Turbofan

The Pratt & Whitney JT9D-7A is representative of a modern high bypass turbofan engine. This engine has a dry weight of 8850 lb (39,365 N) and delivers a maximum continuous static thrust at SSL of 39,6501b (176,363 N). The static dry takeoff rating of 45,500 lb (202,384 N) is flat rated up to 27 °С (80 °F). The diameter of the engine is 2.43 m (95.6 in.) with a length of 3.92 m (154.2 in.). The compressor incorporates one fan stage, three low-pressure stages, and eleven high-pressure stages. The turbine has two high-pressure stages and four low-pressure stages. The bypass ratio equals 5.1 at the dry takeoff rating, with a total airflow of 1545 lb/sec (6872 N/s). The dash 7 A model of the JT9D is installed on several versions of the Boeing 747, including the 747-100, -200B, C, F, 747 SR, and 747 SP.

Figure 6.36 presents the rated takeoff thrust for this engine as a function of ambient temperature for altitudes up to 6000 ft. The net thrust is seen to drop approximately 14% in going from sea level to 6000 ft and to decrease rapidly with increasing Mach number.

Figures 6.37 and 6.38 give the rated maximum climb thrust and cruise thrust, respectively, as a function of Mach number for constant values of altitude up to 45,000 ft (13,700 m). Both figures also include lines of constant TSFC values. At the lower altitudes, the net thrust is seen to decrease rapidly as фе Mach number increases. However, at the higher altitudes, T is nearly constant and even increases slightly with Mach number above 30,000 ft and M values greater than 0.7.

The range of operating Mach number decreases in the preceding figures at the higher altitudes. This is a reflection of the limitations of the operating envelope presented in Figure 6.39. Such an envelope can result from several limitations, including temperature restrictions, stress limits, surge, and com­pressor stall.

-20 0 20 40 60 80 100 Ambient temperature, Tam, °F Figure 6.36 JT9D-7A net takeoff thrust. Dry, 100% ram recovery, no airbleed, no power extraction, Pratt & Whitney Aircraft standard exhaust. (Courtesy, Pratt & Whitney.)

9D-7A maximum cruise thrust. One hundred percent ram reco – , no power extraction, Pratt & Whitney Aircraft reference exhaust ss for ICAO standard day +10°C and below. (Courtesy, Pratt &

0 0.2 0.4 0.6 0.8 1.0 Flight Mach number, M Figure 6.39 JT9D turbofan engine. Estimated engine operating envelope. One hundred percent ram recovery. (Courtesy, Pratt & Whitney.)

Temperature restrictions are normally associated with the turbine inlet temperature (TIT). High-pressure turbines in the latest high bypass turbofan engines operate with gas temperatures in the 2000 to 2300 °F (1094 to 1260 °С) range. Various techniques have been developed that keep blade metal tem­peratures equal to those of uncooled blades used in earlier turbine designs. Normally, blade cooling is only required in the first or first and second turbine stages. After these stages, sufficient energy has been extracted from the burner exhaust to cool the hot gases to a tolerable level.

Three forms of air cooling are described in Reference 6.7; these are used singly or in combination, depending on the local temperatures. The air for this cooling is bleed air taken from the compressor section. Even though this air is warmer than ambient air, it is still considerably cooler than the burner exhaust.

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Convection Cooling Cooling air flows inside the turbine vane or blade through serpentine paths and exits through the blade tip or through holes in the trailing edge. This form of cooling is limited to blades and vanes in the area of the lower gas temperatures.

Impingement Cooling Impingement cooling is a form of convection cooling, accomplished by directing cooling air against the inside surface of the airfoil through small, internal, high-velocity air jets. Impingement cooling

is concentrated mostly at critical sections, such as the leading edges of the vanes and blades.

Film Cooling Film cooling is a process whereby a layer of cooling air is maintained betwediklhe high-temperature gases and the external surfaces of the turbine blades and vanes.

Of the three forms of air cooling, film cooling is the most effective and the least demanding as far as airflow is concerned.

These types of cooling are illustrated in Figure 6.40, which shows their application to both stationary and rotating turbine stages.

Surge and compressor stall are related but are not the same thing. Surge refers to oscillations in the rotational speed of the entire engine. This surge is usually related to compressor stall, where the local angles of attack of the rotor blades, for various reasons, achieve sufficiently high values to cause local stalling. Some of these reasons include inlet airflow distortion from gusts, inlet design or uncoordinated maneuvering, rapid power changes, water ingestion, and Reynolds number effects.

A typical compressor map is given in Figure 6.41. This map shows qualitatively the relationship among the corrected rpm, corrected airflow, and total pressure ratio across the compressor. A small insert in the figure illustrates an airfoil on the compressor rotor under the influence of two velocities, one proportional to the, airflow and the other proportional to the rotational speed. At a fixed blade angle, the angle of attack of this section obviously increases as N increases or Wa decreases. This is reflected in the map, which shows one approaching the surge zone as N increases for a constant Wa or as Wa decreases for a constant N.

As the altitude increases, the surge zone drops down, mainly because of Reynolds number effects. At the same time, the steady-state operating line moves up. Thus compressor stall and surge are more likely to be encountered at the higher altitudes.

Accelerating the engine can also lead to compressor stall. Suppose, in attempting to get from the steady operating point A to point B, the rpm is suddenly increased. The airplane may be unable to accelerate rapidly enough to follow the rpm, so the airflow is less than the steady-state value. Surge can be alleviated by unloading the compressor during certain operating conditions. This is accomplished by bleeding air near the middle or end of the com­pressor. Stators having variable blade angles are also used to delay com­pressor rotor blade stall. The JT9D, for example, has variable stators automa­tically positioned by hydraulic actuators on the first four stages of the high-pressure compressor. Their function is to provide an adequate surge margin during engine starting, acceleration, and partial thrust operation.

A small, modern turbofan engine typical of others of this size is shown in Figure 6.42a and 6.42b. Only the rotating assemblies of the JT15D-1 engine

Figure 6.40 Types of air cooling of turbine vanes and blades. (Courtesy General •Electric Co.)

are shown in order to emphasize its relative simplicity and general configura­tion. The JT15D-9 is a growth version of the -1 model and was achieved by adding one additional axial stage to the low compressor. Although this decreases the bypass ratio, the pressure ratio across the compressor is increased so that the rated thrust of the -4 engine is approximately 14%

Figure 6.41 Typical compressor map.

greater than that of the -1 engine. Notice that the high-pressure section of the compressor for the JT15D employs a centrifugal compressor similar to that of the Garrett TPE331/T76 engine. Specifics on the JT15D-1 and -4 turbofan engines are given in Table 6.2.

Turboprop

The PT6A-27 turboprop engine manufactured by Pratt & Whitney Air­craft of Canada, Ltd. is representative of the latest technology for turboprop engines of this size. Airplanes in which this engine is installed include the Beech 99A, the de Havilland Twin Otter, and the Pilatus Turbo Porter. The arrangement of this engine is shown in Figure 6.43. Compared to a turbojet, the core engine is reversed with the air being taken toward the rear of the engine and flowing forward through the compressor, burner, turbines, and then exhausted through stacks at the front of the engine.

A typical nacelle installation for this engine is illustrated in Figure 6.44.

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An inlet duct channels the air to the rear of the engine, where it must turn in order to enter the engine. In good weather, the vane is raised from the position shown to lie flush with the surface above it, and the bypass door is closed. When ice or other types of particles are encountered, the bypass door is opened and the vane is lowered, as pictured. The air-particle mixture is then deflected downward. Because of their inertia, the heavier particles continue

Table 6.2 Characteristics of the JT15D Turbofan Engine Manufac­tured by Pratt & Whitney Aircraft of Canada, Ltd. Installation JT15D-1 Cessna Citation JT15D-4 Cessna Citation II Aerospatiale Corvette Thrust—takeoff, lb 2,200 2,500 Maximum continuous 2,090 2,375 Maximum cruise 2,065 2,345 TSFC—takeoff 0.540 0.562 Maximum continuous 0.538 0.556 Maximum cruise 0.537 0.555 Mass flow, lb/sec 75 77 Bypass ratio 3.3 2.7 Pressure ratio 8.5 10.1 Engine dry weight, lb 514 557 Engine diameter, in. 27 27 Engine length, in. 59 63 Compressor stages 1 f&n, 1 fan, 1 axial, 1 centrifugal 1 centrifugal Turbine stages 2 power, 2 power, 1 compressor 1 compressor

on out through the bypass door, while the air is turned and drawn into the engine.

The specifications for this engine are presented in Table 6.3, and its performance is given in Figures 6.45 and 6.46a and 6.46b. These curves, taken from the installation handbook, assume no losses. Figures 6.45 and 6.46a are

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Table 6.3 PT6A-27 Turboprop Engine Specifications

Static Sea Level Ratings	shp	eshp	SFC (Ib/eshp/hr)
takeoff	680	715	0.602
Maximum continuous	680	715	0.602
Maximum climb	620	652	0.612
Maximum cruise	620	652	0.612
Mass flow	6.1 Ib/sec
Pressure ratio	6.3
Dry weight	3001b
Diameter	19 in.
Length	62 in.
Compressor—3 axial and	1 centrifugal	stage
Turbine—1 power stage and 1 compressor stage

Standard Standard + 10 Standard+20 Standard + 30 Ambient temperature, Tam, °С Figure 6.45 PT6A-27 takeoff performance. Prop speed—2200 rpm. (Courtesy, Pratt & Whitney of Canada.)

600 500 400 300 200 100

6.46a PT6A-27 maximum cruise performance. •2200 rpm. (Courtesy, Pratt & Whitney of Canada.)

1800 2000 2200 Prop speed, rpm Figure 6.46b PT6A-27/28 maximum cruise data. Ap­proximate ASHP versus prop speed, 100 to 300 kts. (Courtesy, Pratt & Whitney of Canada.)

for the rated propeller rpm of 2200, while Figure 6.46b presents a correction to the power for operation at lower rpm values. Only the maximum cruise and maximum takeoff ratings are given, since these are the same as the maximum climb and maximum continuous ratings, respectively. The power curves are for actual shaft power and do not include an effective power increment for the residual thrust. Statically, from Table 6.3, this thrust appears to equal approximately ЩЖ This represents about a 5% correction to the shaft power.