Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

At higher altitudes the air density is lower which means there is less air to freeze andless water vapour to ice over components. It also means that there is less air resistanceand drag to impede the aircraft. However the propeller’s need air in order to move theplane forwards, so while less air means less resistance, it also means less propulsion.In order to keep the plane flying at the correct altitude and speed, the power needs tobe increased to overcome the lack of air.1.2.2 TemperatureThe troposphere, which reaches about 36,000 feet in altitude, contains approximately75% of the atmospheres mass and it is where the majority of our weather occurs. Thehigher the altitude the lower the temperature gets, at a rate of approximately 2ºC per1000ft. The coldest temperatures of -80ºC are found in an isothermal layer between36,000ft and 65,000ft, where the troposphere meets the stratosphere. Above 65,000ftthe temperature begins to rise despite the rising altitude. This is because of the ozonelayer which absorbs ultraviolet sunlight and emits it as heat. Zephyr 7 is set to flywithin the isothermal layer and will therefore be experiencing warm temperaturesduring the day, but very cold temperatures in the night. The cold temperatures and thedaily cycling over such a large range is the biggest problem that faces Zephyr.The cold temperatures mean that there is a high probability that critical componentssuch as sensors, motors and servos will malfunction. The cold temperatures can causeelectronics to fail and structures to become brittle. Metal components can alsocontract in the cold temperatures and if two metal objects need to remain in contact in10