Zephyr was originally constructed to accompany and capture images of “Qinetiq-1” – a giant manned helium balloon that attempted to break the world altitude record in 2003. After the Qinetiq-1 balloon developed a leak, the company continued to work on the UAV for other purposes, including military, communication and Earth observation applications. Paul Davey, Zephyr’s Business Development Director, said that most importantly, it was proven that the aircraft is able to fly using its solar electrical power system through two complete diurnal cycles. "The aircraft was flown on solar power and charged its batteries during the day, discharged its batteries during the night, and remained aloft the following dawn when the cycle was repeated" – he said.

Using conventional instrumentation that is placed on the ground, the UAV is launched and must be manually flown to a height of approximately 3km. A control team on the ground piloted the aircraft using a basic instrument panel, according to telemetry data and the forward-looking view from the camera attached to the UAV. An autopilot took over the controls during a number of tests.

The 54-hour flight wasn't officially announced as a world record because the Federation Aeronautique Internationale (FAI) was not notified of the attempt prior to the flight, as rules of the official world record dictate. However, the Qinetiq team hopes that air traffic controllers will be willing to verify a second flight, which the Zephyr completed on August 31^st. This time the FAI was notified of the event, but no official representative was present at the spot to witness the 33-hours and 43-minutes-long flight.

In recent years, various companies have designed several other solar-powered UAVs. “SoLong”, an aerial vehicle developed by a US firm “AC Propulsion”, has succeeded in flying for over 48 hours, powered by solar energy in daytime and using rechargeable batteries at nighttime. However, the aircraft did not maintain a constant power supply and occasionally had to glide or soar. In the framework of NASA's extensive solar UAV program, the Pathfinder and Helios UAVs were developed.  

TFOT recently published an article covering the history of solar powered UAVs, including details of a different attempt to break the world record for solar based flight performed by Israeli students from the Technion. 

More information on the Zephyr can be found on the Qinetiq’s official website.

Further discussion on the Zephyr  solar UAV can be found on the TFOT forums.