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

WWW.SURVIVALEBOOKS.COMFM 1-506The term "turbo" means "turbine." Therefore, atruboshaft engine is an engine that delivers powerthrough a shaft, which, in turn is powered by a turbinewheel. Army aircraft gas turbine engines are generallythe free-type power turbine turboprop or turboshaftengines. Having a free power turbine enables the poweroutput shaft to turn at a constant speed while the powerproducingcapability of the engine can be varied to accommodatethe increased loads applied to the poweroutput shaft. Turbine engines may also be classified intotwo general groups, centrifugal-flow and axial-flow,depending on the type of compressor used. However,most gas turbine engines in Army aircraft employ acombination of both types.AIR INLET SECTIONThe amount of intake required by a gas turbine engineis approximately 10 times that required by a reciprocatingengine. The air entrance is designed to conduct incomingair to the compressor with minimum energy loss resultingfrom drag or ram pressure loss, that is, the flow of air intothe compressor should be free of turbulence to achievemaximum operating efficieney. Proper design contributesmaterially to aircraft performance by increasingthe ratio of compressor discharge pressure to duct inletpressure.The amount of air passing through the enginedepends on the–Speed of the compressor RPM.Forward speed of the aircraft.Density of the ambient air.Inlets (Figures 3-3, 3-4) are classified as–Nose inlets located in the nose of the fuselage,power lant Pod, or nacelle.Winq inlets– located along the leading edge ofthe wing usually at the root for single-engineinstallations.Annular inlets – encircling in whole or in partthe fuselage or power plant pod or nacelle.Scoop inlets -projecting beyond the immediatesurface of the fuselage or nacelle.Flush inlets – recessed in the side of thefuselage, power plant or or nacelle.Bellmouth inlets– a bell-shaped funnel withcarefully rounded Shoulders, mounted to frontof engine.There are two basic types of air entrances in use:single entrance and divided entrance. Generally, it is bestto use a single entrance with an axial-flowengine to obtainmaximum ram pressure through straight flow. Singleentrance is used almost exclusively on wing or externalinstallations where the unobstructed entrance lends itselfreadily to a single short, straight duct (Figure 3-5).3-3