直升機(jī)教員手冊 Helicopter Instructor’s Handbook
時(shí)間：2014-11-10 08:35來源：FAA 作者：直升機(jī)翻譯 點(diǎn)擊：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. Antitorque Drive System Explain to the student that the drive system may be exposed or placed inside of a covered tail boom depending on the type of helicopter. Point out to the student the different parts of the tail rotor drive system, (if installed) such as the hanger bearings, flex couplings, input seal, and output seal. Also, point out what to look for during preflight (leaks, loose fittings, or obvious damage). The instructor should ensure the student understands the common failure modes and weak links. For example, the witness pins on the shafts at the couplings, coupling packs, slippage marks, and metal particles indicating a movement between the surfaces (around a loose rivet). The tail rotor gear box should also be covered at this time. Fluid levels and attaching hardware are important preflight items to check. [Figure 5-26] Clutch Explain to the student the purpose of the clutch (if installed) on the helicopter, and how a centrifugal clutch works. A centrifugal clutch is a clutch that uses centrifugal force to connect two concentric shafts, with the driving shaft nested inside the driven shaft. The input of the clutch is connected to the engine crankshaft while the output may drive a shaft, chain, or belt. As engine rpm increases, weighted arms in the clutch swing outward and force the clutch to engage. The most common types have friction pads or shoes radially mounted that engage the inside of the rim of a housing. On the center shaft, there are assorted extension springs, which connect to a clutch shoe. When the center shaft spins fast enough, the springs extend, causing the clutch shoes to engage the friction face. It can be compared to a drum brake in reverse. When the engine reaches a certain rpm, the clutch activates, working. This results in waste heat but, over a broad range of speeds, it is much more useful than a direct drive in many applications. Those using the belt clutch system must be very careful to ensure full engagement and engagement procedures. Excessive throttle can quickly ruin an engine because there is no load during the initial starting, so the engine can speed past its rpm redlines very quickly. Those events require expensive teardowns and overhauls. Most large helicopters use a clutch during the start sequence and then gradually engage the rotor system to normal operating rpm. Free-turbine engines do not need a clutch because there is little load from the rotor system. The rotor slowly starts turning during the start sequence and gradually achieves normal operating rpm. Explain to the student that there are three main types of clutch found on reciprocating helicopters. 1. Centrifugal clutch—briefly explain how the centrifugal clutch operates and how to determine if the clutch is operating normally, using the rotor tachometer. 2. Belt drive clutch—briefly explain how the belt drive clutch operates and how to determine if the clutch is operating normally using the rotor tachometer. Show the student the location of the pulley belts and that the pilot must check for frays, tears, or cracks on the belt(s) during preflight of the helicopter. 3. Sprag clutch—explain how the sprag clutches have inclined ramps and rollers. If the drive shaft is faster than the driven shaft, the rollers are forced against the ramps and the clutch locks up and transmits full power. If the driven shaft is turning faster than the driving shaft, the rollers retreat down the incline and allow the driven shaft to rotate freely, hence the freewheeling clutch. NOTE: A clutch is used to disconnect the engine from the rotor load to enable a starter motor to turn the engine for starting. Some turbine helicopters have a centrifugal clutch (Gaszelle) that engages the rotor system above about 28,000 rpm. Also, the R-22, R-44, and HU-269 series use belt clutches to allow the engine to be started without excessive loading. The older Hillers and Bell 47 series machines used centrifugal clutches mounted above the engines. Freewheeling Unit Explain to the student that all helicopters are fitted with a form of freewheeling unit. Also, explain the purpose of the freewheeling unit and where it is located on the helicopter. The freewheeling unit makes autorotations possible by disconnecting the dead or failed powerplant from the transmission and removing the drag from the rotor system. One of the most popular types is the sprag clutch. The freewheeling unit allows the engine to drive the rotors but does not allow the rotors to turn the engine. When the engine(s) fail, the main rotor still has a considerable amount of inertia and still tends to turn under its own force and through the aerodynamic force of the air through which it is flying. The freewheeling unit is designed to allow the main rotor to rotate now on its own regardless of engine speed. This principle is the same as being in a car and pushing the clutch in, or putting it into neutral while the car is still moving—the car coasts along under its own force. This occurs regardless of what is done to the accelerator pedal. Fuel System Explain to the student the parts and functions of the fuel system. Figure illustrates a typical gravity feed fuel system. Show the student how to properly check the fuel for water or other contaminants. Also point out to the student the location of the fuel shutoff valve in case of an emergency. If installed, show the student how to operate the hand-operated fuel primer and why a primer if installed for a carburetor engine must be closed and locked for proper engine operation. Explain to the student the purpose and part(s) of the engine fuel control system and the location of the system. Each type of helicopter (reciprocating or turbine engine) requires a different type of fuel control, and each one also has a different type of delivery for the fuel control. 1. Reciprocating engines have a carburetor or are fuel injected. 2. Turbine engines have several types of fuel control systems: a. Full Authority Digital Engine Control (FADEC)— engine is electronically controlled with no mechanical connections. Requires electricity to fully operate and function. b. Mechanical Units—no power is needed, it is all mechanical and is reliable but not as efficient. c. Hydro/Mechanical hybrid units have some characteristics of both. Usually older versions of early attempts at FADEC type systems. Many had a manual reversion capability. |
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