Free Turbine Turboprop

Free Turbine Turboprop

Figure 1.21 differs from Fig. 1.20 in that the turbine functions are
separated--one turbine drives a compressor and another the propeller. The
turbine that drives the compressor behaves as it does in a single-shaft
turbojet and the propeller turbine has the pressure ratio and flow characteristics
of a nozzle. Curve C-C of Fig. 1.16 again represents an operating line.

This engine offers better efficiency at reduced specific power than the
single-shaft engine running at constant speed. The speed of the gas generator
is reduced when the ETR is lowered; however, since there is no
propeller or large gear train on this shaft, the inertia is comparatively low.
The finite time required to respond to a demand for more power is small
and usually not objectionable.

The free turbine engine is the preferred choice for helicopters, which
require the main rotor to spin freely if the engine should stop. The load
imposed by the turbine, gears, and accessories is small enough to allow this
to happen without the use of a clutch or a free-wheeling device.
When the engine is idling, both the engine and propeller speeds are low.
This is a desirable quality for ground operations because of the low noise
level. Low engine speeds at part power are also useful when extended
periods of time for loitering are necessary.

Single-Shaft Turboprop

Single-Shaft Turboprop

Simplified analysis. This type of engine is represented by the sketch of
Fig. 1.20. The design differs from the turbojet in that the engine speed can
be independently set by adjusting the propeller pitch. The magnitude of
(N/x/~)C.l is then known when the compressor inlet temperature is given.
The speed parameter of a compressor map (i.e., Fig. 1.12) is thus established
as an independent variable. A match point is defined on a compressor
map by the intersection of the given corrected speed line with a line for
a given ETR such as curve A-A in Fig. 1.16. The corrected speed of the
turbine is then determined. If we assume that the pressure ratio through the
engine inlet to be about equal to that through the exhaust nozzle, we can
say that turbine pressure ratio is approximately equal to that of the
compressor. A match point on the turbine map is thus located and the
estimate of the powers developed by the turbine, compressor, and engine
are routine.
Performance trends. Adjustable propeller pitch provides for good
propulsion efficiency at a variety of flight speeds and engine powers.
Constant engine speed is the preferred mode of operation, however, even at
reduced power levels. The engine then quickly responds to demands for
higher power. (If rapid increases in engine speed were necessary at lower
speeds, the large polar moment of inertia of the propeller and gears would
delay the response. This can be dangerous.) Engine efficiency at reduced
power with constant speed is inferior to that obtainable along curve C-C of
Fig I. 16, but this situation is seldom necessary and usually has only a small
effect on most missions. Optimum altitudes are usually selected for cruise to
get good efficiency.
A single-shaft turboprop engine has a characteristic that is useful when
steep descents into tight airports are made. The propeller can be used as a
windmill to absorb power from the velocity of the airstream to drive the
engine. In this way, the flight speed is reduced to low levels in spite of a
steep angle of descent. This drag force can also cause problems in a
two-engine airplane when one engine fails.

Single-Shaft Turbojet

Single-Shaft Turbojet

Previous remarks have noted that this engine is now used only for special
purposes in modern aircraft because of the conflicting requirements of
engine and propulsive efficiencies. The component arrangement is, however,
a building block for many engines. Studying a simple jet engine, which is
sketched in Fig. 1.14, provides an understanding of the behavior of more
complicated ones. A picture of a production engine, which includes an
afterburner, is compressor and turbine maps of Figs. 1.12 and 1.13. This is where these
components are supposed to operate when the design value of the ETR is
imposed. Performance of the components is assumed and conventional
cycle analysis enables the design point performance to be estimated. An
approximate technique can be used for anticipating the way that the
compressor and turbine operating points change when the ETR is raised
or lowered. Such a procedure is now described. It ignores Reynolds
number effects, but one should recognize that anything that changes the
Reynolds number has the potential of producing other noticeable changes
in performance.

Method of approximate analysis. We shall assume an ideal gas having
the properties expressed in Sec. 1.3. This allows the major trends to be
easily revealed. To establish these trends, it is also convenient to make the
reasonable assumption that the gas flow rate in the turbine just behind the
combustor is about the same as that in the compressor immediately in front
of it. Moreover, if we confine our observations to the cases where the
turbine is choked (see Fig. 1.13a), we can plot straight lines on the

coordinates of a compressor map to show the constant corrected turbine
We may use a constant value (e.g., 0.95) for Pr, l/Pc, o without obscuring
any important trends. The equation then indicates that compressor pressure
ratio varies linearly with the corrected weight flow when the turbine is
choked. The slope of the line is proportional to the square root of ETR.
Lines A-A and B-B of Fig. 1.16 thus represent the requirements of
continuity for two values of ETR. An operating point would be indicated
by the intersection of a line representing the given ETR with the curve for
an appropriate corrected speed.

OFF-DESIGN PERFORMANCE OF GAS TURBINE PROPULSION ENGINES

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.

Turbofan Engines

Turbofan Engines

The objective of modern turbofans, see Fig. 1.10a, is to achieve acceptable
thrusts and efficiencies at higher flight speeds than a turboprop can. As
with a turboprop, the main engine can operate in any regime of Fig. 1.2
without disturbing the effectiveness of the thrust-producing components.
Similar to a propeller, a fan imparts energy to a large quantity of air: the
quantity is less than that of a propeller, however, being limited by factors
affecting the installation weight and drag, as well as fan efficiency.
Figure 1.10a differs from Fig. 1.9 only by the fact that the turbine located
between stations 5 and 7 drives an axial flow fan or compressor. The fan
raises the pressure of air passing through it so that a portion of this flow,
~VrF, can be discharged with the velocity VFj, which is lower than that
prevalent in turbojets, but higher than that behind propellers. The rest, of
the flow into the fan blades, I~E, passes through the engine. We call the
ratio WF/W ~ the bypass ratio.

The tolerable Mach numbers of the tips of the fan rotors are much
higher than those of conventional propeller blades. Available compressor
technology can be applied to produce the desired fan pressure ratios with
high efficiencies at elevated Mach numbers (the relative close spacing of the
fan blades and the presence of a casing surrounding the fan are the
principal reasons for this difference).

The designer of fan engines has the responsibility, however, of selecting
the energy delivered to the fan and the mass flow through it. The ensuing
discussion calls attention to the principal elements governing that decision.

Afterburning Turbojet Engines

Afterburning Turbojet Engines

The term afterburning, or reheat, applies to engines in which a second
combustor is placed between stations 5 and 7 (see Fig. 1.7) to increase the
temperature of the gas downstream of the turbines that drive the compressors.
Afterburners are stationary and are consequently subject to far less
stress than turbine rotors. The metal parts are much more easily cooled
than turbine airfoils and stoichiometric temperatures can thus be approached
unless unwanted disassociation intervenes. Engine thrust can be
augmented by about 50% at takeoff and by over 100% at high speeds. If
we again assume constant specific heat, the output specific energy may be
expressed as

Effects of Real Gases on Calculated Performance

Effects of Real Gases on Calculated Performance

Recall that we have assumed Cp and y to be constants. This concept is
true when the average square of the linear speeds of the molecules accounts
for all of their energy. When molecules consist of two or more atoms,
however, the atoms rotate about each other and vibrate; this energy is
absorbed by the gas. More and more energy is diverted this way as the
temperature is raised; thus, the value of Cp continually increases while ~,
decreases. We note that air consists almost entirely of the diatomic gases
oxygen and nitrogen and that Cp increases and 7 therefore decreases with
increasing air temperatures. After combustion, we also find important
quantities of triatomic elements--carbon dioxide and water vapor. Combustion
thus causes a further increases in Cp and reduction in y. The gas
constant is also lowered. We now need to determine how these changes
affect our thoughts about engine thermodynamic design.
The result of a spot check of the effect of real-gas properties is presented
in Table 1.1. These results are typical. The assumed fixed conditions for the
cycle are noted. The calculated gas properties are: the enthalpy increase for
compression, the energy added by fuel to obtain the stipulated turbine inlet
temperature, the enthalpy converted into mechanical energy during the
expansion process, the energy produced by the engine, and the engine
efficiency. The first column shows the results of assuming Cp to be 0.24 and
to be

EFFECT OF THERMODYNAMIC VARIABLES ON ENGINE PERFORMANCE

EFFECT OF THERMODYNAMIC VARIABLES ON ENGINE PERFORMANCE

Figure 1.1 presents a sketch of an elementary gas turbine engine. A
compressor converts mechanical energy into pneumatic energy and raises
the total pressure of the air between stations 2 and 3. (The station numbers
conform to a standard form, see Fig. 5.1 of Ref. 1.) When the engine is in
forward motion, additional pneumatic energy converts the kinetic energy of
the relative motion into pressure. Combustion of fuel in the burner adds
heat and raises the air temperature between stations 3 and 4. The turbines
between stations 4 and 5 convert part or nearly all of the available energy
at station 4 into mechanical energy. Part of this mechanical energy is
transferred to the compressor to effect the compression between stations 2
and 3. Additional mechanical energy may be transferred through a propulsion
device such as propeller or fan. Any pneumatic energy remaining at
station 5 is used to accelerate the gas to the velocity Vj and the kinetic
energy of ~V 2 per unit mass of air represents additional power output
from the engine. Heat may be added in an afterburner to further increase
and the output power.

AIRCRAFT ENGINE DESIGN OBJECTIVES

ENGINE DESIGN OBJECTIVES

All aircraft represent an investment of money for an anticipated profit. It is evident that commercial aircraft are built and bought to earn a monetary return on an investment. Although the revenue derived from investment is less tangible for many military aircraft, these too are expected to provide
some kind of an identifiable payoff that can be expressed in terms of money. An appropriate economic analysis, therefore, always precedes a decision to design, develop, and manufacture a new or a modified type of aircraft engine. The costs considered include all of the expenses of acquiring
and servicing the aircraft involved as well as the engine.

A necessary part of the economic study is the execution of a preliminary design of the proposed vehicle and its powerplant. Chapter 3 of this volume as well as Ref. 3 review the many details that must be considered during such a preliminary design.

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