6.1.2. HFC Sizing

The hydrogen consumption during the flight and the motor power are related by

$$P\_{\rm mot} = \eta\_c \cdot \eta\_{\rm FC} \cdot LHV \cdot HC\_{\prime} \tag{41}$$

From the discharging time given in Equation (21) in the case of an HFC, the multirotor aerial vehicle flight time is related to the motor/propeller-specific efficiency by

$$t\_{fllight} = \frac{\eta\_e \cdot \eta\_{FC} \cdot LHV \cdot m\_{H\_2}}{(m\_{STACK} + m\_{others}) \cdot \mathcal{g}} \cdot \eta\_{MP} \tag{42}$$

where *mSTACK* is the stack fuel cell mass, which is given by *mSTACK* = *mFC* + *mH*<sup>2</sup> + *mtank*. The evolution of the flight time in terms of the battery mass or hydrogen mass is given in Figure 12a,b. For both cases, it is observable that the flight time increases at first, and then decreases as the battery mass or the hydrogen mass increases from 0 to ∞. The decrease in the flight time is caused by the decrease in the motor/propeller efficiency when the drone weight is too heavy. Usually, the flight time maximum is not reached, because the energy storage system mass is limited by the *GTOW*. Thus, the optimum weight of the battery must be sought in the permitted region given in Figure 12a,b. The optimized parameters of the battery and the hydrogen fuel cell are presented in Table A4.

**Figure 12.** Flight time evolution.
