Understanding and improvement of eVTOL UAV performance through high-fidelity analysis of interactional aerodynamics

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Authors
Misiorowski, Matthew P.
Issue Date
2019-08
Type
Electronic thesis
Thesis
Language
ENG
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Aeronautical engineering
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Abstract
The reduced cost and complexity of these designs has lead to research endeavors attempting to model the flight dynamics of the vehicle with low-order, computationally efficient models. The lowest of these models simply assuming rotor thrust is proportional to the revolutions per minute (RPM) of the rotor, while more complex methods use a dynamic inflow model. These reduced-order models, as well as others unmentioned, presently fail to capture the interactional flow physics that necessarily occurs between multiple rotors or rotors adjacent to nearby lifting/control surfaces. All current eVTOL UAVs have limited performance (especially in metrics like range and endurance) due to the low energy density of the lithium-polymer (Li-Po) batteries they carry, relative to hydrocarbon fuels. Additionally, when these vehicles are produced, they are typically designed with a very large margin on rotor power. This ensures the vehicle is controllable even when the model of the flight dynamics neglects any interactional aerodynamics. For the performance of these aircraft to improve, a fundamental understanding of the interactional flow physics must exist and when defined, will allow the vehicle designs to perform more efficiently in their missions.
Interactional aerodynamics were also analyzed on a stacked co-rotating system in hover. In addition to the high-delity CFD analysis, the computed aeromechanics of the rotor system were compared to a lower-order Viscous Vortex Particle Method (VVPM) with lifting line. This comparison served to illustrate the dierences that still remain between the costly high-delity analysis and more ecient models that may not capture all the important physics. In addition to the aeromechanics comparison of the methods, the acoustic properties of the rotor system were also investigated as the index angle between the stacked rotors varied.
Rotor-rotor and rotor-wing interactions were investigated, applying the high-fidelity analysis to a conventional quadcopter and a quadrotor bi-play both in hover and edgewise flight. These studies analyzed the rotors and wings in isolation for comparison against the individual components in the full vehicle simulations. This comparison derives the effect of the interactional aerodynamics on performance. The study analyzed a quadcopter in different flight orientations and the quadrotor bi-plane at various angles of attack, rotor speeds, and varied the size and number of rotors on the conguration. The produced thrust and required power of each rotor were the primary performance metrics determined by these studies. Supplemental information was obtained regarding rotor pitching moment, and wing lift to drag ratios.
Investigations of ducted rotors in hover and edgewise flight were completed on a representative eVTOL fixed-pitch rotor and duct. With baseline characteristics established, geometry modifications to the duct inlet and diffuser were analyzed. The performance of the ducted rotor system predominantly focused on the system thrust, H-force, and pitching moment. In addition, a comparison of the rotor-induced dynamic loads was made specifically focusing at the blade passage frequency. Flow visualization was used to explain performance observations by attributing specific flow features to azimuthal locations around the rotor and duct as well as explain the dynamic loads induced by the rotor.
Electric Vertical Take-Off and Landing (eVTOL) Unmanned Aerial Vehicles (UAV) are growing in popularity for both civilian and military applications. These electrically-powered rotorcraft were initially used by hobbyists but now are starting to ll roles in commercial delivery, surveillance, disaster relief, and troop resupply, amongst others. The increased use of distributed electric propulsion has given way to novel designs and the smaller scale of some of these vehicles has simplified the design. In mane cases, these aircraft use fixed-pitch rotors, eliminating the weight, drag, and complexity of a rotor swashplate (designed to provide collective and cyclic pitch control to rotors). In addition to eliminating the swashplate, the electric aircraft can use variable rotor speed amongst the multiple rotors to control the vehicle.
High-fidelity computational fluid dynamics (CFD) was used to solve the Navier-Stokes equations and analyze multiple rotor platforms exhibiting interactional aerodynamics. A finite element method with a sliding mesh interface as well as a finite volume scheme with overset meshes were used to fully resolve rotor motion. Convergence studies were completed to verify both schemes approached stable solutions and validation was performed against experimental data when available.
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August 2019
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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