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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.
The reverse current circuit breaker in the system shown is of the electromagnetic type and its purpose. In order to record the hours run. Explain the term Power Factor and state how it is affected by a circuit containing inductance and capacitance. The title Aircraft instruments and integrated systems is therefore most apposite. The angle the lines of force make with the earth's surface at any Figure 3.
ELequals x J. As rar a Fig 2. In its rotation. As only one phase winding is connected between each pair of lines then.. The primary reason for this is due to the fact that in calculaUng the power.
This may voltage levels.. I ol then as we have already seen p. The vector diagram for a current I lagging a r-. The in.
The importance of these components will Pig. Is less than unity. The Urie curren t. The current is resolved into two components at right angles. The stator winding The generator is cooled by ram air see also is star connected. The fcrence in the reception of radio signals. IQr SlalOr b. Although such frequency variations are not suitable for the direct operation of all types ofa. Three spring-loaded brushes are equispaced trated In Fig. At the drive stator assembly is made up of high permeability lam.
Three small current transformers are fitted finally discharged.. An air-collector ring encloses the slots and is terminal box mounted on the end frame of the connected to a vent through which the cooling air is generator. Provision is made for the installa- into the terminal box and form part of a protection tion of a lherrnal y.
The brushes are electrically connected to d.. The other three ends the main housing through ventilation slots at the drive. The generator mounted above the brush-gear housing. POUi tion be applied directly to resistive load circuits such rmg as electrical de-icing systems.
Below Hz the field current is limited input terminals housed in an excitation terminal box and the output relatively reduced.
The terminal consists of two major assemblies: It has a three-phase output of on each slip Ting and are contained within a brush-gear 22 kV A at volts and it supplies full load at housing which also forms a bearing support for the this voltage through a frequency range of to rotor.
Oil for system operation Is The unit employs a hydromechanical variable. A constant frequency is inherent in an a.! Such a mechanism is referred trolled and transmitted through the combined effects to as a constant-speed drive CSD unit and an of the three units.. The power used to drive the generator is con.
Constant speed drive unit Some form of conversion equipment is therefore required and the type most widely adopted utilizes consists of a variable displacement hydraulic unit. Gcrni ml or mo. The engines cannot be relied upon to do this directly and. The differential gear consists of a carrier shaft carrying two meshing 1: The shaft and the output ring gear "housing". Since the input ring gear "housing" is The variable displacement unil consists of a geared to the fJXed displacement hydraulic unit.
In this way. Underdtive phase the output ring gear "housing" serves as the con- tinuous drive transmission link between engine and generator. The fixed displacement unit is similar to the variable displacement unit.
It therefore responds to changes in transmission output speed. Each housing also has! The placement unit. When oil is pumped to the fixed displacement unit by the variable one. Oil same direction as. The cylinder block and input ring governor control valve.
In this con. The electromagnetic the overdrive phase and this is shown in Fig. The cylinder block of this unit and the output ring gear "housing" do not. Because the input ring gear ''housing" is now rotating in the same direction as the carrier shaft then the speed of the freely rotating planet geaI meshing with the 6 housing will be reduced. This hydromechan1cal process of Fig 2. In this condition. The control valve changes the angula.
The pressure of the pistons against the inclined face of the unit now ' causes its cylinder block to rotate in the same l. The speed of the second i To P'!
In this system see. Any unbalance in real load operation. ResetUng of disconnect mechanisms can only be Fig 2. In some mechanisms a magnetically. CSD oil filter 6. In the system illustrated. CSD oil cooler 2. CSD oil scrvico port 5. CSD 3. When a disconnect has ta. When the solenoid is energized. Gonorator 4. The excite armature is mounted on the same shaft as the main A sectioned view of a typical constant frequency generator rotor and the output from its three-phase generator is illustrated in Fig.
A mounting flange. Al the same time. The assembly components are contained within a cast aluminium is contained within a tubular insulator located in the casing made up of an end bell section and a stator hollow shaft on which the exciter and main generator Output leminol Oulpul term.
All three ate the problems associated with them. The exciter. It consists of windings is fed to the rotating rectifier assembly The main generator stator. A suppression p. In isolated generator Further information on the circuit arrangement operation. The air in longitudinal slots in the pole faces. Large copper is exhausted through a pcrfora tcd screen around the bands. The complete stator phases arc brought directly to the upper surface shaft is supported at each end by pro-greased sealed of an output terminal board.
In addition to the field coils. The stub shaft. A thermally-operated overheat purpose of the damper windings is to provide an detector switch is screwed directly through the stator induction motor effect on the generator whenever frame section into the stator of the main generator.
Its purpose synchronism. The leads from the three taining the rotating rectifier assembly. In parallel operation see subjected to excessive centrifugal forces. The field current. The principal components and socUons of the sequent operational running period. Is controlled by a voltage regulator system.
Frequency -Wild Generators Figure 2. Ung system. In The production of a desired output by any type of this case. The essential difference relates feature. Rectified o. Jess space. The fundamental the arrangements of which are essentially varied to construction and operation of both the generator suit the particular type of gener. This configuration reduces weight. At the higher due.
At low or normal ambient equal current to flow in both windings. The three-phase voltage produced in the windings ii connected. The magnets of the a. This current is fed int mately double. The output is tapped and is fed back to the shun1 regulator must be supplemented by some other medium field windings of the exciter.
When the generator running. As a result of the initial voltage regulator which supplies a sensing current current Dow. When the main output reaches the rated value. In turn. A Fig. The rectifier assembly consists of 1ix silicon the compounding rectifier.
The exciter stator of the generator described on During the initial stages of generator operation. When position of the permanent magnetic poles. This is provided by the regulator system. A thennistor is located in series with one As the temperature of the winding increases. Voltage regulators nom1ally form are connected to a bridge "signal" rectifier.
When of load current delivered by the generator. The two current plished by a network of magnetic amplifiers or signals. Regulation is accom. To mo. After rectification. In addition to the control output windings in the error magnetic amplifier. As already explained on same function as a d. The secondary that the voltage regulation principles adopted for d. The output side of the bridge is The regulation of the output of a constant-frequency connected to an "error" control winding of tl1e pre- system is also based on the principle of controlling amplifier and then from this amplifier to a "signal" field excitation.
It is made up of a three. Since under this condition 1 the voltage dr is to monitor the generator output voltage. J error to the pre-amplifier. If the a. At this age of the three a. The function of the voltage error detector two tubes. This results in an increase or decrease.
Pora positive error signal the core flux and excitation current will be reduced. Ln rectified a. When current flows through the "error" control paralleling or.. Real load is the actual shown. There When current flows through phase "C" of each are. It is generators falls proportionately. Lncreased to eight amps a difference of three amps fore when two or more operate in parallel they lock will flow through the error sensing clement of its t9gether with respect to frequency and the system relevant load controller.
A circuit diagram of the Thus. The error signals are then applied as d. Since this is controlled the output of each corresponding current transformer by speed-governing settings then it means that the is reduced by one amp. Each load controller is capacitive currents and voltage in the system made up of a two-stage magnetic amplifier controlled expressed in kilovolt-amperes reactive kV AR.
The current and magrtetic field two principal sections: If now a paralleled generators is determined by the real relative generator. In accordance under which the current output from each transformer with signals from a reactive load-sharing circuit.
A difference Reactive Load. Whereas in the real load-sharing circuit magnitudes of their output voltages which vary. The direction of the current and field in increased while the-loads of the other generators the load controller sensing elements of the remaining are reduced and unbalance in reactive load sharing generators is such that the governor flyweights in exists.
Sharing The sharing of reactive load in the nature of the circuitry should however be noted between paralleled generators depends on the relative at this point. The resulting d. Voltages ing of the corresponding mutual reactor will be proportional to the magnitude of the differential additive and so the error detector will sense this as currents are induced in the secondary windings and an overvoltage.
I To pre -ornpli!
I generator. These are. I generator takes the associated with the real load-sharing circuit and they greater share of the load. In the case of the other three generator. The field relays are A schematic diagram of the method based on that similarly operated. The generators are connected to their respective load busbars and the synchronizing busbar.
The breakers also trip automatic- puts fomu part of the paralleling system. Synchronizing Lights each breaker being closed or tripped by manual In some power-generating systems a method of operation of switches on a panel at the Flight indlcatlng synchronization between generator out. Z Synchronizing. SI voltages induced in their mutual reactors will have The lights are connected into phases ''A" and lagged behind the currents from the generators.
Indicator lights are located adopted for the triple generator system of the Boeing adjacent to all switches to indicate either of the is given in Fig. Engineer's station. The the secondary windings. The generator is of a similar type to the to the synchronizing busbar.
Phase "B" is impose heavy loads on the generator or CSD and connected to both the voltmeter and frequency meter possibly cause damage to them. The number 1 generator is then con. Since at synchronizing lights flash alternately. Such aclion would busbar via the synchronizing lights. This action also connects the generator generator. With all three of the complete unit. After deploymen is connected to its load 'busbar.
As the first engine is started. A typical nominal fan As the third engine is started. The complete unit is stowed on a special wiase difference between the two generator outputs. It there. Semi-conductors are also further defined by types. Chapter 2. As a result of this exchange.
In concept. The principal items which may be grouped under this If a p-type semi. Jectrons trode on the other.
Fig 3. In the rectification of main a. This will. A thin layer of a low- melting point alloy. Rectifiers employing germanium a Typical rectifier stacks metallic element are also available but as their operating temperature is limited and protection against short duration overloads is difficult. Contact with the two elements of the rectifying junction. Toe greater the difference in the resistance to current flow in the two directions the better is the rectlfylng effect.
Some rectifiers. In selenium In the same manner as a normal silicon diode. The voltage at which. The other face is soldered to a base. This is because once H! Aluminium at a sufficiently high voltage the resistance in the reverse direction breaks down completely and reverse Rec t. The mean value of rectified voltage can be essential requirement and is normally taken care controlled by adjusting the phasing of the gate signal of by blower motors or other forced air methods such with respect to the applied voltage.
It should be noted place can be varied by applying small current signals that these figures represent lhe actual temperature at between the gate and the cathode. When reverse voltage is applied an S. For germanium the temperature is about tically zero until a forward er! Once conduction has been initiated it can a complete unit. It Is a three- other deleterious materials. These rectifiers are called Zener Diodes n.
The voltage at which breakover takes without destroying the rectifier. IS gate signal currents or "firing point" is varied.: R switching is in the battery charger unit already would be required In a variable speed motor circuit. A typical application of produces a variable d.
For half-wave rectification of a three-phase a. Figure 3. In the former arrangement the d. For a single-phase a.. The output from the operations in a little more detail. Examples of three- input. Input throughout a full cycle. II 2 and a smoother output waveform is obtained. From "B" to "C" the line voltage corresponds to that Since the negative half-cycles art1 Included. Let us consider the points "A" rectifiers which conduct being as tabulated in Fig. Also the voltage applied to continues through the remaining tluee conducting the bridge network Is that between two of the phases.
QUlpu l. This process other on the negative side. These The output voltage.. Between the points "C" and times that of the a.. S and "B" on the three phase voltage curves. I Between phoses 3. The cores are then cut into two C- shaped parts to allow the pre-wound coils to be fitted. A single. On a shell. The second. In some designs the cores are formed of strips 3.
Circuit Connections. Transformer windings are of enamelled copper wire or strip. Part of both primary and secondary windings are wound on each side of the core. An exception to this normal arrangement is in a variant known as an auto-transformer. After assembly of the windings the core parts are clamped together by a steel band around the outside of the core. Transformers for windings is sometimes undesirable because this pro- three-phase circuits can be connected in one of several vides an external path for the flow of certain harmonic combinations of the star and delta connections see currents which can lead to interference with radio also Chapter 2.
This ratio of turns N When an alternating voltage is small excitation current sufficient only to magnetize applied to the primary winding an alternating the core. It consists of three main parts: O Step-down. When the transformation ratio is such that the P. The flux cuts across the secondary winding and In one the laminations are L-shaped and are by mutual induction in practice both windings are assembled to provide a-single magnetic circuit.
The circuit arrange- ments for both types are also shown in Fig. For example.. When the load is disconnected.. These materials have the characteristics of fairly and almost equal to the applied voltage.. There are two classes of trans. Two different winding to set up an alternating magnetic flux in the forms of construction are in common use. The three main parts are shown schem.. The core of Transformer pri11ciple a voltage transformer is 'laminated and conventionally is built up of suitably shaped thin stampings..
The differ. Its circuit arrangement ls shown load. One such assembly Is illustrated in therefore. If this is junction with a. Those Fig. The cable ls wound with a single tum if it ln the example illustrated the tappings provide a carries high currents.
It is designed with only a secondary winding on a toroidal strip-wound core of silicon-iron. Secondary leads from the various be additive. The busbars terminate in the flexible induced in the remaining turns of the winding will insulated straps. These transformers have not done.
It consists of seven transformers which are turns between the primary terminals act In the same supplied with primary voltage via the three feeder way as the primary winding of a conventional trans- terminals and by insulated busbars passing through former. Tho operating principle is the primary turns is less than that of the secondary turns.
A typical unit is shown in Fig. When a voltage is applied to the primary terminals 1n some aircraft generating systems. The voltage three sets. The circuit arrangement of a typical and output circuits alike they carry the difference step.
The secondary tappings are so arranged that up to four output voltage levels may be utilized. The three windings are star-connected and are supplied with the "primary voltage of volts from the alternator system.
When a load circuit between the induced current and primary current. As in the case of an a. The increased current will.. The unit designed.. This consists of a transformer and two three-phase bridge is due to the fact that the resistance of primary wind. The differs from that for which the transformer was circuit is shown schematically in Fig.
Each secondary winding is current will rise. J These terminals. TI1e switches tions of static transformers and rectifiers. The transformer has a conventional star.
An anunetcr shunt dropping 50 mV at leading to still larger current. Cooling of the unit in this case the primary current will be reduced. C QfJno To 11S-V e A rotary inverter consists of a d. Since the frequency of the a. In order to control the voltage at thls zav tiv. When the inverter is switched on.
Although now largely superseded by inverters preset resistor is also connected in series with the of the solid-state circuit or static type. This resistor is preset the inverter designed to produce either 26 volts or to give the required excitation current at the regulated volts Hz a..
Is shown in the block tor. The function of an annaturc winding. It will be noted from the diagram that the d. This output is voltage coil of the regulator which varies the pile then supplied to a pulse shaper circuit which controls resistance in the usual manner.. The a. They are and field system. The power driver As In the case of other types of generators. The power driver also shorts itself out each is done by means of a voltage sensor and a current time the voltage falls to zero.
The reason for this is to cause the pulse age and frequency required for operating the systems shaper to delay its output to the power driver stage connected to the inverter. In the example illustrated. This output is fmally pulse shaper output tluough the medium of a regula- fed to a filter circuit which reduces the total odd tor circuit and a notch control circuit.
Power seleelor In the majority of large public transport aircraft. J I Gro AJ the name suggests of a connector located in the aircraft at a con. In addition to the external power supply system. Pig 4. The batteries of an aircraft are.
Bollery busbor f I system. Systems are restricted to the supply of power under emergency conditions. I' some types of aircraft. The reason for connect the supply direct to the "vital" and No. Indication that both busbars are also "tied" to the ing of the supply takes place at the heavy-duty ground power supply is provided by magnetic indica contacts of the relay thus preventing arcing at the tors ''A'' and "B" which are energized from the vital main pins.
Tl1is ensures that break. It consists of two positive pins and one negative pin. The pins are enclosed by a protective shroud. RQV fit c. This require! One set of Ex ternal power supply connection auxiliary contacts complete a circuit to a magnetic indicator which then indicates that the external sup1 The circuit of a th ree-pin receptacle system is is connected and on ''C" in Fig..
In some aircraft d. To An example of a current type of unit ls shown In bo tttry system Fig. The pins were of different diameters to prevent a reverse polarity condition. Before external power is applied. If the phase sequenco is correct the pro. A single-phase supply Is also In aircraft which from the point of view of electrical fed to an amber light which comes on to indicate that power are principally of the "a.
Systems tection unit completes a circuit to the control relay coil. External power I bus-he No. The circuit arrangement is shown in tween aircraft types but in order to gain some under- Fig. The positive pin of the When external power Is coupled to the receptacle a receptacle is coupled directly to the battery busbar three-phase supply is fed to tho main contacts of the via circuit breaker "A".
The T. The circuit unit in the event that the aircraft's battery is in. When the.. Figure 4. Thus, assuming that an overdrive condition arises an increased pressure will be exerted by the motor pistons on the centre plate and there will be a tendency for it to be squeezed out from between the plates. However, since the plate is restrained to rotate independently about a fixed axis. In an underdrive condition, the pressure on the eccentric centre plate. The use of brushes and.
Thus, the brushless a. A sectioned view of a typical generator is illustrated in Fig. It consists of three principal components: A mounting flange, which is an integral part of the stator frame, carries twelve slots reinforced by steel inserts, and key-hole shaped to facilitate attachment of the generator to the mounting studs of the constant-speed drive unit. The exciter, which is located in the end bell section of the generator casing, comprises a stator and a threephase star-wound rotor or exciter armature.
The exciter armature is mounted on the same shaft as the main generator rotor and the output from its three-phase windings is fed to the rotating rectifier assembly. The rotating rectifier assembly supplies excitation current to the main generator rotor field coils, and. The assembly. A suppression capacitor is also connected in the rectifier circuit and is mounted at one end of the rotor shaft. Its purpose. The main generator consists of a three-phase starwound stator, and an eight-pole rotor and its associated field windings which are connected to the output of the rotating rectifier.
The leads from the three stator phases are brought directly to the upper surface of an output terminal board, thus permitting the aircraft wiring to be clamped directly against the phase leads without current passing through the terminal studs. In addition to the field coils, damper amortisseur windings are fitted to the rotor and are located in longitudinal slots in the pole faces.
Large copper bands, under steel bands at each end of the rotor stack, provide the electrical squirrel-cage circuit. The purpose of the damper windings is to provide an induction motor effect on the generator whenever sudden changes in load or driving torque tend to cause. In isolated generator operation, the windings serve to reduce excessively high transient voltages caused by line-to-line system faults, and to decrease voltage unbalance, during unbalanced load conditions.
In parallel operation see p. The drive end of the main rotor shaft consists of a splined outer adaptor which fits over a stub shaft secured to the main generator rotor. The stub shaft, in turn, fits over a drive spindle fixed by a centrally located screw to the hollow section of the shaft containing the rotating rectifier assembly.
The complete shaft is supported at each end by pre-greased sealed bearings. The generator is cooled by ram air which enters through the end bell section of the casing and passes through the windings and also through the rotor shaft to provide cooling of the rectifier assembly.
The air. A thermally-operated overheat detector switch is screwed directly through the stator frame section into the stator of the main generator, and is connected to an overheat warning light on the relevant system control panel.
In some types of aircraft, brushless generators are combined with the constant speed drive units such that they can be installed and removed as complete assemblies, such assemblies being referred to as integrated drive generators I. The fundamental construction and operation of the generators follow that of the generator described in the preceding paragraphs, but they differ essentially in the method by which they are cooled. Instead of air being utilized as the cooling medium, oil from the constant speed drive hydraulic system is pumped through the generator for cooling of the rotor windings, diodes, and the stator windings.
The oil itself is, in turn, cooled by means of an external cooler through which air is passed via a tapping from a compressor stage of the engine. In one type of aircraft Concorde in fact the oil is cooled by passing it through a section of the cooler of the main engine oil system, this cooler, in turn, utilizing the engine fuel itself as the cooling medium.
Pressure and temperature sensing probes are included in an I. The purpose of the sensing system is to provide. The disconnection of a C. In this system see Fig. When the solenoid is energized, a spring-loaded pawl moves into contact with threads on the input shaft and then serves as a screw causing the input shaft to move away from the input spline shaft driven by the engine thereby separating the driving dogs of the clutch.
In some mechanisms a magnetically-operated indicator button is provided in the reset handle, which lies flush with the handle under normal operating conditions of the drive. When a disconnect has taken. Resetting of disconnect mechanisms can only be accomplished on the ground following shutdown of the appropriate engine. In the system illustrated, resetting is accomplished by pulling out the reset handle to withdraw the threaded pawl from the input shaft, and allowing the reset spring on the shaft to re-engage the clutch.
At the same time, and with the solenoid de-energized, the solenoid nose pin snaps into position in the slot of the pawl. The production of a desired output by any type of generator requires a magnetic field to provide excitation of the windings for starting and for the subsequent operational running period.
In other words,. The field current, as it is called, is controlled by a voltage regulator system. The excitation of a. However, they all have one common feature, i. In this case, excitation of the rotor field is provided by d. The principal components and sections of the control system associated with excitation are: The primary windings of the compounding transformer are in series with the three phases of the generator and the secondary windings in series with the compounding rectifier.
When the control switch is in the "start" position, d. The output is tapped to feed a magnetic amplifier type of voltage regulator which supplies a sensing current signal to the excitation rectifier see p. This is provided by the compounding transformer and rectifier, and by connecting them in the manner already described, direct current proportional to load current is supplied to the rotor field windings.
When d. As the. The level of voltage is regulated by a transistorized type of. The exciter stator of the generator described on page 33 is made up of two shunt field windings, a stabilizing Winding and also six permanent magnets; the latter provide a residual magnetic field for initial excitation. A temperature-sensitive resistance element thermistor is located between two of the stator terminals to compensate for changes in shunt field resistance due to temperature variations.
The stabilizing winding is wound directly over the shunt field windings, and with the permanent magnet poles as a common magnetic core, a transformer type of coupling between the two windings is thereby provided.
The rectifier assembly consists of six silicon diodes separated by insulating spacers and connected as a three-phase full-wave bridge. The excitation circuit arrangement for the generator is shown schematically in Fig. When the generator starts running, the flux from the permanent magnets of the a. 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.
The three-phase voltage produced in the windings is supplied to the rectifier assembly, the d. A rotating magnetic field is thus produced which induces a three-phase voltage output in the main stator windings. The output is tapped and is fed back to the shunt field windings of the exciter, through the voltage regulator system, in order to produce a field supplementary to that of the permanent magnets. In this manner the exciter output is increased and the main generator is enabled to build up its output at a faster rate.
When the main output reaches the rated value,. During the initial stages of generator operation,. As the temperature of the winding increases, the thermistor resistance decreases to allow approximately.
In the event that excitation current should suddenly increase or decrease as a result of voltage fluctuations due, for example, to switching of loads, a current will be induced in the stabilizing winding since it acts as a transformer secondary winding. This current is fed into the voltage regulator as a feedback signal to so adjust the excitation current that voltage fluctuations resulting from any cause are opposed and held to a minimum.
The control of the output voltages of a. Voltage regulators normally form part of generator system control and protection units. Frequency- Wild Generators Figure 3. Regulation is accomplished by a network of magnetic amplifiers or transducers, transformers and bridge rectifiers interconnected as shown. In addition to the control of load current delivered by the gener-. As already explained on page 36, the compounding transformer and rectifier provides excitation current proportional to load current, therefore the sensing of error voltages and necessary re-adjustment of excitation current must be provided by the voltage regulation network.
It will be noted from the diagram that the threephase output of the generator is tapped at two points; at one by a three-phase transformer and at the other by a three-phase magnetic amplifier. The secondary winding of one phase of the transformer is connected to the a. A" containing a device known as a barretter the characteristics of which maintain a substantially constant current through the arm, the other arm.
The two current Signals, which are normally equal at the desired line voltage, are fed in opposite directions over the a. When there is a change in the voltage level, the resulting variation in current flowing through arm "B" unbalances the sensing circuit and, as this circuit has the same function as a d.
After rectification, the signal is then fed as d. This results in an increase or decrease, as appropriate, of the excitation current flow to the generator rotor field winding, continuing until the line voltage produces balanced signal conditions once more in the error sensing circuit.
Before going into its operation, however, it will be helpful at this stage, briefly review the primary function and fundamental characteristics of the device known as the transistor.
The primary function of a transistor is to "transfer resistance" within itself and depending on its connection within a circuit it can turn current "on" and "off' and can increase output signal conditions; in other words, it can act as an automatic switching device or as an amplifier.
It has no moving parts and is made up of three regions of a certain material, usually germanium, known as a semiconductor see also p.
Some typical transistor contact arrangements are shown in Fig. The letters "p" and "n". A transistor has three external connections corresponding to the three regions or elements known as the emitter which injects the current carriers at one end, the collector which collects the current at the other end, and the base which controls the amount of current flow. The three elements are arranged to contact each other in sandwich form and in the sequence of either n-p-n or p-n-p.
When connected in. Thus, the emitter of an n-p-n transistor has a negative voltage applied to it with respect to base so as to repel negative electrons in the forward direction, while a positive voltage is applied to the emitter of a p-n-p transistor so as to repel positively charged "holes" in a forward direction. Since reverse bias is always applied to collectors then the collector of an n-p-n transistor is made positive with respect to the emitter in order to attract.
Similarly, the collector of a p-n-p transistor is made negative with respect to the emitter so as to attract positively charged "holes".
This is indicated on the symbols adopted for both transistor arrangements, by arrows. Any input voltage that increases the forward bias of the emitter, with respect to the base, increases the emitter-to-collector current flow, and conversely, the current flow is decreased when an input voltage decreases forward bias.
The characteristics of transistors are such that small. By alternately connecting and disconnecting the bare circuit to and from a forward bearing voltage, or similarly, by alternately applying a forward and.
In this manner, a transistor can thereby also function as a switching device. In the regulator circuit shown in Fig. When the system control switch is "on", excitation current flows initially from the battery to the base of TRz and through a voltage dividing network made up of resistances R" Rz and RV l' The purpose of this network in conjunction with the Zener diode "Z" see also p. With power applied to the base of TRz, the transistor is switched on and battery current flows to the collector and emitter junction.
The amplified output in the emitter circuit flows to the base of TR3 thereby switching it on so that the battery current supplied to the field winding can be conducted to ground via the collector-emitter junction of TR3.
When the generator is running, the rotating magnetic field induces an alternating current in the stator and this is rectified and supplied to the d. When the generator output voltage reaches the preset operating value, the current flowing in the reverse direction through the Zener diode causes it to breakdown and to allow the current to flow to the base of. TR1 thus switching it on. The collector-emitter junco tion ofTR1 now conducts, thereby diverting current away from the base ofTRz and switching it off.
This action, in turn, switches off TR3 and so excitation current to the generator field winding is cut-off, The rectifier across the field winding Dj provides a path so that field current can fall at a slower rate and thus prevent generation of a high voltage at TR3 each time it is switched off. When the generator output voltage falls to a value which permits the Zener diode to cease conduction, TRl will again conduct to restore excitation current to the field winding.
This sequence of operation is repeated and the genera tor output voltage is thereby maintained at the preset operating value. The regulation of the output of a constant-frequency system is also based on the principle of controlling field excitation, and some of the techniques thus far described are in many instances applied. In installations requiring a multi-arrangement of constant-frequency generators, additional circuitry is required to control output under load-sharing or parallel operating conditions and as this control also involves field excitation, the overall regulation circuit arrangement is of.
At this stage, however, we are only concerned with the fundamental method of regulation and for this purpose. The circuit is comprised of three main sections: The function of the voltage error detector is to monitor the generator output voltage, compare it with a fixed reference voltage and to transmit any error to the pre-amplifier.
It is made up of a threephase bridge rectifier connected to the generator output, and a bridge circuit of which two arms contain gas-filled regulator tubes and two contain resistances. The inherent characteristics of the tubes are such that they maintain an essentially constant voltage drop across their connections for a wide range of current through them and for this reason they establish the reference voltage against which output voltage is continuously compared.
The output side of the bridge is connected to an "error" control winding of the preamplifier and then from this amplifier to a "signal" control winding of a second stage or power amplifier.
Both stages are three- phase magnetic amplifiers. The final amplified signal is then supplied to the shunt. The output of the bridge rectifier in the error detector is a d. A balanced condition of the bridge circuit concerned is obtained when the voltage applied across the bridge points "A" and "B" is exactly twice that of the voltage drop across the.
Since under this condition, the voltage drop across resistors R, and Rz will equal the drop across each tube, then no current will flow in the output circuit to the error control winding of the pre-amplifier. If the a. The direction and magnitude of current flow will depend on whether the variation, or error in line voltage, is above positive error signal or below negative error signal the balanced nominal value, and on the magnitude of the variations.
When current flows through the "error" control winding the magnetic flux set up alters the total flux in the cores of the amplifier, thereby establishing a proportional change in the amplifier output which is.
If the error signal is negative it will cause an increase in core flux, thereby increasing the power amplifier output current to the generator exciter field winding.
For a positive error signal the core flux and excitation current will be reduced. Thus, the generator output is controlled to the preset value which on being attained restores the error detector bridge circuit to the balanced condition. Frequency- Wild Systems. In systems of this type, the a.
In most applications this is by design; for example, in electrical de-icing equipment utilizing resistance type heaters, a variable frequency has no effect on system operation; therefore reliance is placed more on generator dependability and on the simplicity of the generating system.
In rectified a. Constant-Frequency Systems. These systems are designed for operation under load-sharing or paralleling conditions and in this connection regulation of the two parameters, real load and reactive load, is required. Real load is the actual working load output in kilowatts kW available for supplying the various electrical services,. See Fig. Since the real load is directly related to the input power from the prime mover, i. There are, however, certain practical difficulties involved, but as it is possible to reference back any real load unbalance to the constant-speed drive unit between engine and generator, real load-sharing control is effected at this unit by adjusting torque at the output drive shaft.
Reactive load unbalances are corrected by controlling the exciter field current delivered by the voltage regulators to their respective generators, in accordance with signals from a reactive load-sharing circuit. The sharing of real load between paralleled generators is determined by the real relative rotational speeds of the generators which in turn influence the voltage phase relationships. As we learned earlier see p. It is. Therefore when two or more operate in parallel they lock together with respect to frequency and the system frequency established is that of the generator whose output is at the highest level.
Since this is controlled by speed-governing settings then it means that the generator associated with a higher setting will carry more than its share of the load and will supply energy which tends to motor the other machines in parallel with it. Thus, sharing of the total real load is unbalanced, and equal amounts of energy in the form of torque on the generator rotors must be supplied. Fundamentally, a control system is comprised of two principal sections: A circuit diagram of the system as applied to a four-generator installation is shown schematically in Fig.
The current transformers sense the real load distribution at phase "C" of the supply from each generator, and are connected in series and together they form a load sharing loop.
Each load controller is. The output side of each load controller is, in turn, connected to a solenoid in the speed governor of each constant speed unit. When current flows through phase "C" of each generator, a voltage proportional to the current is induced in each of the current transformers and as they are connected in series, then current will flow in the load sharing loop. This current is equal to the average of the current produced by all four transformers.
Let us assume that at one period of system operation, balanced load sharing conditions are obtained under which the current output from each transformer is equal to five amps, then the average flowing in the load sharing loop will be five amps, and no current circulates through the error sensing elements. If now a generator, say No. The share of the load being carried by the other generators falls proportionately, thereby reducing the output of their current transformers, and the average current flowing in the load sharing loop remains the same, i.
If, for example, it is assumed that the output of No. The three amps difference divides equally between the other generators and so the output of each corresponding current transformer is reduced by one amp, a difference which flows through the error sensing elements of the load controllers. The error signals are then applied as d. The current and magnetic field simulate the effects of centrifugal forces on the flyweights and are of such.
Thus, in the unbalanced condition we have assumed, i. I generator running at a higher governor setting, the current and field resulting from the error signal applied to the corresponding load controller flows in the opposite sense and repels the flyweights, thereby simulating a decrease of centrifugal force. The movement of the flyweights causes oil to flow to underdrive and the output speed of the constant speed unit drive decreases, thereby correcting the governor setting to decrease the load being taken by No.
The direction of the current and field in. The sharing of reactive load between paralleled generators depends on the relative magnitudes of their output voltages which vary, and as with all generator. If, for example, the voltage regulator of one generator is set slightly above the mean value of the whole parallel system, the regulator will sense an under-voltage condition and it will accordingly increase its excitation current in an attempt to raise the whole system voltage to its setting.
However, this results in. Thus, its load is increased while the loads of the other generaton are reduced and unbalance in reactive load sharing exists. It is-therefore necessary to provide a circuit to correct this condition.
In principle, the method of operation of the reactive load-sharing circuit is similar to that adopted in the real load-sharing circuit described earlier. A difference in the nature of the circuitry should however be noted at this point. Whereas in the real load-sharing circuit the current transformers are connected directly to.
These are, in fact, transformers which have i a power source connected to their secondary windings in addition to their primaries; in this instance, phase "C" of the generator output, and ii an air gap in the iron core to produce a phase displacement of approximately 90 degrees between the primary current and secondary voltage.
When a reactive load unbalance occurs, the current transformers detect this in a similar manner to those associated with the real load-sharing circuit and they cause differential currents to flow in the primary windings of their associated mutual reactors. Voltages proportional to the magnitude of the differential currents are induced in the secondary windings and will either lead or lag generator current by 90 degrees.
When the voltage induced in a particular reactor secondary winding leads the associated generator current it indicates that a reactive load exists on the generator; in other words, that it is taking more than its share of the total load.
In this condition, the. The secondary winding of each mutual reactor is connected in series with an error detector in each voltage regulator, the detector functioning in the same manner as those used for voltage regulation and real load-sharing see pp.
The voltage induced in the secondary winding of the corresponding mutual reactor will be additive and so the error detector will sense this as an overvoltage. The resulting d. In the case of the other three generators they will have been carrying less than their share of the reactive load and, therefore, the voltages induced in their mutual reactors will have.
Thus, the output of each power amplifier will be adjusted to increase the amount of exciter current being delivered to their associated generators until equal reactive load-sharing is restored between generators within the prescribed limits.
However, an air-drive can serve. The drive consists of a two-bladed fan or air turbine as it is sometimes called, and a step-up ratio gear train which connects the fan to a single a.
The generator is of a similar type to the main generator see also p. The complete unit is stowed on a special mounting in the aircraft fuselage, and when required is deployed by a mechanically linked release handle.
When deployed at airspeeds of between to knots, the fan and generator are driven up to their appropriate speeds by the airstream, and electrical power is delivered via a regulator at the rated values. After deployment of the complete unit, it can only be restowed when the aircraft is on the ground. Explain the term Power Factor and state how it is affected by a circuit containing inductance and capacitance.
State the factors upon which the frequency output of an a. With the aid of a sketch, describe the construction of a typical aircraft generator and determine mathematically the output frequency of the machine which you have illustrated. With the aid of a schematic diagram, describe how a generator can be excited and how its output voltage can be controlled.
With the aid of a sketch, explain the construction and operating principles of a three-phase brushless generator. What factors must be controlled when constantfrequency a. State the functions which current transformers perform in controlling load sharing between constant-frequency generators.
In connection with a transistor, what do the letters "p" and "n" refer to? In aircraft electrical installations a number of different types of consumer equipment are used which require power supplies different from those standard supplies provided by the main generator. For example, in an aircraft having a 28 volts d. It therefore becomes necessary to employ not only equipment which will convert electrical power from one form to another, but also equipment which will convert one form of supply to a higher or lower value.
The equipment required for the conversion of main power supplies can be broadly divided into two main types, static and rotating, and the fundamentals of construction and operation of typical devices and machines are described under these headings. The principal items which may be grouped under this heading are rectifiers and transformers, some applications of which have already been discussed in Chapter 3, and static d. The latter items are transistorized equivalents of rotary inverters and a description of their construction and operating fundamentals will be given at the end.
The process of converting an a. As a result of this exchange, a barrier layer is formed which exhibits different resistance and conductivity characteristics and allows current to flow through the element combination more easily in one direction than in the opposite direction.
Thus, when the applied voltage is an alternating quantity the barrier layer converts the current into a un directional flow and provides a rectified output. One of the elements used in combination is referred to as a "semi-conductor" which by definition denotes that it possesses a resistivity which lies between that of a good conductor and a good insulator.
Semi-conductors are also further defined by. Thus, an element having. When a voltage is applied. If, on the other hand, the semi-conductor is made negative to the metal, further electrons are drawn from the metal to fill more positive holes and the "reverse" resistance of the barrier layer is thus increased.
The greater the difference in the resistance to current flow in the two directions the better is the rectifying effect.
A similar rectifying effect is obtained when an ntype semi-conductor is in contact with metal and a difference of potential is established between them, but in this case the direction of "easy" current flow is reversed. In practice, a small current does flow through a rectifier in the reverse direction because p-type material contains a small proportion of free electrons and n-type a small number of positive holes.
In the rectification of main a. Rectifiers employing germanium a metallic element are also available but as their operating temperature is limited and protection against short duration overloads is difficult, they are not adopted in main power systems. The selenium rectifier is formed on an aluminium sheet which serves both as a base for the rectifying junction and as a surface for the dissipation of heat.
A cross-section of an element is shown diagrammatically in Fig. A thin layer of a lowmelting point alloy, referred to as the counter electrode, is sprayed over the selenium coating and insulating varnish. Contact with the two elements of the rectifying junction, or barrier layer, is made through the base on one side and the counter electrode on the other.
Mechanical pressure on the rectifying junction tends to lower the resistance in the reverse direction and this is prevented in the region of the mounting studs by the layer of varnish. In practice a number of rectifying elements may be connected in series or parallel to form what is generally referred to as a rectifier stack.
Three typical stacks are shown in Fig.
When connected in series the elements increase the voltage handling ability of a rectifier and when connected in parallel the ampere capacity is increased. This will be apparent from Fig. The silicon is in the form of an extremely small slice cut from a single crystal and on one face it has a fused aluminium alloy contact to which is soldered an anode and lead. The other face is soldered to a base, usually copper, which forms the cathode and at the same time serves as a heat sink and dissipator.
The barrier layer is formed at the aluminium-silicon junction. The limiting factors in the operation of a rectifier are: In selenium rectifiers the maximum temperature is of the order.
It should be noted that these figures represent the actual temperature at the rectifying junction and therefore the rectifier, as. Proper cooling under all conditions is, therefore, an essential requirement and is normally taken care.
Voltage ratings are determined by the ability of a rectifier to withstand reverse voltage without passing excessive reverse current, and the characteristics are such that reverse current does not increase proportionately to the applied voltage.
This is because once all the current carriers have been brought into action there is nothing to carry any further current. However, at a sufficiently high voltage the resistance in the reverse direction breaks down completely and reverse current increases very sharply. The voltage at which breakdown occurs is called the Zener voltage, and as. For power rectification, rectifiers must have a high Zener voltage value and each type must operate at a reverse voltage below its designed breakdown value.
Some rectifiers, however, are designed to breakdown at a selected value within a low voltage range between 2 and 40 volts is typical and to operate safely and continuously at. It is a threeterminal device, two terminals corresponding to those of an ordinary silicon diode and the third, called the "gate" and corresponding to the thyratron grid.
The construction and operating characteristics of the device are shown in Fig. The silicon wafer which is of the "n-type" has three more layers formed within it in the sequence indicated. When reverse voltage is applied an S. The voltage at which breakover takes place can be varied by applying small current signals between the gate and the cathode, a method known as "firing".
Once conduction has been initiated it can be stopped only by reducing the voltage to a very low value. The mean value of rectified voltage can be. Thus, an S. A typical application of S. Rectifiers are used in single-phase and three-phase supply systems and, depending on the conversion requirements of a circuit or system, they may be arranged to give either half-wave or full-wave rectification.
In the former arrangement the d. The single-phase half-wave circuit shown in Fig. The output from the single rectifier is a series of positive pulses the number of which is equal to the frequency of the input voltage. For a single-phase a. For half-wave rectification of a three-phase a. This arrangement is comparable to three single-phase rectification circuits, but since the positive half-cycles of the input are occurring at time intervals of one third of a cycle degrees the number of d.
Figure 4. Examples of threephase bridge rectifier applications have already been shown in Chapter 3 but we may now study the circuit operations in a little more detail.
In this type of circuit only two rectifiers are conducting at any instant; one on the positive side and the other on the negative side. Also the voltage applied to the bridge network is that between two of the phases, i. Let us consider the points "A" and "B" on the three phase voltage curves. These points represent the line voltage between phases I.
Between the points "C" and "D" the line voltage corresponds to that between. This process continues through the remaining three conducting paths, the sequence of the relevant phases and the rectifiers which conduct being as tabulated in FigA.
The output voltage, which is determined by the distance between the positive and negative crests, consists of the peaks of the various line voltages for phase angles of 30 degrees on either side of their maxima. Since the negative half-cycles are included, then the ripple frequency of a bridge rectifier output is six times that of the a. A transformer is a device for converting a.
It consists of three main parts: The sequence of alerting is shown at b of Fig. As an aircraft descends or climbs to the preselected altitude the difference signal is reduced, and the logic circuit so processes the input signals that, at a pre-set outer limit H 1 typically ft above or below preselected altitude, one signal activates the aural alerting device which remains on for two seconds; the annunciator light is also illuminated.
The light remains on until at a further pre-set inner limit H2 typically ft above or below preselected altitude, the second. As an aircraft approaches the preselected altitude, the synchro system approaches the 'null' position, and no further alerting takes place. If an aircraft should subsequently depart from the preselected altitude, the controller logic circuit changes the alerting sequence such that the indications correspond to those given during the approach through outer limit Hi, i.
Angle of attack The angle of attack AoA , or alpha a angle, is the angle between sensing the chord line of the wing of an aircraft and the direction of the relative airflow, and is a major factor in determining the magnitude of lift generated by a wing. Lift increases as a increases up to some critical value at which it begins to decrease due to separation of the slow-moving air the boundary layer from the upper surface of the wing, which, in turn, results in separation and turbulence of the main airflow.
The wing, therefore, assumes a stalled condition, and since it occurs at a particular angle rather than a particular speed, the critical AoA is also referred to as the stalling angle. The manner in which an aircraft responds as it approaches and reaches a stalled condition depends on many other factors, such as wing configuration, i. Other factors relate to the prevailing speed of an aircraft, which largely depends on engine power settings, flap angles, bank angles and rates of change of pitch.
The appropriate responses are pre-determined for each type of aircraft in order to derive specificaliy relevant procedures for recovering from what is, after all, an undesirable situation. An aircraft will, in its own characteristic manner, provide warning of a stalled condition, e.
It is, therefore, necessary to provide a means whereby a can be sensed directly, and at some value just below that at which a stalled condition can occur it can provide an early warning of its onset. Stall warning systems The simplest form of system, and one which is adopted in several types of small aircraft, consists of a hinged-vane-type senso. The vane is protected against ice formation by an internal heater element. When a reaches that at which the warning unit has been preset.
Control switches for normal operation and for testing are also provided in this unit. In larger tyges of aircraft.
Since the pitch attitude of an aircraft is also changed by the extension of its flaps. If the aircraft's attitude changes such that a increases. The circuit of a typical system is shown in basic form in Fig. When the aircraft is on the ground and electrical power is on.. Sensing relays and shock strut microswitches on the nose landing gear are included in the circuit of a system to permit operational change-over from ground to air.
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. In normal level flight conditions. The complete unit is accurately aligned by means of index pins at the side of the front fuselage section of an aircraft. Sensor signals. It consists of a precision counter-balanced aerod namic..
The only signal now supplied to the amplifier and demodulator is the modified a signal. The output is then supplied to a demodulator whose circuit is designed to 'bias off the ac voltage from the contacts of K 1. The demodulator then produces a resultant voltage which triggers the switch SS 1 to connect a 28 V de supply direct to the stick-shaker motor.
V to the circuit module amplifier. Bias of! In normal flight. During take-off. When such aircraft first get into a stalled condition then.
The positions are: The comparator is also supplied with signals from a central processor unit also within the module which processes a programme to determine maximum a angles based on the relationship between flap position and three positions of the leading edge slats. If the latter is higher than a computed maximum. The manner in In certain types of aircraft the sensor signals are transmitted to an air data computer. A confidence check on system operation may be carried out by placing the circuit module control switch in the 'TEST' position.
This energizes a relay which switches the sensor signal to the motor of an indicator. Since the switch isolates the sensor circuit from the amplifier. In order to prevent the development of a deep stall situation. The aircraft then sinks rapidly in the deep Slalled attitude. In aircraft having computerized flight control systems. When selected for installation. Whenever stick-push is activated. Another type of indicator currently in use has a pointer which is referenced against horizontal yellow.
In some cases a conventional pointer and scale type of display is used. Indicators are connected to the alpha sensors of a stall warning system. Indicators There is no standard requirement for angle of attack indicators to be installed in aircraft.
In the more sophisticated types of aircraft. The field differs from that of an ordinary magnet in several This partly explains the fact that the magnetic poles are relatively large areas. That this is so is obvious from the fact that a magne. A plane passing through the magnet and the centre of the earth would trace out on the earth's surface an imaginary line called the magnetic meridian as shown in Fig.
Terrestrial magnetism The surface of the earth is surrounded by a weak magnetic field which culminates in two internal magnetic poles situated near the North and South true or geographic poles. The origin of the earth's field is still not precisely known. The operating principle of a direct-reading compass is based on established fundamentals of magnetism.
As far as present-day aircraft are concerned. It would thus appear that the earth's magnetic field is similar to that which would be expected at the surface if a short but strongly magnetized bar magnet were located at the centre. Its points of maximum intensity. If a map were prepared to show both true and magnetic meridians.
The horizontal angle contained between the true and the magnetic meridian at any place is known as the magnetic variation or declination. Figure 3. Magnet'ic variation As meridians and parallels are constructed with reference to the true or geographic North and South pcles..
BB and CC are isoclinals. I Terrestrial magnetism. While the variation differs all over the world. At some places on the earth. The angle the lines of force make with the earth's surface at any Figure 3. Lines are drawn on the charts. Information regarding variation and its changes are given on special charts. It will not. These lines emerge vertically from the North magnetic pole. The angle of dip at all places undergoes changes similar to those described for variation and is also shown on charts of the world.
The pivot point is above the centre of gravity of the magnet system which is balanced in such a way as to minimize the effects of angle of dip over as wide a range of latitudes North and South as possible.
This total force is resolved into its horizontal and vertical components. If stated as a relative value. As in the case of variation and dip. Earth's total force When a magnet freely suspended in the earth's field comes to rest.
The earth's magnetic force may be stated either as a relative value or an absolute value. It is filled with a silicone fluid to make the compass aperiodic. The majority of compasses currently in use are of the card type. Places on these charts having the same dip angle are joined by lines known. The relationship between these components and dip is shown in Fig.
The system is pendulously suspended by an iridium-tipped pivot resting in a sapphire cup supported in a holder or stem. The bowl is of plastic Diakon and so moulded that it has a magnifying effect on the card and its graduations.
Compass Direct-reading compasses have the following common principal construction features: Lines of equal H and Z forces are referred to as isodynamic lines. The fluid also provides The bowl is in the form of a brass case which is sealed by a front bezel plate.
The compass shown at b of Fig. Its magnet system is similar to the one described earlier except that needle-type magnets are used. Values are quoted by manufacturers as part of the operating data appropriate to their equipment.
Changes in liquid volume are compensated by a capsule type of expansion device. Acceleration error This may be broadly defined as the error. Changes in volume of the fluid due to temperature changes. A small lamp is provided for illuminating the card of the magnet system. In this connection it is usual to apply the compass safe distance rule which.
Compass location The location of a compass in any one type of aircraft is of importance. Errors in indication The pendulous suspension of a magnet system. A permanent-magnet deviation compensator is located at the underside of the bowl. Compensation of the effects of deviation due to longitudinal and lateral components of aircraft magnetism see page 87 is provided by permanent magnet coefficient 'B' and 'C' corrector assemblies secured to the compass mounting plate.
There are two main errors that result from such components. The distance is measured from the centre of a compass magnet system to the nearest point on the surface of equipment. I hemisphere: RLY W!: In either the northern or southern hemispheres. R c I s d s system when its centre of gravity is displaced from its normal position.
The forces brought into play will be as shown in Fig. Consider now an acceleration on a northerly heading in the northern hemisphere. Since both the point P and centre of gravity are in the plane of the magnetic meridian.
The reaction to this force will be equal and opposite and must act through the centre of gravity. The reverse effects occur during a deceleration. The two forces constitute a couple which. When an acceleration occurs on an easterly heading in the northern hemisphere. When accelerating or decelerating on any fixed heading. The extent and direction of the error is dependent upon the aircraft's heading.
As soon as the system is tilted. In order to form a clearer understanding of its effects. Turning from a nonherly heading towards east or west Figure 3. In the southern hemisphere the results will be reversed in each case.
For a correctly banked tum. Turning errors During a turn. If the change in attitude is also accompanied by a change in speed. As The system's centre of gravity. Let us assume that a change in heading to the eastward is required. If an aircraft flying level is put into a climb at the sa: One further point may be mentioned in connection with these errors.
In the southern hemisphere diagram b the effects are somewhat different. We may again consider the case of an aircraft turning eastward from a northerly heading. Since the centre of gravity is now north of point P.. The south magnetic pole is now the doll inant pole and so the offset dip angle of the magnet system changes to displace the centre of gravity to the north of point P The same effects will occur if the heading changes from N to W whilst flying in the northern hemisphere.
Turning from a southerly heading towards east or west If the turns are executed in the northern hemisphere Fig. Turning through east or west When turning from an easterly or westerly heading in either the northern or southern hemispheres diagrams e - h no errors will result because the centrifugal acceleration acts in a vertical plane through the magnet system's centre of gravity and point P.
In all the above cases. For this reason the term nonherly turning error is often used when describing the effects of centrifugal acceleration on compass magnet systems. Hard-iron magnetism is of a peramenent nature and is caused. Aircraft magnetism Magnetism is unavoidably present in aircraft in varying amounts. The two types of magnetism can be further divided in the same W'iJ. The centre of gravity is merely deflected to the north or south of point P. In turning from a southerly heading in the southern hemisphere Fig.
A point which may be noted in connection with turns from E or W is that when the N or S end of the magnet system is tilted up. Y that magnetic materials are classified according to their ability to be magnetized. Components of hard-iron magnetism The total effect of this type of magnetism at a compass position may be considered as having originated from equivalent bar magnets lying longitudinally.
The deviations caused by each of the components are set out in Table 3. Q and R. There is also a third type of magnetism. Soft-iron magnetism is of a temporary nature and is caused by the metallic materials of an aircraft which are magnetically 'soft' becoming magnetized due to induction by the earth's field.
The resulting deviations are termed easterly when positive. The effect of this type of magnetism. The components are respectively denoted as P. Such magnetism depends. The strength of these components does not vary with heading or change of latitude. The polarities and strengths of components X and Y vary with changes in aircraft heading relative to the fixed direction of the earth's component H.
Components of soft-iron magnetism The effect of this type of magnetism may be considered as originating from a piece of soft-iron in which magnetism has been induced by the earth's field. Components X. Component R effective only in the aircraft altitudes indicated.
A change in the polarity of component Z will only occur with a change in magnetic hemisphere. This field. Y and Z also change with geographical location because this results in changes in the earth's field strength and direction. Table 3. Total magnetic effect The total effect of the magnetic fields that produce deviating forces relative to each of the three axes of an aircraft is determined by algebraically summing the quantities appropriate to each of the related components..
Table Each of the three components produce three soft-iron components that are designatej aX The deviations caused by such components are set out in Table The polarities and direction of components cZ and jZ depend on whether an aircraft is in the northern or southern hemisphere..
In practice it is not necessary to distinguish between them. The coefficient is calculated by taking the average of the algebraic differences between deviations measured on a number of equidistant D and E.
The relationship between them and the components of aircraft magnetism is shown in Fig. There are five coefficients designated A. Coefficient A This represents a constant deviation and may be termed as either real A.
Coefficient C This represents the resultant deviation due to the presence. The coefficient is calculated from the formula: When these components are of like signs. When of like and unlike signs these components cause deviations whose directions are the same as those caused by components P and cZ. Coefficient D This represents the deviation due to the presence.
Deviation on W 2 Since components P and cZ cause deviation which varies as the sine of an aircraft's heading 8. In the case of A.
These adjustments are effected by compensator or corrector magnet devices which. A compensator forms an integral part of a compass see Fig. Adjustment for coefficient A is effected by repositioning the compass in its mounting by the requisite number of degrees.
The total deviation on an uncorrected compass for any given direction of an aircraft's heading by compass may be expressed by the equation: One pair of magnets is positioned laterally to provide a variable longitudinal. It is calculated from the formula: Coefficient E This coefficient represents the deviation due to the presence of components bY and dX of like signs. Deflection of the compass magnet system would be obtained in a similar manner with the aircraft heading west.
Variation of the field strength by rotating the magnets will. It will be apparent from the foregoing operating sequences that maximum compensation of deviation on either side of cardinal headings is obtained when the magnets are in complete alignment. The north-seeking pole of the compass magnet system will. The coefficient C compens. If the magnets are rotated so as to strengthen the field between poles N 1 and S 2. This would also be the case if the aircraft was heading south.
The manner in which compensation is carried out may be understood by considering the case of an adjustment having to be made for coefficient B. Since tl1e intensity of a field varies in inverse proportion to the square of the distance from its source.
When the appropriate compensator magnets are in the neutral position. The total field of the magnets. At b of Fig. The gears on which magnets are mounted are connected to operating heads which. Indication of the neutral position of magnets is given by aligning datum marks. The gyroscope and As a mechanical device a gyroscope may be defined as a system Its properties containing a heavy metal wheel or rotor. There are three such instruments. The gimbal system is mounted in a frame as shown in Fig.
Both these properties depend on the principle of conservation of angular momentum. The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings. The system will not exhibit gyroscopic properties unless the rotor is spinning.
When the rotor is made to spin at high speed. The complete group constitutes what is tenned the 'basic six' arrangement. The whole assembly is known as the gimbal system of a free or space gyroscope. The three additional instruments utilize a gyroscopic type of sensing element. It will also be found. I Elements of a gyroscope. If a weight is now suspended from the inner gimbal ring with the rotor spinning it will be found that the ring will support the weight.
The two gyroscopic properties may be more closely defined as follows: If we lift the front wheel off the ground. These rather intriguing properties can be exhibited by any system in which a rotating mass is involved. Other familiar mechanical systems possessing gyroscopic properties are aircraft propellers. Figure 4. The property v. Determining the direction of precession The direction in which a gyroscope will precess under the influence of an applied force may be determined by means of vectors and by solving certain gyrodynamic problems.
Precession of a rotor will continue. The other segments will be affected in the same way. The angular change in direction of the plane of rotation under the influence of an applied force.
The greater the force. The axis about which a force is applied is termetl the input axis. Each segment has motion m in the direction of rotor rotation. The change in direction takes place. At a in Fig. Let us assume for a moment that the rotor is broken into segments and concern ourselves with two of them at opposite sides of the rim as shown at c.
In transmitting this force to the rim of the rotor. This motion is resisted by rigidity. It is done by representing all forces as acting directly on the rotor itself.
The rate ot' precession also depends on three factors: At this point there will be no further resistance to the force and so precession will cease. This time. I I X d el In the example illustrated in Fig. As in the previous case this results in the direction of motion changing to the resultant of motion m and force F..
F x--'1. These datums are established by using vertical and horizontal spin-axis gyroscopes respectively as shown in Fig. Both types utilize their fundamental properties in the following For this reason. It will also be noted from Fig. When a Limitations of a free In flight. Each one has three degrees of freedom and. AXIS Dll! Thus, to an observer on the earth having no sense of the earth's rotation, the gyroscope would appear to veer or drift.
This may be seen from Fig. S a which illustrates a horizontal-axis gyroscope at a latitude A: At 'A', the input axis is aligned with the local N-S component of w,; therefore, to an observer at latitude A When the input axis is aligned with that of the earth 'B' , drift would also be apparent, but at a rate equal to w,, i.
In order to further illustrate drift, we may consider diagram b of Fig. If the same gyroscope were to be positioned so that its input axis ZZ 1 was aligned with the E-W component of w, at any point, its spin axis would then be vertical; in other words, it becomes a vertical-axis gyroscope. Since the plane of rotation is coincident with that of the earth, there will be no apparent drift. Real drift Real drift results from imperfections in a gyroscope such as bearing friction and gimbal system unbalance.
Such imperfections cause unwanted precession which can only be minimized by applying precision engineering techniques to the design and construction.
Transport wander Let us again consider a horizontal-axis gyroscope which is S!! In this position it will exhibit an apparent drift equal to w,. Assume now that it is carried to a lower latitude, ,and with its input axis aligned with the local vertical component of w,. During the period of transport it will have appeared to an observer on the earth that the spin axis has tilted in a vertical plane, until at the new latitude it appears to be in the position shown at c of Fig.
This apparent tilt, or transpon wander, would also be observed if, during transport, the input axis were aligned with either a local N -S component, or a local E-W component of w,. Transport wander will, of course, appear simultaneously with drift, and so for a complete rotation of the earth, the gyroscope as a whole would appear to make a conical movement. The angular velocity or transpon rate of this movement will be decreased or increased depending on whether the E-W component of an aircraft's speed is.
The N -S component of the speed will increase the maximum divergence of the gyroscope axis from the vertical, the amount of divergence depending on whether the aircraft's speed has a N or S component and also on whether the gyroscope is situated in the northern or southern hemisphere.
The relationship between w. Transport wander u V i! If the input axis of a gyroscope were to be positioned such that its spin axis was vertical, then during transport it would only exhibit transport wander. Control of drift and transport wander Before a free gyroscope can be of practical use, drift and transport wander must be controlled so that the plane of spin of the rotor is maintained relative to the earth; in other words, it requires conversion to what is termed an earth gyroscope.
The control of transport wander is normally achieved by using gravity-sensing devices which automatically detect tilting of the gyroscope's spin axis, and applying the appropriate corrective torques. The operation of some typical control methods will be described later under the headings of the appropriate flight instruments. Displacement Depending on the orientation of its gimbal system, a displacement gyroscope limitations gyroscope can be subject.
Gimbal lock This occurs when the gimbal orientation is such that the spin axis becomes coincident with one or other of the axes of freedom which serve as attitude displacement references. Let us consider, for example, the case of the spin axis of a vertical-axis gyroscope shown in Fig.
If, in this 'locked' condition of the gimbal system, the gyroscope as a whole were to be mrned, then the forces acting on the gimbal system would cause the system to precess or topple. Gimbal error This is an error which is also related to gimbal system orientation, and it occurs whenever the gyroscope as a whole is displaced with its gimbal rings not mutually at right angles to each other.
The error is particularly relevant to horizontal-axis gyroscopes when used in direction indicating instruments see page Methods of operating There are two principal methods of driving the rotors of gyroscopic gyroscopic flight flight instruments: A typical vacuum system is shown schematically in Fig.
A vacuum indicator, a relief valve, and a central air filter are also provided. In operation the pump creates a vacuum that is regulated by the valve at a value between 3. Some types of tum-and-bank indicator may operate at a lower value Vacuum-operated system. Relief valve. Vacuum connection fl'; Each instrument case has two connections: When vacuum is applied, the pressure within the cases of the instruments is re,duced to allow surrounding air to enter' and emerge through the spinning jets.
The jets are positioned adjacent to a series of recesses commonly called 'buckets' formed in the periphery of each gyroscope rotor, so that as the airstreams impinge on the 'buckets', the rotors are rotated at high speed. An example of a relief valve is shown in Fig.
During system operation the valve remains closed by compression of the spring, the tension of which is pre-adjusted to obtain the required vacuum so that air pressure acting on the outside of the valve is balanced against spring tension.
If for some reason the adjusted value should be exceeded, the outside air pressure would overcome spring tension, thus opening the valve to allow outside air to flow into the system until the balanced condition was once again restored. A pressure-operated system is, as far as principal components are concerned, not unlike a vacuum system but, as will be noted by comparing Figs 4. Electric In electrically-operated instruments, the gyroscopes are special adaptations of ac or de motors that are designed to be driven from the appropriate power supply systems of an aircraft.
In current applications, ac motors are adopted in gyro horizons, while de motors are more common to tum-and-bank indicators. Gyroscopes used for the purpose of direction indicating can also be motor-driven, but they normally form part of a magnetic heading reference system, or of the more widely adopted flight director systems.
These systems will be covered in later chapters. Gyro horizon A gyro horizon indicates the pitch and roll attitude of an aircraft principle relative to its vertical axis, and so for this porpose it employs a Pressure regulator In-line fitter. Pressure gauge. Pump Gyroscopic instruments. Supplementary indications of roll are presented by the position of a stabilized pointer and a fixed roll angle ,scale.
Two methods of presentation are shown in Fig. The gimbal system see Fig. In operation the gimbal system is stabilized so that in level flight the three axes are mutually at right angles.
When there is a change in an aircraft's attitude, it goes into a climb, say, the instrument case and outer ring wm move about the axis YY I of the stabilized inner ring. The horizon bar is pivoted at the side and to the rear of the outer ring, and engages an actuating pin fixed to the inner ring, thus forming a magnifying lever system.
The pin passes through a curved slot in the outer ring. In a climb attitude the bar pivot carries the rear The front end of the bar is therefore moved downwards through a angle than that of the outer ring, and since the movement is relative to the symbolic aircraft element, the bar wili indicate a climb attitude. Changes in the lateral attitude of an i.
Hence, lateral attitude are indicated by movement of the symbolic aircraft element relative to the horizon bar, and also by relative movement between the mil angle scale and pointer.
The reason for this restriction is to lock see page A typical instrument of the vacuum-driven type is shown in Fig. The upper bearing of the rotor is to for the effects of differential expansion between the rotor shaft and casing under varying temperature conditions.
A plate which. I I Pneumatic type of gyro horizon. O pendulous vane unit, 11 buffer stops, l2 bank pointer. The air issuing from the jets impinges on the rotor buckets. When power is applied. One of the essential requirements of any gyroscope is to have the mass of the rotor concentrated as near to the periphery as possible. The ac power supply is fed to the motor stator via slip rings.
This is overcome by designing the rotor and its bearings so that it rotates on the outside of the stator.
Electric gyro horizon This instrument is made up of the same basic elements as a pneumatic type. After spinning the rotor. The latter is positioned over the outer ring rear-bearing support and pjvot which are drilled to communicate with a channel in the outer ring.
This presents no difficulty where solid. An induction motor normally has its rotor revolving inside its stator. A vacuum supply connection is provided at the rear of the instrument case.