~ • Automatic Flow Control B11 J. 11'. TtUILllfSt1!\· IT CAN BE said without contradic tion that the fuel system of a jct engine is the most complex section of the entire power unit and the m~st difficult to understand. Like all other complicated mechanisms. the jct engine fuel system had a humble beginning. Not many years ago, a system con sisting of a single fuel pump. a baro stat, a pressurizing \'ah-e, torch ignit ers, and a set of burners would suffice. \Vith the tremendous increase in po\\' er, speed, and altitude, derh·ed from present day engines. these now out oi date systems would be hopelessly in adequate. During the early days of jct dcn:1- opment. the trend was to design separ ate units and connect up with external plumbing. \Vith more and more units IN JET ENGINE FUEL SYSTEMS being addl·d. tud system designers re sorted to incorpowting several of these into one major assembly. called the Fuel Control Unit. While this had the ad\'antage of cutting down the external plumbing. it added to the complexity of the system to all except the initiated. The reason for this article is to describe briefly in simple language how this major unit operates automatically to adjust the fuel flow to meet the changes in air /fuel requirernents due to rapid changes in altitude and forward speed and during acceleration. Anglo-American: One oi the most used fuel control units in Canada today is the Lucas-Bendix type. made by Ren line to the f._l) _ filt<.:r of the r.c.u .. thus n.:durill,! th<.: pump lkli,·ery to that requi red f;•r idling. The au10111;11ic schnluling of fuel. \\'heth<.:r it is for chang<.:s in altitude or forward speed. or for rapid accder .11io11s. is .d i taken care of through this pump sen·o line which connects the F.C.U. to the pumps. \ Vhen the pilot opens the throttle from idling. the pressure on the down str<.:am side of the throttle ,·:ih·e will d rop in relation to the upstre:1111 pres sure from the pumps. Since these pres sures act on either side of the throttling control pressure drop piston, this change will cause the piston to mo,·e to keep the ser\'O ,·ah·e in the T.C.L'. closed. With i11cre:1si11g pump speed. and the sen·o line sealed off in the T.C.L" .. the ,en·o pr<.:ssur<.: will h11 ild 11p in the pumps, and 111n,·<.: the: p11111p ,en·o pis tons 10 i11crc:1se d<.:li,·c:ry to 111eet the: new th rottle k,·er po,ition. Thus :1 correct How al a constant pressure 1s .-I pril, 1?56 Uuth photo;,!ra ph aud drawi n ;,! illustrate th e Luca,;- 13 cndix fue l ,·nr11 1·o l unit. .ATMOS?HERE TO ;enows CM;.M&EP ACCEWt;...TOil: co-.... tQOl UNtr AlflTUOE METERIMC VALVE AOJUSIM[NI FUEL INlET FROM r CU DRAIN PtUG TI-O~OTHE lEVER STO? sci:re..-, THROTHE lEVEli! fU EL lNlET FROM PUMP .\.llllUOE VALVE SERVO flllER H!ll011L( CONIROL UNII H PRfSSURE CUT on VAlV[ tHROTILE VAlV£ COMPR~SSQR lNlEf PC:ESSUQ[ mai111ai11<.:d to 111<.:<.:t the full range of throttle openings. Overspeed Control: \\'hen the m:1xi- 111um throttle op<.:ni ng is reached. the serYo line is bled o ff inside the pump to pre,·cnt the pump o,·c:r-speed ing and increasing the rpm beyond the s:1fe limit. From the prc.:c<.:ding par:1gr:1phs. it will be conc<.:i,·ed that the pressure drop piston in the T .C.L' .. auto111.11ic:dly con trols the· ,·uel prc·"urt: .111d pump d<.: li,·ery. through thl' p11111p 'l'r\O line. lo meet ch.111gn 111 throttll' 11pl'11i11g,. Adjus1 me11t for Altirnck: .\1110111:11ic :u lj11s1111c·111 11111,1 hL· 111.1,k 10 co111 pc·11- sate for d 1.111gcs in thl· .1ir fuel r:11io ,lue 10 ,·.1ryi11g .d1i1u,k, .111d forw;ird spcnls. This is 1.1krn ci r<.: of hy using the ch:rnging compressor inlet pr<.:ssur<.: to operate .111 altitude metering ,·:1h·<.: .-\. pipe line is taken lrom th<.: air intak<.: to the unit. feeding compressor inlet pressure in to a bello\\'s assembly. .-\.s the lor\\':1rd speed 01· the aircraft increases. or the :iltitude dccrc:1s<.:s. the bellows expand through incn:as<.:d inlet 1m:ssurc. This ,,wscs :1 le,·cr :111:1chcd to the bellows 10 mo,·c and permit th<.: .dtitudt.: mctning ,·:1he ,<.:no ,·:1h·e in the T .C: .L-. to un,c:11. ,, ith ,uh,equ<.:111 hkt.:di11,! o!I of flll·I from the h.1,·k of the a l1i111ill' llll'tl'ri11:-: ,.,hl' pi,1011. This .1nio11 pl'r111i1, the pi,11111 .1 111 I .111.1, hnl ,·ah-c to moYc.: :111.! i11crc1Sl' the orifice -Ii FUEL CONTROL around the tapered end of the.: valve, pc:rrni tting more fuel lo Aow. This opening and closing of the ~e rvo valve and movement of the piston is taking place automatic:illy while the pilot is Aying the.: aircraft. thus the f11c.:I Aow is trimmed to suit the.: changing conditions of Aighr. Acceleration Control: Still another ad justment hJs to be made, this is to control tht: iuel flow during accekra tion . when there is a danger of cm:r fuc.:ling at tht: ea rly stages of throttle opt:ning. This could cause a si tuation where the rndden increase in fuel burn ing results in the setting up of J back pressure at the rea r end of the com prc%or. sutlicient to damage the com pressor bl:ides. This condition is called "compressor stall .. and is avoided by making use of the.: pressure diffrrenti:il across the compn:ssor in con j u:1ct ion with a pressure modula tor. The modu- 1:ttor is a sub-assembly of the accelcr:1- tion control 11nit ( .\ .<'.C .) cont.11111r.~ a capsule ~tack. one.: ~,ction of which is open to compressor inlet pressure. A series of rt:strictors and a half-ball val\'e, modul:tlc the compressor inlet and out let press11rt:s which arc passed on to a second chamber containing a closed bellows :isst:mhly. \,Vhcn the ::1od ulated pressure is sufficiently high, the closed bellows contract to open the metering vah-e servo valve and bleed fuel from the back of the metering va lve piston. T his permits the metering valve to move to allow an increased Aow of fuel. sufficient to acceleratt: tht: engine within rhe specif-ieJ time lim it and yet avoid 01·crfueling at the critical engine speed. The , \.C.L". also cont:iins a pressure drop piston and sen·o vah·e mechanism which bleeds the pump servo line to reduce pump ddivcry should the A.C.U. fuel inlet pressure get too high in relation to the outlet pressure. The foregoing is only a 1·ery brief description of how the Lucas-Bendix How control un it operates. However it should be sufficient to provide an in sight into the subject wi thout getting too inrnl1·ed. PILOT'S NOTES ( CrJ11/i11t1rd Jr,,m p«l(t:' J:i ) rt:mt:mber not to commit tltat part icular nror himself. Cartoons, howt:ver, fail if they do not get the point over to tht: rcadt:r. Used with discretion, tht:y arc a useful attention-getter. Used freely they can reduce the book to an inferior type of comic strip. \,Vhen the manual has hecn drafted and the illustrations are in a sufficiently advanceJ stage of prepar:ttion. the con tents of the book are agreed. paragraph by parag raph, with the company t<:st pi lots. This may be a lengthy procedure since opinions on operating tech niques may vary widely. and test pilots. par ticularly when the we:1the r is good. arc busy people. Eventually agreement is reached with them and with other in terested parties within the company and then a conference is called at which RCAF representati\'es go 01·er the draft and make their comments and sugges tions. A technique ad,·ocated by the aircraft manufacturer may not be agree- ~ Simmonds AVIATION IN SURANCE Complete Coverage and Service offered by BRITISH AVIATION INSURANCE COMPANY LIMITED MONTREAL TORONTO VANCOUVER \__ Head Office: Insurance Exchange Bldg., Montreal .} )iJll IJ.llt1f li.n and d e rive the bene fit of a large and widespread circula tion. . lpril, 1956 AI/ICIIA>T 341 Church Street, Toronto 2 , Ont., Canada COWLING LATCH Simmonds High-Strength Cowling Latch, of simp le two-piece, toggle type construction provides posi tive, flush-fitting closure of cow lings over a w ide range of curva tures. Easy to install. ACCESS LATCH Simmonds All- Purpose Access Latch is de signe d for fla t panels and fo r uses on cargo doors, radomes, armament compartme nts and other access paraels. It has a lso been fo und to be adaptable to a wide variety o f industrial purposes. Jtmmondt Aeroctffsories O F CANADA l l l.<\TEO SS00 MONKLAND AVE. MONTREAL, OUE. WA. 0247 SA 53/ 5. 81

Automatic Flow Control in Jet Engine Fuel Systems

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