Electric aircraft face a steep tradeoff between the demand for runway performance and range. While fuel based propulsion technologies typically increase in specific power with increasing size, electric propulsion is typically much more scalable. This system scalability enables alternative designs including distributed propulsion, optionally powered propulsion units, and vectored thrust, which can all contribute to better runway performance and range. In this paper, we explore how continuously powered distributed propulsion can reduce takeoff distance while still satisfying range constraints. We use a combination of a blade element momentum method, a vortex lattice method, experimental data, and nonlinear optimization techniques to model and explore the design space. We have found that for this conceptual design study, a fully blown wing with propellers at the optimal diameter for the load (8 propellers for a 300 km range constraint) can reduce the takeoff distance by over 80% when compared to the optimal 2 propeller case using the same models. There is over a 2x increase in the wing lift coefficient which leads to a 36% reduction in liftoff speed. Also, the optimal fully blown case produced 2.9 more thrust during takeoff with only an 11% increase in total aircraft mass. Using propeller tip speed as a surrogate for noise, we found that the propeller tip speed decreased takeoff performance in an exponential manner: the tip speed could be decreased from Mach 0.8 to Mach 0.5 with only a 2x increase in takeoff rolling distance while decreasing the constraint to Mach 0.3 produced an 8x increase.
展开▼
