Answers - Turbine Engine (Part 2) | Aircraft Systems

Answers - Turbine Engine (Part 2)

61. c
Almost all of the components on a turbine engine, including the hot section, are required to be inspected on a time o? cycle basis. Additional times when a hot section must be inspected include during an overhaul or when an overtemperature or overspeed incident occurs.

62. a
The use of shrouded turbine rotor blades reduces blade vibration and improves turbine efficiency. With shrouded blades, the tips of the blades contact each other and provide support. This added support reduces vibration substantially. The shrouds also prevent air from escaping over the blade tips making the turbine more efficient.

63. a

64. a
If a turbine engine catches fire during an attempted start, you should immediately turn off the fuel and continue to turn the engine with the starter. By continuing to rotate the engine, the fire is likely to be drawn into the engine and discharged out the tailpipe.

65. b
The first step in starting a typical turbine engine is to engage the starter. Once this is done, the ignition is turned on. Then, when the N1 compressor obtains a predetermined rpm, the fuel lever is moved to the idle position. Normal lightoff is indicated by a rise in the exhaust gas temperature (EGT).

66. b

67. b
Airflow through some turbine engines during low thrust operations must be stabilized to prevent the compressor from stalling. To do this, variable inlet guide vanes or compressor bleed valves are used. Variable guide vanes rotate to maintain the correct angle of attack relationship between inlet air flow and compressor speed. Compressor bleed valves, on the other hand, dump away unwanted air.

68. c
Most modern gas turbine engines use a dual-spool compressor that utilizes two axial-flow rotors or one axial and one centrifugal-flow rotor. An advantage of the dual-spool compressor is the ability of the first compressor (N,) to seek its own best operating speed. Therefore, when the engine is operated at altitude where the air is less dense, the reduced drag on the first stage compressor allows the compressor to speed up thereby increasing efficiency.

69. a

70. a
When inspecting the hot section of a turbine engine, the exhaust cone and tailpipe should be inspected for cracks, warping, buckling, or hotspots. Hotspots on the tail cone are a good indication of a malfunctioning fuel nozzle or combustion chamber. For example, if a fuel nozzle is spraying a solid stream of fuel instead of an atomized spray, the fuel continues to bum as it passes through the exhaust section, producing bum marks on the tail cone. By the same token, a combustion chamber which is not properly controlling the flame zone may allow the flame to come in contact with the tail cone.

71. a
Each set of rotor blades within an axial-flow compressor has a corresponding set of stator vanes. The stator vanes direct the airflow to the next set of rotor blades at the proper angle and partially convert velocity energy to pressure energy.

72. a
According to Bernoulli's Principle, any time a fluid passes through a constriction at subsonic speeds pressure decreases while velocity increases. The diameter of a convergent nozzle decreases as the exhaust gases move aft. Therefore, as exhaust gases pass through a convergent nozzle the velocity of the gases increases while the pressure decreases.

73. a
A supersonic flow of air differs from a subsonic flow in that as a supersonic flow passes through an expanding tube its speed increases while pressure decreases. The diameter of a divergent nozzle increases, or expands, as exhaust gases move aft. Therefore, as supersonic gases pass through a divergent nozzle, gas velocity increases and pressure decreases.

74. b

75. b

76. b

77. a

78. c
Although an axial-flow compressor does not provide a high pressure rise per stage, it is capable of greater peak pressure ratios. The higher peak ratios are made possible by increasing the number of stages.

79. a
A compressor stall occurs when the angle of attack between the compressor vanes and the air moving through a turbine engine's compressor becomes excessive. When this happens, the compressor blades can no longer move air at a sufficient rate. To prevent this, some turbine engines are equipped with a bleed valve located at the first compressor stages that vents air overboard to maintain the correct effective angle of attack and prevent a compressor stall.

80. c

81. b

82. c
Each set of rotor blades within an axial-flow compressor has a corresponding set of stator vanes. The stator vanes help prevent swirling as they direct airflow to the next set of rotor blades and decrease air velocity by converting velocity energy to pressure energy.

83. c

84. c
The most common type of turbine blade used in jet engines is the impulse-reaction type. This type of blade is constructed with an impulse section at its base and a reaction section at its tip. This design distributes the workload evenly along the blade's length.

85. a
The primary factor in determining the pressure ratio in an axial-flow compressor is the number of stages within the compressor. Additional factors that affect pressure ratio include overall compressor efficiency and the pressure ratio produced by each stage.

86. b

87. c

88. a
As air leaves the compressor and enters the combustion section it is divided into a primary and secondary path. The primary path consists of approximately 25 to 35 percent of the total airflow and is routed to the area around the fuel nozzle to support combustion. The secondary path consists of the remaining 65 to 75 percent of the total airflow and is used to form an air blanket on either side of the combustion liner that cools the engine and centers the flames so they do not contact any metal.

89. a
The fixed stator vanes in the turbine section of a gas turbine engine are located ahead of the turbine rotor. The turbine stators act as nozzles to increase gas velocity and decrease pressure.

90. a
A set of stator blades is placed immediately behind each set of rotor blades in an axial-flow compressor. The stators act as diffusers to decrease air velocity and increase pressure before the airflow is allowed to continue to the next stage or to the burners.

91. b
Compressor field cleaning is the process of removing an accumulation of contaminants from compressor blades. Dirty compressor blades reduce aerodynamic efficiency and engine performance. Two common methods used for removing dirt deposits are a fluid wash and an abrasive grit blast.

92. c

93. b
In the early stages of engine main bearing distress, increased friction can cause oil temperatures to rise while power parameters remain within normal limits. However, as a main bearing gets closer to failing, the engine's power parameters will change.

94. c

95. b
Due to the extremely high temperatures and vibration that exist in a hot section, cracking is the most common problem encountered.

96. a
As air passes through a compressor, centrifugal force throws particles of dirt, oil, soot, and other foreign matter outward so that they build up on the casing, guide vanes, and compressor blades. However, because of the high temperatures present in the hot section, the turbine blades are not susceptible to this problem.

97. c
Galling is a transfer of metal from one surface to another usually caused by severe rubbing.

98. a
Some of the variables that an automatic fuel control unit senses include the power lever position, engine rpm, either compressor inlet pressure or temperature, and burner pressure or compressor discharge pressure.

99. c
If the compressor blades of a turbine engine are dirty or damaged, the engine will run at a higher internal temperature. Whenever an engine's internal temperature increases, the corresponding exhaust gas temperature (EGT) also increases. Under these circumstances, an engine's EGT limits may be reached before its maximum or takeoff engine pressure ratio (EPR) is obtained.

100. a

101. b
Creep, or growth, are terms used to describe the permanent elongation of rotating parts. Creep is most pronounced in turbine blades because they are continually subjected to extreme heat and centrifugal loads.

102. a
Although the orientation of compressor blades in a turbine engine is fixed, their angle of attack relative to the airflow is variable. Angle of attack is affected by the speed of the compressor, the direction of the airflow coming off the stator vanes, and the velocity of the airflow coming off the stator vanes.

103. a

104. b
The power produced by a turbine engine is directly proportional to the density of the air at the inlet. The factors which affect air density at the inlet are the speed of the aircraft, the altitude at which the aircraft is flying, and the ambient air temperature.

105. b
Thermal efficiency refers to the ratio of net work produced by a turbine engine to the chemical energy supplied in the form of fuel. The three most important factors affecting thermal efficiency are turbine inlet temperature, compression ratio, and the efficiency of the compressor and turbine.

106. c
The number of turbine wheels used in a gas turbine engine is determined by the amount of energy that must be extracted to drive the compressor and all accessories. Both turbofan and turboprop engines require more turbine wheels than a turbojet, because more energy is required to drive the fan or prop.

107. a

108. c

109. a
Prior to shutting down some turbine engines, a cool-off period is required to allow the turbine wheel to cool and contract before the case contracts around it. Although the turbine case and turbine wheels operate at approximately the same temperature when the engine is running, the turbine wheels are relatively massive compared to the case and, therefore, cool and contract more slowly.

110. b
Some turbine engines are equipped with low tension ignition systems that typically utilize self-ionizing or shunted-gap igniter plugs. Shunted-gap igniters contain a semiconductor material between the center and ground electrodes. The plug fires when current flows from a storage capacitor in the ignition exciter through the center electrode, the semiconductor, and to the plugs outer casing.

111. a
The term shrouded turbine refers to a gas turbine engine that uses shrouded turbine blades. The use of shrouded turbine rotor blades reduces blade vibration and improves turbine efficiency. With shrouded blades the tips of the blades contact each other, thereby providing support. This added support reduces vibration substantially. The shrouds also prevent air from escaping over the blade tips making the turbine more efficient.

112. c
Creep, or growth, are terms used to describe the permanent elongation and deformation of rotating parts. Creep is most pronounced in turbine blades because they continually must operate in extreme heat while being subjected to excessive centrifugal loads.

113. a

114. c

115. c
The jet nozzle of a gas turbine engine is attached to the rear of the tailpipe or rear flange of the exhaust duct and represents the last component the exhaust gases pass through. Therefore, the jet nozzle is part of the exhaust section.

116. c

117. c
Hot spots within the combustion section are possible indicators of a serious condition, such as malfunctioning fuel nozzles or other fuel system malfunctions. Therefore, whenever hotspots are present they must be interpreted carefully.

118. c
Fan blade shingling is the term used to describe the overlapping of midspan shrouds on fan blades. Any time rotating fan blades encounter a resistance that forces a blade sideways shingling occurs. Shingling is typically caused by an overspeed, FOD, a bird strike, or a compressor stall.

119. b

120. a The term hot streaking describes a condition where a fuel nozzle shoots out an unatomized stream of fuel which can contact the combustion liner or other components creating hot spots.