Conceptual Design of High-Lift Propeller Systems for Small Electric Aircraft
Patterson, Michael Dobson
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The maturation of electric propulsion technologies presents intriguing new degrees of freedom for aircraft designers including the ability to distribute many small propellers upstream of the wing's leading edge to augment the lift produced. These propellers are termed "high-lift propellers" because they act collectively as a form of high-lift device. This dissertation focuses on how high-lift propellers should be designed and how the design of high-lift propeller systems can be incorporated into the wing design process. One contribution is a novel method for quickly assessing wing lift augmentation based on changes in the dynamic pressure and circulation due to the propeller's influence. The method predicts lift augmentation based on the propeller loading, diameter, installation angle, and upstream distance from the wing. The analysis method is based on consideration of momentum theory and thin wing theory, with semi-empirical corrections abstracted from computational fluid dynamics simulations. The overall approach provides not only lift augmentation predictions but also practical and transparent insights into effective design strategies. A second contribution is a high-lift propeller design method based on blade element momentum theory that is capable of generating blades that produce the specified lift augmentation performance while requiring on the order of 15% lower power than conventional minimum induced loss propeller designs. Additionally, existing Federal Aviation Administration stall and reference approach speed regulations are examined, and guidance for developing regulations appropriate for aircraft with high-lift propellers is presented. Finally, example trade studies are performed for the upcoming flight demonstrator aircraft being developed as a part of NASA's Scalable Convergent Electric Propulsion Technology Operations Research (SCEPTOR) project.