Answers - Turbine Engine (Part 1) | Aircraft Systems

Answers - Turbine Engine (Part 1)

1. c

2. c
When high energy gases leave the combustion section of a turbine engine, they enter the turbine section. The turbine section is made up of stationary and rotating airfoils, or vanes. The stationary vanes, sometimes called a nozzle diaphragm, direct the high energy gases leaving the combustor into the rotating turbine blades. The nozzle diaphragm also increases the velocity of the gases.

3. b
When looking at the profile of a compressor blade you will see that the tip of each blade is cut out to reduce tip thickness. This shape allows the blade to wear rather than break if the blade tip should come in contact with the case.

4. a
On a dual axial or dual spool turbofan engine, the forward fan is typically bolted to the first compressor making the fan part of the low-pressure compressor.

5. c
The abbreviation P stands for pressure, and subscript t7 indicates a total at station 7 which is immediately aft of the last turbine stage. Therefore, Pt7 is the total pressure aft of the turbine.


6. b
Minor damage to turbine engine blades and vanes can usually be repaired if the damage can be removed without exceeding the allowable limits established by the manufacturer. However, all repairs must be well blended so that the blade's surface is smooth. Blending is almost always done by hand using crocus cloth, fine files, and stones. Furthermore, whenever possible, blending is performed parallel to the length of the blade to minimize stress points. Cracks are normally not allowed, in any area.

7. a

8. b
During the combustion process in a turbine engine, burning fuel provides heat to expand the compressed air coming from the compressor. Throughout this process the pressure remains relatively constant.

9. b
In a typical turbine engine, a small increase in rpm produces a relatively proportional increase in thrust when operating at low engine speeds. However, at high engine speeds a small increase in rpm produces a large increase in thrust.

10. a
A two-spool or dual spool turbine engine is one in which there are two independently rotating units. The front compressor is called the low-pressure compressor, and the rear compressor is called the high-pressure compressor. This type of engine has more operating flexibility than a single spool engine because the two compressors are free to find their own optimum rpm. This allows the low pressure compressor to increase in rpm at altitude because of the reduction in drag caused by the decrease in air density.

11. c
When high energy gases leave the combustion section of a turbine engine, they enter the turbine section. The turbine section is made up of stationary and rotating airfoils, or vanes. The stationary vanes are grouped together to form a nozzle which increases the velocity of the gases and directs the high energy gases leaving the combustor into the turbine's rotating blades.

12. c
The gas pressure in a turbine engine reaches its highest value as compressed air leaves the compressor and enters the burner. Once in the burner section, the air expands due to the heat produced by the burning fuel. From here, the gases pass through a nozzle diaphragm where they are accelerated prior to entering the turbine blades.

13. a
A jet engine exhaust cone collects the exhaust gases discharged from the turbine buckets and gradually converts them into a steady stream. In doing this, the divergent shape of the exhaust cone causes the velocity to decrease and the pressure to increase.

14. a 
As air passes through the compressor section of a typical axial-flow compressor, it becomes extremely turbulent. To help prevent turbulent air from flowing into the combustion section, the air passes through a stator vane which straightens airflow and eliminates turbulence.

15. c
In all turbine engines the turbine transforms a portion of the kinetic energy of the exhaust gases into mechanical energy to drive the compressor section.

16. a
One of the critical factors to observe when starting a turbine engine is the exhaust gas temperature. A hot start is characterized by the exhaust gas temperature exceeding the specified limits during an attempted start and can cause substantial damage to the combustion and turbine sections. The most likely cause of a hot start is an excessively rich mixture.


17. b

18. a
Certain materials may be used to mark combustion and turbine components during disassembly and assembly. For example, layout dye, chalk, and some commercial felt-tip markers are considered acceptable for use in marking parts that are directly exposed to an engine's gas path such as turbine blades and disks, turbine vanes, and combustion chamber liners.

19. b
A hung start occurs if a turbine engine starts normally but the rpm remains at some low value rather than increasing to the normal idle rpm. Hung starts are generally a result of shutting off the starter too soon, or by insufficient starter power.

20. c

21. c

22. c
The two parts that make up a centrifugal compressor are the impeller and the diffuser. The impeller accelerates the flow of air to the diffuser which is designed to direct the flow of air to the manifold at an angle that returns the maximum amount of energy.

23. c
After a fuel control has been replaced on a turbine engine, it is often necessary to retrim the engine. Retrimming consists of adjusting both the idle and maximum speed. On some newer turbine engines, such as the GE T700, retrimming may not be necessary after the fuel control is replaced.

24. c
If a bearing becomes magnetized, metal particles would be attracted to the bearing surfaces and cause premature wear. Therefore, if a bearing has magnetism present, it must be removed with a suitable degausser before the bearing can be reused.

25. a

26. c

27. a
Turbine discharge pressure is identified in service manuals and on engine instruments by the standardized abbreviation Pt7.

28. a

29. c

30. c
Welding and straightening of rotating airfoils typically requires very specialized equipment. Furthermore, only authorized overhaul facilities and manufacturer are typically authorized to weld or straighten a damaged rotating airfoil.

31. b
Wax or grease pencils, when used on turbine engine components, can cause hot spots to occur, and graphite lead pencils can cause dissimilar metal corrosion.

32. c

33. a
Turbine main bearing oil seals are generally either the labyrinth or carbon rubbing (carbon ring) type. The labyrinth seal relies on pressure to prevent oil from leaking along the compressor shaft. Carbon rubbing seals, on the other hand, are usually spring loaded and are similar in material and application to the carbon brushes used in electrical motors. These seals rest against the surface provided and create a sealed bearing cavity or void that prevents oil leakage.

34. b
One of the advantages of a dual spool axial compressor over a single spool is the ability to have two separate compressors rotate at their own optimum rpm. By having two compressors rotate at different speeds, higher compression ratios are obtained.

35. b

36. b

37. c
Although an axial-flow compressor does not give as high a compression rise per stage as a centrifugal compressor, its multiple stages and ability to take advantage of ram air pressure allow it to produce higher peak pressures.

38. b
In an axial-flow compressor, the stator blades are fixed airfoils that are placed at the discharge end of each compressor stage. Their purpose is to control the direction of airflow (answer B) into the next compressor stage or combustion section and eliminate turbulence. The stationary airfoils in the axial flow compressor are most appropriately called stator vanes.

39. a
In a centrifugal-flow compressor, the diffuser is placed at the outlet of the compressor. The purpose of the diffuser is to reduce the velocity of the gases and to increase their pressure. This prepares the air for entry into the burner cans at low velocity so combustion can occur with a flame that will not blow out.


40. c
Stress rupture cracks on turbine blades usually appear as minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length. Stress rupture cracks located on the first stage turbine indicate either an over-temperature condition or centrifugal loading.

41. a

42. c
The diffuser section of a centrifugal-flow compressor is located between the outlet of the compressor section and the inlet of the burner section. The purpose of the diffuser is to reduce the velocity of the air exiting the compressor, thereby increasing air pressure. This prepares the air for entry into the burner cans.

43. b

Stress rupture cracks on turbine blades usually appear as minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length. Stress rupture cracks located on the first stage turbine indicate either an over-temperature condition or centrifugal loading.

44. b

45. b
The materials within the turbine section of an engine will deteriorate rapidly if exposed to extreme temperatures. Therefore, the turbine inlet temperature is the limiting factor for a turbine engine.

46. c

47. c

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

49. b
The split-spool, axial-flow compressor offers the greatest advantages. For example, since the compressor and turbine are split, starting speed is easily obtained. Furthermore, high altitude performance is better because the two separately rotating compressors are able to seek their own optimum rpm.

50. c
In order to maintain the balance of the turbine assembly, when a turbine blade is removed for inspection, it must be reinstalled in the same slot.

51. a
Although peak efficiency of the centrifugal compressor is not as great as in the axial-flow type, it does give a higher pressure rise per stage (answer A). Modern day centrifugal compressors produce as much as 8 or 10 to 1 compression ratios, while axial-flow compressors produce approximately a 1.3 to 1 compression ratio.

52. b
The highest heat-to-metal contact in a turbine engine occurs as the heated gases leave the combustion section and enter the turbine inlet vanes.

53. a
An axial-flow compressor assembly is made up of two principle elements, the rotor and the stator. The rotor consists of a set of blades installed on a spindle that rotates at a high speed and impels intake air through a series of stages. The stator blades, on the other hand, act as diffusers at each stage, changing high velocity to pressure.

54. a
The two types of centrifugal-flow compressor impellers are the single entry and the double entry. The single entry has vanes on only one side of the impeller, while the double entry has vanes on both sides of the impeller.

55. c
Between each row of rotating blades in an axial-flow compressor there is a set of stationary airfoils called stator vanes. The stator vanes direct the air between stages and diffuse, or slow down the air causing pressure to increase.

56. c

57. a

58. c
When turbine blades are subjected to excessive temperatures, stress rupture cracks are likely to develop. Stress rupture cracks usually appear as minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length.

59. a
At the discharge end of an axial-flow compressor, the air is extremely turbulent. To help eliminate this turbulence, as well as slow the air flow, stator vanes are installed. These vanes are sometimes called straightening vanes or the outlet vane assembly.

60. c
Accumulation of dirt on the compressor blades reduces the aerodynamic efficiency of the blades, with resultant deterioration in engine performance. Furthermore, dirt deposits can retain moisture and other chemicals that cause corrosion.



YOU MAY ALSO LIKE