## 3 Experiments and Results
### Experimental Setup  
The experimental modes involved flying the drone in an altitude hold mission, or in a loiter mode at a particular GPS co-ordinate mission for both cases i.e., with or without the fuel cell unit and the solid fuel stick onboard the drone. As shown in Figure 1, the fuel cell holder is a box attached to the bottom of the drone chassis. For experimental runs with only the battery, this fuel cell holder box was removed and also, the fuel stick was removed from the drone. The electrical power circuits for both modes of operation with/without the fuel cell are also shown in Figures 2 and 3. The details of the contents within the fuel cell holder box from Figure 1 are visible in Figure 6.  
The white and blue cylindrical object in Figure 6 is the 'Hydrostik Pro' metal hydride fuel stick, the detailed specifications for which are available in Table 2. It is a metal hydride storage system for hydrogen. The top of the fuel stick is connected to a pressure regulator, which is further connected to the hydrogen input tube, which connects the fuel stick to the Horizon H-30 PEM fuel cell stack. Note that once the fuel stick is connected, the fuel cell starts working immediately. The fuel stick can be connected by simply screwing the pressure regulator tightly onto the fuel stick, which starts the flow of hydrogen. The fan on the side of the H-30 fuel cell stack is used to absorb air, and also to maintain the  
Figure 5: Mission Planner screenshot during flight.  
internal temperature of the fuel cell stack. A tube is connected to the exhaust port of the fuel cell stack; this water exhaust tube passes through the purge valve. The purge valve is connected to the purge cycle controller, which is seen in black behind the fuel cell stack, and it controls the opening and closing of the purge valve for the removal of accumulated water vapor within the stack. The two electrical output wires from the H-30 fuel cell stack are connected to the BEC; the details of the electrical systems are available in Figures 2 and 3. Also, a battery used is shown in the bottom-left corner of Figure 6. For details related to the operation of the parts of the fuel cell stack shown in Figure 6, readers are directed to [23].  
In Section 3.2, it is shown that the fuel cell stick shown in Figure 6 cools as it supplies hydrogen to the fuel cell stack. The prototype battery cooling assembly used to take advantage of this cooling effect is shown in the left half of Figure 7. The fuel cell stick and battery are enclosed in an all around jacket of aluminium foil; this is labeled in Figure 1 as the FSB pouch. This is done so that the cooler fuel stick could absorb heat from all over the surface of the battery. This prototype battery cooling assembly is then strapped to the bottom of the drone chassis, as shown in the right half of Figure 7. The prototype battery cooling assembly shown in Figure 6 is not necessarily an optimized design; however, it is an initial prototype. Optimization of the design for the cooling assembly is beyond the current scope of the paper and will be handled in future efforts.  
For the experiments where the fuel cell was not used, the aluminium cooling jacket and the fuel stick were removed, the fuel cell holder box and its components were not attached to the drone chassis, and the battery was strapped into the bottom of the drone by itself. At this point it is worth making a comparison of the drone weight with/without the fuel cell stack and required peripherals; these details are shown in Table 3. As seen in Table 3, without the fuel cell, when the drone was powered by the battery alone, it weighed 1.2 kg. With the fuel cell and all associated components, the drone weighed approximately 1.75 Kg. Recall from Section 2.1 that each motor provides a thrust equivalent to a weight of 0.7 kg, so even after using the fuel cell stack and associated peripherals on board, the drone should easily be able to lift at least an additional payload of 500 g.  
Figure 6: Components of the drone power supply and battery cooling system.