Reza Youneszade - Northern Research Center for Science & Technology, Malek Ashtar University of Technology, Iran
Kamran Dadashi - Northern Research Center for Science & Technology, Malek Ashtar University of Technology, Iran
Majid Rahgoshay - Northern Research Center for Science & Technology, Malek Ashtar University of Technology, Iran

Unmanned aerial vehicles (UAVs), as remote-controlled or autonomous flying devices characterized by high flexibility and mobility, were usually employed to conduct remote sensing and surveillance missions. Recent developments in fuel cell (FC) technologies show great potential to increase flight duration of UAVs with satisfactory fuel economy. For UAVs powered by FC propulsion systems, the maneuverability and the power performance may be considerably affected, since the FC has drawbacks such as the long start-up time, delayed response and weak power performance. The integration of FCs with other power sources can significantly improve the dynamic load-response, the power performance, and the energy storage capacity of UAV propulsion systems. FC hybrid with the battery is a well-known and wonderful scheme. In this paper, a flowchart for sizing different components of a UAV with PEMFC as main power source is presented. The flight time estimated by this flowchart showed a good agreement with actual flight time.UAVs have been widely applied in practice as one of the best applicable candidates to conduct remote sensing and surveillance missions. UAVs can achieve higher fuel economy, more reliability, and much safer than conventional piloted helicopters [i,ii]. Because the power density and energy density of power sources determine the propulsive force and the flight endurance, respectively, propulsion systems are particularly important for UAVs [iii,iv]. Typically, UAVs are powered by conventional internal combustion engines (ICEs) and electric motors [v]. Compared with pure ICE-based UAVs, the UAVs powered by electric motors are more lightweight, reliable, and efficient [vi]. Furthermore, the responsiveness to the dynamic load from the electric motors is faster than that of ICE. In recent years, using fuel cells (FCs) as the major power source has become very popular for electric propulsion systems in aircrafts or UAVs, [vii]. Three typical categories of FCs are used in UAVs: ۱) proton exchange membrane FC (PEMFC); ۲) solid oxide FC (SOFC); and ۳) direct methanol FC (DMFC)As a summary, the PEMFC is the preferred choice for small commercial UAVs due to its small size, high operating efficiency, and suitable operating temperature range. The critical issues faced by PEMFCs are the expensive PEM, high-cost noble metal catalyst, and relatively low energy density of the hydrogen fuel under normal circumstances [۲].Although FCs used as power sources in UAVs have many advantages, they have certain common defects: excluding the relatively low power density, their operating efficiency at the low/high power output mode are unsatisfactory and load response is not rapid. To overcome these shortcomings of the FC, a practical approach is to hybrid FC with other power sources. The battery is an essential energy storage device to achieve the stable output for hybrid propulsion systems. [viii]By inverting the propulsion system model of a multi-rotor UAV, a new model can be obtained allowing to estimate fuel cell characteristics based on thrust generated by motor and propeller. This allows for an iterative approach in order to determine the time required to deplete hydrogen stored in the cylinder at constant power draw, which effectively serves as a flight time estimate. Thus, two distinct subsystems can be distinguished in the system model: the actuating system and the power system. Figure ۱ shows a simplified view of the methodology. The inputs of this flowchart are UAV total weight and thrust-to-weight ratio. With these inputs in hand, the motor and propeller are selected. The fuel cell selection is based on the continuous power draw of motors in hover flight (in no-wind conditions). On the other hand, the battery sizing is based on maximum power draw at take-off or in wind conditions. The hydrogen Figure ۱. Multi-rotor UAV flight time estimation flowchart with fuel cell power system.For validation purpose, a multi-rotor UAV configuration with PEM fuel cell as main power source wasselected and its reported flight time was compared to flight time estimated by flowchart presented in thispaper. Table ۱ shows the validation results for Phoenix, Spectronik. There was a ۱۰% difference betweenreported and estimated flight times In this paper, a flowchart for estimating a multi-rotor UAV Flight time with PEMFC as main powersource was presented. In this flowchart, size and characteristics of each part of propulsion chain can bederived. The validation showed that this flowchart can estimate the actual flight time with acceptableaccuracy

UAV, Multi-rotor, PEMFC, Hybrid construction