Optimal flight trajectories of hybrid electric aircraft with in-flight charging

The transition towards electric aircraft is particularly challenging due to the relatively low specific energy densities of electrical energy storage systems. If electric aircraft are to be realised, flight paths must be optimised to take advantage of the unique features of electric propulsion systems. In this paper, the problem of determining the optimal flight trajectories for aircraft powered by a combination of a fuel cell stack and lithium-ion battery pack is considered. Particular emphasis is given to the role of the battery pack's temperature and in-flight charging requirements on the results. Solutions that minimise the fuel consumption and the flight time are first considered. For both cases, the optimal solution was observed to discharge the battery during the climb with only minimal in-flight recharging of the battery by the fuel cell. A scenario that requires a fast climb and descent and the battery to be charged upon arrival was identified and shown to lead to an oscillatory profile for optimal in-flight charging. These results demonstrate the potential of solving optimal control problems to generate tailored electric aircraft trajectories.