Effects of the Unsteady Blade Loading on the Aero-Acoustic Performance of a Boundary Layer Ingesting Propeller

Abstract

In recent years, the civil aviation sector has increasingly focused on optimizing propulsion designs to improve efficiency and reduce CO₂ emissions. Propellers have regained attention for their superior efficiency, particularly in applications like full-electric aircraft, though high noise emissions remain a significant challenge. This thesis investigates the aerodynamic and acoustic performance of a Boundary Layer Ingesting (BLI) propeller installed at the end of the fuselage of a transport plane, behind the vertical tail. The study focuses on the impact of unsteady blade loading on performance and explores whether increasing blade count or modifying blade sweep can mitigate drawbacks in a BLI configuration.
The development of numerical tools was the first step in this investigation. The aerodynamic tool, an enhanced Unsteady Non-Linear Vortex Lattice Method, accounts for viscosity and flow separation, while the acoustic solver uses Hanson's Helicoidal Surface Theory to model noise emissions from propellers in both isolated and installed conditions. Simulations were conducted on a modified 6-bladed XPROP-S. The velocity profiles at the propeller disk, used as inflow for the installed BLI analysis, were taken from CFD results of the fuselage with the vertical tail installed.
The complex inflow field led to periodic increases in blade loading, especially behind the vertical tail, causing oscillations in thrust and power. Increasing the blade count from two to six resulted in steadier performance and reduced force oscillations. In the installed configuration, the propellers exhibited a thrust-to-power ratio increase of 2% to 5%, with in-plane forces scaling with thrust and remaining within 3% of it. Noise emissions rose significantly after installation, with peak emissions shifting from the rotational plane to the propeller axis, where unsteady loading noise sources became more prominent. The noise level increase reached up to 46 dB during cruise for the 6-bladed baseline. While increasing the blade count reduced noise levels in isolated conditions due to lower propagation efficiency of steady loading noise, in installed conditions, unsteady loading noise sources counterbalanced this, leading to overall increased noise. Maximum noise emissions ranged from 32 dB (isolated, six blades) to 78 dB (installed, six blades). Sweeping the baseline blade backward at the tip and forward near the hub increased loading and thrust by up to 20%, particularly at the tips. In installed conditions, backward sweep reduced overall blade loading changes, while forward sweep increased them. Noise levels varied, with isolated conditions showing a slight increase and installed conditions experiencing up to a 4.7 dB reduction with backward sweep.
The analysis underscores the importance of accounting for unsteady blade loading and how the dominance of unsteady loading noise sources is influenced by loading oscillation amplitude, propeller geometry, and operating conditions.