1. Basic Mechanics
1.1 Introduction
1.2 Principles of jet propulsion
1.3 Methods of jet propulsion
1.1 Introduction
The development of the gas turbine engine as an aircraft power plant has been so rapid that
it is difficult to appreciate that prior to the 1950s very few people had heard of this method of
aircraft propulsion.The possibility of using a reaction jet had interested aircraft designers for a
long time, but initially the low speeds of early aircraft and the unsuitably of a piston engine for
producing the large high velocity airflow necessary for the " Jet " presented many obstacles.
A French engineer, Rene Lorin, patented a jet propulsion engine in 1913, but this was an
athodyd and was at that period impossible to manufacture or use, since suitable heat resisting
materials had not been then developed and, in the second place, jet propulsion would have
been extremely inefficient at the low speeds of the aircraft of those days. However, today the
modern ram jet is very similar to Lorin's conception.
Lorin's Jet Engine.
In 1930 Frank Whittle was granted his first patent for using a gas turbine to produce a propulsive
jet, but it was eleven years before his engine completed its first flight. The Whittle formed the
basis of the modern gas turbine engine, and from it was developed the Rolls-Royce Welland,
Derwent, Nene and Dart engines. The Derwent and Nene turbo-jet engines had world-wide
military applications.
A Whittle-Type Turbo-Jet Engine
Although appearing so different from the piston engine-propeller combination, applies the same
basic principles to effect propulsion. Both propel their aircraft solely by thrusting a large wwight
of air backwards.
1.2 Principles of Jet Propulsion
The earliest known example of jet engine is that of Hero's engine. This device showed how the
momentum of stream issuing from a number of jets could impart an equal and opposite reaction
to the jets themselves, thus causing the engine to revolve.
Jet propulsion is a practical application of Sir Isaac Newton's third law of motion which states
that, " for every force acting on a body there is an opposite and equal reaction." For aircraft
propulsion, the " body " is atmospheric air that caused to accelerate as it passes through the
engine. The force required to give this acceleration has an equal effect in the opposite direction
acting on the apparatus producing the acceleration.
Propeller.
The mechanism rotates by virtue of the reaction to the water jets.
due to the reaction of the water jet, the hose cannot be held or controlled by one fireman.
Jet reaction is definitely an internal phenomenon and dose not, as in frequently assumed,
resulted from the pressure of the jet on the atmosphere. In fact, the jet propulsion engine,
whether rocket, athodyd, or turbo jet, is a piece of apparatus designed to accelerate a stream of air or gas and to expel it at high velocity.
There are, of course, a number of ways of doing this, but in all instances the resultant reaction or thrust exerted on the engine is proportional to the mass or weight of air expelled by the engine and to the velocity change imparted to it. In other words, the same thrust can be provided either by giving a large mass of air a little extra velocity or a small mass of air a large extra velocity. In practice, the former is preferred, since by lowering the jet velocity relative to the atmosphere a higher propulsive efficiency is obtained.
1.3 Methods of Propulsion
The types of jet engines, whether ram jet, pulse jet, rocket, gas turbine, turbo/ram jet or
turbo-rocket, differ only in the way in which the " thrust provider ", or engine, supplies and
converts the energy into power for flight.
The ram jet engine is the athodyd, or aero-thermodynamic-duct to give it its full name. It has no
major rotating parts and consists of a duct with a divergent entry and a convergent or
convergent-divergent exit. When forward motion is imparted to it from an external source, air
is forced into the air intake where it loses velocity or kinetic energy and increase its pressure
energy as it passes through the diverging the duct. The total energy is then increased by the
combustion of fuel, and the expanding gases accelerate to atmosphere through the outlet duct.
A ram jet is often the power plant for missiles and target vehicles; but is unsuitable as an
aircraft power plant " because it requires forward motion imparting to it before any thrust is
produced."
Pulse Jet Engine
The pulse jet engine uses the principle of intermittent combustion and unlike the ram jet it can be
run at a static condition. The engine is formed by the aerodynamic duct similar to the ram jet,
but due to the higher pressure involved, it is of more robust construction. The duct inlet has a
series of inlet " valves " that are spring-loaded into the open position. Air drawn through the
open valves passes into the combustion chamber and is heated by the burning of fuel injected
into the chamber. The resulting expansion causes a rise pressure, forcing the valves close, and
the expanding gases are then ejected rewards. A depression created by the exhausting gases
allows the valves to open and repeat the cycle.
Pulse jets have been designed for helicopter rotor propulsion and some dispense with inlet
valves by careful design of the ducting to control the changing pressure of resonating cycle.
The pulse jet is unsuitable as an aircraft power plant because it has a high fuel consumption and
unable to equal the performance of the modern gas turbine engine.
Although the rocket engine is a jet engine, it has one major difference in that does not use
atmospheric air as the propulsive fluid stream. Instead it produces its own propelling fluid by the
combustion of liquid or chemically decomposed fuel with oxygen, which it carries, thus enabling
it to operate outside the earth's atmosphere. It is, therefore, only suitable for operation over
short periods.
The application of the gas turbine to jet propulsion has avoided the inherent weakness of the
rocket and the athodyd, for by the introduction of a turbine-driven compressor a means of
producing thrust at low speeds is provided. The turbo-jet engine operates on the
" working cycle ". It draws air from the atmosphere and after compressing and heating it.
At aircraft speeds below approximately 450 miles per hour, the pure jet engine is less efficient
than a propeller-type engine. The propeller efficiency decreases rapidly above 350 miles per
hour due to the disturbance of the airflow caused by the high blade-tip speeds of the propeller.
Turbo/Ram Jet Engine
The turbo/ram jet engine combines the turbo-jet engine (which is used for speeds up to Mach 3)
with the ram jet engine, which have good performance at high Mach numbers. The engine is
surrounded by the duct that has a variable intake at the front and an afterburning jet pipe with
a variable nozzle at the rear. During take-off and acceleration, the engine functions as a
conventional turbo jet with the afterburner lit; at other flight conditions up to 3 Mach, the
afterburner is inoperative. As the aircraft accelerates through Mach 3, the turbo-jet is shut down
and the intake air is diverted from the compressor, by guide vanes, and ducted straight into the
afterburning jet pipe, which becomes a ram jet combustion chamber. This engine is suitable
for an aircraft requiring high speed and sustained high Mach number cruise conditions where
the engine operates in the ram jet mode.
2. Working Cycle and Airflow
2.1 Introduction
2.2 Working Cycle
2.3 The Relations between Pressure, Volume, and Temperature.
2.4 Changes in Velocity and Pressure
2.5 Airflow
2.1 Introduction
The gas turbine engine is the essentially a heat engine using air as a working fluid to provide
thrust. To achieve this, the air passing through the engine to be accelerated; this means that
velocity of kinetic energy of the air is increased. To obtain this increase, the pressure energy is
the first of all increased, followed the addition of heat energy, before final conversion back to
kinetic energy in the form a high velocity jet efflux.
2.2 Working Cycle
The working cycle of the gas turbine engine is similar to that of the four-stroke piston engine.
However, in the gas turbine engine, combustion occurs at a constant pressure, whereas in the
piston engine it occurs at a constant volume.
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