Major components of the gas turbine engine
Types of jet engines: ramjets and pulsejets
The simplest jet engine of all is the ramjet, which has no moving parts. Such an engine equipped with a fuel metering and injection system. Because a ramjet must be accelerated to a very high speed before it will operate, the engines have limited use.
A pulsejet is a ramjet with a set of shutters, spring-loaded to remain in the closed position normally, placed across the engine’s air inlet. When the engine is launched at a speed sufficient to maintain operation, ram air pressure forces the shutters open. Fuel is injected and burned continuously in the combustion chamber. As soon as the combustion chamber pressure equals the ram air pressure, the shutters close. The combustion gases are ejected through the jet nozzle at the rear, generating thrust. When the pressure in the combustion chamber drops off, the shutters open again, admitting more air. The cycle repeats itself with great rapidity.
Exhaust system
The exhaust system of a jet engine passes the turbine discharge gases to atmosphere at a velocity, and in the required direction, to provide the resultant thrust. Great care must be taken in the design of the exhaust system at the rear of the engine. If the flow of exhaust gases is impeded by too small an exit, temperatures and pressures will be built up inside the engine, while too large an exit will make them fall, and create a loss of thrust.
When afterburning is in operation, the area of the exhaust nozzle can be increased by opening two eyelids that partially obstruct the nozzle aperture when closed. The pilot actuates these eyelids by pneumatic rams which in turn are linked to the fuel supply system. As they open the supply of fuel is increased.
Bypass engines can benefit spectacularly from the use of afterburning. Thrust can be increased by 70% or more for short periods of time. This enables the airplanes to reach an economical cruising height far more quickly than planes not fitted with afterburners.
How airplanes fly
The word «aircraft» means any kind of craft or vehicle which air can support. Airplanes, helicopters and gliders are heavier-than-air craft. They are supported by the dynamic action of the air upon their aerodynamic surfaces. Rockets do not need air for support. They use the power of their reaction engine to propel them through space, and are called «spacecraft».
All heavier-than-air craft use aerodynamic surfaces or airfoils to develop the necessary supporting force. These airfoils are usually in the form of fixed or rotary wings. In order to develop the required lift, the airfoils must move through the air with sufficiently high speed. This speed is imparted to the aircraft by the thrust of its power plant. The thrust may be developed by rotating the pulling or pushing propellers, or by throwing back masses of air by means of gas turbine engines.
To change the altitude and direction of flight aircraft use control surfaces or controls. These comprise the rudder, the elevator, and ailerons. The rudder is used to deflect the movement of the aircraft to the left or to the right. The elevator makes the aircraft climb or dive. The ailerons produce rolling movement.
Tricycle landing gear
Many of the current planes are using a tricycle-type landing gear. In this gear the main landing wheels are located behind the center of gravity and a smaller nose wheel is provided instead of the more conventional tail wheel. In landing the plane comes in with a tail-down attitude, but after contacting the runway and losing speed the plane falls forward until the nose wheel contacts the runway and balances the plane.
For light planes the tricycle gear gives somewhat better visibility especially in taxying and for that reason has become quite popular. The larger planes also have better taxying visibility and in addition the plane is in a more level position for loading. The more level attitude of the plane on the ground is also a good feature for passenger-carrying planes, as the passengers are not inconvenienced by excessive changes in position, particularly annoying in sleeper planes.
The larger transports use the tricycle gear almost exclusively, for normally it puts less stress on the fuselage in landing. Also, the level landing attitude of the plane presents less frontal area than the tail-down-plane, thereby reducing the initial drag when starting the take-off. Usually these larger planes are equipped with an emergency tail wheel or bumper to prevent damage to the aft fuselage sections in case the tail accidentally hits the ground.
Retractable landing gear
Most retraction systems are hydraulically operated, though some are electrically operated or even manually operated. This adds weight and complexity to the design. In retractable gear systems, the compartments where the wheels are stowed are called wheel wells, which may also diminish valuable cargo or fuel space.
Red lights indicate the gear is in the up-locked position; amber lights indicate that the landing gear is in transit (neither down and locked nor fully retracted).
Multiple redundancies are usually provided to prevent a single failure from failing the entire landing gear extension process.Whether electrically or hydraulically operated, the landing gear can usually be powered from multiple sources. In case the power system fails, an emergency extension system is always available. This may take the form of a manually operated pump or a mechanical free-fall mechanism which allows the landing gear to fall due to gravity.
Environmental protection
Environmental problems people face globally are too serious and too complex to be ignored. They may affect different communities, population groups and countries in different ways. Aircraft emissions were recognized as a problem for the aviation industry in the early 1980s when interest in the acid rain and ozone depletion increased.
There are four main types of emission from aircraft engines: carbon monoxide (CO), unburned hydrocarbons (UHC), carbon dioxide (CO2) and nitrogen oxides (NOx). CO2 is produced directly from the combustion of fuel. Modern engines have better fuel economy and hence produce less CO2 than early jet engines or engines built without regard for efficiency. NOx is the most significant emission. During an average flight, 80% of the pollution is NOx, produced by heating air to high temperatures as it passes through the engine combustion chamber.
We are permanently exposed to noise levels, which endanger our health and quality of life. Noise is becoming a major pollutant: the constant noise in the street, and even inside buildings, laying loud music, noise from transport means etc. People’s activities produce aggressive noises that cause an enormous stress on the nervous system; it is the cause of health hazard for entire population of the world.
Turbojet engines
Turbojets are classified according to the kind of compressor they use. Centrifugal compressors operate by taking in air near a hub at the centre and rotating it with an impeller.
As the impeller whirls the air at high speed, centrifugal force carries the air to the perimeter of the impeller at a considerable velocity. Here the air is collected in a diffuser to increase the pressure, and then led to a manifold which, in turn, feeds it to the engine’s burners.
The majority of today’s turbojets use an axial compressor. Axial compressors are used especially in the larger engines, because they are capable of producing high compression ratios, sometimes as high as 13:1, or more. An axial compressor, as the name implies, compresses air as it flows in an axial direction through an engine. A series of rotating blades and stationary vanes work on the air as it passes through a series of stages inside the compressor. Each stage adds to the compression process.
There are two types of axial-compressor engines, those with so-called single compressors and those with dual compressors. In dual-compressor engines (sometimes called twin-spool engines) there are two compressors that are mechanically independent of one another, although they are related as to airflow.
Rocket engine
There are two general types of rockets: the solid-propellant rocket and the liquid or bipropellant rocket. Rocket fuels and oxidizers are called propellants.
Solid-propellant rockets are simplest in arrangement and least susceptible to control. Liquid-propellant rockets are somewhat more complex than the solid type, but they are susceptible to a much greater range of control during operation. This control is accomplished by varying the rate of flow of the fuel and oxidizer to the combustion chamber.
The rocket engine consists of a propellant injector, combustion chamber surrounded by a cooling jacket, and a nozzle to allow the natural expansion of the combustion gases.
The rocket engines all operate on the same principle whether they are solid-fuel or liquid-fuel types.
Aircraft maintenance
Aircraft maintenance is the overhaul, repair, inspection or modification of an aircraft or aircraft component.
In Canada, maintenance includes the installation or removal of a component from an aircraft or aircraft subassembly.
Maintenance may include such tasks as ensuring compliance with Airworthiness Directives or Service Bulletins.
At the completion of any maintenance task a person authorized by the national airworthiness authority signs a release stating that maintenance has been performed in accordance with the applicable airworthiness requirements. In the case of a certified aircraft this may be an Aircraft Maintenance Engineer or Aircraft Maintenance Technician, while for amateur-built aircraft this may be the owner or builder of the aircraft.
Major components of the gas turbine engine
Any gas turbine engine consists of three main sections: the compressor section, the combustion section and the turbine section; air enters the engine through the inlet duct and guide vanes.
The function of the inlet duct and guide vanes is to supply air to the engine inlet under all conditions of operation. From the inlet duct the air flows to compressor. The compressor is located after the inlet duct. The function of the compressor is to compress air and to provide the required pressure ratio. From the compressor section through the diffuser compressed air flows to the combustion section. There the combustion chamber is located.
The function of the combustion chamber is to expand the air by burning fuel in the air stream.
The turbine section of the engine is located after the combustion chamber between two additional components: the nozzle diaphragm and exhaust cone. The turbine section is designed to extract power from the jet stream to drive the compressor and accessories.
The diffuser, the nozzle diaphragm, the exhaust cone, afterburner and the accessory section serve to provide transition from one main section to another. The diffuser directs air from compressor to the combustion chamber and serves to change pressure and velocity.