直升机教员手册 Helicopter Instructor’s Handbook
时间：2014-11-10 08:35来源：FAA 作者：直升机翻译 点击：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. Turbocharged or supercharged piston engines can operate well at high altitudes. Weight per horsepower and reliability are the main factors favoring the turbine engine. Explain that the working cycle of the turbine engine is similar to that of the piston engine (i.e., induction, compression, combustion, and exhaust). One other difference is the fact that the piston engine combustion (power) is intermittent; in the turbine engine, each process (cycle) is continuous. The manufacturer may be able to provide a diagram of the internal components of the turbine engine. This will allow further discussions with the student regarding the internal workings of the engine. Figure is an example of a turbine engine. The instructor should also explain the increased fuel usage in a turbine engine is due to the continuous combustion process and the fact that approximately the first percent of the engine’s power is required to sustain the engine’s induction and systems such as the oil system and electrical generator. This accounts for a turbine engines high idle speed. A turbine engine may idle at 16,000 rpm and generate maximum power at 38,000 rpm. Turbine engine power curves are very steep and it may need seconds or longer to begin generating large increased power demands. There is very little extra power available at close to idle settings from turbines. Usually, turbine engines must be above 80-90% rpm to develop moderate power output. This is the reason to keep turbine engines under power loads to have the power available if needed. Show the student the main parts of the turbine engine (compressor, combustion chamber, turbine, and accessory gearbox assembly). Then, discuss what each section is doing during flight or 100 percent power, as depicted in Figure 5-24. Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main difference between a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine (turboshaft engine) rather than producing thrust through the expulsion of exhaust gases (turbojet engine). The instructor should fully understand and be able to explain that the turbine and four-stroke helicopter engines both have four cycles: intake, compression, power, and exhaust. This continuous combustion process is the main limitation due to material limitations. The extreme heat and centrifugal forces place tremendous stress on the rotating parts of the combustion section. When operating helicopters with turbine engines, instructors should teach the student about starting batteries and the different characteristics of a lead acid and Ni-cad (nickel-cadmium) batteries. Lead acid batteries generally do not have the energy density per pound of the Ni-cad batteries, but cost much less and have much lower maintenance costs. Lead acid batteries also tend to have a sloping power output curve that can allow the operator to perceive impending failure and replace the battery; however, lead acid batteries must be specially designed to withstand the deep charge that happens during a turbine engine start. The student should be reminded of the differences between the start times of a reciprocating engine (a relatively short period of time) and the prolonged turbine starting sequence (lasting 30–60 seconds not counting a cooling period if the internal engine temperatures are initially too high). Additionally, the battery for a turbine engine installation must be designed with sufficient residual reserve to furnish cooling rotation in the event of an aborted or hot start. Ni-cad batteries have much higher energy densities for their weight and, most significantly, can withstand the long, very high current drain necessary to start a turbine engine in cold temperatures. One advantage of Ni-cad batteries is the almost flat output power curve. The uniform output provides consistent turbine starter activation. Unfortunately, Ni-cad batteries produce a very flat consistent discharge power output which suddenly and rapidly decreases at the end of its charge, and this means that it can be very difficult to determine if the battery is at full capacity or towards the end of the charge curve. To receive proper service and consistent turbine starts, battery voltage and battery charge indications must be closely and consistently monitored for long-term, gradual changes and be maintained in accordance with the manufacturer’s recommendations. This usually requires completely discharging and charging the individual battery cells. Most manufacturers then require that batteries be reassembled with equal output cells for best results. For more information on starter batteries, the instructor should review chapter 10 of the Aviation Maintenance Technician—General Handbook. As a reminder: 1. The compressor draws air into the plenum chamber and compresses it. 2. That air is directed to the combustion section where fuel is injected into it. 3. The fuel-air mixture is ignited and allowed to expand. 4. This combustion gas is then forced through a series of turbine wheels, causing them to turn. 5. Turbine wheels provide power to both the engine compressor and the accessory gearbox. 6. Power is provided to the main rotor and tail rotor systems through the freewheeling unit, which is attached to the accessory gearbox power output gear shaft. 7. During the starting process, follow the manufacturer’s requirements closely for hot or slow starting procedures. A fully charged battery will help in most cases. 8. Always follow the manufacturer’s cool down procedures to allow internal parts to settle to cooler uniform temperatures as much as possible before engine shut off. Now, briefly explain what each section comprises and any emergency actions related to each one. Compressor The compressor is similar to a fan. As air is drawn inward, stator vanes act as a diffuser at each stage, decreasing air velocity and increasing air pressure. The high pressure air then passes through the compressor manifold where it is distributed to the combustion chamber via discharge tubes. |
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