Urban Air Mobility: Effects of increasing three-dimensionality on fixed and rotary wings in unsteady aerodynamic environments

Abstract

The rapidly growing field of electric vertical takeoff and landing aircraft, air taxis, and urban air mobility vehicles promises transformative solutions to alleviate urban congestion, accelerate deliveries, and revolutionize transportation systems. Central to the successful integration of these futuristic modes of transportation is a comprehensive understanding of their aerodynamics, particularly in the context of unsteady airflow encountered in urban environments. This work explores the foundational aspects essential for achieving efficient and safe urban air mobility operation. The focus lies on the integration of rotary and translatory wings in gusty and unsteady flow environments since – unlike conventional fixed-wing aircraft – many urban air vehicles utilize rotor systems for both vertical takeoff and forward flight. The research framework is structured around three interconnected pillars: advancing rotary wings, fixed-wing-gust encounters, and the synthesis of rotary wings in gusty conditions. The combined results from these three pillars are fundamental in reaching the future goal of efficient and safe urban air mobility. The first pillar investigates the aerodynamic characteristics of advancing rotary wings, particularly concerning flow structures, blade loading, and the influence of the trailing edge geometry using experimental, numerical, and modeling techniques. A comparison between a standard NACA0012 airfoil profile and an elliptical profile is conducted at advance ratios ranging from 0.00 to 1.00 at pitch angles from 7 deg to 25 deg. Four main vortex structures were detected in reverse flow. At the aerodynamic leading edge, a strong interference of the tip vortex with the reverse flow dynamic stall vortex was identified when blade flapping was restricted. Dynamic stall vortices advect closer to the blade surface for the blunt elliptical airfoil, thus reducing the wake area in reverse flow. Overall, the vortex structures that form on the ellipse are more coherent than those on the NACA0012. A 29% pitching moment increase was measured in the reverse flow region with sharp trailing-edged blades compared to blunt blades. The blunt trailing-edged blade delayed flow separation and thus prevented the formation of a reverse flow dynamic stall vortex, reducing the pitching moment. The second pillar delves into the three-dimensional dynamics of fixed-wing-gust encounters, aiming to understand the formation of leading-edge vortices and their impact on lift generation. Emphasis is placed on exploring strong transverse gust encounters and the effects of sideslip angle on leading edge vortex formation, with the objective of devising predictive models for lift generation under varied gust scenarios. Experimental investigations in a towing tank and the employment of a strip theory Küssner model show a peak lift coefficient decrease with decreasing gust ratios and increasing sideslip angles. The model accurately predicts the experimental results at gust entry as well as within the gust. Flow reattachment is delayed due to the formation of a leading-edge vortex inducing reverse flow on the wing suction side, resulting in a non-zero wing forcing at gust exit. The third pillar examines the effects of gusts on both hovering and advancing rotors. It synthesizes the findings from the previous two pillars, mirroring real-world conditions occurring on urban air mobility vehicles. Gusts cause an increase in blade flapping and lagging moments, and a nose-down pitching moment in both hovering and advancing rotors. In forward flight, the moment response mirrors a wing-gust encounter. A lower advance ratio broadens the moment peaks. Reverse flow shows a smaller moment response but a wider azimuth angle impact. Increased gust and advance ratios amplify moment disturbances, with gust encounters on the retreating blade more sensitive to gust ratio changes. By integrating insights from rotary wings and gust encounters, this research provides a comprehensive understanding of aerodynamic phenomena crucial for the development of efficient and safe urban flight vehicles. Through this multidisciplinary approach, this thesis contributes to advancing the fundamental understanding of aerodynamic challenges in urban air mobility, paving the way for the development of innovative solutions to propel the future of urban air mobility.