Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.4. Engine Performance and Technology
Other reports in this collection
For the past 50 years, the principal propulsion source for military and civil aircraft has been the gas turbine. For a variety of technical reasons, this situation is likely to
continue into the foreseeable future. The earliest military aircraft gas turbines, developed toward the end of World War II, opened the way to high-speed flight by
providing high power from low weight, compact engines. For military use, these "jet" engines offered an escape from the aerodynamic limitation of the propeller,
despite their relatively low thermodynamic efficiency at the time. Early civil aircraft gas turbines continued, at first, to use the engine to drive a propeller in so-called
turbo-prop form (engine shown in Figure 7-7a). Several types of turbo-props are still used for short-haul operations where cruise speeds are less important, but larger
and faster aircraft have dispensed with the propeller.
Since those early days, huge strides have been made in the critical and pacing technology fields that influence the key design and performance characteristics of
engine design. In particular, major advances have been made in the fields of turbo-machinery aerodynamics, combustion, turbine blade cooling, and materials. For
military engines, these advances have been realized mainly in increases in the ratio of engine thrust to engine weight. For civil aircraft engines, the benefits have led to
high bypass ratio engines with substantially lower fuel consumption, which have contributed to the rapid growth in air transportation over the past 3 decades.
It is common to compare engines in terms of specific fuel consumption (SFC), which is the fuel flow rate per unit thrust at cruise. However, the ultimate goal is to
minimize total fuel burned per unit payload, rather than SFC; this computation involves engine weight, installation drag, and their effect on the total fuel required to
complete a flight mission. For subsonic transport aircraft, the weight of the engines is on the order of 10-15% of the empty weight of the aircraft; a reduction of one unit
of total engine weight translates to a reduction of between 1.5 and 4 units of aircraft empty weight, depending on the design. The relatively larger reduction in aircraft
weight derives from concomitant reductions in requirements for supporting structure. The benefits are further magnified by the fact that the reduction in fuel burn
attributable to engine weight savings is proportional to increasing aircraft range.
The following subsections present a brief outline of engine performance issues from an historical perspective (7.4.2) and a look forward into the future (7.4.3). The link
between performance considerations and emissions is discussed in Section 7.4.4. Because the gas turbine is expected to remain the principal power source for aircraft
propulsion well into the future, however, Section 7.4.1 presents a simplified review of the engine's fundamental principles to explain, among other things, why there is
no reasonable alternative to the gas turbine or derivatives thereof in the foreseeable future.
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