**6. Energy Storage Sizing and Flight Time Comparison**

The sizing of the energy storage system is focused on maximizing the flight time while minimizing the GTOW. This paper considers five different energy storage configurations. The first two configurations utilize either a battery or an HFC as the primary energy source. The remaining three configurations are hybrid setups, including combinations of Bat/SC, Bat/HFC, or Bat/SC/HFC. By exploring various energy storage structures, the impact on the autonomy of the multirotor aerial vehicle can be assessed. Figure 11 provides a general configuration of the propulsion chain based on the Bat/SC/HFC hybrid setup. In the hybrid cases, it is possible to consider, during the cruise phase, the recharging of the battery, by the energy surplus of the HFC in the Bat/HFC or Bat/SC/HFC configurations, or the supercapacitor, by the energy surplus of the battery in the Bat/SC configuration; or by the HFC in the Bat/HFC/SC configuration. In both cases, oversizing of the battery or HFC is required.

After the optimal pair motor/propeller has been selected by the optimization algorithm, the sizing of the ESC part will be conditioned by the maximum current imposed by the motor. The fuselage sizing part is not considered in this paper, it is assumed to be ready. The sizing of the energy storage system makes it possible to maximize the flight time of the drone while keeping a minimum mass. For this, it is assumed that:

$$m\_{\text{copter}} = m\_{\text{StorageEnergy}} + m\_{\text{orthers}}.\tag{38}$$

**Figure 11.** Multirotor propulsion chain based on a hybrid energy storage system, Bat/SC/HFC.

#### *6.1. Sizing of the Battery and the Hydrogen Fuel Cell*

When using a simple energy storage system, the sizing of the latter is performed in a way that the flight time is maximized by respecting the constraint of the *GTOW*.

#### 6.1.1. Battery Sizing

The LiPo battery and the motor power are related by

$$P\_{\rm mot} = P\_{\rm lat} \cdot \eta\_{\rm c} \cdot \eta\_{\rm b} \tag{39}$$

From the discharging time given in Equation (15), the multirotor aerial vehicle flight time is related to the motor/propeller-specific efficiency by

$$\mathbf{t}\_{flight} = \frac{\eta\_{\mathbf{c}} \cdot \eta\_{\mathbf{b}} \cdot \rho\_{\mathbf{b}} \cdot m\_{\text{flat}}}{(m\_{\text{flat}} + m\_{\text{others}}) \cdot \mathcal{g}} \cdot \eta\_{MP}. \tag{40}$$
