Takeoff and Performance Trade-Offs of Retrofit Distributed Electric Propulsion for Urban Transport

Abstract

While vertical takeoff and landing aircraft have shown promise for urban air transport, distributed electric propulsion on existing aircraft may offer immediately implementable alternatives. Distributed electric propulsion could potentially decrease takeoff distances enough to enable thousands of potential inter-city runways. This conceptual study explores the effects of a retrofit of open-bladed electric propulsion units. To model and explore the design space we use blade element momentum method, vortex lattice method, linear-beam finite element analysis, classical laminate theory, composite failure, empirically-based blade noise modeling, motor and motor-controller mass models, and gradient-based optimization. With liftoff time of seconds and the safe total field length for this aircraft type undefined, we focused on the minimum conceptual takeoff distance. We found that 16 propellers could reduce the takeoff distance by over 50% compared to the optimal 2 propeller case. This resulted in a conceptual minimum takeoff distance of 20.5 meters to clear a 50 ft (15.24 m) obstacle. We also found that when decreasing the allowable noise by approximately 10 dBa, the 8 propeller case performed the best with a 43% reduction in takeoff distance compared to the optimal 2 propeller case. This resulted in a noise-restricted conceptual minimum takeoff distance of 95 meters.

Noise of Future Aircraft Propulsion: Sources and Design Considerations

“…However, when the noise was constrained, the 8 propeller configuration showed the optimum takeoff distance with a reduction of 43% compared to conventional two prop case. Furthermore, according to the results presented in the work of Moore and Ning (2019), the following conclusion can be drawn: the 2 propeller configuration will always be noisier than the 4 or 8 propeller for a given thrust to weight ratio. However, results are questionable since the work focused more on aerodynamics for takeoff distance reduction and only a low fidelity semiempirical model was used for noise assessment.…”

“…When noise constraints are imposed, multiple smaller propellers allow for smaller tip speeds, still being able to comply with thrust requirements (Moore and Ning, 2019). Reducing tip speed has a beneficial effect on noise, as already mentioned.…”

“…However, thrust-to-weight ratio must be taken into consideration when choosing the number of propellers, because a large amount of propellers rotating at lower speeds will have a significant amount of weight penalty. Moore and Ning (2019) in their study, found that the optimum propeller number is 8, for an aircraft of the size similar to Tecnam p2006t. Another ben efit of multiple small propellers, is that they have higher BPF, which have larger sound power dissipation in the atmosphere (Synodinos et al, 2018a).…”

Flight-Test Validation of a Takeoff Performance Uncertainty Model

“…Interest in a better understanding of field performance uncertainty is likely to increase in the coming years, carried by the rising tide of the quest for the most effective urban mobility solution. Discussions around this topic are often dominated by multirotor vertical takeoff and landing (VTOL) concepts, but there is a school of thought that advocates fixed-wing solutions with distributed electric propulsion as a faster, more viable route to effective urban mobility [11], which can be built on an existing airworthiness framework (such as 14 CFR Part 23) and may suffer less from the noise issues and technological risks of VTOL ideas. Such concepts hinge, however, on their feasibility in tight, cluttered, complex environments, often featuring "tabletop" runways on the roofs of buildings, and complicated failure and abort scenarios.…”

Cross-Validation of Hybrid-Electric Aircraft Sizing Methods

“…Some studies retrofit an existing aircraft with hybridelectric propulsion (HEP) to analyze a specific aspect of the technology, maintaining takeoff mass constant (e.g., Refs. [7][8][9]). Others actively resize the aircraft to allow a fair comparison with a conventional reference aircraft (e.g., Refs.…”