US2851113A - Control system for a variable pitch propeller and its driving turbines - Google Patents

Description

E. CONTROL SYSTEM FOR A VARIABLE PITCH PROPELLER Sept. 9, 1958 M. IRWIN ET AL AND IIs DRIVING 'IURBINES 9 sheets-sheet 1 Filed Nov. 8, 1950 Sept. 9, V1958 E. M. IRWIN x-:TAL 2,851,113 CONTROL SYSTEM FOR A VARIABLE PITCH PRQPELLER AND ITS. DRIVING TURBINES Filed Nov. 8, 1950 9 Sheets-Sheet 2 Cttornegs Sept. 9, 1958 E. M. IRWIN ET'AL 2,851,113

CONTROL- SYSTEM FOR A VARIABLE FITCH PRPELLER f AND ITS DRIVING TURBINES Filed Nov. 8, 1950 9 Sheets-Sheet 3 'lrwentors S M L Am T EN Nm .II Ww. mm Mm .I ED N A sept. 9, 19584 CONTROL SYSTEM FOR A VARIABLE FITCH PROPELLER 9 Sheets-Sheet 4 Filed Nov. 8, 1950 E. M. IRWIN ETAL 2,851,113 CONTROL SYSTEM FOR A VARIABLE FITCH PROPELLER AND ITS DRIVING TURBINES Filed Nov. 8, 1950 9 Sheets-Sheet 5 Sept. 9, 1958 .III III Il III I IIIIIIIIJ 9 Sheets-Sheet 6 E. M. IRWIN ET AL CONTROL SYSTEM FOR A VARIABLE PITCH PROPELLER AND ITS DRIVING TURBINES Sept. 9, 1958 I Filed Nov. 8, 195o Sept. 9, 1958 E,

CONTROL SYSTEM Fo AND ITs DRIVING TURBINES Filed Nov. 8. 1950 M. IRWIN ET AL R A VARIABLE FITCH PROPELLER 9 Sheets-Sheet 7 Sept. 9, 1958 E. M. IRWIN ET AL CONTROL SYSTEM RoR A VARIABLE PIICH RROPELLER AND IIs DRIVING TuRBINRs 9 Sheets-Sheet 8 Filed Nov. 8, 1950 /DLE i y Gttornegs WMM sept. 9, 1958 Filed Nov. 8, 1950 E. M. IRWIN ETAL CONTROL SYSTEM FORA VARIABLE PITCH PROPELLER AND ITS vDRIVING TURBINES 9 sheets-sheet 9 United States Patent C CONTROL SYSTEM FR A VARIABLE PTCH PRO PELLER AND ITS DRTVING TURBINES Edmund M. Irwin, Floyd J. Boyer, Arthur W. Gauhatz, and Robert I. Wente, lndianapoiis, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application November 8, 1950, Serial No. 194,716

66 Claims. (Cl. Uli-135.72)

This invention relates to control systems for power plants and, more particularly, to a control system for an aircraft power plant in which a propeller is driven by two gas turbine engines.

The control system, in its preferred embodiment, is particularly adapted to the control of a particular propulsion unit embodying a variable pitch governing propeller which may be feathered and which may lbe operated in direct blade angle control in both forward and reverse pitch. The propulsion unit lcomprises clutches by which either or both power units may be utilized to drive the propeller. It comprises, moreover, a starting system, fuel supply apparatus, fuel feed controlling means,

fuel ignition, and other engine auxiliaries.

A primary object of the invention is to assure and facilitate operation of the power plant most safely and eiiiciently, and with a wide choice of operating conditions. Another primary object of the invention is to provide a system by which the various components and accessories of the engine may be organized into a unitary system and the ultimate control by the pilot or flight engineer of an aircraft made as simple as possible. Another important purpose of the control system is to provide a maximum of llexibility of operation of the power plant with a minimum of controls requiring attention from the pilot. Another important object of the invention is to provide what may be termed a supervisory control to insure that conicting and harmful control operations will not be possible; in other words, to provide proper scheduling and interlock of various functions relating to the power plant.

A further object of the invention is to provide for harmonious operation of the two power units of the engine and of one or both units with the propeller' under various conditions.

Further and more specific objects of the invention are to provide a control which cuts out the power unit if the propeller is feathered; to provide a control adaptable to various propeliers of the most advanced types; to facilitate starting `of the units; to provide for safe, efcient, and automatic control of power clutches; to interloci: :the power control levers of the two units to prevent improper operation. of each relative to the operation of the other and the operating condition of the power plant; and to coordinate the power control levers of the unit for joint control of both power units and the propeller to eliminate conflicts.

Many other objects and advantages of the invention will be apparent to those skilled in the art from the subsequent detailed description `of the preferred embodiment of the invention, The importance of these will be apparent to those cognizant of the demands upon and responsibilities `of aircraft iiight personnel, which make it extremely important that the operation of the power plant of the aircraft provide for various procedures and con-- tingencies, that it be accomplished by the simpliest possible controls, and that safeguards be provided against improper operation. t

" ice Referring to the drawings, Figure l is a schematic diagram of a dual power unit gas turbine propeller aircraft propulsion plant incorporating the control system ofthe invention; Figure 2 is a somewhat schematic View of the mechanical structure of a power unit control and a pilots control lever assembly; Figure Za is a detail of the yunit control; Figure 3 is a sectional View taken on the plane indicated in Figure 2, further illustrating a power control lever; Figures 4 to 9, inclusive, comprise a circuit diagram of the electrical system of the invention, which has been subdivided into separate figures according to function in view of the complexity of the system and to facilitate exposition thereof; Figure l0 is a diagram illustrating the operating sequence `of the power control lever and throttle switches; Figures 11 and l2 are diagrams of the operating sequences of the limit switches ofthe actuators for the clutches and for the propeller, respectively; and Figure 13 is a chart of a preferred control schedule of an illustrative power plant.

More particularly, with reference to Figures 4 to 9, Figure 4 shows the power circuits, Figure 5 the starter circuits, Figure 6 the ignition, fuel control, and intake shutter circuits, Figure 7 the clutch circuits, Figure 8 the propeller control circuits, and Figure 9 the interlocking circuits of the power control levers. It will be understood that these circuits are interdependent. However, since the entire system cannot be shown properly on a single sheet, it is believed that clarity of exposition will best be served by dividing the control circuit betweeny the various figures on a functional basis.

Introduction in view of the complexity of the system, it is believed desirable to preface the detailed description by a general account of the nature of the system and ofthepower plant.

Referring to Figure l, the power plant comprises two identical gas turbine engines A and B, which will be referred to as power units. Since these units may be of known type, and since the invention is not restricted in its application to a particular type of power unit, detailed description of these units is unnecessary. ltmay be pointed out, however, that each gas turbine power unit comprises a turbine driving a compressor. Atmospheric air taken in through intakes. 1l is compressed by the compressor, the compressed air is heated by combustion therein of fuel, and the heated gases drive the turbine` The exhaust gases from the turbine leave the power units through exhaust cones l2 providing a rearwardly-directed exhaust which contributes to the propulsive eifect. Each power unit drives a power loutput shaft 13 connected t0 a reduction gear assembly C. Reduction gearing of any suitable type, indicated schematically by the power unit pinions 16 and the gear 17, drives a propeller shaft 18 on which is mounted the propeller D. The power shafts 13 drive lthe pinions 16 through clutches 19 which are illustrated schematically in the gure. It will be understood that the details of the reduction gear are, in gleneral, immaterial to the invention. The reduction gearing C, and particularly 'the clutches 19, are preferably of the type more fully disclosed in the copending Iapplication of Victor W. Peterson and Herbert H. Schnepel, Serial No. 174,052, tiled July 15, 1950, now U. S. Patent No. 2,838,913.

An external source of power for starting the gas turbine engine is required. Preferably, this comprises a starter motor 2l energized from a sourcey of compressed air indicated as 22 by an electrically-operated valve 23. One starter serves both power units, being coupled by shaft 2.4 to a starter selector 26 illustrated more fully in the aforementioned Peterson et al, application by which the starter is clutched to either power unit. The shafts 27 and bevel gears 23 indicate schematically the power transmission from the starter selector to the power units.

Each power unit has associated with it a number of auxiliary and control instrumentalities which are shown in the ligure only for the power unit A. Fuel for the unit is supplied through a line 31 from a source such as a fuel booster pump (not shown) and forced by a pump 32 driven by the unit through a fuel line 33, fuel control 34, line 35, and solenoidoperated shutoff valve 36 to the fuel burners of the unit (not shown). Details of the fuel control are immaterial to the invention. ln general, the functions of the fuel' control are to meter the flow of fuel in proper relation to the desired power output from the engine and conditions such as temperature and pressure of the incoming air, the temperature of the heated gas, and engine speed to provide the desired power output without the necessity of detailed attention from the pilot; to promote efficient operation of the power unit; to safeguard the unit against overheating or overspeed; to insure the maintenance of combustion; to supervise acceleration and deceleration of the power unit; and to control the engine speed directly by a governor in the fuel control through a part of the operating range.

Such fuel controls are known to those skilled in the art. Basically, the fuel control operates by bypassing a portion of the pump output through a line 37 to the pump input, passing the remainder to the engine. The

amount of fuel allowed to go to the engine is controlled by the factors and considerations mentioned above. Pressure and temperature of the air at the compressor inlet are transmitted to the fuel control from appropriative sensitive devices (not shown) through a pressure connection 38 and a temperature line 39. Pressure may be sensed by at Pitot tube and temperature by a fluid-filled thermal bulb, in known manner. The utilization of these signals by the fuel control may be accomplished by mechanisms known to those skilled in the art, the details of which are irrelevant to the invention. The control requires a power unit speed input for the speed-responsive mechanism or governor therein, which may be derived in any suitable manner, as by the gearing indicated schematically by bevel gears 40, shaft 41, bevel gears 42, and shaft 43. The fuel control also receives an input indicative of temperatures in the turbine inlet of the unit from thermocouples 44 and amplifier 46.

The fuel control receives two primary control input signals, lever 47 receiving an input indicative of the desired power output of the unit relative to the maximum power available under the existing conditions, and lever 48 receiving a speed signal to set the governor mechanism of the fuel control. These power and speed signals originate in a pilots power control lever 49 which is coupled mechanically, as by link 51 and arm 52, to a unit control 53. The details of the internal mechanism of the fuel control are immaterial to this invention, which may be practiced with any fuel control having characteristics suited to the engine utilized. A fuel control having both power and speed inputs is shown in British Patent 729,201.

The unit control is an important part of the control system, having for its principal functions the transmission of power and speed signals to the fuel control and pitch or speed signals to the propeller mechanism. As will be explained more fully, the unit control schedules the signals to provide for efficient operation of the power unit without the necessity of calculation by the pilot or flight engineer. The transmission from the unit control to the fuel control may be of any suitable type, preferably a simple mechanical linkage such as push rods or links 57 and 58 coupled to the crank arms 47 and 48, respectively.

The unit control also supervises other functions of the power unit. It controls the energizatioin of an ignition generator 59 which, through a lead 61, energizes spark 4. plugs or other ignition devices by which combustion s initiated. The ignition apparatus is preferably of known type and need not be described. The amplifier 46 is coupled to the ignition apparatus to cut off the ignition when combustion is established and re-energizc the ignition if the flame goes out.

The power unit air intake 11 is provided with shutters 62 which are ordinarily closed when the unit is out of operation. These shutters are opened and closed through any suitable connection such as the linkage illustrated at 63 by an electro-mechanical actuator 64. This actuator is energized to open or to close the shutters by a shutter switch 66 by way of the unit control 53, which prevents closing the shutters when the power unit is in operation.

The power unit clutch 19 is controlled by a clutch actuator 67 which is indicated for the purpose of illustration in the drawing as operating the clutch through a mechanical linkage 68. Preferably, in practice, the clutch actuator is an electrical actuator operating valve by which hydraulic fluid for engaging and disengaging the clutch is supplied to a double-acting hydraulic motor and a valve by which cooling fluid is supplied to the clutch during the period of engagement. The preferred clutch and the valve mechanism are disclosed in the abovementioned Peterson and Schnepel application, and the disclosure need not be duplicated here. The clutch actuator comprises an electric motor energized from the unit control 53. The operation of the clutches is interlocked so that the clutches may not be brought into engagement simultaneously and so that control of clutch engagement and cooling is dependent upon the speed of the driving and driven shafts. The speed control is effected in part by speed switches 69, preferably centrifugal switches, driven by the power unit, which transmit speed signals to the unit control.

Such speed switches are readily available articles of commerce. Examples of centrifugal switches are illustrated in U. S. Patents Nos. 2,452,298, 2,457,192 and 2,621,267. The specific structure of the speed switch is entirely immaterial to our invention.

The fuel pumping mechanism indicated schematically by the pump 32 is preferably in practice a system including a normal and an emergency fuel pump which may be, for example, of the type disclosed in an application of Floyd J. Boyer, Serial No. 139,216, tiled January 18, 1950, for Fuel Systems (Patent 2,640,423). The system involves means for bypassing the main pump, to check the functioning of the auxiliary pump, by a check switch 71 operable by the pilot.

The auxiliary and control apparatus identified by the numerals 31 to 71, inclusive, is duplicated for the power unit B.

Additional control instrumentalities common to both engines are a coordinating control 72, a start switch 73, and a feather switch '74. As previously stated, the unit control serves to coordinate the various control operations for one power unit. The major function of the coordinating control is to coordinate the two power units of an engine with each other and with the propeller, as will'be more fully explained.

The start switch 73 controls the starter air valve 23 by way of the coordinating control, which prevents operation of the starter under improper conditions.

As previously stated, the propeller D is of a feathering type. Feathering and unfeathering of the propeller are initiated by the pilot-operated feather switch 74 and the coordinating control which energize an electric -motor in a propeller actuator 76. This actuator is the primary control for the propeller and is coupled to the propeller through a mechanical linkage indicated at 77. By means of the propeller actuator, the propeller may be feathered, may be operated as a speed-governing variable pitch propeller, may be operated in direct blade angle control through a range of positive and negative pitches for ground operation, and may be set at zero thrust when the propeller load 1is picked up by the power umt.

The coordinating control energizes the solenoids S8 to close the fuel shutoff valves 36-and also deenergizes certain of the unit control circuits when the propeller is feathered. The propeller actuator also receives a signal from the power control lever 49 of each unit by way of the u'nit controls S3 and the coordinating control, which coordinates these signals to transmit a single speed or pitch signal to the propeller actuator.

The control of the power plant also involves a speed switch device 7S, driven from the propeller shaft 18 by suitable gearing, indicated schematically at 79, which transmits signals to the unit controls 53.

The operation ofthe propeller as a constant speed variable pitch propeller is directly controlled by an electrical propeller governor S1 which transmits signals to a hydraulic mechanism in the propeller, which increases and decreases pitch, through an electrical circuit indicated at 82. This propeller governor receives desired speed signals from the propeller actuator thro-ugh an electrical circuit 33. lt also receives an input indicative of the actual speed of the propeller from an alternator 84 driven by the propellershaft through suitable gearing indicated at 86.

The power control lever 49 operates through a normal power control range and additionally through a reverse thrust range in which the power plant control is modified to secure both positive and negative thrust for ground operation. The reverse thrust control requires different coordination of the various elements of the mechanism. This also is achieved by the unit control, which is transferred to reverse thrust control by an electrical signal transmitted through a circuit S7 from the power control lever.

The unit control actuates a manual fuel shut-off valve located in the fuel regulator when the power control lever is moved to the stop position.

Power control levers and unit controls A suitable structure of the unit controls 53 and the power control levers 49 is indicated in a general or schematic manner in Figures 2, 2a', and 3. lt will be understood that structural details of these mechanisms are omitted from the drawings, since such details are unneces sary to an understanding of the principles and application of the invention, and may be varied widely. Figure 2 illustrates the arrangement of the two power control levers and one of the unit controls. The power control levers 49 operate in a quadrant 100, each lever operating through aligned slots 101 and 102 in the quadrant which constitute the normal operating path of the lever. Figure l0 shows the operating positions of the control lever more fully. It may be noted that slot 102 is shortened in Figure l0, the actual length being preferably about twice that of slot 101. When the lever is at the left hand or zero degree end of slot 101, the power unit is stopped. As the lever is moved through the slot 101, the engine is controlled in starting and idling conditions. At fifteen degrees, conditions are correct for starting and idling on the ground, and `at thirty degrees for idling under operating conditions. Detent mechanism (not shown) may be provided to locate the lever in the Stop, Start, and Operational Idle positions. As the lever is progressed through the slot 102, the engine is operated in normal power operation with power increasing as the lever approaches the right hand end of the slot. Each lever may be moved through a gate 103 into a slot 104 in which the engine is operated in what is termed the reverse thrust control range, which is intended for ground operation, and in which both forward and reverse propeller thrust are available. .When the lever is in 'the slot 101 or the slot 104, the propeller is under direct blade angle control, so that 5 Y the operation in the slots v*101 and 104 is referred to as the blade angle control range.

On the other hand, when the lever 49 is in the slot 102, which is referred to as the governing range, a speed signal is transmitted to the propeller governorso that the propeller pitch is varied to maintain the desired propeller R. P. M.

Each lever 49 may be mounted for rotation on an axis 106 fixed in a bracket 107 and for sliding movement along the axis (or otherwise) for passage through the gate 103. The lever is coupled to the pull rod or link 51, the other end of which is coupled to the input arm 52 of the unit control 53 so that rotation of the power control lever rotates a shaft 108 journaled in the casing 109 of the unit control. The angles indicated in Figure l0 refer to rotation of shaft 108.

The power control levers are mounted side by side so that they may be operated together conveniently for n'ormal aircraft operation or singly for starting or operation on one power unit.

The unit control 53 includes cam mechanism by which properly coordinated signals governing the operation of the power unit in both the normal and reverse thrust conditions of operation are transmitted. These signals, as previously indicated, are a power signal through rod 57 to the fuel control, a speed signal through rod 58 to the governor in the fuel control, and an electrical signal (via the coordinating control 72) to the propeller actuator controlling propeller pitch in the blade angle range and propeller speed in the governing range. Since the conditions of operation are entirely different in the reverse thrust range from those in the starting and idling portions of the normal control range, the systemis set up for alternative operation so as to transmit properly vcoordinated signals in either type of control.

These signals are transmitted by cams 111, 112, and 113 rotated by the shaft 108. As illustrated, these are plate or disk cams with grooves in the faces of the cams engaged by cam followers. The cams are vfixed to a sleeve 114 rotatively coupled to the shaft 108 as -by conventional splines 116. The sleeve 114 is shiftable axially of the shaft 108 by a fork 117 pivoted on the'casing 109 and engaging between flanges V118 on the sleeve. The sleeve 114 is biased into the axial position shown, which is that for the normal range, by aspring 119 acting on the lever 117, and is shifted axially for the reverse thrust range by a solenoid 121 which, vas illustrated schematically, pulls on the lever 117 as an armature. The power control rod S7, which is slidable in the casing, is Vforked to provide cam followers 122 vand 123 alternatively engageable in cam slots in both faces of power cam 111. The speed control rod 5S is similarly forked for engagement with the two cam slots of cam 112. One face provides for normal control, the other for control in the reverse thrust range. A third cam follower 124 actuated by the cam 113 is suitably guided for reciprocation in the casing 109, as by a guide member A126 fixed to the casing. The cam follower 124- transmits electrical signals to the propeller actuator by means of potentiometers 127 and 128, the movable contacts of which are shifted by the cam follower 124.

In the preferred control system, the ymovement of cam follower 124 is the same in both the starting and reverse thrust ranges. This cam has only one groove in which the follower engages in both ranges, as illustrated. Alternatively, this cam may be mounted directly onvshaft 108 so as not to be shifted by solenoid 121.

The forms of the slots in the controlling cams are calculated to provide the desired operatingcharacteristics, dependent upon the nature of the power units, the fuel control, and the propeller and its control mechanism.

The solenoid -121 is energized whenever the corresponding control lever 49 is moved through the gate 103 into the reverse thrust slot by a switch 131 which Dmay be actuated by the power control lever in any suitable manner. As illustrated in Figure 3, the switch 131 is a normally open switch which may be closed by a bellcrank lever 132 pivoted on the fixed structure of the throttle quadrant assembly and urged into switch-closing position by a spring 133. The lever 132 is normally held from engaging the switch by a bracket 134 extending from the throttle lever and of sufficient arcuate extent to remain in engagement with the lever 134 throughout the angular movement of the lever 49. When the throttle lever is shifted through the gate, the lever 132 closes the switch, as indicated by the broken lines in Figure 3. With the solenoid 121 thus energized, the cams 111 to 113 are shifted downwardly, as viewed in Figure 2, so that the followers engage in the slots in the reverse thrust faces of the cams. This shift takes place as the throttle lever moves through the gate 103; the radius of the cam slots is the same on both faces of the cams at the point corresponding to this angular position.

The supervisory control functions of the apparatus are regulated in accordance with the position of the power control levers under control of a number of switches closed through various ranges of angular position of the lever 49. These switches, referred to as throttle switches, may be operated in any suitable manner by rotation of the shaft 108. As indicated schematically in Figure 2, an appropriate number of switch-operating cams 136 fixed to the shaft S provide for actuation of throttle switches 137 fixed in any suitable manner to the housing 109. These switches may be of a well-known type operated by reciprocation of a plunger which may be actuated by raised or depressed sectors of the peripheral portion of the disks 136. The operating cycles of these switches are illustrated in Figure 10, the bars corresponding to the closed position of the contacts.

The unit control 53 also houses a throttle block mechanism which controls movement of the throttle lever out of the governing range in which it remains in normal fiight conditions. This throttle block may comprise a disk or sector 138 (Figures 2 and 2a) formed with a dog 138 by a solenoid 142. The solenoid 142 is energized in a manner to be described so that the power control may be moved into slots 101 or 104 under proper safeguards but may not be so moved improperly. The latch is formed to permit free advance of lever 49 into the governing range.

The switches 137 operate whether the control is in the normal or the reverse thrust range. A switch T12, olerated only when the power control unit is in the normal range, is actuated by a cam 146 mounted on the cam disk 111. As will be apparent, when cam 111 is shifted downwardly bythe solenoid 121, cam 146 will not engage the operating plunger of switch T12.

lt does not seem practicable to discuss the structure and the details of operation of the control system more fully than has been done in advance of a detailed disclosure of the operating circuits. Details of the structure and operation will be considered in terms of the various circuits as they are described.

General scheduling of the power plant It may be helpful in understanding the details of the system to examine, in a general way, the schedule or conditions of operation of the power plant for use with which the preferred embodiment of the control system is intended. Figure 13 is a scheduling diagram of a dual power unit gas turbine propeller engine employing the control of this invention. It will be understood that values given in this diagram are illustrative; they are to a certain extent a matter of choice for a given power plant, and may vary greatly from one power plant to another. The abscissa of the curves of Figure 13 is the control setting, which is the rotation of shaft 108 of the unit control. Scheduled engine R. P. M. and propeller shaft horsepower are indicated by solid lines in percent of the maximum value. The governor setting, by which is meant the setting of the governor in the fuel control, and the throttle setting, by which is meant the power control setting of the fuel control, are indicated by broken lines, likewise in percent of the maximum setting. Blade angle in the blade angle control range is plotted in degrees. As will be seen, the total travel is indicated as 90 degrees, zero degrees being the Stop position (sce also Figure l0), the range from 2 degrees to 28 degrees being the blade angle control range, and that from degrees to 90 degrees being the governing range in which the propeller pitch governor is in operation.

The curves for shaft horsepower and R. P. M. are for normal power operation through the reverse thrust and governing ranges, and do not include the starting range.

The curves for governor setting and throttle setting are two-valued in the blade angle control range, the broken lines indicating the setting in the reverse thrust range and the broken crossed lines indicating the values when the power control lever is in the slot 104 in which starting is effected.

Perhaps the most fundamental setting isthe throttle setting which is advanced from zero or complete cutoff in the Stop position to about for starting the engine, and then increases, first gradually and then more sharply, as the power control is moved to the degree position for taking off. The Start and Ground Idle throttle settings are set in accordance with the operating characteristics of the engine to give sufficient power for consistent operation and to handle the propeller load. The throttle setting curve through the governing range is preferably such as to give a substantially straight line curve of shaft horsepower, as indicated. In the reverse thrust range, the throttle setting remains substantially constant through the positive thrust portion of the range but increases sharply toward the maximum negative thrust position of the control to provide suicient power for braking. The shaft horsepower curve rises accordingly in the negative pitch range. The effect of the governor in the fuel control on operation in and near the blade angle control range will be discussed presently.

As will be noted from the blade angle curve, the blade angle decreases from about l2 degrees at the 28 degree control position to zero at about the l5 degree control position and increases negatively to minus 20 degrees pitch at the two-degree position. Throughout this range, blade angle is determined directly and solely by the position of the power control lever. No curve is given for blade angle in the governing range since, in this range, there is no fixed blade angle, the angle being adjusted by the propeller governor to maintain the desired engine speed.

The engine speed or R. P. M. curve shows only slight variation because of the characteristics of the gas turbine` which lis essentially a constant speed power plant; that is, it does not admit of wide variation in speed like an internal combustion engine. Basically, engine R. P. M. is controlled in the blade angle control range by the governor in the fuel regulator and, throughout the governing range, which is Operational Idle and above, by the propeller governor.

In a limited range above Operational Idle, the control of engine R. P. M. and propeller shaft horsepower is effected by both governors, the actual operating points varying within the shaded areas bounded by the lines identified as Static and M. P. H. Under static conditions the engine is governed to deliver about 15% rated power. However, assuming that the plane is landing at a speed of 100 M. P. H. with the control in Operational Idle for minimum operational power, the propeller governor can reduce the pitch only to a minimum value. The 100 M. P. H. relative wind acting on the propeller generates energy which is transmitted to the engine, so that at the Operational Idle point and 100 M. P. H. speed the propeller actually furnishes power to the engine amounting to about 2% of maximum engine rating. Above about a 40 degree control setting, the propeller governor takes over, the increased engine power at this setting being sufiicient to require governing action by the propeller governor. The area between the engine R. P. M. curves for 100 M. P. H. and static conditions represents the deviation in engine speed from the scheduled speed which occurs at reduced throttle conditions. Maximum deviation occurs at static conditions, zero deviations at 100 M. P. H., and proportional deviation at speeds between these two values.

It will be noted that the fuel regulator governor setting is at about 90% rated engine speed through the negative pitch portion of the reverse thrust range, then decreases to approximately 80% rated speed at Operational Idle. The governor setting increases rapidly with further advance of the throttle so that, in normal operation in the governing range, the speed governor does not control the engine and does not conflict with the propeller governor. The governor setting at Start is in the neighborhood of 75%.

The blade angle at Start is the value corresponding to minimum torque requirements of the propeller which, in the particular installation chosen for discussion, is approximately minus 6 degrees as indicated on the figure. Operating conditions when the engine has started depend upon the throttle and governor settings, resulting in a speed for the engine when it is brought up to speed with the control in Start position of about 80% rated speed. The speed differs slightly with the clutch engaged due to the propeller load from that of the engine running unloaded, as indicated by the width of the shaded Speed (Start) area on the diagram.

It will be understood that Figure 13 is illustrative of the conditions with both power units in operation. The control is such that it is adapted to operation on a single power unit. With a single power vunit in operation, the propeller governor, the fuel regulator governor, the throttle, and the blade angle are set to the ysame values as when two units are in operation. The horsepower available will, of course, be only one-half that for two units. Throughout the governing range, the principal effect of the reduced horsepower is simply to cause the propeller governor to reduce the propeller pitch so that the single unit can drive it at the scheduled speed. In the reverse thrust range, the scheduling is such that one power unit may handle the entire load. It will be noted that the maximum braking Vload is considerably under the full power of one unit. The power or throttle setting is sufiicient throughout the blade angle control range for one unit to handle the propeller load. When both units are in operation, as is usually the case, an excess of power is available. In this case, however, the governors in the fuel regulators reduce the fuel supply to the individual units so that the speed and power of the two units follow the indicated schedule. Any variation between single and double-unit operation due to governor regulation characteristics is immaterial.

The principles of scheduling involved in the control will `be clear to those skilled in the art fro-m the above. The absolute values 'of the quantities indicated in Figure 13 and the relative values at different points through the range of the control will vary with the desired operating characteristics and with the type of power plant and load involved. The actual cam contoursA will also vary, of course, with the specific characteristics of the fuel regulator including the governor, the propeller governor, and the linkages or other connections between the cams and these controlling devices. Determination of the actual values for a particular installation is simply a matter of engineering design.

Power circuits As a feature of the organization -of the control system, several busses are provided to furnish power to the con- 10 trol devices, the energizationof these busses being related to the operating setup at any given time. To avoid unnecessary duplication in the circuit diagrams, the organization of this bus system or power control circuit is illustrated in Figure 4.

Power for the operation of the control is derived from a D. C. power bus 150 in the aicraft. A nacelle bus 151 for each engine or dual power unit is energized from the power bus by a relay 152 energized by a manually-operable switch 153 under. control of the pilot or ight engineer. This bus may energize fuel booster pumps, fuel tank shutoff valves, etc. (not shown). Alternating current for the thermocouple amplifiers and propeller governor is generated by an inverter 154 connected to the D. C. power bus 150. The output of the inverter is fed to the 400 cycle bus 155 through front contacts of a relay 156 energized directly from the nacelle bus 151.

The additional D. C. busses are identified as 1, 2, 3, and A4, bus 1 being energized from the nacelle bus 1951 through a fuse 157 so that this bus is constantly energized whenever the power plant is set up for operation by energization of the nacelle bus.

It may be noted here that, throughout this specification, the various switches, relays, and other electrical apparatus common to both power units and those individual to the A unit will be indicated by unprimed reference characters, and devices individual to the B power unit by corresponding primed reference characters. Throttle switches are indicated by T and speed switches by S. Relays in general are identified by R, relays in the clutch control circuits by C and relays in the propeller circuits by P.

Proceeding with 'the description of the power circuits, bus 2 is energized from bus `1 through the normally open contacts of a power relay RP energized by throttle switches T1 and T1- in 4parallel which, as indicated in Figure l0, are closed except when the power control lever is in the'Stop position. Thus, bus 2 is energized when either unit is in operation, but is cut out whenever the power controls for both-units are moved to the Stop position.

Bus 3 is energized from bus 2 through back contacts RF1 of relay RF, the feather relay, which is energized whenever the controls are actuated to feather the propeller. The relay remains energized as long as the propeller remains feathered and bus 2 is energized.

Bus 4 is energized from bus 3 through the back contacts RR1 and RRl, in series, of relays RR and RR', the reverse thrust relays of each unit. Relays RR and RR are energized by the power control lever switch 131 of the corresponding unit when the lever is shifted through the gate 103 into the reverse thrust slot.

In summary, it may be pointed out that bus 2 is energized except when both power units are cut out; bus 3 is energized whenever bus 2 is energized unless the propeller is feathered or the feathering movement has been initiated; and bus 4 is energized whenever 'bus 3 is energized except when either control lever is shifted into the reverse thrust slot.

Starter circuits The starter circuits shown in Figure 5 are vsolely those concerned with the operation of the starter selector to connect the starter to one or the other power unit and with the energization -of the starter actuator to supply compressed air to the starter. The starter selector 26 comprises a coil 161 for engaging the clutch by which the starter is connected to power unit A and a coil 162 for clutching the starter to power unit B. The clutching mechanism is described in the above-mentioned Peterson et al. application; however, an understanding thereof is not necessary for the understanding of this invention. Coil 161 is energized by throttle switch T2 and coil 162 by throttle switch T2', these-switches being closed except when the corresponding power lever is in Stop position.

The energizing circuits proceed from bus 4 through T2 or T2', line 163 or 163', front contacts RS1 or RSZ of the starter relay RS and coil 161 or 162 to ground. The starter relay RS, which may be in the coordinating control 72, is energized from bus 4 through throttle switches T3 and T3 (in parallel), speed switches S1 and S1', the pilot-operated start switch 73, and theRS relay coil to ground. The start button 73 is held closed by a solenoid 164 coupled to the movable contact and energized in parallel with relay RS.

Switches T3 and T3 are closed only when the corresponding power lever 49 is in Stop position, Switches S1 and S1' are closed below a low but self-sustaining running speed of the units A and B respectively, which speed may be taken as 5900 R. P. M. Thus, in order to energize the starter relay, the power control of the unit being started must be advanced from Stop, the other power control must be at Stop, the propeller must be unfeathered (to energize busses 3 and 4), both units must be below 5900 R. P. M., and the pilot must actuate the start switch 73. The starter relay closes contacts R51 and RSZ in the circuits to the starter selector 26, energizing the coil 161 or 162 corresponding to the unit the control of which has been advanced.

Movement of the unit control to Start position closes its throttle switch T4 or T4' to actuate the starter. These switches are connectedin parallel in a circuit from bus 4 through front contacts RSS of the starter relay lead 165, switches T4 and T4', lead 166, and valve-opening iield coil 23h and armature 23a of the motor of the air valve actuator 23, which opens the air valve and thus supplies power to the starter motor 21. The starter motor cranks the selected power unit, accelerating the unit until its speed reaches 5900 R. P. M., at which point the speed switch S1 opens, deenergizing the starter relay and the solenoid 164, releasing the start button 73. When the starter relay deenergizes, it closes back contacts RS4 which make a circuit from bus 2 through line 167 and the valve-closing iield coil 23e and armature 23a of the air valve actuator, operating it in the reverse direction to shut off the air supply to the starter. The actuator motor 23 is a well-known type of reversible motor, and includes limit switches 168 and 169 which operate in the usual manner to open the circuit to the motor when the actuator has traveled through its full range, thus deenergizing the opening circuit when the valve is open and the closing circuit when the valve is closed.

Since all the starter selector and starter energizing circuits are supplied from bus 4, they are inoperative if the propeller is feathered or either unit control is in the reverse thrust range.

As will be apparent, the starter air valve circuit could easily be modified to provide a relay circuit for control of an electric starter motor if such were desired. The significance of the starter circuits will be more fully apparent after the description of related circuits. Lines 177, 177', 206, and 206', indicated on Figure 5, relate to other portions of the system, to be described. Switches T2 energize a number of circuits relating to the corresponding unit when the power control is moved from Stop. This common energizing circuit may be regarded as a unit bus.

Intake shutter circuits Figure 6 illustrates the ignition, fuel, and shutter circuits. The circuits shown are for one power unit only, being duplicated for the other unit.

Shu ttcrs 62 (Fig. l) are usually closed except when the pov/er unit is in operation. Particularly when a power unit is shut down in ight, it is desirable to close the shutters so that ram air will not rotate the idle power unit. The shutters are opened and closed by the actuator 64, which is a standard reversing electric motor actuator with limit switches, under control of the shutter switch 66 and throttle switch T5, which is closed when the control is at Stop.

Switch 66, which is actuated by the pilot or flight engineer, is normally open and may be closed on one contact to energize the shutter opening field winding 64b and armature 64a of the actuator 64 from the nacelle bus 151, or on the other contact to energize the shutter closing field 64e and armature 64a through throttle switch T5. These circuits are made through the usual limit switches actuated by the motor, of the same nature as those previously described for the air valve actuator. Switch T5 is closed only when the power control of the unit is at Stop, thus providing an interlock to prevent closing of the shutter when the power unit is in operation.

Fuel system circuits The elements in Figure 6 which are included in the fuel system are enclosed in a broken line. They include the solenoid 83 which closes the normally-open fuel shutoff valve 36 (Figure l) when energized by a circuit from bus 2 through front contacts RF 2 of the feather relay` line 170, and solenoid 88 to ground. The fuel line is thus closed when the propeller is feathered, regardless of the position of the power control lever 49.

The pump checkout solenoid 171 in pump 32 is energized from the nacelle bus through the pilot-operated check switch 71 and line 172.

A pump failure warning light 173 indicates incipient failure of the main or primary fuel pump. This light is actuated by a change in pressure within the fuel system, which is all that need be known about it for the purpose of understanding the present invention; it is more fully disclosed in the abovementioned Boyer application. The light 173 is energized by a switch 174 actuated by a fluid pressure responsive device 176 in the fuel system. An energizing circuit for the switch 174 proceeds from bus 4 through throttle switch T2, which is closed except when the control lever is at Stop, and lead 177. Thus, the warning light is energized whenever bus 4 is energized, the power control lever is away from Stop, and the pressure switch is closed.

Switch T2 also energizes a circuit from bus 4 to the fuel shutoff solenoid 88 through lead 177, speed switch S2, lead 178, back contacts RF3 of the feather relay, and lead 170. Switch S2, which is one of the speed switches in the unit 69, is closed below a minimum suitable speed for initiation of combustion in the power unit, which may be 1700 R. P. M. Thus, when the propeller is unfcathered and the power control is moved away from Stop so that the unit can be started, the solenoid 8S is energized to keep the fuel shut oif until the unit is accelerated to 1700 R. P. M., at which point the unit will provide sufficient air circulation for proper ignition of the fuel. This arrangement prevents possible ooding of the unit by supplying fuel at too low a speed, or improper combustion in the absence of suiicient air. At 1700 R. P. M. the power unit is capable of assisting the starter in bringing the unit up to a speed at which the unit is self-sustaining. Thus, the starter, aided by the turbine in the unit, continues to accelerate the unit until 5900 R. P. M. is reached, at which point the unit is more than self-sustaining and the starter is cut out by switch S1 or S1' (Figure 5).

Ignition system control The ignition system preferably includes spark plugs in the unit (not shown) and an ignition current generator which supplies a high potential to the spark plugs. Such systems are well known, and the control system of thc invention is applicable to various ignition systems of this and other types. The invention is concerned only with control of the energization of the ignition generator, which is identified as 59 in Figures land 6. The control system provides for energizing the ignition when the starting operation is initiated, de-energizing it when the flame has been established, and automatically reenergizing the ignition if a blowout of the flame occurs. The control includes the thermocouples 44 which respond to tempera-` ture in the turbine inlet, the output of which is amplified by amplifier 46 energized from the 400 cycle kbus 155. The output circuit 181 of the amplifier is energized when the temperature to which the thermocouples respond reaches a value well below the operating range but suiliciently high to indicate the presence of combustion; such, for example, as 800 degrees F. This output energizes temperature relay RT, the front contacts of which close a circuit from line 177 through the coil of ignition relay Rl to ground. The ignition generator 59 is energized from line 177 through the back contacts of the ignition relay RI and line 182.

When starting the unit, the unit is cold, relay RT is deenergized, and the circuit from line 177 to the ignition system is closed by relay Rl. Therefore, when the power control lever is moved away from Stop, switch T2 is closed, providing ignition in the combustion chambers so that, when the fuel is injected at 1700 R. P. M., it is immediately ignited. The ignition circuit stays energized until the thermocouple amplifier responds at 800 vdegrees F., closing the contacts of relay RT and break- ,ing the ignition circuit at relay Rl. As will be apparent,

if the llame goes out, the ignition circuit will be re-established by the thermocouple `amplifier and relays RT and RI unless the power control has been returned to the Off position or bus 4 has been deenergized.

Clutch control circuits The circuits previously described are interrelated with the circuits by which the clutch actuators 67 are controlled, illustrated in Figure 7. These clutch circuits are particularly adapted for use with the power plant and clutch system disclosed in the above-mentioned Peterson et al. application. However, they may, with appropriate modifications, be applied to other clutch arrangements, and it is believed that the principles of the invention are capable of wide application to various power plants and clutch arrangements therefor.

By way of introduction, it may be repeated that the clutches disclosed in the .Peterson et al. application are engaged and disengaged by hydraulic motors controlled by valves operated by actuators. The actuator for each clutch also controls a valve to supply oilto the clutch for cooling during the period of slip after engagement. The actuator has three positions of rest: a clutch disengagement position, a clutch engagement position requiring full travel of the actuator from the disengagement position, and a coolant shutoff position involving movement part way back to the disengagement position. The supply of coolant is initiated by movement to the engagement position and terminated by movement to the coolant shutoff position. The clutch is disengaged by ycompleting the return movement of the actuator.

The purposes and functions of the clutch control of the invention may be generally summarized as follows: The system provides for clutching the propeller automatically to an operating power unit, for clutching the second power unit to the power-driven propeller for starting thereof, and for clutching the idle units in sequence to a windmilling propeller for an air start of the units. The system insures that these operations can take place only under proper conditions of setting of the power control levers and speed of the power units and propellers. lt also interlocks the clutches so that one clutch may not be engaged as long as the cooling oil is being supplied to a previously engaged clutch, because a clutch should not be required to pick up the load of the propeller and an idle engine at the same time, and because the supply of coolant is not adequate for` two clutches,

These functions are assumed by circuits involving connections to the feather relay RF and starter relay RS, a number of throttle switches, speed switches responsive to the rotation of each power unit and of the propeller, and a number of relays in the operating circuit of each clutch.

14 Referring now to Figure 7, it may be pointed Vout that this figure illustrates the circuits involved in the control of the clutch of the A unit. Since the control of the clutches is interlocked, this includes certain of the control instrumentalities of the B unit clutch which 'are enclosed in a brokenline rectangle in the'iigure. 1t will be understood that the control lsystem Afor the lB unit is identical to that shown for the A unit and involves the same cross-connections. It-has, therefore, been omitted from i the drawings to avoid unnecessary duplication and complication.

We may `begin the description of the system by tracing the circuits involved in engagement of the A unit clutch with that unit in operation. Since the propeller Iis 'unfeathered before starting the unit, contacts RF4 of the feather relay RF will be open. In starting the power unit, the starter relay is energized, closing its 'front contacts R85, RS6, and RS7. As vpreviously explained (Figure 5), the starter relay is held in until "the lunit reaches 5900 R. P. M., when it is opened'by speed switch S1. While closed, contacts RSS energize a clutch transfer relay CT through lead 185. Relay CT completes a selfholding circuit from bus 4 through lead 186, propeller speed switch S4, line 187, and front contacts CTI. Switch S4, which is inthe speed switch unit 78 driven by the propeller shaft, remains closed until the propeller shaft reaches a speed somewhat below the normal range for propulsion, which 'may be, for example, a'speed corresponding to 11,200 R. P. M. of the engine, referred to hereinafter as 11,200 equivalent R. P. M.

Contacts R56 ofthe starter relay close a circuit from bus 4 through line 188 to the coil of the ground start relay CG. Relay CG sets up a `self-holding circuit through throttle switch T 6, line 189, and its front contact CG1. Switch T6 is closed only in the lStart position 'and thus holds relay CG energized 'until the power control is advanced from the Start 'setting Lines 185 'and 188' energize transfer relay CT and ground :start relay CG respectively, of the B unit'clutch-control (not shown), and line 187 from speed switch S4 provides the holding circuit for CT.

Energization of relays CT and CG sets up a circuit for engaging the clutch through speed switch S3, which closes when the power unit reaches a speed slightly below that at which S4 opens, say 11,000.12. Thus, with the unit control in Start, as the unit becomes selfsustaining and accelerates to 11,000 R. P. M., at 'which point it is capable of taking on the propeller load, the clutch is engaged. The circuit .is from bus 4 through T6, lead 191, front contacts CTZ, switch S3, front contacts CTS, line 192, front contact CGZ, line 193, throttle switch T7 of the B unit (which is closed only when the B unit control is at Stop), line 194, back contacts CC1 of the coolant control relay CC, line 195, closed contacts 197er, and engaging field winding 67e, brake release coil 67r, and armature 67a of the clutch Valve actuator motor 67 to ground. Switch 197 is one of three limit switches 197, 198, and 199 operated by the armature 67a. Figure 7 shows these switches in their condition when the actuator is in disengaging position. .Figure 11 is a timing diagram of the switches. Switch .197 is closed on contact 197a until the actuator substantially completes its travel to the engaged position, when this switch is thrown to contact 197b. Movement of the switch 197 completes a circuit from the clutch-engaging line through Contact 197b, line 201, and the coil of pilot switch by-pass relay CB. This relay shunts throttle switch T7 at its front contacts CB1. It also completes a self-holding circuit from the engagement line 195 through line 202 and front contacts CB2. Relay CB is provided to prevent deenergization of the clutch-engaging line by moving the power control of the 'other unit from Stop. lt makes possible, if desired, the use of a solenoidtype actuator which must be held energized to continue the flow of cooling fluid to the clutch.

As stated, the energization of engagement line 195 occurs when the unit reaches 11,000 R. P. M. The inertia load and drag of the reduction gear land propeller decelerate the unit slightly, but the dropout point of switch S3 is low enough that this switch remains closed. As the propeller load is accelerated, the clutch synchronizes after an initial period of slip and the power unit regains speed until the propeller is turning at a speed equivalent to 11,200 unit R. P. M. At this point, switch S4 opens, breaking the holding circuit of transfer relay CT. In deenergizing, this relay completes a circuit from bus 4 through lead 203, back contacts CT2, switch S3, back contacts CT3, line 204, and the coil of the coolant contral relay CC to ground. In energizing, relay CC opens back contacts CCI, deenergizing the clutch-engaging line 195 and thereby by-pass relay CB. Relay CC makes a circuit from bus 4 through throttle switch T2, which is closed except in Stop, line 206, contacts CC2, line 207, contact 19811 of the limit switch 198, and disengaging eld winding 67d, brake release coil 67r, and armature 67a of the clutch valve actuator. This circuit energizes the actuator for reverse rotation to terminate the coolant supply. This coolant shutoff circuit is mainn tained energized through switch T2 until the actuator 67a has closed the coolant valve, unless the unit is stopped. When the actuator has rotated in the reverse direction a sucient distance to cut: olf the coolant, switch 198 closes on contact 198a, opening the circuit to the motor 67.

The circuit just described provides for continuing the supply of coolant until the slip period of the clutch is terminated. The engine is now in normal operation with one power unit operating the propeller, and the power control may be advanced into the governing range or into the reverse thrust range for operation of the aircraft, although normally the second power unit would be started before any such operation is undertaken.

When the coolant flow is cut olf, a circuit for energizing the B unit clutch is prepared. This circuit is from bus 4 through T2, line 206, contacts CCZ, line 207, contact 198a, line 213, and the coil of the B unit second clutch engagement relay CS to ground. This circuit, the purpose of which will be explained, is not activated until the lirst clutch is engaged and the coolant supply has been terminated.

Having been engaged, the clutch may be disengaged either by actuating the controls to feather the propeller or by moving the unit control to Stop. If the propeller is feathered, the feather relay RF closes a circuit from bus 2 through front contact RF4, line 208, contact T8b of throttle switch T8 (closed except in Stop), clutch-dis engagement line 209, limit switch 199, disengage field 67d, and armature 67a to ground. The actuator is thus energized to complete its return movement, reversing the clutch engagement valve and disengaging the clutch. This movement is terminated by the limit switch 199. It will be apparent that this control prevents any attempt to drive a feathered propeller. If the unit control is moved to Stop, switch T8 closes on contact TSa, energizing the disengage line 209 from bus 1 through line 210 and contact T8a. Thus, either unit is automatically declutched by bringing the power control of the unit back to Stop.

A circuit is also provided to disengage the clutch when the power unit is started. When the starter relay is energized, contacts RS7 energize line 212 from bus 4, thus energizing contact TSb. When the unit power control is moved out of Stop position, contacts T8b close, energizing the clutch-disengaging line 209.

The system also provides for engaging the clutch of one power unit to start that unit when the other power F unit is already in operation. The manner in which the clutch is engaged when the unit is operating in the Start and Ground Idle power position has been explained for the A unit. Assuming that the A unit has not been started but that the B unit is operating in Ground Idle 16 condition,'we'may now trace the operating circuit and procedure for starting the A unit. The feather relay RF and the starter relay RS will be deenergized. Coolant control relay CC is also deenergized.

When the B unit clutch has completed the engagement cycle, the B unit clutch actuator 67 energizes the second clutch engagement relay CS of the A unit through lead 213', as previously described for relay CS'. For starting the A unit from the B unit, the power control of the B unit is left in the Start position. Ground start relay lGG of the A unit, which was energized by the starter relay, is held energized from switch T6' of the B unit,

' since the holding circuits of relays CG and CG' are encrgized in parallel through leads 188 and 188' and holding contacts CGI and CG1. Also, with the B unit in Start position, switch T7 will be open. As long as the A unit control is in Stop, the disengage circuit of the A unit clutch will be energized from bus 1 through lead 210, contact T8a, and lead 209. This circuit will be broken at T8Q when the A unit power control is moved away from the Stop position. The clutch transfer relay CT, the holding circuit of which is connected in parallel with that of relay CT through lines 187 and 187', will have been deenergized by opening the holding circuit at speed switch S4 when the propeller reaches 11,200 equivaient R. P. M.

The second clutch engagement relay CS sets up the circuits by which the second clutch to be engaged is freed from control by the unit speed switch S3 and the throttle switch T7 of the other unit. Contacts CS1 bridge leads 191 and 192, thereby shunting the front contacts CTZ and'CT3 of the transfer relay. Contacts CS2 connect leads 193 and 194, thus shunting the throttle switch T7'. When the power control of the A unit is brought to Start position, switch T6 is closed. completing a circuit from bus 4 through lead 191, contacts CS1, lead 192, front contacts CG2. lead 193, con tacts CS2, lead 194, contacts CCI., and lead 195 to the clutch valve actuator 67, which functions to engage the clutch as previously described. It may be noted that switch T6 also closes an additional holding circuit for relay CG through lead 189 and contacts CGI.

As the A unit is brought up to speed by the operating B unit, fuel is admitted at 1700 R. P. M. by switch S2 (Figure 6) and the ignition, which was energized by switch T2, is cut out by the ignition relay when the temperature reaches S00 degrees F., just as when the unit is cranked by the starter. When power unit A reaches a speed of 11,000 R. P. M., speed switch S3 closes, energizing a coolant shutoff relay circuit from bus 4 through lead 203, back contacts CTZ, switch S3, back contacts CTS, lead 204, and relay coil CC to ground. Relay CC establishes the coolant shutoff circuit, as before, from bus 4 through switch T2, lead 206, contacts CC2, and lead 207 to the actuator 67, and breaks the clutch-engaging circuit at CCl.

With both units in operation, the power controls may be moved into the reverse thrust range or the governing range for taxiing, and may be advanced in thc governing range for takeoff and flight. If it is desired to declutch one power unit in ight and drive the propeller by the remaining unit only, this may be accomplished by returning the power control on the unit which is to he cut out to Stop to energize the clutch disengagement eircuit from bus 1 through wire 210, contact TSa, and wire 209.

The clutches may not be simultaneously engaged to start both the propeller and an idle unit from a running unit. With the unit control at Stop, the clutch engaging circuit of that unit is broken at T6. lf it is moved from Stop, the engaging circuit of the other unit clutch is broken at speed switch S3 of that unit, which is open below 11,000 unit R. P. M. Switch S3 of the second unit is not bypassed until the coolant has been shut off in the first clutch engaged.

The system also provides for starting the power units one after the other from a windmilling propeller in flight, a feature which is principally for use in multiengined aircraft. Wtih the power units out of operation, all relays will be deenergized and, ordinarily, the propeller will be feathered. If feathered, the propeller is unfeathered and allowed to gain speed. The feather relay when deenergized supplies busses 3 and 4. Assuming that the A unit is to be started, the power control lever is moved into the governing range, closing throttle switch T10, establishing a circuit from bus 4 through T10, lead 216, back contacts CG2 of the ground start relay, lead 193, B unit throttle switch T7', lead 194, and contacts CC1 of the coolant shutoif relay to the engagement line 195, causing engagement of the clutch in the manner previously described. The ignition and fuel circuits operate as previously described, and the A unit is thus brought into operation. The coolant control relay CC is energized to shut off the coolant when the unit reaches l`l,000 R. P. M. under control of speed switch S3 in a circuit from bus 4 through back contacts CTZ and CTS. When the coolant is shut off, the second clutch engagement relay CS of the B unit is energized as previously described.

If the B unit is started first, as soon as engagement is completed and the coolant is shut off, the clutch actuator 67 energizes the second clutch engagement relay CS, contacts CS2 of which bypass throttle switch T7 so that the A unit may be started. The clutch is engaged by a circuit from bus 4 through T10, line 216, back contacts CGZ, line 193, contacts CS2, line 194, contacts CC1, and line 195 to the clutch actuator. The coolant is cut E when switch S3 energizes relay CC.

This sequence of operations provides an alternative method for starting the second power unit on the ground. In this method, the power control of the first unit started is advanced into the governing range. With the rst power unit in the governing range, the second power unit may be started by advancing its control lever into the governing range, the clutching control sequence corresponding to that just described. Lead 319 branching from lead 209 relates to the throttle block circuits, to be described.

Propeller control circuits The functions of the propeller control circuits are, in general, to establish the operating conditions of the propeller, to coordinate these conditions with the operation of the power units, and vice versa, and to Aestablish a system of control eliminating conflicts between the `runit controls. The invention is not concerned with the control elements of the propeller per se except as they are components of the overall control system. It is contemplated that the system may be used with propellers of various types as, for example, propellers with either hydraulic or electrical pitch control, and with `various control arrangements, as long as the characteristics of the propeller control system are compatible with the overall control system of the invention. The principles of the invention also lend themselves to control with loads other than propellers. For this reason, and in the interest of conciseness, we will not concern ourselves with the details of the propeller governor .and controls, and the general nature of these controls will be discussed only sufficiently to explain their relation to the power plant control which is the subject of this invention.

In general, to carry out the purposes and advantages of the invention as embodied in an aircraft power plant, the propeller should have three phases of operation: lt should be capable of being feathered, that is, brought to a blade angle of approximately 90 degrees so that relative wind provides no substantial turning moment and the drag ofthe stationary propeller is a minimum.

Secondly, the propeller should have a variable pitch range for normal propulsive operation in which the pitch of the propeller is regulated by a speed governor. in this type of control, with the speed set at a'desired value, the pitch of the propeller will depend principally upon the power input to 'the propeller, the air speed, and air density, in addition, of course, to the constants of the installation which depend upon the physical' form and dimensions of the propeller. -v

Thirdly, the propeller should have a range of 4eor'itrol in which the speed governor is inoperative and the `'blade angle lis controlled directly through a range of 'positive and negative values. This-control is desirable principally for ground operation. This range also Vprovides `for Asetting the propeller to a blade angle in which the propeller torque requirement is a minimum. Propellers of the characteristics stated are known yand are available for aircraft installations.

A Vpropeller having the three modes of operation-re-y ferred to above and `capable of external rcontrol to pro-` vide these modes of operation is shown in Blanchard-et al. Patent 2,307,102. The preferred propeller foruse finrconnection with this invention is a development lof Vthe Blanchard et al. patent propeller, generally as 'shown in Dinsmore et al. Patent 2,669,312, which discloses a propeller, including an electrical actuator yand an electronic governor, of the typepreferably employed with this invention.

The control Asystem of the invention includes an arrangement by which no conflict arises when the power control levers of `the units are at diiferent positions, either through misalignment or because one unit is operating at reduced power or is cut out. The system 'further includes interlocks between the power control levers which, however, will be discussed in connection with Figure `9. The coordination of the power unit control with the propeller depends in large measure upon 'coordination of the slots in the controlling cams 111, 112,

and 113 (Figure 2), and also upon certain interlocks yto feather the propeller, also controls the -energization of busses 3 and 4 yfrom which the starting, ignition, fand clutch circuits are energized, as previously described;

and, in addition, the feather relay controls `the energization of the fuel shutoff valve solenoid and certain `of the Aclutch circuits by means of contacts 'of the relay.

The Apreferred embodiment of this 4portion of the system is illustrated in Figure l8. The pilots feather switch 74 controls feathering and unfe'athering-of thepropeller, energizing the propeller actuator 76v through appropriate circuits. The actuator is coupled to limit fswitches '76f, 76g, 76p, and'76n :which yprovide limits for-its operation over 4the various vportions `of the propeller `actuating range. p 'These switches are closed :over the'ranges indicated 'by the bars in Figure -12. AIn general, apart yfrom the feather switch, actuator 76 is controlled by potentiometers 127 and 128 in the unit controls S3-and 53' which transmit to the actuator'signals establishing'blade angle or propeller speed governor setting. The actuator is controlled by these potentiometersthroug'h a discriminator and follow-up circuit to be described.

The actuator also transmits electrical speedtsignalsto the electronic ygovernor 81 'for the propeller which reeeives an input from `the propeller-driven alternator 84 ysignalling actual propeller* Shaft speed. The propeller governor may be of the type disclosed in U. S. T Patent 2,669,312. The propeller governor actuates hydraulic valve controlling solenoids HD andfHl iin-the propeller which act to 'increase and decrease the pitch, respectively, when the propeller is under governorcontrol. The Ypropeller control solenoids vI-ID 4.and 'Hl and the valve operate'd thereby may be of the typetdescribed in U. LS. 'Patent No. 2,630,136, 'Brandes et al. (filed Tune 8, Y19219). Normally, the governor controls the solenoids'thro'ugh lines 283 `and 284. Power for operating a pump 'toivary the propeller pitch is taken from the propeller shaft.

For feathering and unfeathering, an electric motor 300 drivesa pump, which is thus operative when the propeller is not rotating. Motor 300 is energized from the aircraft A. C. power system, represented by a threephase generator 301, under ,control of a transfer relay PI and a centrifugal switch S5, which is closed on the ,motor contacts below a low propeller speed. Switch S also breaks the circiut to solenoids HD and HI below the operating point of the switch. A governor disconnect rrelay PL also is provided to cut the circuits from the governor to coils HD and Hl. The manner in which relays PJ and PL are energized will be made clear.

; We may start by tracing the circuits by which the propeller is unfeathered, since this is a prerequisite to bringing the power plant into operation. First, the power Control lever of one power unit must be advanced into Vthe governing range to close throttle switch T11 or T11.

Then, by pushing feather switch 74 (that is, moving it to lthe left in Figure 8) yand holding the switch in this position, a circuit is established from the nacelle bus 151 through the feather switch, line 231, throttle switch T11 or T11', and line 232 to energize the unfeather relay PE. The pilot switch 74 also energizes transfer relay PJ through line 233, connecting the feathering pump tuator 76 is energized from the unit controls 53 and 53.

Considering the unit control 53, for example (the two being identical), cam follower 124 (see also Figure 2) adjusts the movable contacts of two potentiometers 127 and 128. Potentiometer 127 is connected in a series circuit from 'bus 2 through a fixed resistance 246 and potentiometer resistor 127 to ground. Potentiometer 128 is connected between ground and bus 2 through a variable adjusting resistance 247. The movable contact 128a of potentiometer 12S transmits a position signal for the actuator 76. The corresponding potentiometer 128 of the B unit control transmits a signal in the same manner. Potentiometers 127 and 127', which are varied concurrently with potentiometers 128 and 128', respectively, transmit signals to a discriminator relay PR in the coordinating control 72 which selects for transmission to theactuator motor the signal from potentiometer 128 or 128 which is of greater magnitude, thereby preventing conicts and facilitating the cutting out of one power unit. The discriminator relay PR, which is of a polarized type, comprises coils PRl and PR2 connected in series between the contacts of potentiometers 127 and 127. As will be apparent, if the contact 127a is farther advanced from ground than Contact 127e', current will v ow from contact 127a `to contact 127a', and vice versa.

When current thus ows from contact 127a, the relay contact is closed on xed contact PRa which is connected to potentiometer contact 128:1. The circuit from Contact 128:1 to contact PRa passes through throttle switch T12, which is closed on contact T1251 except in the Start position. The discriminator relay movable con-- tact is biased to remain closed on one or the other of contacts PRa and PRb, and will not remain open. The movable contact of relay PR is connected through line 248 to a follow-up control relay PF for the actuator motor 76.

Since the propeller has been in feather or maximum pitch, the actuator is at the maximum pitch point of its range of movement. When the power lever 49 of the A unit is advanced, an electrical signal is transmitted by contact 128a through switch T12 and contacts PRa to line 248 which calls for a movement of the actuator toward decreased pitch. The follow-up system by which the motor 76 is controlled includes a potentiometer 249 ture 76a of the actuator motor. Contact 249a is connected through lead 251 and the coils of the polarized follow-up relay PF to signal line 248. The potential tapped 0E by contact 249:1 is thus balanced against the potential of contact 128a or 12811. When the potential at 128a is higher, current ilows from line 24S to line 251, engaging the movable contact of relay PF with the increase pitch contact PFi. If the current flows through the relay PF in the opposite direction, the movable contact engages the decrease pitch contact PFd. Relay PF is biased to open position, and leaves the motor-energizing circuits open unless the relay coils are energized. These contacts energize circuits to operate the armature 76a in the appropriate direction to match the signal transmitted by the unit control. When the position of the actuator corresponds to that transmitted, the potentials at 249e and 123a balance, and the motor circuit is opened at the movable contact of follow-up relay PF.

The motor energizing circuit to unfeather may be traced from bus 2 through line 252, contact PFd, line 253, limit switches 76n and 76p in series, line 254, the contacts of unfeather relay PE, line 256, back contacts P11 of the increase pitch relay PI, the coil of the decrease pitch relay PD, and line 257 to ground. Relays PI and rPD are referred to as increase pitch and decrease pitch relays in terms of their function in blade angle control. The front contacts of relay PD close a circuit from bus 2 through line 25S, front contacts of relay PD, line 259, armature 76a and brake release coil 761' of the actuator motor, line 261, back contacts P12 of the increase pitch relay, and line 257 to ground. Coil 76r, which is energized with the armature, releases a normally engaged brake which holds the motor shaft. The eld 76s of the actuator motor is energized directly from bus 2 through line 258. When current ows as just described from line 259 through the armature to line 261, the motor is rotated in a direction to operate the actuator to decrease the pitch of the propeller. The exact position at which the motor stops will depend upon the position of thc power control lever, but is immaterial. The motor will bring the propeller actuator into the governing range, and therefore the propeller out of the feathered position into the range of pitch angles in which the governing control is effective, by a mechanical signal from the actuator to valves in the propeller which control the unfeathering A' operation, fluid being supplied to effect the operation by the feathering pump motor 300. The function of relay PE is to bypass contacts PH1 of the feather limit relay which are open when the propeller is completely feathered. This arrangement prevents unfeathering unless the switch 74 is closed.

The feather limit relay PH is energized from bus 2 through line 291 and limit switch 76j when the actuator is at the feather limit position (Figure 12). When the actuator leaves this position, relay PH is deenergized at switch 761. Back contacts PHl shunt the contacts ci' unfeather relay PE and maintain the decrease circuit to the motor energized.

It will be further understood that, by appropriate movement of the contact 12ga, for example, the propeller may be taken out of the governing range into the blade angle control range in which blade angles preferably from about plus twelve degrees (just below the minimum pitch in the governing range) to about minus twenty degrees may be set by the power control and actuator without reference to propeller speed. The maximum negative pitch is obtained by movement of the control lever to the two degree position in the reverse thrust slot. As the lever is moved forward in this slot, the pitch increases plus twelve degrees just below Operational Idle. At and above Operational Idle the propeller is in the governing range in which the pitch may vary, depending upon the operation of the governor, but is always greater than twelve degrees. In the governing range, the actuator operates through speed control rather than direct blade angle control. The feather position is beyond the gov- .erning range and is obtained by action of the actuator when the unit is started. This control is elected by the throttle switches T12 and T12', which are closed in he FStart position of the power control on contacts T1-2b and T125', respectively. As previously `pointed out, switches T12, unlike the other throttle switches which respond only to the angular position of the shaft 108 (Figure 2)., 'are actuated only when the control lever is in slot 101 and remain closed on contact Tl2a in the reverse thrust range. Thus, when the A unit control is in Start, contact PRa of the discriminator relay is connected through contact T12band line 263 with an adjustable contact point of voltage dividing resistor 264. Resistor 264 is connected in a circuit from bus 2 through fixed resistor 266, tapped resistor 264, and yfixed resistor 267. By adjusting the variable contact of the voltage divider 264, it may be set up to transmit a potential signal to the actuator which will bring thepropeller exactly to the minimum torque posion. The B unit control 53 is identical in this respect with the A unit control, so that movement of either unit to the Start position with the other unit in Stop'will energize the discriminator relay PR and shift the throttle switch T12 to transmit an exact minimum torque signal to the follow-up relay PF.

Actuator limit switch 76p opens 'at the actuator position corresponding to minimum governed speed, just before the actuator enters the position corresponding to maximum positive pitch in blade angle control. However, throttle switches T13 and T13 are closed inthe blade angle control range (below Operational Idle and above- Stop), and bypass switch 76p. Thus, When the control is shifted to Start, and the signal calling for decrease of blade angle to zero closes contacts PFd, the actuator motor is energized as previously described except that the circuit is maintained through switch T13 a'fter limit switch 76p opens. The actuator motor thus drives until the follow-up signal from potentiometer 249 opens the circuit at contact PFd with the propeller in starting condition.

A quadrant lock prevents the actuator from leaving rthe governing range at either end unless `the lock is released by energizing a solenoid QL. When the power control levers are below Operational Idle and the propeller is unfeathered, a circuit is 'completed from bus 2 through line 2811, throttle switches T14 and T15l or T14 and T15., line 282, and back contacts 15H3 o'f the `feather limit relay to energize a quadrant lock release solenoid QL. Switches T14 are clo'sed in the blade angle control frange-and switches Tre' are rclosed in this range and Stop. Thus, this 'circuit will be completed at the throttle 'switches vif both power controls are in the blade angle control range, 'o'r 'if 'either 'is in this lrange and 'the other y :is at Stop.

"Whenthe quadrant lock is released, the governor circuit to thep'ropeller through lines 283 and 284 is also broken'at relay PL, which is energized 'from bu's 2 through throttle `switches T114 and T15 and line 282. i Thus, when the power control is moved from the gove'rnng range 'intoe'ither the 'reverse `thrust or the starting range, the quadrant lock is released and 'the -governor cir- *cu'it to 'coils HD and Hl is opened.

"When the propeller has been set at minimum torque blade angle, b'o'th power units maybe lb'rough'tinto operafti'on, a's previously described. Assuming that the units have 'been started andorre 'or both power controls is moved To 'Operational Idle, which is the lowest 'point ofthe governing range, "o'r above, a signal 'is transmitted to the acltuator 76 to drive into the governing range. The signal calling -for 4rotation yin the increase pitch direction operates Y Ythe relay 'PFto complete a circuit "from'bus`2 4through line tion.

tional position of the actuator motor may control av 22 252, contact P Fi, line 271, contact 76g, line 272, contacts PH2 of the feather limit relay, line 273, coil PI ofthe increase pitch relay, and line 257 toground. Relay PI completes a circuit from bus 2 through line 258, front contacts P12, line 261, coil 76r, armature 76a,.line 259, .back contacts of relay PD, and line 257 to ground.' This supplies current through the armature in the reverse direct to that previously described, driving the actuator in the increase pitch and increase speed direction. The armature 76a stops when the follow-up relay is balanced.

When either power control is moved into the governing range, the governor disconnect relay PL and quadrant lock release solenoid QL are deenergized by throttle switches T14 and T15 or T14 and T15.

As the actuator enters the governing range, it ceases'to set propeller blade angle and 'operates by way of .the governor 81 to which it transmits a'propeller speed signal. `It` the power control is advanced fartherto call for higher power output and higher propeller speed, the actuator motor 76 will follow the signal up to the maximum propellet speed setting, at which point limit switch'76g opens.

The operation of the `'propeller governing system may now be outlined, so far as it relates to the present inven- Although many arrangements by which the rotapropeller governor, and many Ways by which propeller speed may be transmitted tothe governor, are available, a suitable one is illustrated diagrammatically The actuator motor 76, through a mechanical connection `schematically indicated at 275, moves the contact point of the potentiometer 27 6 energized from the electronic governor 81, supplied by bus 155. The potentiometer tap takes oft a fraction of the potential across the potentiometer and feeds this potential into thev electronic governor through line 277. The ratio of 'the potential on line 277 to that across the resistor 276 constitutes a desired speed signal input to the governor 81. The alternator 84, which is driven by the propeller shaft, `feeds an actual speed signal into the governor. By'mechanismsin the governor, which are immaterial 'to the present invention, the desired speed and actual speed signals are compared and any discrepancy affects the output signal of the governor, which is transmitted through lines 283 and 284, the contacts Ior relay PJ, and switch S5 tothe solenoidcoils HI and HD in the propeller. These coils actuate a hydraulic valve which, by mechanismrimmat'efria'l to the invention, controls the transmission of 'uid from a pump in the propeller, which is preferably driven by the vpropeller shaft, to hydraulic motors which vary the pitch of the propeller. If the propeller speed exceeds that vsignalled for, the coil HI acts to increase the propeller jpi'tch and thus its resistance to rotationuntil the speed .is reduced to the desired value. Tf 'the speed is too low, coil HD decreases the pitch so that the speed increases. As will be apparent, the coils vHD and HI could be field coils of an electric motor to adjust the propeller pitch, or could be relay coils controlling an electricalsys'tem. They may be energized alternately during a cycle, with the resultant eiect depending upon 'the relative time of energization of the two solenoids during the cycle. The structure of the propeller and the ineans by `which the governor regulates the vpropeller fare immaterial to the invention except that thecharacteristi'cs ofthe propeller and its governor must 'accommodate themselves to the control 'system of the invention. The 'speed selected for the propeller is coordinated with thepower lcontrol `of `the units by cams V111 and 113 4in the unit controls, .as previously described in connection with yFigure 13.

We may now consider the circuits involved in blade angle control of thepropeller in the reverse thrustrrange employed for ground operations. With the Vgovernor disabled and the quadrant block released `by movementof the power control levers into the reverse `thrust .slot 104, the potentiometer contact 128a or 128a' transmits a signal to the actuator to drive into the blade angle control range and .setpropeller pitch, which is .accomplished -in

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