South Uist, Outer Hebrides, Scotland--In an aviation first, an autonomous aircraft has flown nonstop across the Atlantic Ocean, a feat first done by humans in 1919. And at 9.5-ft wingspan, the Aerosonde craft, developed by The InSitu Group (Bingen, WA) and built by Environmental Systems and Services (Melbourne, Australia), is the smallest aircraft to ever cross the ocean.
InSitu and the University of Washington (UW, Seattle) conducted the flight, supported in part by the US Office of Naval Research. Starting from Belle Island in eastern Newfoundland, the journey took 26 hr 45 min to cover 2,031 miles, for an average speed of 76 mph. The Aerosonde took off from a car-top cradle after being accelerated to 50 mph. After 30 sec under manual control, and a systems checkout over the airfield, the aircraft, named Laima for the ancient Latvian deity of good fortune, began following its specified transatlantic flight plan. An autonomous Global Positioning System (GPS) satellite-navigation system furnished guidance. Direct UHF contact was broken around 25 miles out. To satisfy air traffic control restrictions, pressure altitude for the flight was 5,520 ft, which was lowered to about 400 ft in UK airspace, 25 min before landing under manual control at the Benbecula Military Range.
Satellite imagery of Laima's ground track indicated that the vehicle experienced moderate rain for up to 18 hours, which led to some water accumulation in the fuselage of the combination fiberglass, carbon-composite, and wooden structure aircraft. The aircraft collected and stored location, altitude, and winds aloft data during the flight, but temperature and humidity information was not observed, according to aircraft designer and InSitu president Tad McGeer. "We decided not to take the time to install this hardware as it was peripheral to the mission," he adds.
Onboard instruments provide scalar airspeed, McGeer notes. But without a compass, S-turn maneuvers are done, and the subsequent ground-speed variation obtained via GPS navigation determines wind velocity using an algorithm he developed.
McGeer says, "There was no great trick to the aircraft design." But he does cite the 70-gram GPS system from Trimble (Sunnyvale, CA). "GPS was one of several important pieces. The aircraft tracked the GPS-defined course within a few meters throughout the flight." The GPS was itself "accurate to tens of meters," he adds.
The control system was custom made by InSitu and includes an autopilot loop, using airspeed, altitude, and yaw and pitch rates, and the GPS navigation loop. Dynamometer testing was used to allow the 1-hp, single-cylinder, highly modified Enya (Japan) model airplane engine to run at the lean fuel burn necessary for long-range flying. On the crossing, the engine burned 4 kg (1.5 gal) of aviation gasoline, leaving 1.0 kg (0.38 gal) in the tank.
The successful flight was actually the fourth attempt made during one week in August. The first and third tries were lost at sea, presumably to hardware failures in the engine or fuel system. Inconsistent quality in the fuel pumps had been experienced. The second crashed after launch when a communications failure prompted an autonomous switch from manual to automatic control. While this happened occasionally during previous test flights, this time the changeover turned up an improper initialization in the autopilot software, causing a large roll input, according to McGeer. "We changed our preflight procedure to avoid the problem, and fixed the code after returning from Newfoundland."
Each Aerosonde currently costs about $25,000. McGeer sees production models being "substantially less," in the $10k to $20k range.
Beyond the crossing milestone, the flight demonstrated the robot aircraft's ability to operate at great distances and make weather-related observations. InSitu and UW are looking to field an Aerosonde weather-reconnaissance demonstration project off the Pacific Northwest coast. The goal: improve forecasting for the region by data gathering in over-ocean areas not easily monitored up to now. The developers hope to use Iridium low-orbiting satellites for direct long-distance aircraft communications. Overland applications may also include geographical and wildlife surveys, utility-line monitoring, and traffic reporting. Future Aerosonde versions are targeted to have ranges up to 4,300 miles at altitudes to 40,000 ft.
Atlantic crossing takeoff weight 13.1 kg (28.8 lb)
Landing weight approx. 9.1 kg (20.0 lb)
Service ceiling 14,000 ft
Longest flight to date 31 hr