impose a twisting moment, reducing the wash-out of the propeller blades. High bending deformations, or the addition of dihedral, allow the moment arm of the centrifugal loads to grow enough such that the twisting moments of the aerodynamic forces can be overcome. This can greatly diminish the aerodynamic performance loss. On the other hand, the associated increase of the peakiness of the local aerodynamic loading distributions may increase the noise
emissions even when a net loss in aerodynamic performance is still present.
Clear relations between the aerodynamic, acoustic, and structural performance of elastic, swept blades were identified. These changes in aerodynamics and acoustics are too great to be ignored for highly swept blades. Analyses of such flexible, highly swept blades clearly require the inclusion of structural calculations to accurately predict the aerodynamic and acoustic performance.