Project 2 : Joby Lotus
Another Joby Aviation project is the Lotus aircraft, shown in Figure 3, which is exploring a novel VTOL configuration on the 55-pound UAV scale. In this aircraft, two-bladed propellers on each wingtip provide thrust for vertical takeoff. After the aircraft picks up enough forward speed for sufficient wing lift, each set of two blades scissors together and the individual blades become wingtip extensions, forming a split wingtip. A tilting tail rotor provides pitch control during takeoff and landing and propels the aircraft in forward flight. As one may expect, the design of these wingtip blades – the span, airfoil choice, twist and chord distribution, pitch, and dihedral – was an interesting compromise between propeller and wingtip performance. Dozens of CFD simulations were run on different combinations of these design variables in the cruise configuration, to maximize the cruise performance within the constraints of the configuration. At the same time, the performance of these blades in the propeller configuration was also analyzed with CFD to validate lower-order design methods. Example results from some of these simulations are shown in Figures 4 and 5.
Project 3: Leaptech
The third project Joby Aviation is participating in is LEAPTech (Leading Edge Asynchronous Propeller Technology), a partnership with NASA and Empirical Systems Aerospace. The goal of this design is to investigate potential improvements in conventional fixed-wing aircraft through electric propulsion. A row of small propellers is located along the leading edge of the wings and, during takeoff and landing, these propellers increase the velocity (and, therefore, the dynamic pressure) over the wings. This increases the lift produced by the wing and allows for a smaller wing to be used for the same stall speed constraint. Since many small aircraft use a wing sized to meet a stall speed constraint but too large for optimal cruise performance, this smaller wing allows for more efficient cruise. Additionally, the ride quality is significantly improved due to the higher wing loading. However, the performance of this blown wing is difficult to analyze with lower-order tools, particularly since much of the required analysis occurs around stalling conditions. Therefore, a large number of CFD simulations were performed in the design process, looking at various combinations of propeller sizes and powers, wing aspect ratios and sizes, angles of attack, etc. To reduce the computational expense, the propellers were modeled as actuator disks with the body force propeller method in STAR-CCM+, which negated the need to resolve the actual blade geometry, drastically decreasing the required mesh size.
The first phase of testing this configuration was to build the full-scale wing, propellers, and motors, and mount them above a modified semi-truck which was run at takeoff speeds on the runway at NASA Armstrong Flight Research Center. An example CFD solution of this configuration is shown in Figure 6, and the experimental test apparatus is shown in Figure 7.
Outside of takeoff and landing, these leading-edge propellers are planned to fold against their nacelles – similar to the S2 propellers – and wingtip propellers, as mentioned above, will provide propulsion. Although lower-order analysis methods were evaluated for estimating the drag and efficiency impact of operating these propellers concentric with the wingtip vortex, unsteady CFD proved to be the most reliable analysis method. A range of design parameters were analyzed, and one such solution is shown in Figure 8. A flight demonstrator is planned for flights beginning in 2017; a rendering of this aircraft is shown in Figure 9.
Joby Aviation is quickly advancing the state of general aviation aircraft with its revolutionary electric propulsion concepts, and simulation is playing a big role in understanding the complex nature of their state-of-the-art ideas and in the design and development of their unconventional systems. The S2, Lotus, and LEAPTech designs show great promise towards an electric future in aviation never before possible.
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