*2.2.3. Distributed propulsion*

The investigation of hybrid-electric or universally-electric system is often coupled with distributed propulsion technology [72-75]. This combination is explained by the nature of the electric energy, which can be easily distributed and by the search for aero-structural benefits through higher integration of the propulsive device with the airframe. The field of Boundary Layer Ingestion (BLI) [47, 75] consisting of re-energizing the low momentum boundary layer in view of aerodynamic efficiency improvement, becomes central. This is mainly the reason why distributed propulsion has been intensively investigated on Blended Wing Body (BWB) configuration [12, 48] as it offers large potential for application of BLI by distributing buried propulsion devices along the trailing-edge of the fuselage (see Section 3). For BLI application on tube and wing configurations, the Propulsive Fuselage configuration, which is characterized by a large fan encircling the rear end of the fuselage, was evaluated as most promising and was the center of several investigations [49–53].

Electric distributed propulsion technology is expected to disrupt the traditional aircraft design paradigms [54–56]. A salient example is the redesign of the wing for optimum efficiency in cruise enabled by distributing propellers along the leading-edge as initiated in the LeapTech Project [57] (see Section 3). Usually, wing design is constrained by low-speed operations in order to achieve according to the properties of the high-lift system acceptable takeoff and landing field performance. Benefiting from the propeller slip-stream effect on the wing, the low-speed requirement on high-lift devices and wing design could be reduced, opening the design space for optimum wing design for en-route operations. Moreover differential thrust could be applied to control the aircraft reducing the requirement on flight control surfaces. The distribution of thrust along the wingspan could enable for instance the control of the aircraft yawing motion resulting in reducing or even conceived the complete removal of the rudder. The one-engine-inoperative case is a very stringent low-speed condition for the sizing of the propulsion system, vertical stabilizer and flight control surfaces in order to comply to the airworthiness regulations and the aircraft top-level field performance requirement. Because of the intrinsic redundancy in thrust production provided by distributed propulsion technology and through a proper redundancy definition in the energy and power system, failure modes either energy/power system inoperative or propulsive device inoperative would result in less severe penalty on system sizing [58, 59]. It is also conceivable that the airworthiness regulations notably with respect to the climb gradient requirements will need to be revisited to adapt to the characteristics of aircraft employing distributed propulsion technology. These highlighted potential benefits in aircraft design demonstrate that the full-benefit of hybrid-electric and universally-electric propulsion system can only be assessed through a holistic integration at aircraft level.
