OFF-DESIGN PERFORMANCE OF GAS TURBINE PROPULSION ENGINES
So far we have been concerned with engine power and efficiency at one
particular operating condition as determined by the ambient inlet pressure
and temperature, turbine inlet temperature, and assigned component properties.
Engines are, however, expected to start from a standstill, accelerate,and run over a range of flight speeds with a variety of compressor and
turbine inlet temperatures. They must eventually decelerate and stop.
Smooth changes from one operating condition to another are essential.
Providing stipulated thrusts with reliability and with the required or higher
efficiencies is expected at all specified conditions.
As Chapter 3 of this volume emphasizes, we must ultimately be concerned
with both engine and airframe performance for a given mission. We
can tentatively assume that an airplane, flight path, and thermodynamic
cycle have been selected and that components have been chosen to provide
a desired efficiency at a given value of thrust. What an engine does at other
levels of thrust (or at off-design) needs to be evaluated. The result is
influenced by the characteristics of the compressors and turbines and their
arrangement in an engine.
Any change in specific power causes the operating point of each component
to move. The principal variable causing this change is the ratio of
turbine inlet to compressor inlet temperature, which we shall call the engine
temperature ratio or ETR. Flight altitude, fuel flow, and flight Mach
number are the determinants of this ratio. Note, however, that power and
thrust are directly proportional to the inlet pressure unless low pressures
reduce the Reynolds number beyond the point where component performance
deteriorates.
The changes in the component operating points with the ETR have a
crucial influence on the effectiveness and reliability of a given engine design.
Anticipating and understanding the physical events governing these
changes are valuable assets for both design and development. This subject
is reviewed in this section. Instead of pursuing the formal attacks described
in Chapter 8 of Ref. 1, we shall use a few approximations that are adequate
for locating possible problem areas on component maps. The relative
behaviors of several designs are then compared.
Two pictures of a turboprop engine provide some background for engine
configuration. Figure 1.9 is a cutaway view showing the compressor,
turbine, gears, and propeller shaft. The provisions for accessories, which
usually consist of an electric generator, oil pump, and hydraulic fluid
pumps, are also shown. The relative sizes of these particular accessories are
indicated by a photograph of the complete engine in Fig. 1.11. Engines
often provide additional power in the form of compressed air, which is bled
from the compressor. In this section, however, we shall ignore the small
power requirements for accessories and other parasitic demands and deal
only with those of the propellers and jets.
So far we have been concerned with engine power and efficiency at one
particular operating condition as determined by the ambient inlet pressure
and temperature, turbine inlet temperature, and assigned component properties.
Engines are, however, expected to start from a standstill, accelerate,and run over a range of flight speeds with a variety of compressor and
turbine inlet temperatures. They must eventually decelerate and stop.
Smooth changes from one operating condition to another are essential.
Providing stipulated thrusts with reliability and with the required or higher
efficiencies is expected at all specified conditions.
As Chapter 3 of this volume emphasizes, we must ultimately be concerned
with both engine and airframe performance for a given mission. We
can tentatively assume that an airplane, flight path, and thermodynamic
cycle have been selected and that components have been chosen to provide
a desired efficiency at a given value of thrust. What an engine does at other
levels of thrust (or at off-design) needs to be evaluated. The result is
influenced by the characteristics of the compressors and turbines and their
arrangement in an engine.
Any change in specific power causes the operating point of each component
to move. The principal variable causing this change is the ratio of
turbine inlet to compressor inlet temperature, which we shall call the engine
temperature ratio or ETR. Flight altitude, fuel flow, and flight Mach
number are the determinants of this ratio. Note, however, that power and
thrust are directly proportional to the inlet pressure unless low pressures
reduce the Reynolds number beyond the point where component performance
deteriorates.
The changes in the component operating points with the ETR have a
crucial influence on the effectiveness and reliability of a given engine design.
Anticipating and understanding the physical events governing these
changes are valuable assets for both design and development. This subject
is reviewed in this section. Instead of pursuing the formal attacks described
in Chapter 8 of Ref. 1, we shall use a few approximations that are adequate
for locating possible problem areas on component maps. The relative
behaviors of several designs are then compared.
Two pictures of a turboprop engine provide some background for engine
configuration. Figure 1.9 is a cutaway view showing the compressor,
turbine, gears, and propeller shaft. The provisions for accessories, which
usually consist of an electric generator, oil pump, and hydraulic fluid
pumps, are also shown. The relative sizes of these particular accessories are
indicated by a photograph of the complete engine in Fig. 1.11. Engines
often provide additional power in the form of compressed air, which is bled
from the compressor. In this section, however, we shall ignore the small
power requirements for accessories and other parasitic demands and deal
only with those of the propellers and jets.
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