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recognized today as the prime attribute of an UA when compared to manned aircraft, and endurance is determined largely by the efficiency of the powerplant, propulsion is, with processors, one of the two key UA technologies.
Two key propulsion metrics are specific fuel consumption (SFC) for efficiency and specific power (SP) for performance. AFRL’s Versatile Affordable Advanced Turbine Engines (VAATE) program aims to achieve a 10 percent decrease in SFC by 2015, while improving thrust-to-weight (T/W) by 50 percent and lowering engine production and maintenance costs. Reciprocating engines for aircraft generally produce 1 hp per pound of engine weight (746 watts/lb), and today’s fuel cells are approaching this same level, while lithium-ion batteries have about half this SP (See Figure 4.3-1). Fuel cells in particular are expected to show rapid advancement over the coming decade due their increasing use in hybrid automobiles. Heavy fuel engine (HFE) technology has advanced over the last few decades to the point where replacement with internal combustion engines on tactical UA is now practicable. However, further HFE development investment needs to be made to make their use on small UA practicable. Additional investment also needs to be made in turbine technology for a J-UCAS class engine with a high thrust to weight ratio and low SFC. Specific power trends in propulsion and power technology are forecast in Figure 4.3-1 and Table 4.3-1.
Recommended Investment Strategy: Focus DoD research on developing diesel reformaters for fuel cell use, enhanced engine durability and time between overhaul, improved specific fuel consumption for enhanced endurance, and alternative propulsive power sources like fuel cells, photovoltaic, and nuclear propulsion systems.
4.3.4 Reliability
Aircraft reliability and cost are closely coupled, and unmanned aircraft are widely expected to cost less than their manned counterparts, creating a potential conflict in customer expectations. The expected benefit of lower unit prices may be negated by higher attrition rates due to poorer system reliability. The impact of reliability on UA affordability, availability, and acceptance is described in detail in Appendix
H. Figure 4.3-2 illustrates how the mishap rates of larger UA compare to that of representative manned aircraft (F-16 and U-2) after similar numbers of flying hours have been accumulated. Since UA fleets are generally smaller than manned fleets, they have accumulated flying hours at lower rates resulting in slower progress down this curve. As an example, the MQ-1 Predator fleet just reached the 100,000-hour mark in October, 2004, 10 years and 3 months after its first flight, whereas the F-16 reached this same mark in one quarter of that time and the 800,000-hour mark in that same time. However, the Figure shows that the mishap rates of the recent, larger UA track closely with that of the F-16 fleet at a comparable point in its career.
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