Gas Turbine Engine - Prelims

Jet propulsion

The force which is generated in the opposite direction to that of a discharge of fluid under pressure, escaping through an opening

Reaction engine

Device utilizing jet propulsion which operates on sir isaac newton's third law of motion

Hero's engine

Consisted of a sphere into which steam was introduced under pressure

Campini Camproni CC-2

One of the first attempts in developing jet powered aircraft resulted in the development of a hybrid design

Heinkel 178

Year earlier than CC-2, the germans had flown their version of a jet powered aircraft with an engine developed by a young german scientist Hans von Ohain

Hans von Ohain

German scientist who designed the engine used for the germans' version of a jet powered aircraft

Sir Frank Whittle

Submitted patents for his jet engine in 1930, but it was not until 1939 that sufficient financial and technical backing was found to enable him to manufacture a flyable version of his engine

Gloster meteor F8

This is powered by later versions of the 'Derwent' engine capable of 3600 lb of thrust

Nene engine

rolls royce culminated this engine which at the time was the most powerful engine in the world, with 5000 lb of thrust

Air breathing and rockets

Two types of propulsion devices

Air breathing

Uses atmospheric air and some fuel as main propellant

Rockets

Carries the entire propellant composed of liquid or solid fuel and the oxygen

Increase the speed of exhaust gases

Increase the quantity of exhaust gases

Jet engine can be increased in two ways

Gas turbine engine

the most practical form of jet engine in use today.

Ramjet

An athodyd which has no rotating parts and consists of a duct with a divergent entry and a convergent or convergent-divergent exit.

aero-thermodynamic-duct

Athodyd

Pulse jet

Unstable as a powerplant due to high fuel consumption and cannot match performance of modern gas turbine engines

Pulse jet

Can be run at static conditions unlike the ram jet and is constructed with a robust construction

Scram jet

is a variant of a ramjet air breathing jet engine in which combustion takes place in a supersonic airflow

Scram jet

Relies on high vehicle speed to compress the incoming air forcefully before combustion with airflow at supersonic throughout the entire engine

Rocket engine

A jet engine that does not use atmospheric air as a propulsive fluid stream (non-air breathing)

Rocket engine

Uses propellants thus enabling outside atmosphere operations

mass

air delivered by the compressor

acceleration

difference between the outlet velocity of the air to that of its inlet velocity due to the addition of the heat energy

Constant pressure

In gas turbine engines, combustion occurs at

turbine of the engine

location where power is developed in a GTE

Brayton cycle

Combustion in the gas turbine occurs at a constant pressure because of the continuous process of the

engine pressure ratio

ratio of turbine discharge pressure to the inlet air pressure

spool

a compressor and turbine are joined on one shaft

turboprop

gas turbine engine that delivers power to a propeller

turboprop

similar in design to turbojet engines except that the power produced is delivered to a reduction gear system that spins a propeller

reduction gearing

necessary in turboprop engines because optimum propeller performance is achieved at much slower speeds than the engine's operating rpm

turboprop engines

used extensively in business and commuter type aircraft because the combination of jet power and propeller efficiency provides good performance characteristics at speeds between 300 and 400 miles per hour

equivalent shaft horsepower

output from a turboprop engine is the sum of the shaft horsepower developed at the turbine, and the residual jet thrust

turboshaft

gas turbine engine that delivers power to a shaft that can drive something else

turboshaft engines

widely used as auxiliary power units and industrial applications to drive electrical generators and surface transportation systems

free power turbine

turboprop employs a _____ to drive the speed reduction gear that drives the propeller at a lower rotation speed

turbofan engine

consists of a multi-bladed ducted propeller driven by a gas turbine engine

turbofans

were developed to provide a compromise between the best features of the turbojet and the turboprop

turbofan engines

have turbojet-type cruise speed capability yet retain some of the short-field takeoff capability of a turboprop

forward-fan engines; aft-fan engines

engines that have the fan mounted in front of the compressor are called _____, while turbofan engines that have the fan mounted to the turbine section are called _____

thrust ratio, bypass ratio, and fan pressure ratio

3 terms that must be familiarized when discussing bypass engines

turbofan engine's thrust ratio

a comparison of the thrust produced by the fan to the thrust produced by the engine core exhaust

turbofan's bypass ratio

ratio of incoming air that bypasses the core to the amount of air that passes through the engine core

fan pressure ratio

pressure leaving the fan to the pressure entering the fan

Low bypass (1:1)

Medium bypass (2:1 or 3:1)

High bypass (4:1 or greater)

turbofans in civil aircraft are generally divided into 3 classifications based on bypass ratio:

unducted fan

utilize titanium, lightweight stainless steel, and composite materials to surpass the fuel economy of several high bypass turbofan engines by more than 15%

ducted ultra high bypass engines

a design encasing the propfan in a conventional cowl-type inlet which can achieve Mach 0.9

hot and cold section

2 groups of the 5 major sections of gas turbine engine

combustion, turbine, and exhaust sections

components under the hot section of turbine engine

air inlet duct and compressor section

components under the cold section of turbine engine

air intake

either built into the airframe itself, if the engine is mounted in the airframe, or is the forward part of the nacelle installation if the engine is wing or rear fuselage mounted

air intake

designed to provide a turbulence free supply of air to the first stage compressor of the engine, with the minimum energy loss occurring through the inlet

uniform pressure

to enable the ocmpressor to operate satisfactorily, the air must reach the compressor at a _____, distributed evenly across the whole of the face of the first stage

pitot type intake

simplest form of intake with a single nentrance circular cross section

pitot type intake

air normally flows directly through the intake when it is fitted to wing mounted engines, but it can form an 'S' shaped duct when used on engines which are mounted at the bottom of the aircraft fin

pitot type intake

maximizes the use of ram effect on the air, due to the aircraft forward speed, and suffers only a minimum loss of ram pressure as the aircraft attitude changes

pitot type intake

its efficiency reduces as the aircraft approaches sonic speed due to the formation of a shock wave at the engine intake lip

divergent duct

the shape of a subsonic intake usually takes the form of a _____

ram pressure recovery point

usually reached between about Mach 0.1 to Mach 0.2 (120-250kph)

secondary intake doors

referred to as auxiliary inlet doors required on certain types of aircraft, to allow a supplementary airflow to reach the compressor face during high power operation when that aircraft is either stationary or at low airspeeds

harrier (harrier jump jet)

an aircraft which must use high levels of engine power when it has either zero or very low forward airspeed during vertical takeoff, hover, or landing

harrier (harrier jump jet)

the engine air intake of this aircraft is designed for relatively high speed flight, and thus, is not necessarily capable of allowing in the large mass of air the engine requires when it is running at high power levels at zero forward airspeed

supersonic aircraft

requires a particular type of air intake, because the engine compressor cannot handle supersonic flow

supersonic diffuser

its task is to control the shock wave which is fundamental to efficiently reduce the velocity of the airflow

central shock cone

some of the earliest types of supersonic aircraft featured a _____, called an inlet cone, which was used to form a shock wave

'convergent divergent' diffuser

this design of supersonic intake works because of the fact that supersonic airflow will slow down as it enters a constricted area

'convergent divergent' diffuser

its objective is to slow the airflow to Mach 1.0 just before the throat of the diffuser

'convergent divergent' diffuser

only suitable for short bursts of supersonic flight, at less than Mach 2.0

center body design

this design has a sharp center body sometimes called a spike

spike

this strikes the airflow producing an oblique shock wave

extended; retracted

the spike will be _____ as the aircraft flies faster and _____ as it slows down. this design is suited to sustain supersonic flight (sr71 blackbird)

bellmouth inlets

have a convergent profile that is designed specifically for obtaining very high aerodynamic efficiency when stationary or in slow flight

bellmouth inlets

typically used on helicopters, some slow moving aircraft, and on engines being run in ground test stands

bellmouth inlet

short in length and has rounded shoulders offering very little air resistance

compressor section

its primary function is to supply air in sufficient quantity to satisfy the requirements of the combustion burners

compressor section

its secondary function is to supply bleed air/customer bleed air for various purposes in the engine and aircraft

compressor pressure ratio

this ratio is the outlet pressure divided by the inlet pressure

compressor speed (rpm)

forward speed of aircraft

density of the ambient air

factors which affects the amount of air passing through the engine

axial flow compressor and centrifugal flow compressor

two types of compressor being used in engines presently available

centrifugal compressor

sometimes called a radiual outflow compressor and is one of the earliest compressor designs and is still used today in some smaller engines and auxiliary power units (APU's)

impeller (rotor)

diffuser

manifold

centrifugal compressors consist of:

single-stage compressors; double-stage compressors

compressors having only one impeller are referred to as _____ while compressors having two impellers are referred to as _____

double-sided or double-entry impeller

two impellers mounted back-to-back

turbine assembly

attached to the compressor shaft, converts the pressure, velocity, and heat of the gases passing through the turbine into mechanical energy, which is used to drive the impeller of the compressor round at high speed

impeller

the turbine assembly, attached to the compressor shaft, converts the pressure, velocity, and heat of the gases passing through the turbine into mechanical energy, which is used to drive the _____ of the compressor round at high speed

diffuser section

a system of stationary divergent ducts which are designed to convert the kinetic energy of the airstream, into potentional energy

axial flow compressor

converts the velocity of the airstream, its kinetic energy, into potential energy or pressure

stage

one row of rotor blades, of airfoil section, which are fastened to a disk, followed by one row of stator vanes also of airfoil section

rotor blades; stator vanes

a stage is one row of _____, of airfoil section, which are fastened to a disk, followed by one row of _____ also of airfoil section

rotor blades

used in an axial flow compressor have an airfoil cross-section with a varying angle of incidence, or twist

twist

this compensates for the blade velocity variation caused by its radius

stator vanes

arranged in fixed rows between the rows of rotor blades and act as diffusers at each stage, decreasing air velocity and raising pressure (fastened to the compressor outer casing)

divergent passages

the spaces between the rotor blades and the stator vanes form _____

inlet guide vanes

the set of stator vanes immediately in front of the first stage rotor blades

inlet guide vanes

direct the airflow into the first stage rotor blades at the best angle while imparting a swirling motion in the direction of engine rotation

pressure stage

each consecutive row of rotor blades and stator vanes constitutes a _____

outlet guide vanes

guide the compressed air into the diffuser to prepare the air mass for combustion

spool

a number of compressor rotor stages on a single shaft, which is driven by a turbine, is termed a _____

air annulus

space between the rotor drum and the compressor outer casing