# Integration and Flight Test of a 7 kW Turboelectric Vertical Take-Off and Landing Unmanned Aircraft
## 5 Discussion
### Active Cooling Requirements for Safe Operation  
Temperature-sensitive electrical components drive cooling requirements. For example, a switching circuit is employed to enable the aircraft flight modes powered by battery, turbine, or both, as well as safe power management based on battery charge level. Mode switching is desirable for a \"low-noise mode,\" where the aircraft operates on battery power alone to ameliorate turbine acoustic emissions. This study utilized large insulated-gate bipolar transistors (IGBTs) to switch between different battery modes. These transistors operate in a similar fashion to metal-oxide-semiconductor field-effect transistors (MOSFETs), but in this case, also offer better protection against rapid changes in electrical current. Similar to issues encountered during early development bench testing, there were IGBT failures in the switching circuitry [16]. There were a few different causes for these failures, but the primary cause was insufficient heat dissipation and temporary discharge of excess current beyond the IGBT rating. These two issues are related to high current loads that require the IGBT to dissipate more heat energy, such that a simple solution would be to use IGBTs with a higher-rated amperage or to use two IGBT's in parallel so that each would receive half of the total current coming from the battery. However, dividing the current between two IGBTs was not effective because one momentarily received the entire battery current before the gate on the other closed. This caused the first IGBT to receive too much current and fail, subsequently causing the second IGBT to then fail. High summertime temperatures further exacerbated the issue of excessive current. During ground testing, outdoor temperaturesreached 40 \({}^{\circ}\)C (104 \({}^{\circ}\)F). Temperatures inside the fuselage were even hotter. The aircraft also experienced sunlight, such that there was significant radiant heating.  
Alternatively, a larger IGBT rated for 1000 A was utilized, significantly higher than the maximum expected load of about 400 A. A large heat sink was also attached to provide adequate cooling. After installation, the aircraft was taken outside for ground testing during high temperatures. After significant time (about an hour) with outdoor heating under no load, the IGBT failed such that continuity was measured across the gate, despite the gate being closed. After the aircraft was brought inside to cool, the IGBT was tested again and did not provide continuity. Therefore, elevated temperatures from the weather alone could result in a \"soft-failure\" in the IGBT, causing the gate to close despite no input voltage. Had the IGBT been under a load with this soft-failure state, it would have been permanently damaged. Thus, there is a clear need to use IGBTs with high temperature ratings, and to provide adequate cooling to the solid-state devices. This allows for extended aircraft ground time for a safe take-off with ambient-heated IGBTs. This will need to be addressed by future designers by either prescribing short runway loiter times that are less than a few minutes or by utilizing active cooling.  
Once the aircraft is in forward motion, external cooling airflow can be utilized to manage the fuselage inside temperature. Thermocouple data showed that fuselage inside temperature was almost 30 \({}^{\circ}\)C hotter than ambient, even with a large amount of cooling flow based on an MFP (Mass Flow Parameter) analysis [18]. From the analysis, it is also clear the aircraft did not reach thermal equilibrium. Therefore, rather than simply increasing the hole for cooling flow, a design should separately address radiant, convective, and conductive heat sources. This could be done with a combination of ducts and thermal resistors, such as fiberglass. Despite disadvantages in weight, active cooling is recommended for key electronics during hot day operation. This active cooling could be as simple as attaching fans on electronic component heat sinks.