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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Turbine Engines

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Turbine Engines

An aircraft turbine engine consists of an air inlet, compressor,
combustion chambers, a turbine section, and exhaust. Thrust
is produced by increasing the velocity of the air flowing
through the engine. Turbine engines are highly desirable
aircraft powerplants. They are characterized by smooth
operation and a high power-to-weight ratio, and they
use readily available jet fuel. Prior to recent advances in
material, engine design, and manufacturing processes, the
use of turbine engines in small/light production aircraft was
cost prohibitive. Today, several aviation manufacturers are
producing or plan to produce small/light turbine-powered
aircraft. These smaller turbine-powered aircraft typically
seat between three and seven passengers and are referred to
as very light jets (VLJs) or microjets. [Figure 6-22]

Eclipse 500 VLJ.
Figure 6-22. Eclipse 500 VLJ.

Types of Turbine Engines
Turbine engines are classified according to the type of
compressors they use. There are three types of compressors—
centrifugal flow, axial flow, and centrifugal-axial flow.
Compression of inlet air is achieved in a centrifugal flow
engine by accelerating air outward perpendicular to the
longitudinal axis of the machine. The axial-flow engine
compresses air by a series of rotating and stationary
airfoils moving the air parallel to the longitudinal axis. The
centrifugal-axial .ow design uses both kinds of compressors
to achieve the desired compression.

The path the air takes through the engine and how power is
produced determines the type of engine. There are four types
of aircraft turbine engines—turbojet, turboprop, turbofan,
and turboshaft.

The turbojet engine consists of four sections: compressor,
combustion chamber, turbine section, and exhaust. The
compressor section passes inlet air at a high rate of speed to
the combustion chamber. The combustion chamber contains
the fuel inlet and igniter for combustion. The expanding
air drives a turbine, which is connected by a shaft to the
compressor, sustaining engine operation. The accelerated
exhaust gases from the engine provide thrust. This is a basic
application of compressing air, igniting the fuel-air mixture,
producing power to self-sustain the engine operation, and
exhaust for propulsion. [Figure 6-23]

Turbojet engines are limited in range and endurance. They
are also slow to respond to throttle applications at slow
compressor speeds.

A turboprop engine is a turbine engine that drives a propeller
through a reduction gear. The exhaust gases drive a power
turbine connected by a shaft that drives the reduction gear
assembly. Reduction gearing is necessary in turboprop engines
because optimum propeller performance is achieved at much
slower speeds than the engine's operating rpm. Turboprop
engines are a compromise between turbojet engines and
reciprocating powerplants. Turboprop engines are most
efficient at speeds between 250 and 400 mph and altitudes
between 18,000 and 30,000 feet. They also perform well at
the slow airspeeds required for takeoff and landing, and are
fuel efficient. The minimum specific fuel consumption of the
turboprop engine is normally available in the altitude range
of 25,000 feet to the tropopause. [Figure 6-24]

Turbofans were developed to combine some of the best
features of the turbojet and the turboprop. Turbofan engines
are designed to create additional thrust by diverting a
secondary airflow around the combustion chamber. The
turbofan bypass air generates increased thrust, cools the
engine, and aids in exhaust noise suppression. This provides
turbojet-type cruise speed and lower fuel consumption.
The inlet air that passes through a turbofan engine is usually
divided into two separate streams of air. One stream passes
through the engine core, while a second stream bypasses the
engine core. It is this bypass stream of air that is responsible
for the term "bypass engine." A turbofan's bypass ratio refers
to the ratio of the mass airflow that passes through the fan
divided by the mass airflow that passes through the engine
core. [Figure 6-25]