U.S. Naval Research Laboratory engineers recently demonstrated Hybrid Tiger, an electric unmanned aerial vehicle (UAV) with multi-day endurance flight capability. The team integrated technologies developed in prior NRL power and energy programs into a single UAV to achieve multi-day endurance with a Group 2 UAV. Group 2 UAVs are typically in the 21-55 pound weight class and normally operate below 3,500 feet above ground level at speeds less than 250 knots. This was the first time Hybrid Tiger flew through a complete 24-hour period, its longest flight to date.
“Extrapolating the flight endurance of Hybrid Tiger from the recent flight data suggests it can reach the program goal flight endurance,” said Richard Stroman, Ph.D., a mechanical engineer from the NRL Chemistry Division. “The results validated our extensive simulation efforts, because the flight endurance is almost exactly what was predicted by our simulation.”
Flight simulations suggested flight endurances predicted for summer conditions are realistic at lower latitudes and with greater solar energy.
“The flight was effectively a performance test in worst-case conditions: temperatures falling below zero degrees Celsius, winds gusting to 20 knots, and relatively little solar energy as we approached the solar solstice Dec. 21,” Stroman said. “Despite all of that, Hybrid Tiger performed well.”
New solar-integrated wings provided daytime power and supplement a redesigned power management system. “This power management system hybridizes solar energy with other on-board energy sources including battery-electric and a hydrogen fuel cell in a lightweight form factor, suitable for airborne craft, as well as ground-based unmanned systems,” said Stearns Heinzen, Ph.D., an aerospace engineer from NRL Tactical Electronic Warfare Division.
The program’s researchers are also developing energy-aware power management algorithms, which vary operational modes and generate a vehicle navigation strategy based on weather forecasts and locally observed opportunities for energy harvesting. Autonomous soaring, for example, is used to gain altitude from thermal updrafts when they are available. Hybrid Tiger combines multiple power sources with different advantages to achieve extreme endurance. A high-pressure hydrogen fuel tank and fuel cell system provides nighttime power. High-efficiency photovoltaics provide power during daylight hours.
Hybrid Tiger was developed and demonstrated with Department of Defense needs in mind, yet its technologies are also valuable for non-defense scientific applications such as atmospheric research and commercial applications.