*2.1. Design Iteration Code*

The design process is described in Figure 1. In a first step, assumptions about the propulsion system, the wing configuration, the fuselage shape and the empennage were applied in the pre-design (red box). With these assumptions, preliminary values were defined for the range, passenger and luggage weight, runway take-off length, cruise and landing speed and wingspan. Having these pre-design values, as well as the initial values (blue box) set, the sizing process (green box) began with the weight estimation according to Roskam [14].

In the next step, the sizing diagram was used to identify the design point and, thus, the power and wing area. This calculated wing area was used to size the wing, resulting in the required aerodynamic lift coefficients *CA*,*Land* and *CA*,*To*. Further, the lift coefficient was used to obtain the drag-polar and, thus, the lift-to-drag ratio. This ratio was, in turn, used to derive the weight of the aircraft components and, thus, the center of gravity. With the calculated maximum takeoff mass, the payload-range diagram was created, which then led to the required fuel mass. Bringing the iteration to an end and moving on to the analysis (yellow box), a CAD model was adjusted, and the three-side view was drawn, bringing the mechanical concept to a final freeze.

**Figure 1.** Schematic representation of the sizing code with the pre-design in red, the initial values in blue, the sizing process in green and the results and analysis in yellow.

Range Calculation for Hybrid-Electric Aircraft

Conventional fuel burning aircraft can be sized using the well-known Breguet equation, while battery electric aircraft can be sized using the modified Breguet Equation [15]. It is possible to simplify the mission segments as "equivalent stationary horizontal flight

phases" [16]; however, for this application, a more detailed incremental time step approach was chosen. This allows the split between the energy sources to vary from one moment to the next. With this approach, the normal flightpath of an aircraft can be simulated in small time increments, calculating the required energy for each time step, splitting that energy between battery and fuel and deriving the resulting mass change from it. This allows for a very refined energy management strategy between the primary and secondary energy source.

The input parameters for the incremental calculation are the basic aircraft characteristics regarding mass, aerodynamics, efficiencies and specific energy. The output is fuel and energy used per flight segment. This incremental approach will also be used in Section 3.1 to create the payload-range diagram for the aircraft.

#### *2.2. Design Process of the Aircraft*

To feed the design iteration code with information about new technologies, they must be thoroughly investigated. For each new component of the aircraft, a literature study was conducted to derive equations and reference values to apply to the design. The reference aircraft was used to calibrate the masses, empennage and sizing diagram.
