## 3 Experiments and Results
### Experimental Results
#### 3.2.2 Results of Cooling Batteries Onoard Drones with/without a Fuel Stick
For the gain in flight time computations, the flight times were recorded for purely battery powered drone flights, and drone flights powered by a battery, fuel cell stack, and supercapacitor. The differences between these flight times are shown in the last column of Table 4, and these results show that the mean flight time gained by the use of a fuel cell stack and supercapacitor along with the onboard battery was 26.8 s with a standard deviation of 11.09. Please note that for flight time gain calculations, only the data for tests 1 to 6 is used because tests 7 and 8 were both interrupted halfway by severe gusts of winds, forcing emergency landings. Figures 13a,b show the temperatures of a battery after the drone landed in flight tests 5 and 6, respectively. As seen in Figure 13b, the temperature of the battery was lower than the temperature of the battery in Figure 13a. This can also be verified by noticing that a larger portion of the battery surface in Figure 13a appears to be closer to 'white' in color (i.e., higher temperature) compared to the battery surface in Figure 13b. Also Figure 14a,b provide pictures of the fuel stick temperature before take-off and after landing, respectively, for flight test 6. The cooling effect of the fuel stick is very clearly observed in this case, because despite managing to cool the battery by around 4.6 \({}^{\circ}\)C (i.e., absorbing heat) as seen in Table 4, the fuel stick surface temperature after landing appeared to be approximately the same temperature (around 31 \({}^{\circ}\)C) as before take-off. Further it was also observed that, even after the drone had landed in the above tests, and the battery and fuel cell stick were completely exhausted, the use of the supercapacitor allowed the flight computer to stay online for around five minutes. Thus, the proposed system not only helps to cool batteries, but as a side-effect, also enhances flight time a little, and allows the flight computer to stay online longer, all of which are critically important in retrieving a drone in case it makes an emergency landing due to unavoidable situations like a battery running out of power. Note that the 26.8 s (on an average) of flight time gained may seem small, but this is gained with the use of a 30 W fuel cell unit with a small capacity fuel stick. These results could possibly be enhanced much further if better capacity fuel sticks were used. Also, the fuel cell stack and peripherals used in these experiments cost  
Figure 11: (**a**) Temperature of battery before flight; (**b**) temperature of battery after flight. Flight test 2—increase in battery temperature (cooled by contact with fuel stick, captured by thermal imaging device 1.) about 1500-2000 USD, whereas even entry level professional quality fuel cell stacks for flying drones can cost several tens of thousands of USD. Also, as seen from the altitude plots in Figure 12c, Flight tests 1 and 2 with the fuel cell experienced turbulence causing the altitude to fluctuate; this can be verified by looking at the roll plots in Figure 12b, which show that the drone was encountering stronger winds during the test with the fuel cell compared to the test without the fuel cell. In contrast, relatively calmer conditions prevailed during tests 5 and 6 and the flight times increased. Thus, the above observed gain in flight time may be more pronounced in calmer weather conditions.