[1]Un'l'ted States Patent Office P a t e n t e d A p r . 4 , 1 9 6 1 tures encountered during landing approaches, and the operating characteristics of gas turbines, the prior art fuel control metering fuel to the engine so as to main2,978,034 tain turbine inlet temperature substantially constant will TUP.BOPROP ENGINE IODLING CONTRDL 5, not maintain constant idling horsep6wer. The control system of this invention will provide sub. Robert L Wente, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Anch., a corporation of DelaVvare Filed Aug. 16, 1955, Ser. No. 528,703 10 8 c iaims. (Cl. 170- 135.72) 'This@'invention is directed to improved controls for 15 gas turbind propeller aircraft engines, commonly called turboprop engines. It is particularly directed to improving the landing characteristics of airplanes propelled by such engines. In gener al, the: problen@ of controlling a turboprop 20 engine is a rather com licated and difficult one. In flight, .P it is advantageous to control engine speed by a variable pitch speed governing propeller and control the power develope d, and hence the propulsive effort, by varying the flow of fuel t,o the engine. Such a control requires 25 safeguards, however., It must, if ii is to function acceptably, be such that it prevents overtemperature conditions@'in the engine, @revents cutting fuel flow below the ' nunimum at which coriibustion will be maintained, and schedules acceleration or deceleration of thek'engine. SO 30 as to prevent surge, stall, transient excess - temperature conditions, or flame6ut@ of the engine. @ A suitable control also must compen sate for changes in temperature and pressure of the air entering the engine so that the engine will function consistently and satisfactorily over 35 a wide range of altitude and temperature without constant adjustment of the control by the @pilot or flight engineer LAdditionally, such controls frequently include some mechanism directly responsive to turbine inlet 6r outlet 40 temperature which serves as an overriding control to liniit fuel flow whenever turbine temperature reaches a danger point, and, in most cases, an overspeed governor. There are known controls for gas turbines, - operating 45 as indicated above, which meter fuel flow to the @ engine iii@ response to three control parameters; engine speed, total pressure of inlet air, and total temperature of.inlet air.' Such @ontrols, however, so far as I am aware, @have' heretofore c6nverted these parameters into fuel 'flow so 50 as.to maintain turbine inlet temperature,constant,at any given control setting. Of course, it is possible to c"ontrol an engine on the basis-of other parameters but this is the preferred SysIt is satisfactory for flight at full power I or, - under 55 cruising conditions, but it has. been found to @ be unsatisfactory during landin g approaches @vhich sh6uld be. m@de with the engine idting. In one installation it has'-,been found 'desitable'to make the landing @approach with t e h engine developing slightly les's: than zero output;@. @pecif- 60 ically, taking from the propeller. @about five,, er cent of @p the maximu'm engine power rating. For best performance in @landing, th6. power 6utput 6f the engine should @ be maintained constafit @ during @the' low speed flight 6r landing %approach i@haracteristies of @the par@@illar aircraft. In this di@cu@sion,.. it should be understood -that the@ term @poyyer output may refer to a negative power output as well as@a@positive@one,,and. that: idffng. operatiori is op6rati6n 1)ower @ ou@put 'near 70 zero. @be d@fi ent ncipally@ @@q4use@ gf temperastantially constant idling power output of the engine during a landing approach on the basis 6f the three controlling parameters of inlet pressure and temperature and engine speed. By virtue of this improved mode of operation, the aircraft may be brought in on a landing approach at a constant power lever setting and it is not necessary for the pilot to devote constant attention to adjusting the power output of the engine to keep the aircraft in the rather narrow range of speeds slightly above stalling speed suitable for landing. The principal objects of the invention are to improve the operating characteristics of gas turbine powered aircraft and t6 improve controlling methods and means for turboprop engines. The manner in which these objects are achieved and the advaiitages of the invention will be apparent to those skilled in the art from the succeedin-g detailed description of the preferred mbde of carrying out the invention and the preferred embodimont of means embodying the invention, illustrated.by the accompanying drawings, in which: Figure 1 is a chart illustrating the fuel flow and turbine temperature characteristics of a typical turboprop engine as a function of ambient temperature at constant idlinghorsepower" Figure 2 is a graphical illustration of the variation with.@ power lever position and ambient temperature of engine shaft horsepower and turbine inlet temp . erature of a turboprop engine controlled in accordance with the invention; Figure 3 is a diagram illustrating variation in fuel ffow with power lever position and ambient temperature; Figure 4 is an elementary schematic diagram of a gas, turbine fuel system; and Figure 5 is a schematic diagram of the relevant partsof a fuel control embodying the invention. Referrin.@ first to Figure 1, this is a diagram illustrating the variation of turbine inlet temperature and fuel@ flow to maintain constant idling power output of an engine, as ambient temperature varies from -60' to 120' F.,@ engine r.p.m. being constant. There are two curves for each of these variables, one representative of variatiom, at zeto air speed and the other indicating variation at an. air speed of three hundred knots. As will be apparent,. as ambient temperature.increases, turbine inlet temperature for constant idling power Output increases quite substantially. , Contrariwise, to maintain constant power output, fiiel flow must decrease as the ambient temperature increases. The fuel required and turbine inlet temperature will both be lower at the three hundred knot air speed because of the contributio n of rani effect of air entering the compressor to engine power output. Looked at another way, this means that when the ai reraft slows down the fuel requirement increases slightly; if this' requirement is not met, power will decrease. However over% the landing approach speed range, this effe i nct s i consid,erable. As will be apparent, since turbine inlet temperatL7re stant, idling power output, a control set up to maintain turbine inlet temperature @constant @would dee r e a s e t h e owdr of the engine as ambi p e n t t d m p e r a t u r e i n c r e a s e s . f c r e a s i n g This, w o u l d b e a c c o m p l i s h e d , o c o u r s e , b y . d e fuel flow. a s t u r b i n e i n l e t t e m p e r a t u r e t e n @ d s @ t o i n c r e a s e with h i g h e r a m b i e n t @ t e m p e r a t u r e s o @ a s t o m a i n t a i n t h e ant. In othe words, the fuel flow teinperature const r landing approach@ at a value which is consonant with the' (;5 increases with increasing ambient temperature for con-
[2]3 curve would have a steeper slope than that in Figure 1. This condition is also very likely to be encountered in landing where air temperature increases rapidly as the ground is approached. Of course, at higher power settings of the engine, it is not practicable to maintain constant shaft horsepower because of the resulting wide variations in turbine inlet temperature, the primary limiting factor of en,@ine oper,ation at such normal power output levels. Referring now to Figure 2, there is illiistrated a schedule of turbine inlet temperature and shaft horsepower as a ftinction of power lever position. The power .lever is the iever moved by the pilot or flight engineer .to determine engine output and, as is customary, its movement is referred to in terms of angles from zero to 90'. In the example illustrated, the maximum poaer output is at 90'. Cruising positions are in the ran.ae from 90' down to about 45', and idling position is at 30'. Below 30' may be a ground maneuvering range of operation of the power plant at fixed propeller blade an-le which is not affected by the present invention and, therefore, does not require analysis. It will be understood that above the 45' power lever position the power control sets a turbin@ inlet temperature. The fuel control acts to meter fuel to maintain thL- turbine inlet temperature and the power output of the engine is what-.ver the characteristics of the engine determine. It will be noted from Figure 2 that above the 45' position of the power lever the shaft horsepower of the engine increases with advance in the power lever position and that the power output is very strongly affected by ambient temperature. The characieristic of the engine is such that for constant turbine inlet temperature the power increases with decreasing ambient temperatlire. Thus, for example, the power output at the 45' position at -60' ambient temperature is about 60 percent of the maximum power output at 120' ambient temperature. With 120' ambient temperature and 45' power lever pos-'@tion, power output is substantially zero. The temperature curve illustrates the turbine inlet temperature correspondirg to all of the four cur-ves of shaft hgrsepoiwer above the 45' control position. It will br-- noted that the increase in power is a function of a gradua@lly increasin.- turbine inlet temperature. If this turbi-@ie in,et temperature ciirve were extrapolated beIG,@N, the 45' control position, it would correspo-@id to the temperature line rlarked "60"' correspoiiding to 60' F. ambient temperature. In this case, the various shaft hatsepower curves would remain widely separated below the 45' control position. In the control accordin-. to the invention, the mode of cor@trol gradual.'ty char@ges between the 45' control position and the 30' or idlin.- control position so tbat. at the 30' Position the control meters fuel to maintain the same povier, otitput regardl6ss of ambient temperature. In order to do this it must, of course, vary the turbine inlet temperature. This is illuslrated by the convergence of the shaft horsepower, curves from the 45' to the 30' control position a-@id the divergin@. temperature curves for -60', zero, 60' and 120' ambient temperature over the same control lever range. Thus, the control no longer attempts to miintain a constant turbine inlet temperature. This involves no hazard to the engine, since the idlin.- is in a low and safe temperature rarge. Figure 3, which is a graph of typical fuel metering co.,iditions as a functio-n of, power lever,positiops and ambient temperature, shows a fuel flow control schedule to obtain the restilt illustr-,ited' by Fi.-ure 2. Fuel is controlled to operat -e the engine at constant horsep,ower at the idling co@idition while -onerating it at constant turbine inlet temperatiire thrgtigh the notmal flight,, ran,-.-. As will be apparent, the curve for 60' teriiperature, ,vhich is@ standard, ;s a gr,,idtial. consistent ciirve throu.-h the entire range from maximum paiver to idle. However, the fuel' flow; curves for the.'Iower; amb ictit.temperature-s break downwardly to reduce flo@w, 2,978,034 4 and thus decrease power at idling, whereas the curve for 120' ambient temperature slopes less steeply and rises toward the standard or 60' curve as the 30' power lever position is approached. 5 It will be apparent from the foregoing how the control method of the invention effects a transition from control to maintain desired turbine inlet temperature under normal Right conditions to a control to maintain substantially constant power output during idling conditions lo such as are suitable for landing approaches. As a result, the performance of the aircraft in landing is consistent and safe and does not reguire undue attention from flight or ground personnel ' An embodiment of the invention in fuel control struc15 tures is illustrated in Figures 4 and 5. Referring first to Figure 4, there is illustrated an engine E coupled to a propeller W of the speed governing variable pitch type thtough a shaft 10. Shaft 10 is -eared to an accessory drive shaft 11 which in turn is geared to a speed input 20 shaft 12 of a fuel control 14 and to drive shaft 16 of a fuel pump 17. The fuel control also has an input of total presure in the engine air inlet, indicated by the line P, and an input of temperature in the air inlet, indicated by the line T. There,is also a power control in25 put to the fuel control indicated by the lever C coupled by linkage 18 to the pilot's engine control lever 20. Fuel from a suitable source is supplied through line 21 to the pump, which discharges it through line 22 to the fuel control 14, which meters the fuel and delivers the 30 required quantity to the engine through line 23. The fuel control includes a throttle or metering valve through which the fuel passes.to the engine and a by-pass valve which maintains constant pressure across the throttling valve. For reasons of clarity, the by-pass valve 24 is 35 shown as a separate element in Figure 4. This by-pass valve communicates with the upstream and downstream sides of the throttle valve through lines 26 and 27, respectively, and acts to maintain constant pr@ssure across the throttle valve by returning the necessary portion of 40 the pumped fuel to the pump inlet through a line 28. Such fuel meterin,g systems are generally known and the relation @ of a by-pass valve to fuel meterin.- valves is illustrated, for example, in British Patent 727,201. The mode in which the invention is carried into effect 45 wili be more cleariy apparent from further consideration of the preferred nature of the fuel control 14, the significant parts of which are illustrated schematically - in Figure 5. Referring to Figure 5, the control input lever C of the 5o fuel control operates a throttle setting lever 30 through suitable means such as a cam (not shown). The s@eed input shaft 12 operates two speed. responsive devices of the fly-ball type (not shown) one of which exerts a governor weight force against a plate 31 on a vilve operr)5 ating shaft 32. The governor weight force is resisted by spring 33 -@ariably loaded by a governor setting lever 34 suitably coupled to input lever C, as by cam mechanism (not illustrated):. The second speed response device exerts a speed weight force indicative of engine speed C,( against a disk 35; on a reciprocable shaft @6 biased by spring 37. The -governor which moves shaft 32 is a govern6r iii the usual sense of the word in that it may act to limit or c<)ntrol engine shaft speed under taxiing or fixed propeller- pitch - conditions. It also may ,ibt as 65 an emergency limiting. governor in ev--tit of failure of failure of the propeller governor. The speed weight force inout to the shaft 36 is a control input for the fuel rnetering@ Fuel metering is :also responsive,,a previot@,@ll,,@ stated-., 70 to . temperature in the-engine inlet throtigh st,,;Iab, h' 'e mec anism such as a bulb in the -air inlet (not - .hoNvn) corinected through a capilla@y tube 41 @to ab@.Ilows- 42. Lellows 42, when it expands urges a retatable and axially shiftable camshaft 43. to t@e left in the figure -agginst 75 the@ a@ction of@@a spring '44@actihg through a Toilcer arnr 46
[3]2,978,084 5 on the other end of the camshaft. The camsbaft 43 receives als6 an input from the speed weight thr6ugh a speed servo cylinder 47 controlled through a follower v,alve and feedbac;k linkage 48 of suitable type by the speed shaft 36. The speed servo piston. acts to rotate the camshaft through a rack 38 and a wide pinion 49 fixed on the camshaft. Therefore, camshaft 43 is turned in response to engine speed and is shifted axially in response to inlet temperature. Camshaft 43 bears a part throttle cam 50 and an acceleration limit cam 52. The acceleration limit cam is provided to, control transient conditions of the engine during changes in power control settings. It may be enaag ed by a follower 53 extending from ii sleeve 54 totatably rilounted on a shaft 56. The sleeve 54 is integral with an arm 57 which has a lost. motion connection with an arm 58 on a second sleeve 59 rotatably mouhted on shaft 56. A cam follower 61 is integral with a slidable collar 62 splined to the sleeve 59. Collar 62 is shifted axially of the slee@,e 59 by a pin 63 extendi]19 from the throttle s6tting lever 30. As will be appareint, axial movement of camshaft 43 by the tem-' perature resr)onse and, likewise shifting the followet 61 through operation of the control input C, changes the zone of cam 50 which is engaged by follower 61. Cam 50 is normally coupled through follower 611 coller 62, sleeve 59 and engaging arms 58 and 57 to Sleeve 54, which has an arm 66 which normally bears against a disk 67 on valve shaft 32. A rotatable and ax.ially shiftable meterir-g valve member 70 mounted on sh@ft@32, is formed with a gene I rally rectangular opening 71 v,,hich cooperates with a rectangular opening 72 in an outer valve sleeve 73 to define a variable orifice which is the fuel metering orifice or throttle valve of the fuel , control. Fuel flows radially from within,the valve sleeve th,rough the orifice defined by the openings @l and 72, which is in the path between lines 22 and 23 of Figur .4 @p @. The area of the orifice may be varied by either @axial or rotary movement of sleeve 70. Axial m6vement ' is controlled by the speed governor weight force, the force of spring 33, and the action of the cams on shaft. 43. Rotary @novement is responsive to the pressure input P of Figure 4 which preferably acts throl-igh a suitable servomechanism, the inlet pressure compensating servo 78 which is controlled in any known manner so that it acts to move the piston rod 79 of the servo in respons@6 to tot &I p ressure in the air,inlet. T'hz) piston.rod 79 lias@ rac@-teeth on it to cooperate with@a gear sector 81-.on'lthe slee@-e 70 so,that it is rotated to open thkp,-v,alve as inle@t pressure@increases. The action, of inlet pressure is, relat,iy@@ly simple,in that it acts alone to vary one dimensiozi of the o.rifice. The @axial shifting of the valve is m6re-@complek.-,:.How.@ ever, normally 'iis valve is controlled by cam 50 acting through,cam@f6llower:61 as.previously@described against th@, @ resistance of spring 33-., @,-Under these @onditions, @ the",acceleration limit cam is@ not effective. However, unde . r@ tfansient conditions of @ operation of, the ehgin.-, the ac'--@ celeration: @ limit @cam @',may en.-age follower 53 to rot'ate sleev,e 54-@clockwise as illustrated in Figurd 51 separatiric abutting arms 57 @ And 5@8 and overridin the @ction . bf 9 cam 50 in a @ dire'ction@ to close the @ l@7- va e and thus limiifuel, flo,@r. r If, 7 f6r @;any @ reason such. as fAilur6@ of th6' propeller go-Vern6r;@@ engine speed iiicreasds t6'the, poi'ntwher'e the goveinbr'weight force will overcome the set,ting of spring, 33,,r @the governor acts, to mbve h@fi 32 aiid: val@e slee've@ 70,,tol.th6 right to decrease fuel flow, the disk 67 mo'vin'g,a-way from arm 66 in this'c@se. In,fli-ht the @'d&'d 'g6vernot is sef'higli6i th,2[ii'the pi6-@ peller @ governo:t and - is, opera ive on y @ @in@ the event of failure @o@f@@ th'e'p'io @pelle@ g'ov-ernor. However, thi@ govemor also pro@lides @for operation 6f the power plant in' fix@d' 1)1@@e , iigld op6ration 'for t@xiing aid brakini, s h whi6h -eq e@ t p go@@ernor, n@@ @41y Oayiiinit engiiie; @peed 6 and thus be in command of the throftle. This mode of operation of the engine is not material tb the preseiit invention, which is usable under different condition@ and whether or not any fixed blade angle operation of the engine is available. Considering th6 operation of the system and assuming that the airplane is in n6rmal flight, the pil6t will have his engine control lever 20 in a flight or cruise settin.- calling for normal power output of the engine. This 10 setting of control lever 20 operates through the input arm C of the fuel control, and fuel control will be at a high power setting with cam follower 61 toward the left or near end of cam 50 as shown in Figure 5. If the pilot wishes to operate the -Dlane in flight at low speed, 15 he moves the contro.I 20 to call for lower power and cam follower 61 will be moved toward the intermediate zone of the cam for lower power settings. The combined effect of the inlet pressure, inlet temperature, and speed senses acts @to supply fuel so as to maintain constant tur20 bine inlet temperature. Engine speed normally does not vary because it is determined by the propel-ler governor and, in the power plant described herein, constant speed Dperation of the propeller throughout the available range of power for flight purposes is the prei'erred mode of op25 eration. It will be, seen, therefore, that in nornial flight the pilot acts to maintain a variable engine power level conditioned primarily on a scheduled turbine temperature. Such contrlol cou!d, of dourse, be exercised by response 30 by the pilot to an indication of turbine tei-nperature and corresponding movement of the control 20 to hold the temperature, at the desired value. Such a mode of operation, depending oli d-IrCCt TCSponse bill tht-- pilot to tLrbine temperature, is impractical, particu@larly for multi35 engine aircraft. 'fherefore, all practical installatl'ons embody power contro I means which may be such as that described herein in which the turbine temperatur e is maintained more or less accurately at a value determined 40 -by the pilot by setting the control lever 20. Assuming now that the plane approaches for a landing, t@ie throttle will be cut. back by the pilot, the power control lever being moved to the 30' position w-hich corresponds to flight idle and which, for example, may call for a negative power output of 175 H.P. from the 45 engine. The cam follower 61 is moved farther to the right on cam 50 to call for lower fuel input and thus less power and moves into the zone represented by the 30' power le-ier position values in the curves of Figures 2 50 and 3. The effect of inlet temperature on flel flow is modified so as to maintain constant power instead of constant@ turbine inlet temperature. With the airplane in the landing glide the propeller remains in goveming operatioii and windmills so as to provide some power to the engine. Because the power output of the engine is kept 55@ constant, the windmilling drag of the propeller remains constant at the various ambient conditions. This cojistant engine,power output could be raaintained with any elementary fuel throttlin- means by providing the pilot with@ an indication of engine power output and having. co the pilot adjust the control lever 20 for each engine dur- ing @he landing approach to hold the power at.the des-ired level. This would be impractical, since it would call for too much attention to engine control iat a time 65 wh&@ there are ma,ny other things to - b,. attend@d to @ by the pilot. @Oidinarily, at about toucl@.dbwn, tile speed of the aircraft decieases t6 a point at which the governor 'brings the propeller against a low pitch stop so that the propeller goverrior loses cOn I trol. As tho spe6d of the aircraft decrease@ on the runway the power itiput from 7 the propeliei to the engine decreases and, therefore, the engine speed 'tends to decrease. In this 1)hase of operation, however, the part, throttlle cam 50 is so contoured thai s@bstantial decrease df ensine s@6ed below th@ n@@al @fli@ht r@p@@.' acts througli:
[4]the speed weight servo 47, rotating camshaft 43, to rotate cam 50 under follower 61. The cam is so con-toured that this rotation increases fuel flow to the engine with decreasing cngine speed. In this regime the spe-.d weight input and the mechanism actuated thereby thus serves as an underspeed governor to keep engine power output up to the demand imposed by the propeller. - This underspeed governor mode of operation also may be used in taxi - ing and braking operation of the engine at fixed blade angle, since increase in load on the engine by increasing pitch _of the propeller tends to slow the eng'ine, which causes the underspeed goverenor to increase fuel and minimize the drop in speed consequent upon the imposition of the higher load. It may be noted that the response of the fuel control to engine inlet pressure remains the same in all modes of op@-ration. Increase in ambient pressure tends to increase the power output of the engine, but since such increase in ambient pressure is accompanied by an ihcrease in the drag of the aircraft these two effects are substantially mutually compensatory. As stated, the value of the constant power operation is primarily in the improved control and safety of the aircraft during the landing approach. This valuable feature has been added to the previously known turbine control systems without injuring the p@-tformance of the eiigine or hazarding its components in the higher powered ratiges used for normal flight. As will be apparent to those skilled in the art, the principles of the invention may be embodied in various fuel control systems and the scope of the invention is not to be considered as limited by the detailed description herein of the preferred embodiment thereof. I
