FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506CHAPTER 8ACCESSORY SYSTEMSAccessories for gas turbine engines can be dividedinto two categories those driven by bleed air taken fromthe compressor section of the engine; those drivenmechanically by an accessory drive shaft and gearboxconnected directly to the turbine shaft. The mechanicalconnection from the turbine shaft may be through anengine-mounted gearbox or through a power takeoffshaft to a remotely mounted gearbox.BLEED-AIR-DRIVEN ACCESSORIESGas turbines are unique among engines. High­-pressure air is available for driving aircraft accessoriesby air motors or air turbines. Compressor dischargeair at high pressure and temperature is bled from theengine through ports provided. This air is ducted as asource of power. It operates the air-conditioning units,hydraulic pumps, thrust reverser actuators, andvarious mechanical actuators in the airplane. Air forcockpit or cabin pressurization is also provided bybleed air from the engine compressor. On multienginedaircraft equipped with pneumatic engine starters, oneengine is usually started from an auxiliary power unitor a ground air source. Air from this operating engineis bled through a system of ducts in the aircraft, topower the starters of the other engines.The Pratt and Whitney dual-axial compressor turbineengine is an example that uses bleed air to operateaccessories. The JT3D turbofan engine is used onaircraft such as the Boeing 707 and B-52 bomber. Thisengine also uses a mechanical accessory gearbox. Itusually has three separate bleed air systems: high pressure,low pressure, and overboard. The high- and lowpressuresystems are used to drive aircraft, enginecomponents, and accessories. The overboard is requiredto preclude compressor instability.Compressor bleed air is also used to anti-ice theengine air inlet guide vanes and, frequently, parts of theair inlet duct. Low-pressure air has a pressure of approximately50 psi and a temperature of more than 300°F.This low-pressure air is taken from bleed air partscompressor mid-section between the low- and high-pressurecompressors. High-pressure bleed air has a pressure ofabout 160 psi and a temperature of more than 650°F whenoperating near sea level. This air is taken from the rearof the high-pressure compressor. The air available fordriving accessories and for other purposes in the aircraftis usually about 3 or 4 percent of the primary engineairflow. Keep in mind the air under pressure that isextracted from the engine is not a bonus. Engine outputand fuel consumption are sacrificed.MECHANICALLY DRIVEN ACCESSORIESThe other method of driving accessories is adirect, mechanical drive operated by gearing from thecompressor-turbine drive shaft. Accessory drives andaccessory mounting pads are provided in an enginemounted,accessory drive gearbox or in a remotelymounted gearbox. On some turbojet engines, accessorypads and mechanically powered drives are also providedin the engine nose section. For dual compressor, axialflowengines, the main accessory drive gearbox usuallyreceives its power from the high-pressure compressordrive shaft. Mechanically driven accessories include:tachometers, generators (alternators), hydraulic pumps,fuel pumps, oil pumps, fuel controls, starters, and (iisome instances) water pumps.LYCOMING T-55 ENGINEThe power extraction system transmits power fromthe N1 and N2 systems to the accessory gearbox locatedat the 6 o’clock position on the inlet housing (Figures 8-1,8-2). Most of these components receive their drivingforce from the N1 system. A minimum amount of poweris extracted from the N2 system.The starter gearbox, mounted at the 12 o’clock position,functions as a centrifuge for air-oil separation duringengine operation. The N1 system provides the drivingforce. A single bevel gear is located at the front end ofthe compressor shaft; meshes with a planetary geartrainhoused in the inlet housing This gear train transmits N1power through two drive shafts: one to the starter gearbox,the other to the accessory gearbox to drive the idlersystem. A gear located on the output power shaft8-1