HYDRAULIC INTRODUCTION AND PLUMBING LINES

2.1 INTRODUCTION  

The term "hydraulic plumbing" refers to process of forming, installing as well as associated repairing/maintenance of the hose, tubing, fittings, and connectors used in the aircraft hydraulic system.
Occasionally it may be necessary to repair or replace damaged aircraft hydraulic plumbing lines. Very often the repair can be made sim ply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabri-cated. Replacement tubing should be of the same size and material as the original line. All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subjected. If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object. All tubing failures should be carefully studied and the cause of the failure determined.
This week will also highlight plumbing connectors.

2.2 PLUMBING LINES :
Aircraft plumbing lines usually are made of metal tubing and fittings or of flexible hose. Metal tubing is widely used in aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines. Flexible hose is generally used with moving parts or where the hose is subject to considerable vibration.
Generally, aluminum alloy or corrosion-resistant steel tubing have replaced copper tubing. The high fatigue factor of copper tubing is the chief reason for its replacement. It becomes hard and brittle from vibration and finally breaks, however it may be restored to its soft annealed state by heating it red hot and quenching it in cold water. Cooling in air will result in a degree of softness but not equal to that obtained with the cold water quench. This annealing process must be accomplished if copper tubing is removed for any reason. Inspection of copper tubing for cracks, hardness, brittleness and general condition should be accomplished at regular intervals to preclude failure. The work ability, resistance to corrosion, and lightweight of aluminum alloy are major factors in its adoption for aircraft plumbing.

In some special high-pressure(3,000 p.s.i.) hydraulic installations, corrosion resistant steel tubing, either annealed or 1/4 -hard, is used. Corrosion resistant steel tubing does not have to be annealed for flaring or forming; in fact, the flared section is somewhat strengthened by the cold working and strain hardening during the flaring process. Its higher tensile strength permits the use of tubing with thinner walls; consequently the final installation weight is not much greater than that of the thicker-wall aluminum alloy tubing.

1.3 THE AEROFOIL AS AN AERODYNAMIC SURFACE

1.3 THE AEROFOIL AS AN AERODYNAMIC SURFACE

1.3.1 General: In its simplest sense an aero foil section may be defined as that profile designed to obtain a desirable reaction from the air through which it moves. In other words, an aerofoil is able to convert air resistance into a useful force that produces lift for flight.
The cross-section of an aircraft wing is a good example of an aerofoil section, where the top surface usually has greater curvature than the bottom surface.

The air travelling over the cambered top surface of the aerofoil shown in Figure 1.4, which is split as it passes around the aerofoil, will speed up, because it must reach the trailing edge of the aerofoil at the same time as the air that flows underneath the section. In doing so, there must be a decrease in the pressure of the airflow over the top surface that results from its increase in velocity (Bernoulli’s principle).

1.3.2 Aerofoil terminology: 
We have started to talk about such terms as: camber, trailing edge and AOA without defining them fully. Set out below are a few useful terms and definitions about airflow and aero foil sections that are frequently used frequently throughout the discussion of generation of aerodynamic forces on aerofoil sections. (Figure 1.5, 1.6)

Camber is the term used for the upper and lower curved surfaces of the aerofoil section. Where the mean camber line is that line drawn halfway between the upper and lower cambers.

edges. Note that this line may fall outside the aerofoil section dependent on the amount of camber of the aerofoil being considered.

Leading and trailing edge are those points on the centre of curvature of the leading and trailing part o f the aero foil section that intersect with the chord line as shown in Figure1.6

Angle of incidence (AOI) is the angle between the relative airflow and the longitudinal axis of the aircraft. It is a built-in feature of the aircraft and is a fixed "rigging angle". On conventional aircraft, the AOI is designed to minimize drag during cruise thus maximizing fuel consumption!

Angle of attack (AOA) is the angle between the chord line and the relative airflow. This will vary, dependent on the longitudinal attitude of the aircraft, with respect to the relative airflow as you will see later.

Thickness/chord ratio (t/c) is simply the ratio o f the maximum thickness o f the aerofoil section to its chord length normally expressed as a percentage. It is sometimes referred to as the fineness ratio and is a measure of the aerodynamic thickness of the aerofoil.

The aerofoil shape is also defined in terms of its t/c ratio. The aircraft designer chooses that shape which best fits the aerodynamic requirements of the aircraft.

Light aircraft and other aircraft that may fly at low velocity are likely to have a highly
cambered thick aerofoil section; where the air flowing over the upper camber is forced to travel a significantly longer distance than the airflow travelling over the lower camber. This results in a large acceleration of the upper airflow significantly increasing speed and correspondingly reducing the pressure over the upper surface.

These high lift aerofoil sections may have a t/c ratio of around 15%, although the point of maximum thickness for these high lift aero foils can be as high as 25-30%. The design will depend on whether forward speeds are of more importance compared to maximum lift. Since, it must be remembered that accompanying the large increase in lift that thick aerofoil sections bring, there is also a significant increase drag. However, thick aerofoil sections allow the use of deep spars and have other

advantages, such as more room for fuel storage and for the stowage of the undercarriage assemblies.
Thin aerofoil sections are preferred on high speed aircraft that spend time flying at transonic and supersonic speeds. The reason for choosing slim wings is to reduce the time spent flying in the transonic range, where at these speeds the build up of shockwaves create stability and control problems. We need not concern ourselves here with the details of high speed flight; this will be addressed comprehensively in outcome 3.

However, it is worth knowing that the thinner the aerofoil sections then the nearer to sonic speed an aircraft can fly before the effects of shockwave formation take effect. What limit the fineness ratio of aerofoil sections is their structural strength and rigidity, as well as providing sufficient room for fuel and the stowage of the undercarriage. A selection of aerofoil sections is shown in Figure 1.7.

Concorde has an exceptional fineness ratio (3-4%) because of its very long chord length resulting from its delta wings. It can therefore alleviate the problems of flying in the transonic range as well as providing sufficient room for fuel and the stowage of its undercarriage assemblies. In general, fineness ratios (t/c ratios) of less than 7% are unusual (Figure 1.8).


With regard to the under surface alterations in the camber have less effect. A slightly concave camber will tend to increase lift, but convex cambers give the necessary thickness to allow for the fitment of deeper and lighter spars. The convex sections are also noted for limiting the movement of the centre of pressure (CP).

This limitation is most marked where the lower camber is identical to that of the upper camber giving a symmetrical section. Such sections have been adopted for medium and high speed main aerofoil sections and for some tail plane sections.

1.3.3 Aerofoil Profiles: There are different Profiles & Shapes of aerofoil. Profile of an aerofoil gives a shape to an aerofoil. According to the profile, shapes may be:

• Double convex symmetrical

• Double convex non-symmetrical

• Convex upper and concave lower surface.

• Convex Upper and flat lower surface

Figure 1.9 shows different profiles/shapes of aerofoil.

1.2 GENERATION OF AERODYNAMIC REACTIONS

  GENERATION OF AERODYNAMIC REACTIONS

1.2.1 Aerodynamic reaction due to airflow: Flight forces are the result of aerodynamic reactions. Aerodynamic reaction is the effect of airflow over the aerofoil. In theory of flight, aerodynamics is mainly concerned with three distinct things:

• Aerofoil
• Relative Air Velocity Or Relative Wind
• Atmosphere

When an aircraft flies, air of the atmosphere flows over its wings. Wings are the aero foils. Air velocity over it is the relative wind. Result of the aerodynamic reaction is the force called Total Air Reaction (TAR) is a force. Lift and Drag are two components of TAR. Generations of these forces are related to the aerodynamic characteristic of the aerofoil in relation to the relative airflow over and under it.

Before explaining production of Lift and Drag forces due to aerodynamic reactions on an aerofoil, effect airflow over a simple flat plate will be explained first. Then, a curbed plate will be taken for better effect and finally aerofoil will be taken as a practical aerodynamic surface.

1.2.2 Effect of airflow Flow over a flat plate: When a body is moved through the air, or any fluid that has viscosity, such as water, there is a resistance produced which tends to oppose the body. For example, if you are driving in an open top car, there is a resistance from the air acting in the opposite direction to the motion of the car. This air resistance can be felt on your face or hands as you travel. In the aeronautical world, this air resistance is known as drag. It is undesirable for obvious reasons. For example, aircraft engine power is required to overcome this air resistance and unwanted heat is generated by friction as the air flows over the aircraft hull during flight.

We consider the effect of air resistance by studying the behaviour of airflow over a flat plate. If a flat plate is placed edge on to the relative airflow (Figure 1.1), then there is little or no alteration to the smooth passage of air over it. On the other hand, if the plate is offered into the airflow at some angle of inclination to it angle of attack (AOA), it will experience a reaction that tends to both lift it and drag it back. This is the same effect that you can feel on your hand when placed into the airflow as you are travelling, e.g. in the open topped car mentioned earlier. The amount of reaction depends upon the speed and AOA between the flat plate and relative airflow.

Figure 1.1: Airflow over a flat plate 

As can be seen in Figure 1.1, when the flat plate is inclined at some AOA to the relative airflow, the streamlines are disturbed. An upwash is created at the front edge of the plate causing the air to flow through a more constricted area, in a similar manner to flow through the throat of a Venturimeter. The net result is that as the airflows through this restricted area, it speeds up. This in turn causes a drop in static pressure above the plate (as explained in the Bernoulli’s principle) when compared with the static pressure beneath it resulting in a net upward reaction. After passing the plate, there is a resulting downwash of the air stream.

The total reaction on the plate caused by it disturbing the relative airflow has two vector components as shown in Figure 1.2. One at right angles to the relative airflow known as lift and the other parallel to the relative airflow, opposing the motion, known as drag.

The above drag force is the same as that mentioned earlier, which caused a resistance to the flow of the air stream, over your hand.

Figure 1.2: Nature of reaction of relative airflow on flat plate

The effect may be summarized as follows: if a flat plate is inclined in a moving stream of air, the air flowing over the upper surface decreases in pressure. This creates a depression over the upper surface which produces a sucking effect on the plate. At the same time, the higher pressure on the underside of the plate produces an upward force.

Lift is the force which overcomes the weight of the aircraft which acts vertically downwards. We are interested in how much of the lifting force is acting vertically upwards. This is done by splitting the total reaction into two component forces, one vertical and one horizontal. The horizontal force adds to the total drag on the aircraft and is referred to as lift induced drag.

1.2.3 Production of lift on a curbed plate:

An improvement on the flat plate is one with a curved front. This tends to produce a smoother flow of air over the upper surface and produces a total reaction, which is nearer to the vertical. This gives higher lift and lower drag. See Figure 1.3.

The airflow across a curved plate remains smooth at higher angle attack than the flat plate.

1.2.4 Lift on an aerofoil: Flat and curved plates have little strength and resistance to bending and torsion and are not suitable for aircraft wings. The modern aerofoil required depth of section to resist bending and must form a box structure to resist torsion whilst still retaining the basic curved plate shape.

Flow maintains the shape of the body over which it is flowing over the aerofoil set at an AOA much greater than that could be set for a flat of curbed plate. As a result, a greater lift force may be created keeping drag to a minimum value. As is illustrated in Figure 1.4, stream line flow is maintained over the surface of an aerofoil where it can be seen that the successive cross sections are represented by lines that run parallel to one another hugging the shape of the body around which the fluid is flowing. Aerofoil is further discussed in subsequent sections.

1.1 INTRODUCTION

1.1 INTRODUCTION

Atmospheric air, aerofoil and the relative movement constitutes the basis of flight forces that keeps the aircraft airborne. Manipulation of the flight forces is the key to flight control and flight manoeuvre.

This week highlights the underpinning principles of flight forces and the factors affecting them.

1.5 THRUST EQUATION

1.5 THRUST EQUATION

1.5.1 Momentum Thrust: If the condition (area A, pressure P and velocity V) at the engine intake and exhaust are designated with the subscripts 'a' and 'j' respectively, then a mass of 'air (m) flowing per unit time through the engine will experience an:

 Increase in velocity   = (V_j - V_a).

The momentum gain = m (V_j - V_a), where m is the mass flow rate of air through the engine under steady condition.              

= rate of change of momentum

= Applied force to the air mass flow as per Newton’s 2^nd Law of motion.

According to Newton's Third Law, for every action, there is an equal and opposite reaction. Therefore as the air mass is accelerated through the engine, there will be an equal and opposite reaction (thrust) acting on the engine in the forward direction. Since the force is obtained due to a change in momentum of the air, this is called the Momentum Thrust of the engine.

Momentum Thrust         = m (V_j - V_a)

=  m V_j - m V_a

Consideration may be given to the fuel mass flow rate (m_f) that is mixing with air at combustion chamber with initial zero velocity relative to the engine, the thrust equation may be modified as follows:

Momentum Thrust         =   (m + m_f )V_j –  m V_a

= m (V_j- V_a ) + m_f V_j

1.5.2 Pressure Thrust: Considering the engine as a physical body in the air, it will be subjected to pressures acting at the intake (P_a) and the exhaust (P_j). The pressures will produce a pressure force of (P_j - P_a)A_j acting on the engine in the forward direction. This force is the result of an unbalanced pressure and is called the Pressure Thrust. Hence,

Pressure Thrust   = (P_j - P_a)A_j

In most practical cases, pressure thrust exists because all of the pressure of the engine cannot be converted into velocity at the exhaust (i.e. gas does not fully expanded to atmospheric pressure). It becomes more pronounced and significant as the speed of the aircraft becomes supersonic and the exhaust nozzle becomes choked. At choked nozzle condition, velocity of exhaust gas cannot exceed M =1, unless it is a C-D duct and invariably there remains significant amount of unconverted pressure.

1.5.3 Total Thrust: The Total Thrust on a jet engine will be the sum of the momentum thrust and the pressure thrust.

Total Thrust         = Momentum Thrust + Pressure Thrust

T_t                = m (V_j- V_a ) + m_f V_j + (P_j – P_a) A_j

In actual practice, fuel flow is usually neglected when net thrust is computed, because the weight of the air that leaks from various section of the engine is assumed to the approximately equivalent to the weight of the fuel consumed. Therefore, the final equation for computing the thrust by a turbo-jet engine becomes:

T_t                = m (V_j- V_a ) +  (P_j – P_a) A_j

This is a general thrust equation and is applicable for all kinds of jet propulsion.

1.5.4 Gross Thrust, Momentum-Drag and Net Thrust: An analysis of the total thrust of a jet engine will show that it can be grouped into two parts.

T_t                = [m V_j + (P_j - P_a)A_j] – [mV_a]

The forward part composed of the exhaust jet momentum [mV_j] and the pressure thrust (P_j-P_a)A_j and is called the Gross Thrust of the engine, i.e. thrust developed by the engine. The rear part is the momentum force of the incoming air impinging on the engine intake and is called the Momentum Drag. Hence the total thrust is the difference of the gross thrust and the momentum drag and it is also called the Net Thrust (actual thrust) of the engine. Hence,

T_gross            = m V_j + (P_j - P_a)A_j

D_momentum         =  mV_a

Net Thrust = Gross Thrust - Momentum Drag

Gross thrust is actually the thrust at the static aircraft, with aircraft speed zero.

1.5.5 Power of aircraft gas turbine engines:  

Turbojet engines are rated on the basis of takeoff thrust generated at standard atmospheric conditions. This is conventional, because output of turbojet engines for the aircraft is THRUST (propulsive force).

Gas turbine engines for turboprop are the torque turbine engines and the output of the engine is in the form of TORQUE on the shaft. Hence, the rating of the engine is the Shaft Horse Power expressed in BHP.

For comparison purpose, thrust of the turbojets may be converted into horse power, called Thrust Horse Power (THP).

THP =  

For turboprop aircraft, total power is the summation of BHP at the engine output shaft (input to the propeller) and the THP from the exhaust thrust. The summation of these two is termed as ESHP (equivalent shaft horsepower).

1.4 PHYSICS OF FORCE, WORK, POWER, VELOCITY AND ACCELERATION

PHYSICS OF FORCE, WORK, POWER, VELOCITY AND ACCELERATION

1.4.1 General: Physics relating to the force, work, power, velocity and acceleration defines the quantities and establishes their relationship with mathematical treatment. This helps us establishing thrust formula, power and work expression for aircraft gas turbine engines. 

1.4.2 Force: Force is defined as external influence acting on a body to make a change in the state of rest or state of motion of the body. It is a vector quantity having a magnitude and direction of action.

1.4.3 Velocity: This is the change in speed of a moving body per unit time in a specific direction of motion. This is a vector quantity having a magnitude and the direction.

1.4.4 Acceleration: This is the rate of change of velocity of a moving body.

1.4.5 Relationship of Force, Velocity and Acceleration: The relationship is established on the basis of Newton’s Laws of motion. There are 3 Laws stating the nature of state of rest and state of motion of a body. These Laws are as follows:

First Law: "A body will continue its state of rest or of uniform motion in a straight line unless compelled by some external force to change its state."

This is actually the law of INERTIA. It is due to the inertia that body at rest tends to remain at rest, and a body in motion tends to continue its motion with the same velocity (speed and direction), in a straight line. The law expresses the necessity of an external force to overcome the effect of inertia.

An aircraft in level flight (cruise) is under zero resultant force, but it continues to fly at constant speed and direction due to inertia. To accelerate (or decelerate) the aircraft, Pilot must increase throttle to create extra thrust as an unbalanced force.

This law has a close relationship with ‘momentum’. Momentum is the product of mass (m) and velocity (V) and is a vector having magnitude and direction. The direction of momentum of a body is the same as the direction of related motion.

From Newton's first law, under no external force, momentum of a body is constant (either ZERO or a non-zero constant quantity). To change the momentum, an external force must act on to the body.

How much force will be required to make a change in motion or momentum, or, how much change in momentum will be effected by a force, is expressed in the 2^nd Law.

Second Law: "The rate of change of momentum of a body is proportional to the applied force and takes place in the direction in which the force acts."

This law states the relationship between the force applied to an object and the resultant change of momentum in that direction.

Normally, the mass of an object is constant and the relation becomes:

F = (mv-mu)/t = m(v-u)/t =ma

Where, m is the mass, u is the initial velocity, v is the velocity after t second, F is 

the applied force acting in the direction of motion, a is the acceleration, mu is the initial momentum, mv is the momentum after t seconds.

This formula has direct application in mathematical treatment of jet-propulsion of an aircraft gas turbine engine.

Third Law: "For every action, there is an equal and opposite reaction."

This law gives the mutual relationship between bodies acting on each other with or without contact. The action and reaction always exist in a pair.

The condition of a book resting on a table will produce an action and reaction pair. The weight of the book will exert a force on the top of the table, and the table will exert a lift on the book to prevent it from falling down under gravity.

In the physics of jet-propulsion, the 2^nd Law is used in mathematical formulation of the action force applied by the engine on to the working fluid (air and gas flow) undergoing change in momentum. According to the 3^rd law, there is a reaction pair of this action force applied on to the engine by the gas. This reaction is the propulsive force or the thrust. Thus, the 2^nd law action force formula is taken as the reaction force (thrust) formula. 

1.4.6 Work: Work is a quantity found by multiplying force acting on a body and the distance through which the body has displaced in the direction of the force due to its action. It is a scalar, having only the quantity. If there is no displacement in the direction of force, it is said that the force has not performed any work, or the work performed is zero.

1.4.7 Energy: This is the capacity of doing work.

1.4.7 Power: Rate of doing work by applying force is called power. 

1.3 THEORY OF JET PROPULSION

 

 THEORY OF JET PROPULSION    

1.3.1 General: Jet propulsion is the method of producing propulsive force in a device by the reaction of an accelerating mass of air (or gas) expelled out through a nozzle in the form of a jet. The generated propulsive force is used to propel the device (or the aircraft) forward in the air.

 

1.3.2 Basic Principle: Jet propulsion is a practical application of Sir Isaac Newton's 3^RD Law of Motion which states that: "For every action, there is an equal and opposite reaction."

For aircraft propulsion, the 'body' is atmospheric air that is accelerated as it passes through the engine. The force applied to the air giving this acceleration (or changing momentum) has an equal effect in the opposite direction onto the engine. The effect by the accelerating air coming out of the engine through its propelling nozzle in the form of a jet is the ‘jet reaction’ which is conventionally termed as the ‘thrust’.

Jet reaction is an internal phenomenon and does not result from the pressure of the jet acting on the atmosphere as shown in balloon example, Figure 1.4, depicting a non-mathematical or mechanical approach of justifying jet-propulsion. A turbo-jet engine could be considered as such an arrangement as the compressor and combustion chamber sections having high pressure air acting on all surfaces, this pressure being dropped through the exhaust pipe, hence, unbalance pressure forcing the engine forward internally similar to the toy balloon. 

1.3.3 Operating Principle: To have jet propulsion based on Newton's Third Law, jet-engines are designed for producing high-velocity gases at the jet-nozzle. To achieve this, a jet-engine first compresses air. Heat is then added to the compressed air in the combustion chamber by burning fuel to produce hot expanding gases that rush towards the rear of the engine and finally escapes through jet-nozzle in a form of high-velocity ‘kinetic jet’. 

All kinds of jet engines, like turbo-jets, ram-jets, pulse-jets etc are designed for the sole purpose of producing high-velocity gases at the jet-nozzle so that reaction forces come into play as a result of jet-reaction. But, propulsive force is also possible by propellers and fans. The basic principle is same, that is, accelerating or changing momentum of air. So, these are also called prop-jets and fan-jets, similar to the turbo-jets. 

Figure 1.4: Toy-balloon experiment (explaining jet-propulsion as in internal phenomena)

1.2 THEORY OF GAS TRUBNINE ENGINE

 THEORY OF GAS TRUBNINE ENGINE  

Gas Turbine Engines used for aircraft propulsion are broadly speaking jet-producing devices where a working fluid undergoes a series of thermodynamic processes. These processes are, if it is taken ideally, isentropic compression (in air inlet diffuser and compressor), heat addition (in combustion chamber), isentropic expansion (in turbine and propelling nozzle). Thermodynamic cycle for gas turbine engines comprising these processes is the Joule/Brayton Cycle.  Ref: Figure 1.1 (a).

Although the diagram in Figure 1.1 (a) is a closed cycle, aero engines works actually on open cycle as it is clear in Figure 1.1 (b).

Figure 1.1

Objective of this cyclic performance of the working fluid is to produce a net propulsive force that is used by the aircraft for its flight through the atmosphere overcoming the drag force. Different types of engines use this working fluid differently to have the same end result.

When a propeller turbine is used, the net shaft work (W₃₄ + W₁₂) is simply supplied to the airscrew (i.e. propeller). If propulsion is by jet, the turbine is required to supply merely the compressor work and it uses only part of the expansion to atmospheric pressure, from 3 to 5. The remaining expansion, from 5 to 4, occurs in the propulsion nozzle. Ref. Figure 1.1(b).

Cyclic processes consisting the Brayton cycle are executed in different and separate working zones or sections as illustrated in Figure 1.2. These sections are the basis of constructional build up of a turbine engine.

Figure 1.2

The mechanical arrangement of the gas turbine engine is simple, for it consists of only two main rotating parts, a compressor and a turbine, and one or more combustion chambers. To these three basic parts are added intake at the front and an exhaust unit at the rear. See Figure 1.3 illustrating a gas turbine engine (turbojet) for the aircraft.

Figure 1.3

How this arrangement of engine sections, producing propulsive force generates propulsive force is the theory of jet propulsion.  

1.1 INTRODUCTION

 INTRODUCTION

An Engine is a thermal device that converts heat energy into mechanical energy. Mechanical energy is principally derived in the form of torque on the output shaft of the engine and is utilized for necessary driving works.

Energy input to an engine is ‘heat’. Heat so used by the engine is derived from different sources, such as: (i) Solid fuel, (ii) Liquid fuel, (iii) Gaseous fuel, (iv) Nuclear fuel. Energy input system of the engine ensures efficient release of heat from the fuel. For the case a chemical fuel (solid/liquid/gaseous), combustion is the process that is to be carried out to liberate heat from fuel through an exothermic reaction. For nuclear fuel, a nuclear reaction is to be carried out in a nuclear reactor so that energy is liberated from atoms through nuclear chain reaction in which ‘nuclear binding’ energy is released as a result of re-arrangement of atomic particles.

Depending upon where the combustion process is carried out, (that is, outside the engine or inside the engine), the engine is classified as External Combustion Engine (ECE) and Internal Combustion Engine (ICE). For an ECE, combustion is carried out externally in a furnace and heat is utilized to produce working fluid, such as, generation of steam from water in a boiler by heating, heat being produced by burning coal. Here, steam is the working fluid that is taken to drive a steam engine. ECEs have applications in the field of industrial electric power generation. In the ICEs, combustion of fuel is carried out in a space or chamber inside the engine itself. This space/chamber is called the combustion chamber. 

Metered and atomized fuel is burnt in compressed air taken in combustion chamber and the hot gas so produced, called the flue gas with heat energy is the ‘working fluid’ that is directly used to stroke a piston or rotate a turbine wheel. ICEs are compact, all sections being integrated into single unit and hence, they have got wide-spread application in industries, locomotives and aircraft. They are in the form of piston engines and gas turbine engines.

A gas turbine engine is an ICE that uses turbines to convert heat energy of a gas into torque; the gas is the combustion-product produced by burning fuel in compressed air in its combustion chamber inside the engine. A turbine is a rotary device with arrangement of series of blades around the periphery of a wheel mounted on a shaft so that energy of the working fluid, when impinged over blades, will rotate the wheel. In short, turbine is an energy transfer mechanism, transferring energy from working fluid to its shaft in the form of rotation or torque.

In aircraft application, an engine is a propulsive device that provides propulsive force (thrust) to propel the aircraft forward overcoming atmospheric drag. Thus, as long as aircraft propulsion is concerned, the objective of the aircraft gas turbine engine is not directly the work output at its shaft but is the propulsive force. This is fundamental difference between the primary objective furnished by the gas turbine engines in industrial applications and in the aircraft applications. However, the basic aerodynamic and thermodynamic considerations are almost the same.  

1.7 LOW PRESSURE PNEUMATIC SYSTEM

 LOW PRESSURE PNEUMATIC SYSTEM  

Many aircraft use air-driven gyro instru­ments as either the primary gyro instruments or as backup instruments when the primary gyros are electrically driven.

For many years all of the air-driven gyro in­struments used an engine-driven vacuum pump to evacuate the instrument case, and filtered air was pulled into the instrument to spin the gyro. The reason for this was that it was much easier to filter air being pulled into the instrument than it was to filter the air after it had been pumped by an engine-driven pump lubricated by engine oil. The output of these pumps always contained some particles of the oil.

Pressurized aircraft created extra problems for suction-operated instruments, and the latest generations of air-driven gyros now almost all use pressure. Turbine-powered aircraft bleed some of the pressure from the engine compressor, regu­late and filter it, and then direct it over the gyros. Aircraft with reciprocating engines use engine ­driven air pumps to provide the airflow for the gyros. This air is regulated and filtered before it is ready for the instrument.

Figure 1.9: Vane-type air pump

There are two types of air pumps used to pro­vide instrument airflow, and both are vane-type pumps. Sliding vanes are rotated by the drive­shaft and as the shaft turns, the chambers located at positions A and B become larger, while those at positions C and D decrease in size (Figure 1.9). Air is pulled into the pump at the position the chambers enlarge, and it is moved out as they decrease. "Wet" vacuum pumps use steel vanes moving in a cast-iron housing and are sealed and lubricated by engine oil metered into the inlet air port. This oil is discharged with the air and is removed with an oil separator before the air is either used for in­flating de-icer boots or is pumped overboard. See Figure 1.10.

Figure 1.10: Vacuum system using a wet-type vacuum pump

The more modern instrument air systems use "dry" pumps (Figure 1.11) that have carbon vanes and rotors and require no external lubrication. These pumps may be used to drive the instruments by produc­ing a vacuum and pulling air through them, as we see in Figure 1.12, or buying using the output of the pump to force the air through the instruments, Figure 1.13. 

Figure 1.11: Dry-type air pump

Figure 1.12: A vacuum system for the instruments of an aircraft using a dry-type air pump

Figure 1.13: A pressure system for the instruments of an aircraft using a dry-type air pump

1.6 MEDIUM PRESSURE PNEUMATIC SYSTEM

MEDIUM PRESSURE PNEUMATIC SYSTEM

1.6.1 General: 

As said earlier, a medium pressure pneumatic system is used on modern wide bodied commercial aircraft. Such aircraft is designed with a very sophisticated pneumatic system that ensures pneumatic supply as source of thermal energy and pressure energy to be used in/as:

v     Environment Control System (ECS): Aircraft pressurization and air-conditioning

v     Wing anti-icing

v     Cargo heat and ventilation

v     Avionics cooling and ventilation

v     Pressurization of hydraulic reservoir

v     Back up to hydraulic system for emergency operation of hydraulically driven components/systems

1.6.2 Supply sources: Medium pressure pneumatic system employs a pneumatic manifold system as illustrated in Figures 1.6 used in typical aircrafts. Such systems may be pressurized from the following sources for pneumatic:

v     Engine Bleed Air

v     APU Bleed Air

v     Ground Pneumatic Cart

Engine Bleed Air: Aircraft pneumatic system may take engine bleed air as a primary source. This is the great advantage to have pressurized air from engine without using air compressor as a separate unit. Air is bled off from convenient stage or stages and routed through suitable ducts to the aircraft pneumatic system. See examples in Figures 1.6 and 1.7.

  

Figure 1.6: Pneumatic sources on a medium pressure pneumatic manifold system (Typical)