AIRCRAFT ELECTRICAL SYSTEMS EHJ PALLETT PDF

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Third Edition. Aircraft. Electrical. Systems. EHJ Pallett. IEng., AMRAeS 10 Abbreviations and Acronyms associated with Electrical Systems. 11 Logic Gates and. Aircraft Electrical Systems Pallett - Free ebook download as PDF File .pdf), Text File .txt) or read Aircraft Instruments & Integrated System by e.h.j Pallett -. Aircraft Electrical Systems by e.h.j. Pallett (n) - Ebook download as PDF File .pdf) , Text File .txt) or read book online. study of aircraft electrical systems.


Aircraft Electrical Systems Ehj Pallett Pdf

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•Produces alternating current. (AC) which is converted to direct current (DC). •Belt driven (engine-driven). Recharges battery while engine is running. • Creates. download Aircraft Electrical Systems (3rd Edition) on tiebuckverpani.ml ✓ FREE SHIPPING on qualified orders. Aircraft Electrical Systems, 3rd Edition. E.H.J. Pallett. © |Pearson | Out of print. Share this page. Aircraft Electrical Systems, 3rd Edition. View larger.

Aircraft Electrical Systems, 3rd Edition. If You're an Educator Additional order info. If You're a Student Additional order info.

Overview Order Overview. Previous editions. Sign In We're sorry! A selection of questions are given at the end of each chapter and the author is indebted to the Society of licensed Aircraft Engineers and Technologists for permission to reproduce questions selected from examination papers. Valuable assistance has been given by a number of organizations in supplying technical data, and in granting permission to reproduce many of the illustrations, grateful acknowledgement is hereby made to the following-.

Finally, thanks are also due to the publishers for having patiently awaited the completion of sections of manuscript and also for having accepted a number of changes of subject. Energy for the operation of most electrically operated equipment in an aircraft is supplied by a generator which may be of the direct current d.

In this chapter we are concerned with generators serving as the source of primary d. A generator is a machine that converts mechanical energy into electrical energy by the process of electromagnetic induction. In both d. Figure 1. AB" arranged to. The ends of the wire are brought together to form a circuit.

When the plane of the loop lies at right angles to the magnetic field position 1, Fig. As the loop approaches the vertical position again the voltage decreases since the rate at which.

At position 3 the induced voltage is zero. If rotation is continued, the number of lines cut gradually increases, until at degrees position 4 it is once again maximum, but. As rotation continues, the number of lines cut decreases and the induced voltage reduces to zero as the loop returns to position 1.

Plotting of the induced voltage throughout the full cycle produces the alternating or sine curve shown. To convert the a. This is shown in Fig. The brushes are set so that each segment moves out of contact with one brush. In other words, a pulsating current increasing to maximum in one direction only.

In order to smooth out the pulsations and to produce a more constant output, additional wire loops and commutator segments are provided. They are so interconnected and spaced about the axis of rotation, that several are always in a position of maximum action, and the pulsating output is reduced to a ripple as indicated in Fig.

Generators are classified according to the method by which their magnetic circuits are energized, and the following three classes are normally recognized -. These generators are further classified by the manner in which the fixed windings, i. In aircraft d. The fixed portion of the armature circuit consists of the four brushes, the links connecting together brushes of like polarity and the. The four field coils are of high resistance and connected in series to form the field winding.

They are wound and connected in such a. The ends of the windings are brought out to the terminals indicated as Z and z',. The characteristics of a generator refer to the relationship between voltage and the current flowing in the external circuit connected to a generator, i.

These are: These relationships are generally shown in the form of graphs, with the graph drawn for one particular speed of the generator. Shunt-wound generators are one of three types in the self-excited class of machine and as already noted are used in aircraft d. The term "shunt-wound" is derived from the fact that the high-resistance field winding is connected across or. Since the field winding is of high resistance, the advantage is gained of having maximum current flow through the external circuit and expenditure of unnecessary electrical energy within the generator is avoided.

When the armature is rotated the conductors cut the weak magnetic field which is due to residual magnetism in the electromagnet system.

A small e. This, in turn, causes a progressive increase in the induced e. The characteristic for this type of generator is shown in Fig.

This is due to the voltage drop IR drop in the armature winding and also to a weakening of the main flux by armature reaction. The fall in terminal voltage reduces the field current, the main flux is further weakened and therefore a further fall in terminal voltage is produced. If the process of increasing the load is continued after the full working load condition has been reached, the terminal voltage will fall at an increasing rate until it can no longer sustain the load current and they both fall to zero.

With reduced excitation the external characteristic of a shunt-wound generator falls much more rapidly so that the point at which voltage collapse occurs will be reached with a much smaller load current.

In practice, field current is adjusted to maintain constant voltage under all load conditions, by a voltage regulator the operation of which will be described later.

Sometimes a generator will lose its residual magnetism or become incorrectly polarized because of heat, shock, or a momentary current in the wrong direction. This can be corrected by momentarily passing current through the field from the positive terminal to the negative terminal; a procedure known as "flashing the field". A typical self-excited shunt-wound four-pole generator, which is employed in a current type of turbo-prop civil transport aircraft, is illustrated in Fig. In its basic form the construction follows the pattern conventionally adopted and consists of five principal assemblies; namely, the yoke, armature, two end frames and brush-gear assembly.

The yoke forms the main housing of the generator, and is designed to carry the electromagnet system made up of the four field windings and pole pieces.

It also provides for the attachment of the end frame assemblies. The windings are pre-formed coils of the required ampere-turns, wound and connected in. The field windings are Suitably insulated and are a close fit on the pole pieces which are bolted to the yoke.

The faces of the pole pieces are subjected to varying magnetic fields caused by rotation of the armature, giving rise to induced e. To minimize these effects the pole pieces are of laminated construction; the thin soft iron laminations being oxidized to insulate and to offer high electrical resistance to the induced e. Since lines of force cannot intersect, the armature field distorts the main field by an amount which varies with the load; such distorting effect is termed armature reaction.

The position of the brushes can be altered to mini-. The windings are such that an interpole has the same polarity as the next main pole in the direction of rotation, and as the fluxes are opposite in direction to the armature flux, they can be equalized at all loads by having the requisite number of turns. In order to provide true correction of armature reaction, the effects produced by interpoles must be supplemented, since alone they cannot entirely eliminate all distortion occurring at the main pole faces.

Compensating windings are therefore connected in series with the interpole and armature windings, and located in slots cut in the faces of the main pole shoes. The sides of the coils thus lie parallel with the sides of the armature coils. The ampere-turns of the winding are equal to those of the armature winding, while the flux due to it is opposite in direction to the armature flux.

The effectiveness of interpoles in minimizing reactance sparking is limited by armature speed, and their application as individual components of a field-winding system is, therefore, restricted to generators operating over a narrow speed range, e. In the case of generators designed for operation over a wide range, e. To counteract this, and for a given load on the generator, it is necessary to reduce the magnetomotive force m. The desired effect may be obtained by winding auxiliary coils over the interpole coils.

An exact balance between reactance e. The armature assembly comprises the main shaft which may be solid or hollow core and main winding, commutator and bearings; the whole assembly being statically and dynamically balanced. In the generator shown, the shaft is hollow and internally splined to mate with splines of a drive shaft which passes through the entire length of the armature shaft.

Armature windings are made up of a number of individual identical coils which fit into slots at the outer edges of steel laminations which form the core of the armature.

The coils are made from copper. The ends of each coil are brought out to the commutator and silver brazed to separate segments, the finish of one coil being connected to the same segment as the beginning of another coil. The complete winding thus forms a closed circuit. The windings are invariably vacuum-impregnated with silicone varnish to maintain insulation resistance under all conditions. In common with most aircraft generators, the commutator is of small diameter to minimize centrifugal stressing, and is built up of long, narrow copper segments corresponding in number to that of the field coils a typical figure is 51 coils.

The segment surfaces are swept by brushes which are narrow and mounted in pairs usually four pairs to maintain the brush contact area per segment - an essential prerequisite for effective commutation. The armatures of all aircraft generators are supported in high efficiency ball or roller bearings, or in combinations of these two types. Where combinations are used in a single generator it will be found. Bearings are lubricated either with a specified high-melting-point grease or lubricating oil and may be of the sealed or non-.

Sealed grease-lubricated bearings are pre-packed by the manufacturer and require no further. Non-sealed grease-lubricated bearings are assembled with sufficient lubricant to last for the period of the generator servicing cycle.

In general the lubricant for oillubricated bearings is introduced into the bearing through the medium of oil-impregnated felt pads. Seals are provided to prevent oil escaping into the interior of the generator. These assemblies are bolted one at each end of the yoke and house the armature shaft bearings. The drive end frame provides for the attachment of the generator to the mounting pad of the engine or gear-box drive see also p.

Inspection and replacement of brushes is accomplished by removing. The brush-gear assembly is comprised of the brushes and the holding equipment necessary for retaining the brushes in the correct position, and at the correct angle with respect to the magnetic neutral axis. Brushes used in aircraft generators are of the electrographitic type made from artificial graphite. The graphite is produced by taking several forms of.

These brushes possess both the robustness of carbon and the lubricating properties of graphite. In addition they are very resistant to burn-.

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As stated earlier, an essential prerequisite for effective commutation is that brush contact area per commutator segment should be maintained. This is accomplished by mounting several pairs of brushes in brush holders; in the generator illustrated in Fig.

The holders take the form of open-ended boxes whose inside surfaces are machined to the size of a brush, plus a slight clearance enabling a brush to slide freely without tilting or rocking.

Contact between brushes and commutator is maintained by the pressure exerted by the free ends. Springs are adversely affected by current passing through them; it is usual, therefore, to fit an insulating pad or roller at the end of the spring where it bears on the top surface of the brush.

The brush holders are secured either by bolting them to a support ring usually called a brush rocker which is, in turn, bolted to the commutator end frame, or as in the case of the generator illustrated, bolted directly to the end frame.

In order to achieve the. Marks are provided on each generator to indicate the normal operating position. When four or more brush holders are provided, they are located diametrically opposite and their brushes are alternately positive and negative, those of similar polarity being connected together by bar and flexible wire type links.

The brushes are fitted with short leads or "pigtails" of flexible copper braid moulded into the brush during manufacture. The free ends of the pigtails terminate in spade or plate type terminals which are connected to the appropriate main terminals of the generator via the brush holders and connecting links. The leads from brush-gear assemblies and field windings are connected to terminal posts secured to a block mounted on the commutator end frame or, in some generators, on the yoke assembly see Fig.

The terminals and block are enclosed in a box-like cover also secured to the end frame. Entry for the output supply cables of the distribution system refer to Chapter 7 is through rubber clamps. The rotation of. A movable link is fitted between two of the terminals which can be connected in an alternative position should it be necessary for the generator to be driven in the reverse direction. Sparking at the brushes of a generator, no matter how slight, results in the propagation of electromagnetic waves which interfere with the reception of radio signals.

The interference originating in generators may be eliminated quite effectively by screening and suppression. Screening involves the enclosure of a generator in a continuous metallic casing and the sheathing of. To prevent interference being conducted along the distribution cable system, the screened output supply cables are terminated in filter or suppressor units.

These units consist of chokes and capacitors of suitable electrical rating built into metal cases located as close to a generator as possible. Independent suppressor units are rather cumbersome and quite heavy, and it is therefore the practice in the design of current types of generator to incorporate internal suppression systems.

These systems do not normally contain chokes, but consist simply of suitably rated capacitors see Fig. The use of internal suppression systems eliminates the necessity for screened output supply cables and conduits thereby making for a considerable saving in the overall weight of a generator installation.

The carbon from which electro- graphitic brushes are made is extremely porous and some of the pores are. The moisture is trapped under the inevitable irregularities of the contact faces of the brushes and forms an outside film on the commutator and it is with this.

Just how vital a part moisture does play was, however, not fully realized until aircraft began operating at high altitudes and the problem arose of brushes wearing out very rapidly under these conditions. Investigations into the problem showed that the fundamental difficulty was the extreme dryness of the atmosphere, this, in its tum, producing three secondary effects: These effects have been largely eliminated by using brushes which have a chemical additive as a means of replacing the function which atmospheric moisture plays in surface skin formation.

Two distinct categories are in general use: Brushes of this category do not wear abnormally at altitudes up to 60, feet providing that generators to which they are fitted have been previously "bench run" for some hours to allow the formation of the protective film. This film, once formed, is very dark in colour and may often give the impression of a dirty commutator. Brushes of the non-film-forming category contain a lubricating ingredient such as molybdenum disulphide which is often packed in cores running longitudinally through the brushes.

Since the brush is self-lubricating it is unnecessary for generators fitted with this type. However, they do have the disadvantage of appreciably shorter life, due to somewhat more rapid wear, when compared with film-forming brushes. A generator armature is coupled to its prime mover, the aircraft engine, via a shaft driven through gearing which forms part of an accessories gear-box.

The required ratio of the gearing depends on the rated output of the generator and load requirements of an aircraft's electrical system and therefore varies. Drive shafts, usually known as quill drives, are metal shafts with serrations or splines either male or female at one or both ends.

The serrations or splines mate with corresponding formations on the generator armature shaft to transmit the torque delivered by the driving gear. One of the requirements to be satisfied by a quill drive is that it must effectively interrupt transmission of the driving torque in the event that the generator armature seizes up.

This is done by designing the drive shaft so that at one section its diameter. Quill drives are usually short and rigid, but in some cases a long drive with one end mating with serrations formed deep in a hollow armature shaft may be specified. This arrangement enables the drive to absorb much of the mechanical vibration which is otherwise passed to a generator from an accessories gear-box. In the mounting flange method, the end frame at the drive end of a generator is usually extended to a larger diameter than the yoke, thus forming a projecting flange.

Holes in the flange line up with and accept studs which are located in the mounting. An alternative form of flange mounting is based on a generator end frame having two diameters. The larger diameter. Another variation of this form of mounting is employed in the generator shown in. In the manacle-ring method of mounting the generator drive end frame has an extension with a recess in the mounting face of the driving unit.

When the generator extension is fully engaged with the recess, a flange on the end frame abuts on a matching flange formed on the driving unit mounting face.

The two flanges are then clamped together by a manacle ring which, after being placed over them, is firmly closed by a tensioning screw.

A spigot arrangement is usually incorporated to provide location of the generator to the drive unit, and to absorb torque reaction when the generator is operating. The maximum output of a generator, assuming no limit to input mechanical power, is largely determined by the ease with which heat arising from hysteresis, thermal effect of current in windings, etc.

With large-bulk generators of relatively low output the natural processes of heat radiation from the extensive surfaces of the machine carcase may well provide sufficient cooling, but such "natural" cooling is inadequate for the smaller high-output generators used for the supply of electrical power to aircraft, and must, therefore, be supplemented by forced cooling.

A typical cooling system is shown in a basic form in Fig. The air is forced at high speed into an intake and is led through light-alloy ducts to a collector at the commutator end. Vibrating Contact Regulator Vibrating contact regulators are used in several types of small aircraft employing comparatively low d.

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Although the coil windings of each regulator are interconnected, the circuit arrangement. A third unit, called a reverse current cut-out relay, also forms part of some types of regulator, and since the relay has a circuit protection function, a description. Voltage Regulator This unit consists of two windings. The shunt winding con-. This con- in series with the current regulator winding and in.

The series winding, on the. Adjustments to this resistor would vary the resistance and is connected in series with the generator shunt-.

The application of the resistor in the system. The contact assembly is comprised of a fixed. The air discharges over the brush-gear and commutator to cool this natural area of high temperature, and then passes through the length of the machine to exhaust through apertures, surrounded by a perforated strap, at the drive end. In order to assist in ram-air cooling and also to provide some cooling when the aircraft is on the ground, many types of generator have a fan fitted at the drive end of the armature shaft.

The efficient operation of aircraft electrical equipment requiring d. It is necessary, therefore, to provide a device that will regulate the output voltage of a generator at the designed value and within a specified toler-. Hal to incorporate a regulating device which will automatically respond to changes of load and speed, and also, automatically make the necessary adjustments to the generator field current. In a few cases, regulation may also be based.

Movement of the armature and, therefore, the point at which contact opening and closing takes place is controlled by a spring which is pre-adjusted to the required voltage setting.

At the same time current passes through the shunt winding of the voltage regulator. As soon as the generator output voltage reaches the pre-adjusted regulator setting, the electromagnetic field becomes strong enough to oppose the tension of the armature spring thereby opening the contacts.

In this equilibrium position, the circuit to the series winding is opened causing its field to collapse. At the same time, the supply to the generator field winding passes through a resistance R which reduces the excitation current.

Aircraft Electrical Systems, 3rd Edition

The reduced output in turn reduces the magnetic strength of the regulator shunt winding so that spring tension closes the contacts again to restore the generator output voltage to its regulated value and to cause the foregoing operating cycle to be repeated.

The frequency of operation depends on the electrical load carried by. In regulators designed for use with twin-generator systems, a third coil is also wound on the electromagnet core for paralleling purposes see p. Current Regulator This unit limits generator current output in exactly the same way as the voltage regulator controls voltage output, i. Its construction differs only by virtue of having a single winding of a few turns of heavy wire. When electrical load demands are heavy, the voltage output value of the generator may not increase sufficiently to cause the voltage regulator to open its contacts.

Consequently, the output will continue to increase until it reaches rated maximum current, this being the value for which the current regulator is set.

At this setting, the current flowing through the regulator winding establishes a strong enough electromagnetic field to attract the armature and so open the contacts.

Thus, it is the current regulator which now inserts resistance R in the generator shunt-field circuit to reduce generator output. As soon as there is Carbon Pile Regulator Carbon has a granular surface and the contact resistance between two carbon faces that are held together depends not only on the actual area of contact, but also on the pressure with which the two faces are held together.

If, therefore, a number of carbon discs or washers are arranged in the form. Since this method eliminates the use of vibrating contacts, it is applied to generators capable of high current output, and requiring higher field excitation current. The necessary variation of pile pressure or compression under varying conditions of generator speed and load, is made through the medium of an electromagnet and spring-controlled armature which operate in a similar manner to those of a vibrating contact regulator.

As the generator starts operating, the progressively increasing ou tput voltage. During the initial "run-up" stages, the combination of low voltage applied to the regulator coil, and the maximum air gap between armature and core, results. This force is far smaller than that of the spring control, hence the armature maintains its original position and continues to hold the carbon pile in the fully compressed condition; the shunt-field circuit resistance is thus maintained at minimum value during run-up to allow generator output voltage to build up.

This condition continues unaltered until the voltage has risen to the regulated. The armature is free to move towards the electromagnet core if the force of magnetic attraction is increased as a result. In these circumstances pile compression is further reduced so that there is more air space between discs to increase resistance and so check a rise in.

Thus, a condition of equilibrium is re-established with the armature in some new position, but with the output voltage still at the required regulated value.

Any reduction of generator speed, within the effective speed range, produces a reduction in generator output voltage thus disturbing regulator armature equilibrium in such a manner that the spring-control force predominates and the armature moves away from the electromagnet core.

The carbon pile is recompressed by this movement to reduce the generator shunt-field circuit resistance and thereby increase generator output voltage, until the regulated output.

When progressive reduction of generator speed results in a condition of maximum pile compression, control of generator output voltage is lost; any further reduction of generator speed, below the lower limit of the effective range, resulting in proportional decrease in output voltage. When a generator has been run up and connected to its distribution busbar system, the switching on of various requisite consumer services, will impose loads which disturb the equilibrium of the regulator armature.

The effect is, in fact, the same as if the generator speed had been reduced, and the regulator automatically takes the appropriate corrective action until the output voltage is stabilized at the critical value.

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Conversely, a perceptible decrease in load, assuming generator speed to be constant and the regulator armature to be in equilibrium, results in the regulator taking the same action as in the case of an increase in generator speed. Construction The pile unit is housed within a ceramic tube which, in turn, is enclosed in a solid casing, or more generally, a finned casing for dissipating the.

Contact at each end of the pile is made by carbon inserts, or in some types of regulator by silver contacts within carbon inserts. The initial pressure of the pile is set by a compression screw acting through the pile on the armature and plate-type control spring which is supported on a bi-metal washer.

The washer compensates for temperature effects on voltage coil resistance and on any expansion characteristics of the regulator, thus maintaining constant pile compression. The electromagnet assembly comprises a cylindrical. A locking device, usually in the form of screws, is provided to retain the core in a pre-set position. Depending on the design of generating system, voltage regulators may be of the single-unit type, shown in Fig.

In multi-generator systems, it is necessary for the generators to operate in parallel, and in order to ensure that they carry equal shares of the system load, their output voltages must be as near equal as possible under all operating conditions.

As we have already. The method most commonly adopted for this purpose is that which employs a "load-equalizing circuit" to control generator output via the voltage regulators.

The principle. The generators are interconnected on their negative sides, via a series "load-sharing" or "equalizing" loop containing equalizing coils eCe each coil forming part of the individual voltage regulator electromagnetic circuits. The resistances RI and R2 represent the resistances. Let us now assume that generator No. I tends to take a somewhat larger share of the total load than generator No.

In this condition the volts drop V I will now be greater than V 2 and so the negative section of generator No. As a result, a current Ie will flow through the equalizing coils which are connected in such a manner that the effect of Ie is to raise the output voltage of generator No.

I, thereby effectively reducing the unbalance in load sharing. The equalizing coils are wound on the same magnetic cores as the voltage coils of the regulators, thus, assuming the same unbalanced conditions as before, the current Ie flows in a direction opposite to that flowing through the No.

I regulator. The magnetic effect of the No. I generator to decrease its output and to shed some of its load. The variations in output of each generator continues until. Describe how generators are classified, naming the three classes normally recognized and the class. In connection with generator brushes, state: Briefly describe the causes of brush wear under high altitude flight conditions and the methods adopted for reducing wear.

With the aid of a circuit diagram, describe the fundamental principle of the carbon pile method of voltage regulation. With the aid of a circuit diagram describe how parallel operation of generators can be obtained. The batteries selected for use in aircraft therefore employ secondary cells and are either of the lead-acid or nickel-cadmium type.

The starting of large d. A similar condition exists should a short circuit develop in a circuit protected by a heavy duty circuit breaker or current limiter. This function possibly applies to a lesser degree on aircraft where the electrical system is predominantly a. Under these conditions the battery could be the sole remaining source of power to operate essential flight instruments, radio communication equipment, etc.

Aircraft Electrical Systems

Both types of cell operate on the same fundamental principle, i. The essential differences between the two lies in the action that occurs during discharge. In the primary cell this action destroys the active materials of the cell, thus limiting its effective life to a single discharge operation, whereas in the secondary cell the discharge action converts the active material into other forms, from which they can subsequently be electrically reconverted,. Thus, a secondary cell can have a life of numerous discharge actions, followed by.

The basic construction of a typical cell is shown in Fig. It consists essentially of a positive electrode and a negative electrode, each of which is, in turn, made up of a group of lead-antimony alloy grid plates; the spaces of the plates are packed with pastes of active lead materials. The two plate groups are interleaved so that both sides of every positive plate face a negative surface. The plates are prevented from coming into contact with one another by means of separators not shown made from materials having.

Single battery cell showing poSitive and negati ve pla1e groups interlocked. Plate separators Ore not shown.

Each group of positive plates and negative plates is connected through a strap to a terminal post. The internal resistance of a cell varies immensely with the distance between the positive and negative electrode surfaces; therefore, to obtain the lowest possible resistance the gap between the plates of each group is made as small as is practicable. A cell contains an odd number of plates, the outermost ones belonging to the negative plate group.

The reason for this arrangement is that unlike a positive plate a negative plate will not distort when the electromechanical action is restricted to one side only.

The plate assemblies of a cell are supported in an acid-proof container. The negative plates are of similar basic structure, but with pure spongy lead Pb forced into the grid.

The electrolyte consists of two constituents, sulphuric acid H2S04 and water, which are mixed in such proportions that the relative density is generally about. During discharge of the cell, that is, when an external circuit is completed between the positive and negative plates, electrons are transferred through the circuit from lead to lead peroxide and the net result of the chemical reaction is that lead sulphate PbS04 forms on both plates. At the same time molecules of water are formed, thus weakening the electrolyte.

For all practical purposes, the cell is considered to be discharged when both plates are covered with lead sulphate and the electrolyte has become quite weak. The cell may be recharged by connecting the positive and negative plates, respectively, to the positive and negative terminals of a d. All the fore-. Two types of lead-acid battery may be found in general use; in one the electrolyte is a free liquid while in the other it is completely absorbed into the plates and separators.

An example of the former type of. The links interconnecting the cells and cell blocks are sealed and suitably insulated to prevent contact with the container. A plastic tray is fitted on to the top edges of the container and is sealed around the cell vent plugs by. A plastic lid combined with an acid-proofed aluminium alloy hold-down frame completely. The battery illustrated in Fig. The plates, active materials and separators are assembled together and are compressed to form a solid block.

The active material is an infusorial earth, known as Kiesel Guhr, and is very porous and absorbent.

Thus, when the electrolyte is added, instead of remaining free as in the conventional types of battery, it is completely absorbed by the active material. The cells are assembled as two l2-volt units in monobloc containers made of shock-resistant polystyrene and these are, in turn, housed in a polyesterbonded fibreglass outer container which also supports the main terminal box.

A cover of the same material as the case is secured by four bolts on the end flanges of the case. In this type of cell the positive plates are composed of nickel hydroxide, Ni 0Hh. Batteries made up of these cells have a number of advantages over the lead-acid type, the most notable being their ability to maintain a relatively steady voltage when being discharged at high currents such as during engine starting.

The plates are generally made up by a sintering process and the active materials are impregnated into the plates by chemical deposition. This type of construction allows the maximum amount of active material to be employed in the electrochemical action. After impregnation with the active materials, the plates are stamped out to the requisite size and are built up into positive and negative plate groups, interleaved and connected to terminal posts in a manner somewhat similar to the lead-acid type of cell.

Insulation is done by means of a fabric-base separator in the form of a continuous strip wound between the plates. The complete plate group is mounted in a sealed plastic container. Basic Electronics-Bemard Grob Basic Electronics and radio Installation by Dale crane Electronics and communication System by George kennedy Principles of Electronics by V K Mehta Integrated Electronics by Millman and Halkias Aviation Electronics by Keith W Bose Aircraft Materials and Processes by George F.

Mechanics of Flight By -A. Kermode 4. Light Aircraft Maintenance-byJ. Heywood 9. Light Aircraft Inspection-by J. Heywood Aircraft Electrical Systems-by E. Pallet Aircraft Instruments-by E. Automatic Flight Controls-by E. Aircraft repair Manual by Larry Reithmaier The helicopter and How to Fly-by John Fay 2. Basic helicopter maintenance-by Joseph Schafer 3. Helicopter Aerodynamics-by R. Prouty 5.

Titterton 6. Rotary Wing Aerodynamics-by W. Stepniewski 9. Basic Helicopter Aerodynamics by J. Seddon Aircraft Electrical System-by E. Pallett Automatic Flight Control-by E.Stick-shaking is accomplished by a motor which is secured to a control column and drives a weighted ring that is deliberately unbalanced to set up vibrations of the column.

When the vacuu. The segment into Lhc desired solid shape by mechanical pressure surfaces are swept by brushes which are narrow and followed by exposure to very high temperature in an mounted in pairs usually four pairs to maintain the electric furnace. In this way high contact pressures. As a result of the initial current flow, armature reaction is set up, and owing to the position of the permanent magnetic poles, the reaction polarizes the main poles of the exciter stator in the proper direction to assist the voltage regulator in taking over excitation control.