New Hybrid UAV Demonstrated 24-Hour Flight

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A new unmanned aerial vehicle was developed and demonstrated with the Department of Defense’s needs in mind, yet its technologies are also valuable for non-defense scientific and commercial applications.

US Naval Research Laboratory (NRL) engineers recently demonstrated Hybrid Tiger, an electric unmanned aerial vehicle (UAV) with multi-day endurance flight capability, at Aberdeen Proving Grounds, Maryland. This was the first time Hybrid Tiger flew through a complete 24-hour period, its longest flight to date.

Technologies developed in prior NRL power and energy programs were integrated 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.

“The results validated our extensive simulation efforts, because the flight endurance is almost exactly what was predicted by our simulation,” according to the NRL.

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,” said Richard Stroman, Ph.D., a mechanical engineer from the NRL Chemistry Division.  “Despite all of that, Hybrid Tiger performed well.”

New solar-integrated wings provided daytime power and supplement a redesigned power management system.

The power management system hybridizes solar energy with other on-board energy sources including battery-electric and a hydrogen fuel cell in a light-weight form factor, suitable for airborne craft, as well as ground-based unmanned systems.

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.

“Autonomous soaring and energy-aware guidance algorithms help maximize energy extraction and minimize energy loss,” Stroman said. “The use of hydrogen fuel not only enables long endurance; it also makes it possible to produce fuel locally.”

Hybrid Tiger was developed as a complete system that can be transitioned directly to industry; however, its technologies can also be used individually to improve the performance of other unmanned systems, according to