- Luis Miguel García-Cuevas González
- Pedro Manuel Quintero Igeño
- Pau Bares Moreno
- Pau Varela Martínez
Distributed electric propulsion (DEP) and boundary layer ingestion (BLI) are becoming a research hot topic as they can be used to reduce the energy or fuel consumption of fixed wing aircraft, from small and medium remotely piloted aircraft systems (RPAS) to big airliners. Furthermore, they can be used to mitigate the risks associated to aircraft loss due to foreign object damage. This work presents a method for optimising the mission profile in RPAS, which presents novel challenges against classic configurations.
A 25 kg of maximum take-off mass, fixed wing, propeller-driven RPAS powered by a 1 kW internal combustion engine is characterised by means of experimental and computational methods. Then, computational fluid dynamics (CFD) simulations are carried out with a DEP+BLI architecture, generating response surfaces that cover the whole flight domain with the minimum number of computations. These simulations are produced assuming that the thrust of the complete set of propellers is equal to the total drag of the aircraft, including all the interactions between the wing and the propellers, and are generated for a wide range of Reynolds numbers and lift coefficients. After that, and taking into account the added mass due to the series hybridisation of the powerplant, specific range and specific endurance maps can be produced and the flight speed can be optimised to maximise either the range or the endurance. The optimisation can be performed for different distributions of propellers in the trailing edge of the wing, so the best configuration can be selected when designing a new RPAS.
Using the precomputed specific range and specific endurance maps, the optimum strategy of internal combustion engine and propellers rotational speeds can be chosen in order to maximise the distance travelled or the amount of time flying over a region, depending on the mission. For a maximum range mission, applying this method achieves fuel savings of 16% against a pure non-hybrid, single-propeller configuration and 4% against a hybrid electric, single-propeller configuration. The complex interactions between the optimum operating conditions of the internal combustion engine, the aerodynamic performance of the aircraft and the performance of the propellers produce some non-obvious results in the optimum flight speed, with changing trends in its evolution against the instantaneous mass of the aircraft.
The generation of specific range and specific endurance interpolation maps require the characterisation of the powertrain, as well as the aerodynamic and propulsive performance of the airframe and the propellers. Due to the effects of BLI and the close position of the propellers to the trailing edge of the wing, there are strong interactions between the aerodynamic performance and that of the propellers. With the method described in this work, these interactions are easily taken into account and the interpolation maps are generated with low experimental and/or computational costs. The specific range and specific endurance maps can be used inflight by the autopilot module to select the optimum flight speed and are also a valuable tool during the design phase of new RPAS.
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