A British-built solar-powered spy plane has set an unofficial world record for the longest continuous flight.
The uncrewed Zephyr aircraft stayed aloft for 82 hours and 37 minutes by storing solar energy collected during the day in a rechargeable battery which powered the plane's two propellers at night.
The flight, at the US army's Yuma ground in Arizona, was aimed at testing Zephyr's ability to relay ground communications, so the flight did not fulfill all the technical requirements of a world record attempt.
But by operating over three day-night cycles, the aircraft's designers at the UK defence firm Qinetiq believe it could essentially operate indefinitely.
"We think the aircraft in future will be capable of weeks or months duration," said Paul Davey, Zephyr's business development director at Qinetiq. The flight – which was partly autonomous and partly controlled from satellite – took place between July 28 and 31.
Davey said the technical challenge in creating Zephyr was to produce an aircraft that is exceptionally light, but strong enough to carry its payload up to a cruising height of around 60,000 feet – twice the altitude of a commercial airliner.
Zephyr has a carbon fibre frame and is covered in super-light solar arrays that are thinner than paper. The rechargeable battery it uses is twice as efficient as any other in the world. To make take-off simpler, the 30kg aircraft with a wingspan of 18 metres is launched be three people holding it above their heads as they run forward.
It flies relatively slowly at 70 miles per hour and because of its low mass and low drag in the thin air at high altitude it needs very little power – equivalent to three or four lightbulbs.
Zephyr can carry a payload of around 2kg – too small to act as a bomber. It is designed to relay radio messages from troops on the ground, but also to give the military enhanced surveillance.
Unlike satellites that get a snapshot of a particular spot just twice in a 24-hour period, Zephyr can circle slowly overhead giving a constant bird's eye view of a location on the ground, such as an enemy military facility.
"They can perform those dangerous jobs, or jobs where you just can't get a manned aircraft," said Davey.
He said Zephyr also will have important scientific applications – for example, it could follow the path of a developing hurricane, flying above the storm and dropping small devices that record wind speed, temperature and air pressure.
Its main limitation though is sun. The shortness of the winter day at high latitudes, means that it cannot fly in winter any further north or south than 40 degrees – the latitude of southern Europe.
The UK Ministry of Defence has funded the £20m development of Zephyr to date. Ultimately, Qinetiq aims to produce the aircraft for around £500,000 per plane. The US military funded the test flight in Arizona.
The current official world record for uncrewed flight stands at 30 hours and 24 minutes, was set by Global Hawk in 2001. That aircraft was the first autonomous vehicle to fly non-stop across the Pacific Ocean. Qinetiq is also involved in a project to develop the Vulture aircraft for the US military which is designed to fly non-stop for 5 years.