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Theoretical and numerical aspects of aerodynamic efficiency of propulsion systems are studied. Focus is on types of propulsion that closely couples to the aerodynamics of the complete vehicle. We discuss the effects of local flow fields, which are affected both by conservative flow acceleration as well as total pressure losses, on the efficiency of boundary layer immersed propulsion devices. We introduce the concept of a boundary layer retardation turbine that helps reduce skin friction over the fuselage. We numerically investigate efficiency gains offered by boundary layer and wake interacting devices. We discuss the results in terms of a total energy consumption framework and show that efficiency gains offered depend on all the elements of the propulsion system.

The development of a wingtip-mounted electric lift propeller concept to enable quiet, efficient VTOL aircraft has yielded a promising UAV design employing two-bladed propellers that lock into a fixed position during horizontal flight to become wingtip extensions, resulting in a low-drag cruise configuration. The desire to reduce dependence on high-fidelity computational simulations which would increase the development timeframe for this concept led to the adoption of a spiral development model in which a series of six increasingly large and complex subscale prototypes were built and tested. Results from these tests were used to validate and update the computational models and led to various modifications of the aircraft configuration. A fully-functional 55-pound airframe designed to demonstrate mechanical feasibility, desired aerodynamic properties, full mission profiles including quiet VTOL and efficient forward flight operations, automatic stabilization, pilot workload mitigation and fully autonomous control was also constructed and is undergoing early stage testing. Static and flight test results as well as conceptual design studies are presented, including aerodynamic and structural simulations. The impact of these results on the design of the aircraft is examined. The feasibility and current performance predictions of this concept as applied to larger scale UAV systems is also touched upon.

Recent advances in electric propulsion technologies have opened up new design options for aircraft through the application of distributed electric propulsion. Notably, many vertical takeoff and landing configurations that were previously impractical or impossible now have the potential to become viable aircraft that provide transformational capabilities. One such configuration, a two-seat fully-electric 200 mph personal aircraft with 200 mile range, is being designed by Joby Aviation to meet unaddressed commuting and transportation desires. The distributed nature of the propulsion system enables unprecedented redundancy and simplicity in a VTOL aircraft, resulting in increased safety and lower maintenance; additionally, other aspects of this configuration reduce noise signatures and drastically decrease energy usage, and the 200 mph cruise speed is significantly faster than comparable existing small VTOL aircraft. Design goals are introduced to constrain the design space to practical and viable configurations. The conceptual design of this aircraft entailed the use of design tools of various fidelity, including vortex-lattice analysis, blade-element momentum theory codes, and full Navier-Stokes CFD analysis. A sizing code integrated results from these tools along with semi-empirical mass estimates to perform mission analysis and analyze trade-offs between various design variables. The resulting aircraft design demonstrates the potential of these new electric propulsion technologies to enable viable and attractive new VTOL configurations that can provide new capabilities to private pilots.

One promising application of recent advances in electric aircraft propulsion technologies is a blown wing realized through the placement of a number of electric motors driving individual tractor propellers spaced along each wing. This configuration increases the maximum lift coefficient by providing substantially increased dynamic pressure across the wing at low speeds. This allows for a wing sized near the ideal area for maximum range at cruise conditions, imparting the cruise drag and ride quality benefits of this smaller wing size without decreasing takeoff and landing performance. A reference four-seat general aviation aircraft was chosen as an exemplary application case. Idealized momentum theory relations were derived to investigate tradeoffs in various design variables. Navier-Stokes aeropropulsive simulations were performed with various wing and propeller configurations at takeoff and landing conditions to provide insight into the effect of different wing and propeller designs on the realizable effective maximum lift coefficient. Similar analyses were performed at the cruise condition to ensure that drag targets are attainable. Results indicate that this configuration shows great promise to drastically improve the efficiency of small aircraft.

ight, becoming lifting surfaces. The design of these blades and their integration onto an airframe pose many challenges, including performing this blade repositioning with a mechanism that is adequately simple, reliable, lightweight, and aerodynamic; determining a blade geometry that strikes the right compromise between performance as a rotor blade and as a wingtip; and providing sucient pitch and yaw control in vertical and transitional ight without resorting to overly complex and heavy mechanisms, such as rotor cyclic. A spiral development model is employed in which a series of increasingly large and complex subscale prototypes are built and tested. Results from these tests demonstrate the strengths, weaknesses, and design tradeos of this conguration. Conceptual design is aided by an aerodynamic and structural mission analysis code. Detailed aerodynamic analysis of these designs is performed in CFD, which illuminates the unusual aerodynamics of this design in vertical and horizontal ight and the eects of these aerodynamics on overall performance. Compelling applications of this concept to the personal air vehicle and on-demand aviation markets are examined.