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Many of the smaller UA (mini- and micro-UA) use battery power instead of two-cycle engines. Low noise signature makes these electric drives attractive in many situations, despite the low efficiency and low power-to-weight ratios compared to reciprocating engines. Recent improvements in the ability to re-charge lithium based batteries have resulted in significant logistics improvements for users in the field. Further improvements are needed in power-to-weight ratios for the next generation of batteries to improve the performance and endurance of these small platforms on a single charge. Currently, most battery-operated MAV have a fraction of an hour of endurance, while mini-UA fair only slightly better, only because they can carry larger numbers of the same lithium-based batteries.
Future-looking efforts for UA propulsion include the use of fuel cell- or nuclear-based power schemes. NASA has pushed fuel cell development for use in UA and by the Army's Natick Laboratory for soldier systems (i.e., small scale uses), and specific energy performance is approaching that of gasoline engines. The gaseous hydrogen fuel cells being used on NASA's Helios UA in 2003 have over 80 percent of the specific energy of a two-cycle gasoline engine (500 vice 600 Watt hours/kilogram) and 250 percent that of the best batteries (220 W hr/kg); further improvement is anticipated when liquid hydrogen fuel cells are introduced. Still in development by NASA are regenerative power systems combining solar and fuel cells in a day/night cycle to possibly permit flight durations of weeks or longer. Additionally, several commercial aviation initiatives are exploring fuel cells for both primary propulsion and auxiliary power units (APUs), see Figure D-4. In the nuclear arena, the Air Force Research Laboratory has studied the feasibility of using a quantum nucleonic reactor (i.e., non-fission) to power long endurance UA. However this remains a concept study, no prototypes or flight worthy hardware are currently planned.
FIGURE D-3. ENGINE EFFECTS ON TAKE-OFF GROSS WEIGHT FOR A DESIRED MISSION
ENDURANCE.
APPENDIX D – TECHNOLOGIES
Page D-5
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本文链接地址：无人机系统路线图 Unmanned Aircraft Systems Roadmap(135)
