the aircraft engineer 1934

7/27/2019 The Aircraft Engineer 1934

1/94

January 25, 1934 Supplement to FLIGHT

FLIGHTENGINEERING SECTION

Edited by C. M. POULSEN

January 25, 1934

CONTENTS PageEthyl . By F. R. Banks, O.B.E.. P.E.Ae.S.. M.I.A.E.. M.Inst.P.T..M.S.A.E 1In the Drawing OfficeLaying outLines andPla t ing . By It. Haley 5Hiduminium E.R.53 B 8Power curves of De Uavilland "Gipsy Six" engine 8

ETHYLBy F. R. BANKS, O.B.E., F . R . A E . S . , M.I.A.E.,

M . I N S T . P . T . , M.S.A.E.The paper on " Ethyl," read by Mr. Banks to theRoyal Aeronautical Society on January 18, presentssome difficulty to a paper like F L I G H T , in that its im-portance, in view of the Air Ministry's decision to useleaded fuel in the future, is such that it ought to bepublished in full. Space does not, however, permit ofthis course being tahen, and we have decided to publisha summary of the first part of Mr. Banks' paper in the

F L I G H T pages this week. The second, and more tecltni-cal, part of the paper is summarised below, but willtake one or two more, instalments to complete.ED.ENGINE DEVELOPMENT

Some information indicating the lines to follow in order tomake full use of fuels containing leadT HE manner in which leaded fuels have been viewed,in some quarters, is extraordinary. If the same diffi-culties had been put in the way of the development ofsuperchargers in this country, one is certain that theywould not yet be in use except in a very experimentalway, and in consequence we would not have been in theposition to produce some of the world's best aviationengines, as we are doing to-day.

With regard to the employment of high Octane fuelsin America, while a large amount of test bench work was,and still is, being done, they appreciated the necessityof obtaining practical flight experience in the earlystages of development and took the view that whateverthe results of bench tests, they would probably come upagains t sundry difficulties in actual service, so the soonerexperience was gained, the better. We, in this country,have admittedly obtained valuable data from the testbed during the past two years or so, particularly onleaded fuels, but little data has been obtained on fuelsof high Octane value and the operation of high dutyengines in actual service.

The problem of the operation of aviation engines onany fuel, because of their relatively high specific poweroutput, is quite peculiar to their class and cannot becompared with the normal operation of automobile

engines, with which little or no trouble is now experi-enced over long periods of use. Theaviation engine hasto deal continuously with a far greater bulk of heatper unit volume of cylinder than practically any otherpetrol engine. It is, therefore, more difficult to obtainreliability ovei long periods of service without payinggreater attention to it between times. Such items asthe pistons and valves are very much larger than thosein normal automobile engines, and it is, therefore, diffi-cult to arrange the design of these parts so that theyare able to dispose of the heat satisfactorily and stillretain their form and material structure. Modernengine development, therefore, demands what may becalled at the present time " super fuels," although infive or ten years' time these will probably be consideredmediocre.The Effect of the Products of Combustion of a Leaded Fuel

upon the Engine PartsThe main products of the combustion of a leaded fuelwith which we are concerned is lead bromide. Underparticular circumstances, the presence of lead bromidemay aggravate certain troubles which, previously, mighthave been latent in the engine, with the possible excep-tion of one, namely, cold corrosion. Theweak links inthe chain connecting satisfactory engine performancewith leaded fuel are as follows : (a) The tendency of the exhaust valve to scale andburn, due to " hot ' corrosion attack of thevalve seat and insert, and/or " mechanical "attack by the pieces of detached scale.(b) " Cold " corrosion attack of the exhaust valve

stems, when the engine is standing idle.(c) " Cold " corrosion of the cylinder bores under the

same conditions as in (b).(d) Corrosion attack of the exhaust pipes and collec-

tor rings.(e) Deposition of lead salts on the sparking plugs.This list appears formidable, but with the materialsavailable to-day, coupled with suitable design, there isno need for these troubles to persist.Exhaust Valves and Valve Seat Inserts, GeneralConsiderations

The exhaust valve is undoubtedly the most vulnerablepart of an engine, since it must perform well at elevatedtemperatures (around 800 deg. C.) when working with-out undue scaling or becoming mechanically weakenedand, in addition, remaining gas-tight.In the case of an exhaust valve which works in thepresence of leaded fuel, a hard, black, polished and


2/94

SUPPLEMENT TOP L I G H T JANUARY 25, 1934THE AIRCRAFT ENGINEERadherent skin generally appears to form on the seat sur-face in the early stages of running. It is not yet clearwhether this skin is a lead product deposited on theseat surface, or is a result of corrosion attack of thematerial itself. One is, however, inclined to the formertheory, that it is in the nature of a glaze, since it hasalso been found on valves which have operated success-fully for long periods. The thickness of the glaze orskin varies somewhat, and is difficult to assess accu-rately, but from photomicrographs appears to be in theregion of 0.0003 in. In cases where the skin has beenfound on valves which have operated successfully forlong periods, it is of even thickness with an unbrokensurface. In the case of a failure (valve burning) thetrouble seems to start when the skin or glaze breaks-down. Thedegree by which the exhaust valve, when inoperation, may or may not be attacked by lead bromideis mainly controlled by its working temperature andthe material used in its construction. It is one's ex-perience that low working temperatures for the valvewith a given material may cause little or no trouble,but an increase in the former may give rise to valvetroubles due to failure of the material to withstand, atincreased temperature, the lead bromide attack, andit will then scale and perhaps burn. Toeliminate thesetroubles at the outset, the broad principle is to ensureby suitable design that the working temperature of theexhaust valve will be kept to an absolute minimum anda valve material chosen which, in addition to highmechanical strength and durability at elevated tem-peratures, will have a good resistance to attack by theexhaust products.

The experience we have in this country, and on theContinent, seems* to indicate that the valve seating* inthe cylinders themselves exercise a considerable influ-ence upon the conditions of the seat of the valve andtherefore, the life and behaviour of the latter. In thisconnection it is interesting to note that the question ofthe conductivity factor of the neat insert materialappears to be of secondary importance and, providedthat the coefficient of expansion of the material is asnear as possible to that of the cylinder head materialin which it is fitted and the method of fitting satis-factory, i.e., good thermal contact maintained, its capa-city for getting rid of heat is much greater than theamount of heat which the valve can give up to it. Theimportant features which a valve seat insert shouldpossess are : a high resistance to corrosion attack andgood surface hardness, or, in any case, toughness.Corrosion resistance appears to be the principal answerto valve-burning troubles and, in addition, the provisionof a hardened surface in one or both cases seems toprevent abrasion of the seat surfaces by any scale whichwould tend to spoil the thermal contact between thevalve and its insert.The rate of development of the modern aviationengine is such that great difficulty is being experiencedin keeping the exhaust valve temperature within reason-able limits. It has been apparent during the* last yearor sothat however efficient the design of the valvesandseat inserts, it is difficult, if not impossible, to get ridof the heat adequately in this manner. Obviously, thehotter the exhaust valve the more limited are thecapabilities of the engine with fuels of given Octanevalue. This has led to the development of the internally-cooled valve, which is one having a hollow stem, andsometimes head, partially filled with a medium, usuallymetallic sodium, which will efficiently transfer some ofthe heat from the valve head to the cooling medium ofthe engine (air or liquid) via thevalve stem and guide.

Suitable Valve andInsert Materials andCombinationsThe material most generally used up to the presenttime, for inserts, is aluminium bronze. This is stillused,, but it will, undoubtedly, be superseded by thespecial alloy steels. These steels are already being tried,

and, in fact, one or two manufacturers have done aconsiderable amount of running with steel inserts andintend to standardise them in all their future models.The materials generally favoured are those of the semi-austenitic or austenitic variety, and in some cases aresimilar to those used for the valves themselves. Thesesteels generally have a good resistance to corrosion andscaling at high temperatures, and there are some par-ticular brands which are exceedingly tough and " workharden " to a high degree. Notable among these isN.M.C. (nickel, manganese, chromium), a high-expan-sion steel produced by Firth's. This steel has a co-efficient of expansion which lies between that ofaluminium bronze and the aluminium alloys generallyused for cylinder head material, being 0.0000223 between200 and 300 deg. C.

It is extraordinary how a change in insert materialafffictB the valve. Many cases have come to one's noticewhere valves have burned when operating againstaluminium bronze inserts and have been completelycured on going over to steel inserts. Thefollowing com-binations could, in the light of recent experience, betried with advantage: (a) A valve of austenitio steel working against aninsert of similar material or N.M.C., preferablythe latter.(b) A similar type of valve as in (a) " Stellited "on the seat and working against insert materialwhich is not " Stellited."(c) Both the valve and insert " Stellited," using thesame materials as in (a).(d) An un--(Stellited " valve against a " Stellited "insert, similar materials used as in (a).In the case of (a), K.E. 965valves have been triedwith success on prolonged tests against inserts ofN.M.C., both on complete air-cooled engines and singlecylinders of the same type.

Some Notes andGeneral Information on theValve andInsertDesign, Construction and FittingThe following deals with the sodium-cooled valve andsteel insert, because it is felt that this combination is adirect line of development to pursue immediately.

ValvesIn Fig. 8 sketches are shown of the sections of twosodium-cooled valves differing in design.(a) shows the latest type of sodium-cooled valvedeveloped in America. Thehollow forged head should benoted, together with the swaged-in hollow plug orthimble at the stem end. The latter is to prevent excessheat from travelling up to the stem end upon which

the valve rockers operate, particularly in the case of theM A T at useDiHPVKCorcup/moM.3Q TO Ft* PLUS.rputto xm/o/r jtiw aauewa

T*/>ER PLUS

SOBiUM

Fig 8.Typical examples of sodium-filled valves7 8 b


3/94

JAN UA RY 25, 1934THE AIRCRAFT ENGINEER

SUPPLEMENT TOF L I G H T

part ial ly lubricated valve gears of air-cooled engines,where there is a risk of excessive heat get t ing to thevalve springs and softening them.(b ) gives a sketch of a similar valve with a hollow stemonly, and it shows a different method of plugging whichis also in use. The tap er plug is pushed in, and thema teria l peaned over i t at the s tem end. A loose,hard ene d cap is the n fi t ted over the stem end, or ahard ene d tungste n-steel but ton welded on. Quite agood idea is to " Stel l i te " over the stem end, and thiswil l ensure that the plugging remains t ight and providessomething of sui table hardness upon which the rockerscan oper ate dir ectly . The valve is of the modified tu lipt y p e . The full tulip head is not desirable since, due toi ts greater exposed surface, i t usual ly runs hotter thanthe o the r type s, and in any case the sodium is too far

properly and cause valve bur ning . Disto rt ion is verydifficult to detect or control, but it valve burning isexperienced, this possibility should be the first to beinvest iga ted. The rim of the valve, and the insert ,should bo regarded as two concentric rings, and bothdesigned to achieve this effect in pra ctic e. Th e designof the valve guide for a sodium-cooled valve is veryimportant , because the stem has to deal with extra heat ,and unless it can get rid of it to the cooling medium inan efficient manner, a high degree of guide wear willresul t .Fig. 9 gives rough sketches of different types of in-serts and the methods of fitting th em . The sh run kscrewed insert is in general use in this country, whereasin America the plain shrunk type is pract ical ly univer-sal . Fu rth er detai ls wil l be found in the main te xt .

SHRU NK NO SCREWED m INSERTS

Fig. 9 Various methods of fitting valve seat in serts. The screwed type is in general use in England while the plain typeis almost universal in Americaremoved from effective contact with most of the headm at e r i a l .The most effective valve seat angle is, in my opinion,one of 30 deg. This gives a be tter ave rage valve ope ningcharacteris t ic than, say, one of 45 deg. , but i t is moredifficul t to ensure maintaining a sat isfactory seat ingwith this than with one to the lat ter angle, and,perhaps, in the case of an exhaust valve, part icularly, i tis more important to ensure that the valve seats i tselfefficiently rather than to benefit by the small increase inporting efficiency offered by the former angle.The quest ion regarding the most sat isfactory width ofvalve seat to employ is a vexed one. For some tim e, th eAmerican engine fi rms have favoured a relat ively wideseat, and in some cases the seat widths have been some-wh at excessive. The tendenc y in the States now app earsto be towa rds a narrowe r seat , part icular ly for enginesof high specific power output using fuels containinglead. The sea t wid ths of some typic al Am erican valvesran ge from ^ in. to 1 in. for a large valve havin g asea t dia me ter of abou t 3 in. and from | in. to & in. forvalves of 1.75 in. to 2 in. seat diame ter. The Britis hengines general ly keep to the lower l imits of the widthsmentioned, and in some cases are only half the width,in pro por tion . One hes itates to speak of " wide " or" narrow " seats as such, and considers that the ques-tion of width is really a compromise which is controlledby the considerat ions of individual engine design. Sat is-factory unit loading, to achieve good thermal contact ,should be considered of prima ry impo rtance. There is arisk, if the seat is very narrow, that any " blow past "which may occur wil l cause " gu t ter ing " r igh t acrossthe seat, whereas with a wider seat* under similar con-ditions, it usually takes longer for this to occur, and itmay only show up in the form of pitt ing . I t will beapp recia ted, the refore, th at i t is unwise to dogmatiseon such mat ters .

Many valve t roubles are occasioned by distort ion ofth e cyl inder head. This is part icu larly liable in thecase of the monobloc arrangement used for the largemodern liquid-cooled engines, and it has also been thecause of a large amount of t rouble in some ordinarymotor-car engine s. Distort ion, so far as i t may affectthe valve, general ly causes misal ignment between thevalve guides and inse rts , or actual " oval is ing " of thelat ter, both of which prevent the valves from seat ing

" Cold" CorrosionThe cause of " cold " corrosion is genera lly accep tedto be as follows: W hen an e ngine comes to re st, a ce r-tain amount of condensat ion takes place, part icularly inthe case of a cylinder in which the exh au st va lve hasstopped in the open posi t ion. Any lead bromide prese ntwil l also condense, and this , with the moisture present ,gives rise to corrosive act ion, part icularly on steel parts .The principal p oints at tack ed by " cold " corrosion a rethe exha ust valve stems and the cylinder barrels . Theextent to which they are at tacked is influenced by thema terials used, and also the working condit ions. Thegeneral t reatment , to avoid a t tack , i s to ensure thatthe parts concerned are well covered by a film of minerallubr icant . Fu r the r deta i l s concern ing par t i cu lar t reat -me nt will be found in the m ain text.The use of " nitriding " in this connection has beenquoted, but our experience here and in Europe seems toindicate that , so long as the parts made from thesematerials are kept well lubricated, l i t t le or no troubleis encou ntered. Adeq uate lubricat ion is nat ura l ly qui tepossible with, say, cylinder barrels, but the use of com-pletely ni t rided e xha ust valves is not recomme nded,al though the valve stems may be t re ate d in this m ann erand are qui te sat isfactory so long as the ni trid ing isnot carried too far down to the hot part of the stem.

Exhaust Pipes and. Collector RingsThe usual material employed in the construct ion ofexh aus t manifolds is mild steel , which after formingis welded and riveted. Mild steel , al thoug h fairly sat is-factory, is not the best al l -round material to use, andis very prone to spl i t , due to a combinat ion of tem-perature var ia t ion and v ibrat ion , which cause fa t igueof the ma teria l . Steels of the stainless and /or aus ten i-t i c var ie ty are gradual ly coming in to use , par t i cu lar lyin America. These are corrosion resis tan t - *1 j a high

degree and do not scale easi ly. Ex ha us t collector ringsof s tainless s teel are, one understands, very general lyused by the mil i ta ry machines in the U.S.A . The useof mild steel rings has been general with the civi l air-craf t operat ing concerns in the S ta tes , and a l though nogrea t trouble was exper ienced f rom thei r con t inued usein conjunct ion with that of leaded fuels , i t is under-stood that they are being replaced with stainless s teelrings as occasion permits .


4/94

SUPPLEMENT TOFLIGHT THE AIRCRAFT ENGINEER JANUARY 25,Sparking PlugsThe re 13 no more difficulty in obta inin g a suitab leplug tor operation with leaded fuel than choosing onefor any eng ine wh atev er the fuel used. In either casea satisfactory choice largely resolves itself into a matterof p rac t ica l ' tes t . The three t roubles which may pos-sibly occur are (1) an excessive rate of build up of de-posit on the insulator; (2) possible breakdown of the

(/HMODIfiED PL UQ

Of H OT 6AS. So*ie Gerosirm u. ftHTM H e # Etrs . v 6000

aezc.- TMEVE M*Y SMI B U I L O I M O f O**osrr\- O>t/s//iG Ore* H e*Tt*e //y ThisO ue Tb /r sS *LL

Fig. 10This sketch shows moditications made to micasparking plugs to suit air-cooled engines using a "lead ed " fuel.insulation; and (3) erosion or corrosion of the electrodet i p . These are not necessarily related, and are dif-ferently influenced by temperature. Mo re complete de-tails will be found in the main text, but let i t be saidthat any t roubles experienced with sparking plugs arenot cri t ical , and the cures are relat ively s t raig htforw ard.

Some Notes on " Stelliting "Although in use for some time, the application ofSatelli te to the valve and seat inserts of aviation enginesis only a relatively recent development, consequentlythe following info rm ation may be of some assis tance ." Stelli te " is a mate rial composed largely of cobaltand chromium, between 50-65 per cent, of the formerand 30 per cent , of the lat te r , togeth er wi th t ung sten ,which may va ry, in propo rtion , from ab out 4 to 20 per

Fig. 11This diagram shows the various stages of stellitinga valve seat.cent . The amo unts of the con st i tuen ts vary accordingto the part icular grade and degree of hardness required.Both at low and elevated temperatures its resista nce towear and oxida tion is excellen t. It is par ticu larl y effec-t ive in res is t ing, at h igh temperatures , corros ion at tackby lead bromide or the products of i ts decomposition.The treatment of valves and inserts consists, briefly, ofapp lying a layer of " Ste lli te " to th e seat surface s,this being done by means of an oxy-acetylene torch." Stelli t ing " is not , actua lly, a welding process, sincefusion of the Stel l i te and the material of the partbeing tre at ed , is not desired. It migh t be described asa high-tem peratu re bra zing process . A descript ion ofthe technique of applying Stelli te will be found in them ain text. The figures 11 to 15, how ever, show th eessent ial points in the process .Fi g. 12 shows a typica l examp le of an exha ust valveof a large American ra dial engine. The valve has ahollow forged head, sodium filled, and both the seatingand valve head are Stelli ted . This valve mea suresabout 3 in . across the outer d iameter of the seat ingand weighs approximately one pound in its completedform. Fi g. 13 gives a sectional view of the sam e valve.Note the hollow swaged in plug at the stem end, andalso the thickening up of the stem section at the neck.This valve is a remarkable example of accurate forging.The il lust ratio n in Fi g. 14 shows Stelli te depos ited on arough machined valve of martens i t ic s teel . The segmentcut from th e valve head shows up the section. Fi g. 15shows a valve of the same type as Fig. 14, but i t isof austeni t ic s teel s imilar to K.E. 965, and is the pro-duct of another ma nufa cture r . Note the di fference inthe Stel l i t ing.

(To be continued.)

Figs. 12 to 15S how further details of different valve types and me thods of " stel liting ." ;' ""


5/94

J A N U A R Y 25, 1934THE AIRCRAFT ENGINEER S Ul 'PL E M FW T T OFLIGHT

IN THE DRAWING OFFICELAYING OUT LINES AND PLATING

B Y K, HALEY.IN getting out a set of lines for either an ordinary" fairing," drawing, or plating up a monocoque fuse-lage, it is impossible to get them correct unless the}7are faired in three views; putting a line in " to eye "is hopeless. Only a few years ago, when 90 per cen t,of the fuselage fairings were done in wood, it was quitea common sight to see a skeleton fairing in position,with pieces glued on here, and pieces cut out there, toallow the stringers to lie fair, proving that the lineswere far from accura te. Imagine plating a ship withf-in. pl atin g and the lines not being fair. No doubtthis procedure was satisfactory to a degree, as whenthe fabric was put on the general appearance was pass-able, but now that we are familiar with metal moiio-coque fuselages the writer is of the opinion that it isworth the trouble to fair the lines properly in the firstplace, and eliminate any error, or endeavour to do so.

In the following article the writer will try to describethe method employed in laying out lines, half-blockmodel, and shell expansion, for a metal fuselage. Thesection on lines will, of course, apply equally to a set of" fairing lines " for an ordinary fairing drawing.LinesTo begin with, endeavour to lay out the drawingpaper for the lines to as big a scale as possible, andthe whole bench can, and should be, utilised, unless themachine is extremely short. A steel straight-edg e, 6 ft.long, is a useful tool when laying out lines and a setof splines or battens, with at least a dozen lead weightsare essential.

If time will permit, the cartridge paper should bepinned down the night before and allowed to stretch.Set down the " datum line ," or " thru st lin e," as inFig. 1, also centre line in plan and " body plan."Draw in outline in profile and plan, the plan beingonly drawn in the bottom half of the lower drawing.Subdivide the total length between the nose and thesternp ost, into an equal number of " stations " (ifpossible, these stations should be a multiple of theframes or fairing formers). N umber these stations0, 1, 2, 3, etc ., from the left-hand side. The numberof stations will depend on the total length, and whetherthe fuselage has a lot of curva ture in its length. A tthe ends, especially the nose, half-stations can be in-serted for fairing purposes only. Hav ing laid out theoutline in profile and half-plan, draw in the shape ofthe widest statio n (usually at half-length) on the

" body pl an ," to the left hand of the centre lin e,station 5, fig. 1.Next, draw lines enclosing the width and total depthon the body plan, as A, B, C, D, Fig. 1, and divide thisfigure into equal part s from " 0, " both vertical andhorizontal, the horizontal lines being numbered fromthe base upwards, and the vertical lines numbered fromthe centre line to the right and left.The horizontal planes will be known as such, and thevertical lines will be called " bu tto ck s." Ma rk off theintersection of these horizontal lines with the profileand the intersection of the buttocks with the plan out-line, as at " XX " and " YY," Fig. 1.Now we are ready to transfer the height above anddepth below the datum of each station to the bodyplan, also the half-width. Ha ving chosen the middlestation and marked same previously, mark off otherstations on the body plan thus:All stations forwardof the middle station mark off to the left of the centreline of the body plan, and all stations ait of the middlestation mark off to the right of the centre line.On the body plan we now have the height and depthof each station on its centre line and the half-widthalong the datum about " 0." These positions are" lifted " off the profile and plan with strips of paperabout in. wide. N ext proceed to lightly outline theshape of each station on the body plan after havingnoted an} - necessary flats that may occur, such as abomber's window on the bottom surface, et c The shapeof the sections will otherwise be largely governed bythe shape of the middle section. A fter all the stationsand half-stations (if any) have been drawn in, proceedto draw in the diagonals as OB, OC, 0A, 0D, Fig. 1.

FairingThe real " fairing " has not yet been carried out ,and we now proceed to fair our " body plan."Taking a strip of paper place same along the hori-zontal plane No. 1, and mark off the centre line on thestrip and all the intersection points of all stations thatcut this line. Transfer these points on the strip to thebottom half of the " plan " and mark off the half-widthof each station. Place a batten throu gh these pointswith weights thereon, and lightly draw a line throughthem . Proceed as above for all the horizon tal planes.Do not attempt to correct shape unless there is anobvious bump, as it may be due to quite another cause.The alteration to another line later on may bring anyirregular lines in the early stages to their correct spots.Now, with another strip of paper placed on ButtockNo. 1, mark off the intersection of all the stations foreand aft, i.e., each side of the centre line on the " body-

pla n." Transfer these points to the profile and markoff these po ints a t each stati on . The ends; of the se

/****

FIG.I.


6/94


7/94

JANU ARY 25, 1934THE AIRCRAFT ENGINEER SUPPLEMENT TOF L I G H T

FIG.YI

LAP JOINT-f -SINGLE Rl Vf S H/KLF BLOCK MOCtEL

# OOUBlt MVETS# TREBLE RIVETS

to mark off the plates on the model, which is the solepurpose of mak ing same. Al though the gauge of theplates will vary from front to rear, the length from themanufacturers is general ly the same.The usual method of plating a fuselage is to ' ' breakjoint," i .e. , never to have two joints opposite one an-other, but to have at least two strakes between eachjo in t measur ing round the g i r th .Cases do occur where the butts are arranged all tofinish on a convenient frame to enable certain lengthsof plate to be util ised. When this is done the stren gthof fuselage is made up by a considerable number oflongi tudinal s t r ingers , hence an increase in weight .As this a rticle is not dealing w ith the pros and cons ofdesign, but the practical side of the work, we will pro-ceed to the next step in the work.Knowing the size and gauge of plates to be used,s tart ing at the middle and working towards the ends ,mark in the plates using as long a length as possible.With a strip of paper having the width of the platemarked thereon, p lace one mark on the top centre l ineof the model and bend the strip round at each frame,m ark ing the width of the plate on the model. We now

have the first l ine of plating marked which will be thetop pla te on the finished fuselage. Of course, the jo intma} 7 not commence on the centre line, but equally oneach side when, of course, the strip of paper will onlyhave half the plate width marked on it .Next, one must decide the width of lap required tojoin the plates longi tudinal ly , and mark same on ourstrip before proceeding to mark off the second line ofpla t ing . These l ines of p lat ing are cal led " s t ra kes ."The first one is " A " stra ke, and w hen m ark ing offB, C, D s t ra kes m ake sure your m ark for th e lap isins ide the s t rake above.All longi tudinal seams must overlap with the s ightedge downw ards (.nee section, Fig . V I). After h avin gmarked off all the width of plates on the frames fromtop to bottom, procure a pinewood batten and pin sameon model through each successive strake and pencil in.On a second strip of paper mark off the length ofthe plate to be used on " A " stra ke, a nd using as longa. leng th as possible st ar t at th e middle with an equallength of plate each side of the middle frame and workfore and aft until you have all the lengths marked on

this s t rake. When carrying out th is operat ion do notomit to allow for the width of the lap joint at eachplate after th e first one. The plate in front of eachadjacent one should lap over, that is, all laps shouldpoint aft .Of course, one may be making all joints and seamsbutt jointed with a narrow plate behind to make thejoin t, w hen the allowance for overlap will IK- om itted .On ma rkin g off the le ngths of pla tes on " B " stra kestart about one-third of the length from the plate aboveand carry on fore and aft . S ta rt " C " stra ke in theopposite directio n, and work as before. This insure sthat no two laps are opposite one another foi" at leasttwo strak es. One mus t arra ng e this " shift of laps "throughout the whole fuselage and towards the ends i twill not be possible to use the total length of platein , and achieve th is end, see Fig . VI at El and K5.At the fore and aft end the plates become congestedif one t r ies to keep the width constan t . To avoid th iscut the plate back in length at a convenient positionto enable one s tanda rd width plate to occupy twostrakes .By now it will be obvious that the fuselage has beenplated with a minimum of waste, and the haphazardmethod of trying a plate on the job in the shop andthe risk of scrap ping has been overcome. Assum ingth a t all the " shell " plates have been m arked on themodel, collect all information re openings, such as sidewindows, doors, hatches, etc;. , and mark them on theside of the mod el. A com plete half-block model shouldhave all at ta chm ents to the " shell " dot ted thereo n,but the extent of detail will remain at the discretion ofthe draug htsm an and the shop requiremen ts . Theprimary object of the model is to obtain the exact siaeof the plat ing.When everything has been marked on the model thatis required, ink in, after final correction, and check,then give the whole surface a coat of clear varnish topreserve the marks thereon.Shell ExpansionThe next step is to pre pa re the " Shell Exp ans ion "drawing for the half-block m odel, Fig . V II . This draw-ing is best described by imagining the model to be madeof tin and flattened out from the round to a flat

Fig. VIINote, all openings, external fittings, doublings and shell drilling should be shown on this drawing


8/94

SUPPLEMENT TOF L I G H T J A N U A RY 25, 1934THE AIRCRAFT ENGINEERsurface. The " girth "' becomes flat, but the length re-mains the same, and is purely a guide to the platingsquad in the shop to locate their plates.Proceed to mark off on the paper for this drawingthe datum and all the frames or formers to the samescale as the model, then with a strip of paper girthround the frames on the model and mark off all plateedges above and below the datum. Transfer thesemarks to the " Shell Expansion Drg." and when all theplates are drawn in we have a view showing the positionof all plates on one side of the fuselage. Note that anyopening, bracket, etc., only occurring on one side ofthe fuselage can still be indicated on the " Half-Block "model and " Shell Expansion " by labelling same " Stb.only " or " Port only " as the case may he.

Having transferred all the plates from the half-blockmodel to the shell expansion and drawn in same, pro-ceed to label all the strakes as Al, A2, Bl, B2, etc., andwhen finished this drawing should be a complete copyof' the " model " and is used hy the foreman in chargeof the plating as a guide for his work.Plate Ordering

Last, but not least, the Plate Order List is preparedfrom the " model." (See Fig. II.)Most of the plates amidships can be scaled off direct,as there is little double curvature, and entered on theorder sheet with a J in. extra in length and J in. extrain width. Where a plate has double curvature, as atthe nose, place a piece of tracing paper over same andmark the outline of the plate on the paper. When thispaper is laid out flat we have the true shape of the flatplate for ordering. This process is called " lifting off.'When these plates come in from the makers or are cutfrom stock, they are labelled as per list, and when allwork is complete thereon they are plated in store, andas they are marked according to the Shell ExpansionDrg., the foreman in charge of the fuselage platingknows exactly where each plate goes. It is not sug-gested here that separate departments should handlethe plates, but one squad does all the rolling and shap-ing, drilling edges for rivets, etc.

With a reasonable amount of good progress workbeforehand, there should he no waiting one depart-ment for the other, and when the plates are requiredfor the final riveting up on fuselage, they should beready in store.Here, perhaps, we should have mentioned that the" Order List " is prepared first from the model, anddelivered to the ordering department, who, presum-ably, order same without delay, while the rest of thework is proceeding. Incidentally, the material for theframes, etc., is also " lifted off " the model, and a listmade out similar to the plate order list. I have pur-

posely refrained from mentioning the frames previouslyas they may he built-up plates with angles, or " Z "bars, etc., and require a separate drawing for eachframe or batch of frames, but it is obvious that anyinformation re the contour of the frames can be obtainedfrom model.In conclusion, the above procedure for lines, plating,etc., is applicable to either a " Monocoque Fuselage "or a flying-boat hull.it HIDUMINIUM R.R.53 B TUnder the name " Hiduminium R.R.53 B," HighDuty Alloys, Ltd., of Slough, have introduced a newlight aluminium alloy. This is the result of many

months of research work in the laboratories of the firm,with the object of altering the standard alloys to suita particular purpose. The new alloy is a slight modi-fication of the well-known Hiduminium R.R.53, whichis one of the series of allovs introduced and patented byRolls-Royce, Ltd. The new alloy is finding wide applica-tion for fast-moving levers, treadles and brackets inthe textile and electrical industries, and should also

Some parts cast in Hiduminium R.R.53 B.be suitable for the smaller forms of castings in theaircraft industry.The analysis of Hiduminium R.R. 58 B is as follows :

Copper ... ... 2.5 per cent.Nickel ... ... 1.5 ,, ,,Magnesium ... 0.8 ,, ,,Iron 1.2 ,, ,,Silicon ... ... 1.2 ,, ,,

The physical properties in the various conditions are asfollows :Vh M Cast.3 percent. MaximumT n L T^L Elongation TJrinell

As cast 'Solution'treatedSolution'treated and artificiallyaged Sand1

As castSolution treatedSolution treated and artificiallyagedThe specific gravity is much th-

i n .Tons81223Cast Text

in . 'Tons1422Bursper cent. Maximum

XJ 4J ^ t . n n .r .0,Tons/ sq.in .Tons812li )

e same as fc

Tons/sq "in .Tons121721

363

1)i '' Hiduminium '

7510 312 9

^nt. naraiie.- ^8 011 012 0' R.R. 53 alloy

Power and throttle curves of De Havilland " Gipsy Six "engine. (See pages 84-86.)78 h


9/94

February 22, 1934 Supplement to FLIGHT

FLIGHTENGINEERING SECTION

Edited by C. M. POULSENNo. 97 (V e 2 I X) 9th Year February 22, 1934

CONTENTS PageEngine Cowling. By J. D. North, F.R.Ae.S., M.I.Ae.E 9E t h y l . By F. E. B acks . O.B.E.. F.K.Ae.s.. M.I.A.E.. M.Inst.P.T.,M.S.A.E 14Technical LiteratureSummaries of Aeronautical Research Committee Report s... ... 16

ENGINE COWLINGBy J. D. NORTH, F.R.Ae.S., M.I.Ae.E.

In FLIGHT of February 8, 1934, we published a sum-mary of the first part of the paper under above titlewhich Mr. J. D. North, Chief Engineer of Boulton &Paul, Ltd., read before the Boyal Aeronautical Societyon February 1. A brief report of the discussion waspublished in FLIGHT last week. Below we publishextracts from the concluding part of Mr. North's veryinteresting paper. It should be pointed out that wehave retained Mr. NorWs numbering of the illustra-tions, which has resulted in certain cases in gaps in the,numbering.ED.

The Townend RingInasmuch as my company have proprietary interestsin the patents covering the Townend Ring, and havedevoted much attention to the development of this

device, I am naturally in a position to give moredetailed information concerning this particular form oflow-drag cowling. I hope that these details will be ofgeneral interest.Although the Townend Ring itself is a simple device,the factors which may influence its performance aremany and various, and time will permit only a verygeneral consideration of some of the more important ofthese factors.Ring Sections

The section of a Townend Ring is an aerofoil sectionin so far as it is required to produce a radial outwardlydirected lift with its consequent downwash. The effec-tiveness of a ring for given conditions is determinedby the intensity of downwash per unit of circumference.Hence it is an advantage to use a section which developsa high lift coefficient in order that the chord lengthrequired may be a minimum.Experience shows that for rings of the usual single-surfaced (plate) type a camber of about 10 per cent, ofthe chord length should be used. Double-surfaced sec-tions usually employed for wings, of the same camberon their upper surface, are less effective than the plain

reduced to one midway between that of upper andlower surfaces. There is some evidence that an increaseof camber to more than 10 per cent, may be advan-tageous under certain conditions.The addition to a plate type ring of a bulbous nosesimilar in form to the leading edge of a moderately thickaerofoil section has been found usually to decrease thedrag of the complete ring installation quite appreciably.Such a bulbous nose has been used by Boulton & Paulas an exhaust collector and provides a method of coolingand silencing the exhaust with no increase, and normallya decrease, in total resistance. Fig. 9 shows ringsections which have satisfactorily been used for Town.end Rings.Angle of Ring Chord

The best angle between the chord line of the ringsection and the thrust axis depends on many factors,such as the exact form of the engine, of the bodybehind the engine, of the section actually used for thering itself and the relative fore and aft position ofthe ring relative to the engine. As a very rough guide,

mr to oFIG. 9

40 *' *EZ WTW CCHNJ3T fOS t


10/94

SUPPLEMENT TOFLIGHT FEBRUARY 22, 1934THE AIRCRAFT ENGINEERuseful for determining the range within which experi-ment may usefully be conducted, the lines of the bodymay be extra-polated forward past the engine to com-plete a reasonable streamline, and the chord of the ringshould then be set at an angle between parallel to atangent to, and at about 4 deg., converging rearwardlyto that tangent, the tangent being taken in a planecorresponding to 50 per cent, of the chord length.

Fig. 10 shows the variation in drag of an engine andstreamline nacelle fitted with a Townend Ring forvarious chord angles of the ring itself. The curvesmarked 1 relate to the ring when the midpoint of thechord lies in the plane of the cylinder centre line.Curves 2 and 3 relate to the same Townend Ring movedforward by successive steps each of about 20 per cent.of the chord length, this chord length in the particularcase being about 48 per cent, of the engine diameter.For the position 1 it will be seen th at ther e is a range ofchord a ngle from 2 deg. to 6 deg. over w hich th edrag is practically constant, and t ha t a t 8 deg. thedrag st arts to rise very rapidly. For positions 2 and 3the minimum drag has increased appreciably and theflat portion of the curve has disappeared, No. 3 showinga sharply marked minimum value of drag, the curverising steeply on either side of this minimum.

The general characteristics shown for the lower set ofcurves without slipstream are retained by the upper setwhich represents the conditions with an appropriate air-screw running at conditions corresponding roughly toclimbing airspeed. W ith very few exceptions, it hasbeen found that the presence of slipstream does notaffect the relative merits of different ring arrangements.Fore and Aft Position of RingThe curves of Fig. 10 also indicate thn nature of the

effect of changing fore and aft position of a TownendRin g relatively to the body. The results relate only toone particular type of ring used on a particular form ofbody, and even for tha t case do not exte nd sufficientlyfar to prove th at position 1 is the best possible. Ex peri-ence, however, indicates that in nearly every case a ringwhich is placed with its chord extending equally aheadof and behind the cylinder centre lines will give betterresults than one placed in any widely different position.

Chord LengthThe chord length required to produce a given degreeof constraint on the tendency of the airflow behind theengine to break away from the body depends mainly onthe lift coefficient which is developed by the ring sec-tion. Sections of the types previously illustrated which

have been found satisfactory apparently develop whenused as Townend Rings lift coefficients of the order of0.5 to 0.6, and with these sections a chord length ofapproximately 0.5 of the engine diameter is found togive the maximum reduction in drag.The flow over the nose of a body engine combination isnecessarily curved and change in chord length of a ringalters the effective angle of incidence between the ringand the airflow, therefore the effect of change of chordis not a simple effect. Fig . 11 shows the measured dragof a nacelle and the engine mounted on a wing whenfitted with thre e different rings, (a) is a ring havin ga chord of approximately 0.33 engine diameter, (b ) isa very similar ring with a chord length of 0.52 enginediameter, while (c) has a slightly increased chord 0.525engine diameter and is fitted with a bulbous nose ex-ha us t collector. The difference between (b ) and (c) haslittle to do with chord length, but that between (o) and(b) indicates the kind of difference which attends onchange of chord length.

120

no

100

90

80

60

50

50

FIG.I!

COST WITH NOTOWN END Rll*5

I WINS KL 2

This figure shows also the increase in drag due to theengine nacelle when no ring is fitted, and it will beseen that with no ring this drag increases very rapidlywith increasing wing lift. W ith any of the rings thisincrease in cost of engine as wing lift increases is verygreatly reduced and within the range covered by thefigures has completed disappeared for the best of thethree rings, i.e., (c). It m ay be remarked tha t it is ageneral characteristic of a good Townend Ring that itmaintains its effectiveness over a considerable range ofconditions.

Polygonal " Rings "Fig. 12 shows two Townend Rings of identical section,chord length, and chord angle, made for use w*ith thesame nine-cylinder engin e. Tested on a stream linenacelle, the measured drag using the polygonal ring was


11/94

FEBRUARY 22, 1934 11THE AIRCRAFT ENGINEERSUPPLEMENT TOFLIGHT

FIG. 12.S EC TION O H M TU W U N g f TP f t ^ - I

found to be slightly less than that with the circularring. For a nine-cylinder engine 54 in. in overalldiam eter , th e equivale nt full-scale drags were 27 lb.for the polygon and 31 lb. for the circular ring, whichis to be compared with 96 lb. with no Townend Ring, atan airspeed of 100 ft./ sec . A very large number oftests have now been made in our wind channel givinga direct comparison between the performance of circularand polygonal rings of otherwise identical form andused on identical engine-body combinations. In no suchcase has the polygonal ring given results inferior to thecircular one, and in the majority of cases the polygonhas shown a definite a dva ntag e. Tesits made with theairscrew running show that slipstream does not adverselyaffect the superiority of the polygon.

Body ShapesSo many practical considerations govern the designof body shapes that it is quite impossible to give anyhard-and-fast rules as to the shape of body whichshould be employed with the Townend Rin g. For-tunately, however, the performance of the TownendRing itself appears to be influenced mainly by the form

of the body for a short distance behind the ring only.Experience to date shows that for current type ofradial engines the maximum reduction in drag with aTownend Ring can be secured if the body immediatelybehind the engine has a diameter of about 0.75 of theengine diameter and if the body lines over a distanceof about one engine diameter behind the engine plateare reasonably fair and do not diverge or converge withabnormal rapid ity. W hat the body form furth er fromthe engine may be may greatly affect the total resist-ance of the body and engine combination, but will notgreatly affect the saving in drag caused by the Town-end Ring.Fig. 13 shows three models of engines and nacelleswhich have been tested. The nacelles themselves ar esolids of revolution having the outline of the standard3:1 streamline strut section and differ only in the posi-tion of the engine on the bodies relative to the maxi-mum ordinate of the basic streamline . The diameterof this maximum ordinate was 0.78 of the maximumdiameter of the engine. Of the three models testedthe intermediate is slightly the best, both with andwithout the Townend Ring, but the differences between

- (mi- FIG. 131 2 to

j - '

c H*ai_eSHAPE

's

7.-&Z,'


12/94

SUPPLEMENT TOFLIGHTFEBRU A RY 22, 1934THE AIRCRAFT ENGINEER

the three arrangements are not large. This interme-diate arrangement is that which, with the polygonalring, gave a total full-scale drag for the 54-in. diameterradial engine of 27 lb. at 100 ft./sec., which is so farabout the minimum resistance which it has been foundpossible to obtain for a radial engine of these dimen-sions with any form of Townend Ring.Although the tests relate directly to engines mountedon streamline nacelles in free air, experience shows thatif the form of body immediately behind the engine

(g ) in this figure shows a case where interference be-tween wing and ring was greatly reduced by cuttingaway a segment of the ring in way of the wing.Generally speaking, cutting of gaps in the ring circum-ference causes the ring to become almost completely in-effective. If, however, a member is provided which willserve to carry the general ring circulation across thegap, the effect of the interruption becomes unimportant.The case illustrated is one in which the wing serves tobridge the gap.

corresponds reasonably closely to that of any of thethree nacelles shown over a length equal to the enginediameter, the drag reduction caused by a given Town-end Ring will be of the same order as that which thesame ring would cause on the streamline nacelle. Nor-mally for best results the body section over the regionabove mentioned should be circular. If a polygon ringis used, a polygon body, sides parallel to those of thering, is as good as, or slightly better than, a circularone.Townend Rings and Interference

The effect of the Townend Ring on the drag of anengine is of the type of phenomenon usually described as" interference." Fig. 11 already shown indicates clearlyhow a Townend Ring may reduce interference betweena wing engine installation and the wing itself, and incases w here, as is usual, such wing engine interference isappreciable, a satisfactory Townend Ring will almostinvariably greatly reduce the interference drag.The Townend Ring itself is very sensitive to certaintypes of interference. If the flow over the outer sur-face of the ring is disturbed, it may be caused to breakaway and local stalling of the ring section provoked.More than a very limited degree of such local disturb-ance is found to produce an effect which spreads roundthe ring circumference and very rapidly destroys theeffectiveness of the ring. The most difficult cases ofinterference with the Townend Ring yet encounteredare those in which interference between the ring anda closely approaching wing occur.Fig. 14 shows at (a) and (b) conditions which almostinevitably lead to serious interference of this typeand should be avoided; (c) and (d), which differ from(a) and (b) only in a relative vertical displacement ofrang and wing so tha t the leading edge of the wingdefinitely cuts the ring periphery instead of being nearlytangent to it, are free from this trouble and give satis-factory results. A variant of (a) in which the engineis dropped below the wing and the nacelle is separatedtherefrom instead of being built on to it may be worsethan (a) itself, and is only satisfactory when the engineis dropped far enough to give a large vertical gap be-tween the ring and the leading edge of the wing. Thearrangement (e) gives excellent results, but it is im-portant not to move the ring and engine so far backtha t the leading edge must be mutilated to clear theengine itself.

The total resistance of a ring-cowled installation isvery little affected by bodies which are within the wingitself. Circular struts for supporting the ring do notincrease the drag as compared to streamline struts.Exhaust collectors within the ring have but a very smalleffect, which is often unmeasurable, and the effect offitting inter-cylinder baffles, various types of air in-takes, or the like, which do not protrude beyond thering is invariably small, and usually negligible.Engine Cooling

Comparison of a typical Townend Ring with any otherform of low drag cowling for use on a similar engineindicates the probability that the ring will interfereless with cooling than will any of its present-day com-petitors. The necessary development of a lift force bythe ring sections and the circulation round the r ingwhich this implies, involves some red uction in thevelocity through the ring as compared to that outside it.This reduction in velocity cannot be of any great mag-nitude and would not be expected to account for anyserious effect on cooling.Practical experience h as shown th at the TownendRing does give cooling superior to tha t of other avail-able types of cowling for radial engines capable ofsubstantially reducing the drag of those engines. Mr.Fedden has published results which show that satisfac-tory cooling can be obtained with a Townend Ring on aparticular engine and aircraft combination to whichthe application of a complete cowling of the N.A.C.A.type was impracticable because it caused overheating.There is a considerable fund of experience in Americaindicating that whereas this complete type of cowlingcan normally be employed successfully on ungearedengines, it leads frequently to difficulties with coolingwhen it is applied to geared and particularly to gearedand supercharged engines. The restricted frontal open-ing of the complete N.A.C.A. cowling is in the regionmainly affected by airscrew boss shielding, and the areaso affected is greatly increased, as has already beenpointed out, when a large diameter slow running air-screw is employed. The Townend Ring, with its widerfrontal aperture, is less affected in this way.Direct measurements of the air velocity close to thesparking plugs of a nine-cylinder radial engine havebeen made in flight (Ref. B. and P. tests 2161 and2173), both with and without Townend Ring, using


13/94

FEB RU AR Y 22, 1934 UTHE AIRCRAFT ENGINEER TOFLIGHT

pitot heads and hot wire anemometers. The pitot headsshow a small rise in velocity of the order of 5 per cent,when the ning was fitted, the hot wire anemometer, onthe other hand, showed a reduction of the same order.Many explanations for this discrepancy are possible, themost probable being that the Townend Ring had sub-stantially changed the direction of air flow, to whichthe hot wire anemometer would be insensitive. The hotwire anemometer being a direct method of measuringcooling and only an indirect measure of air speed, theresults indicate a small loss in effective cooling.Fig. 16 shows temperatures measured on the rear faceof the cylinder heads of a nine-cylinder radial engine

running on the test bed with the standard fan andwind tunne l cooling arrang em ent. Two curves, whichare very nearly identical, relate to the temperatureswith and withou t a Townend Ring . The absence of anyappreciable difference in temperature under the twoconditions may be explained by the artificial coolingconditions. Atten tion is directed, however, to the irre-gularity of temperature distribution round the engines,the maximum variation between individual cylinder tem-peratures being about 50 deg. C.1

7 x \ .

KM~~~^w\ FIG. 17

Frig. 17 shows cylinder temperatures measured on anengine of the same type during climb, with and withoutTownend Rings on the same aircraft.The average temperature when the Townend Ring isfitted has obviously increased apprec iably. The irregu -larity of temperature distribution around the engine ismore marked both with and without ring than is shownin the test-bed case. Repeated tests under similar con-ditions, using the same engines and aircra ft, haveshown that this irregularity, though always present,varied from flight to flight, and it was quite usual tofind th a t a cylinder which had developed an abnorm allyhigh temperature on one flight remained abnormally coolon the next.Fig. 17 relates to a type of engine which had hadits power output boosted to about the limit of its cool-ing capacity when used without a Townend Ring, andeven in this condition at the relatively slow climbingspeed used in this particular case the cooling could notbe regarded as completely satisfactory.Fig . 18 shows in a slightly different form the resultsof similar tests on an engine of generally similar type.In the bottom curves the temperature of No. 3 cylinderwith the Townend Ring is definitely unsatisfac tory.Tests were accordingly made with inter-cylindeir bafflesaround cylinder No . 3, which also half encircled cylinders

300'

tso"

s o o _

V

cruNoea NO.is

FtG.I8

Nos. 2 and 4. As the second set of curves show, the tem -perature of No. 3 cylinder dropped from 285 deg. C. to225 deg. C. No. l2 cylinder only half baffled showed prac-tically the same temperature as No. 3 with the completebaffling. It may be noted th at N o. 1, complete un-baffled, dropped in temperature between the two testsby nearly 70 deg. and No. 5, also unbaffled, by 50 deg. C.Following this test all cylinders except No. 6 were fittedwith half baffles of the type which had apparentlyeffectively cooled No. 2. On a th ird tes t, indicated bythe upper curve, the temperature of No. 3 had returnedto practically its original high figure, and it will beobvious that no sort of connection can be establishedbetween the presence of baffles and cylinder tempera-tures. Further, the irregularity of temperatures roundthe engine is very obvious, and is found to a less degreeeven where cooling is considered satisfactory. Wh ereverthe fitting of Townend Rings has led to actual over-heating, investigation indicates that this irregularityof cylinder temperatures becomes very marked indeed.

Increase in general temperature marked by suchviolent irregularities in temperature distribution canobviously scarcely be attributed to any direct effect ofthe Townend Ring on the effective cooling velocity overthe engine, and many explanations of the effect havebeen considered. Of these, the one which seems to havethe best foundation is that the very considerable change


14/94

SUPPLEMENT TOF LI G H TU

THE AIRCRAFT ENGINEERFEBRUARY 22, 1934

in the direction and general turbulence of the airflowpast the engine caused by the ring may disturb airintake and carburetter conditions and lead to variationsin mixture strength and/or to irregular gas distribution.Serious cooling difficulties attendant upon the fittingof theTownend Ring rarely arise, except in the rapidly-diminishing number of cases in which thecooling margin

of the engine, even without low-drag cowling, is small.The designer of radial engines has realised the import-ance of modern low-drag cowlings and of providing hisengines with cooling capacity which will be adequatewhen such cowlings are fitted, consequently such diffi-culties are steadily growing rarer.Where irregular temperature distribution occurs, andthere is strong evidence that it occurs to some extent inall air-cooled engines, the cooling which has to be pro-vided is that which will keep the hottest cylinder downto permissible limits; and the analysis of the reasons forsuch irregular temperatures and methods for their cureshould be of the utimost interest to the engine makerhimself, since they are one method by which effectivecooling can be appreciably increased.

(To beconcluded.)

ETHYLB Y P. R. BANKS, O.B.E., F . R . A E . S . , M.I.A.E.,

M . I N S T . P . T . , M.S.A.E.(Concluded from, p. 4.)

General Notes on Engine Operation with Leaded FuelsTHE internal appearance of an engine which has runon a fuel containing lead differs somewhat from thatusually associated with the more ordinary fuels. Thedeposit from the use of the former fuel is harder innature and perhaps more adherent than that of thelatter. Its colouration is also different, being white togreyish white on the cooler parts of the combustionchamber and reddish brown on the hotter parts. Thisis due to the presence of lead bromide. There is, some-times a yellowish tinge to the deposit, which may beaccounted for by some lead sulphate present in thedeposit. Where a part, such as an exhaust valve, hasbeen running unduly hot, the deposit is generally ex-ceedingly adherent to the valve head and has a dark" steel " grey appearance. The dyewhich is present inall leaded fuels is particularly useful for the relativelycomplicated fuel systems used in aviation engine instal-lations, since it shows up, almost immediately, anyleakBwhich may be present.

Some queries have arisen regarding the effect ofleaded fuels on ths materials used for aircraft fueltanks. No trouble has been experienced in the; oase oftanks manufactured from the usual aluminium alloys,but with regard to those particular alloys which containa large percentage of magnesium, such as Elektron,there seems to be some doubt as to the advisability ofemploying them for fuel tanks at all.One's personal experience is that corrosion troubleis manifest with high magnesium alloys when water ispresent in the fuel, whether the latter contains lead or

not. If it contains lead, then the corrosion attackappears to be somewhat accelerated. From this onededuces that the presence of water is really the decid-ing factor, but it is almost impossible to avoid a certainamount of water collecting in fuel tanks. It is sug-gested, however, that magnesium alloy tanks could bedesigned with provision for a sump of some materialwhich does not suffer from this corrosion attack, such

as pure aluminium, etc. The sump would be deepenough to prevent anywater reaching the joint betweendt and the tank, in order to avoid the possibility ofelectrolytic action.Engine Tests and the Influence of IncreasingConcentrations of Lead

When considering the duration of engine tests inorder to ascertain the effect of leaded fuel upon theengine parts, one is of the opinion that no tests of lessthan 100 hours' duration are of value.In order to promote rapid engine development onleaded fuels, the knock rating of the finished fuelshould be decided upon in the first place, after whicha basic petrol chosen, having an initial anti-knock valuewhich demands a fairly large amount of lead in orderto attain the required final value. This will ensure thatthe engine is capable of giving satisfactory operationwith any concentration met with in service, evenalthough it may eventually be provided with a fuel,the basis of which only requires a very small amount oflead in order to reach the desired anti-knock value.There are many contentions regarding the influenceof increasing lead concentrations on engine condition,and in general the consensus of opinion appears to bethat an increase in lead concentration gives the engineparts concerned a harder time by increasing the rate ofdeposition of the products of combustion. One is notsubstantially at variance with this view, and has alwaysmaintained that tests should be carried out on the linessuggested in the previous paragraphs of this section.It is quite feasible to suppose that an increase in theamount of lead must generally show up in the form ofgreater rate of deposit build up. However, the follow-ing points are put forward as a matter of interest.Firstly, the American view, backed by six or eightyears of intensive experiment and use of leaded fuels, isthat increasing concentrations of lead tend to increasethe rate of attack and deposit build up, which may leadto troubles previously dormant.Secondly, tests have been carried out by the AirMinistry, at the works of the aviation engine firms inthis country, over the last two years. The tests, of100 hours' duration, were made on representative typesof engines in service, and the results did not completelybear out American experience.The interesting point about these tests is that valvefailure, due to burning, was experienced in some casesand occurred in about 50 to 70 hours of running.Further tests of 100 hours' duration were then madeafter completely reconditioning the engines concerned,

but a fuel having only 1 c.c. of lead per gallon wastried, where previously a " 4 c.c." concentration wasemployed. The same class of petrol was used as thebasis of the fuels, and a similar knock rating to the" 4 c.c." fuel was obtained by the use of added aroma-tics. However, in a directly comparative test withthis and the " 4 c.c." fuel, precisely the same degreeof valve failure in practically similar periods was ex-perienced with both fuels. Therefore, from the experi-ence in this country and in Europe, one would saythat increasing lead concentrations do not necessarilygive rise to trouble or to the same ratio increase ofdeposits in the engine.With regard to the apparent variation between theresults obtained here and in Europe, to those indicatedby American experience, a satisfactory explanationmight be that themodern American engine has been de-veloped over the same period as that of leaded fuels;consequently a certain amount of technique has beenevolved to deal particularly with their use. A greatdeal more flying, with engines using such fuels, hasalso been done in America, while little or none hasbeen done in this country, and practically all our leaded

174/


15/94

FE BRUARY 22, 1934 16

THE AIRCRAFT ENGINEER SUPPLEMENT TOF L I G H Tfuel development has been restricted to relatively severeexperimental running and type tests which, one sub-mits, are more critical than flight conditions. One isof the opinion that any troubles experienced in Americawith leaded fuels of high concentration are not so muchdue to this feature, but rather to the increased " powerper litre " of cylinder capacity at which Americanengines are now running under normal cruising con-ditions in flight.

Knock Testing and Assessing Fuels in Relation toEngine PerformanceThe question of testing fuels for anti-knock value is,admittedly, a subject in itself and hardly comes withinthe scope of this paper, but it has such direct bearingupon the successful development and satisfactory opera-tion of aviation engines that perhaps little excuse isneeded to mention it.The whole essence of knock-testing technique is theability to correlate the results obtained on the fuel-testing unit, with the performance of the fuels in the

engine and to be able to assess the knock ratings of thevarious fuels in their order of merit. It is exeeedinglydifficult to arrange a complete set of conditions for thefuel-testing unit which will imitate, accurately, thosemet with in the engine. Some time ago, the Institutionof Petroleum Technologists appointed a sub-committeeto formulate a suitable programme in order that experi-ments could be carried out and the data obtained there-from used to enable a satisfactory technique to beevolved for the correlation of laboratory knock testresults with actual engine performance. Tribute shouldbe paid to the I.P.T., which is the first body to formu-late a method of knock testing and correlating aviationfuels, to be accepted nationally. This should, however,only be regarded as a preliminary step. These testresults are very clearly and completely described byMr. Pye, who was chairman of the sub-committee, in apaper read before the World Petroleum Congress lastyear.The running tests were carried out at the R.A.E.,Farnborough, on air- and water-cooled units, and atthe engine works of the Bristol Aeroplane Co. Single-cylinder units of representative service engines wereused, and in no case was a complete engine employed.Due, according to the report, to the " extreme diffi-culty of accurately detecting the onset of detonationin a complete aero engine and to the large quantity ofexpensive sub-standard fuel which would be required."One contends that, outside the expense, and particu-larly in the case of the air-cooled engine, the question

of audible detonation is not necessarily important, butthat the temperature effect on the cylinder head, due tothe detonative characteristics of the particular fuelused, is more a measure of that fuel's ability to operatesatisfactorily in the engine.With these large engines of comparatively highspecific power output, a fuel can cause a dangerous risein the operating temperature of the cylinder head, whichon further increase will result in pre-ignition ratherthan detonation. This is markedly so when the non-knocking fuels are used. This is largely the reason whyall the recent fuel tests formulated by the U.S. Army AirCorps specify that the sample tested shall not show ahigher reading of the temperature plug than the refer-ence fuel; rather than taking average bouncing pinreadings.On would say that one or two fuel tests carried outon a complete engine, particularly of the air-cooledtype, heavily thermocoupled at suitable points, wouldhave yielded more valuable information than the single-cylinder tests. A further important point is that thetests at Farnborough were run at varying speeds,whereas general experience would indicate that constantspeed is necessary when matching fuels.

The report also gives an explanation for the testmethods finally adopted as a result of the work done,i.e., C.F.R. Motor Method, modified to use a mixturetemperature of 260 deg. F. , instead of 300 deg. F.,and why the method is less severe than that employedfor correlating automobile fuels (Motor Method). Theexplanation offered is: " that when a read vehicleengine in pulling at low speed on full throttle the con-ditions ara not only severe by reason of the low speed,but that in many types there is also provision for alarge amount of mixture heating which may vary andexceed even the heating provided by a supercharger."A road vehicle engine is certainly severe on the fuelunder the conditions described, but it must be remem-bered that the B.M.E.P. and moan temperatures, whencompared with those of an aviation engine, are not sohigh, and also, the cylinders are not so large with,generally, a greater ratio of surface to volume than theaviation engine.Consequently, audible knock may be severe for thecomparatively short time that the vehicle is runningunder these conditions, but due to the inherent design

of the automobile engine it will take a considerable timebefore detonation becomes great enough to build upexcessive heat and/or bring about actual damage,although, admittedly, the performance of the engine,and consequently the vehicle, is impaired. It is noteasy to see how these conditions can be more severethan, say, those of an air-cooled engine having largecylinders by comparison and running at full rated loadin an aircraft under climbing conditions, where the rateof air flow over the cylinders is at a minimum and asaltitude is increased, with a corresponding decrease inair density, the difficulty of dissipating heat from thecylinders is greater.Actually, the operating speed of an automobile engine,under which detonation usually occurs, is not always

so low as might at first be supposed, when comparedwith the normal speed of an aviation engine, althoughit may be lower in proportion to the maximum speed inthe former case. In fact, the C.F.R. road tests showedthat maximum knock occurred at road speeds varyingfrom about 15 to 40 in.p.h. corresponding to crankshaftspeeds of approximately 900 to 2,500 r.p.m. respectively.The contentions put forward here are not in any wayintended as a criticism of the accuracy of the I.P.T.investigation, since the results of the tests have provedtheir own accuracy and have shown that it is possible toobtain good correlation with a given set of engine con-ditions.Criticism might be made that the engine conditionsspecified did not accurately represent those met with in

actual service, and in addition only fuels of compara-tively moderate knock ratings were tested, whereas,for future engine development, the Air Ministry hasalready brought out a fuel specification, D.T.D. 230, inwhich an Octane value of 87 is called for when testedunder the modified Motor Method and, also, the use oflead is allowed. None of the fuels in the I.P.T. testscontained lead. This fuel will be used for new typesand these engines will undoubtedly produce higherspecific power outputs than the previous models, uponwhich the I.P.T. investigation was made, and willprobably give the fuel a harder time in comparison.It remains to be seen therefore, whether the correla-tion as carried out, applies in this particular case. Somecorrelation tests on complete engines are already in pro-gress in America, and it will be interesting to studythe results and learn the conclusions arrived at, whenthey are published.The latest fuel specification (No. Y-3557-G) evolved bythe U.S. Army Air Corps, calls for a nominal Octanevalue of 92. A. C.F.R. engine is used as the basis, butis considerably modified to conform to Air Corps require-ments. The temperature rise method of assessing theanti-detonation value is retained. The engine speed of


16/94

SUPPLEMENT TOFLIGHT FEBR UAR Y 22, 1934TH E AIRCRAFT ENGINEERth e unit has been increased to 1,200 r.p.m. and a jackettemperature of 165 deg. C. (330 deg. F.) is used.Th e C.F.R. Committee, however, has tentativelyadopted the C.F.R. Motor Method, unmodified, fortesting aviation fuels, pending th e results of the investi-gations now being made. The work of the C.F.R. Com-mittee must not be confused with that of the U.S. ArmyAir Corps, which latter are an entirely separate an dgovernmental body. The C.F.R. Committee representsthe oil concerns, also automobile an d aviation engineinterests .

ConclusionIt is submitted that a fairly comprehensive expositionof the leaded fuel situation was needed in view of thelack of comprehensive an d practical information whichwas available to the engine manufacturers an doperators. It is not too much to say that air supre-macy, whether considered from th e civil or militarystandpoint, will eventually go to the nation whichdevelops engines making th e fullest use of fuels ofreally high anti-knock value.

(28 pages an d

T E C H N I C A L L I T E R A T U R ESUMMARIES OF AERONAUTICAL RESEARCHCOMMITTEE REPORTSThese Reports are published by His Majesty'sStationery Office, London, and may be purchaseddirectly from H.M. Stationery Office at the followingaddresses: Adastral House, Kingsway, W.C.2; 120,George Street, E dinburgh ; York Street, Manchester;1, St. Andrew's Crescent, Cardiff; 15, Donegall SquareWest, Belfast; or through an y Bookseller.

AN APPLICATION OF PRANDTL THEORY TO AN AIRSCREW.By C. N. H. Lock, M.A. R. & M. No . 1521. (41 pagesand 13 diagrams.) August 30, 1932. Price 2s. 3d. net.The velocity field of the helical trailing vortices of an airscrew is obtainedon the basis of Prandtl 's artif ice of replacing the helical vortex sheets byplane vortex sheets, thus reducing the problem to one of two-dimensionalflow of a perfect fluid. Pra nd tl's original trea tm ent was confined to thespecial case in which the tra iling vortex sheets are equivalent to rigid lamina:(the airscrew with minimum energy loss); the present paper solves theproblem for a general distribution of vorticity with radius by means of aFourier expansion, the method being analogous to that used by Trefftz forthe monoplane aerofoil .In its simplest form the method neglects squares and higher powers of theblade incidence and is subject to errors arising from the replacement of helicalvortices by straigh t vortices ; these limitations are afteiwards removed byvarious artif ices which, although they do not represent a r igorous solution ofthe problem, should give results which are sufficiently accurate in all practicalcases.The method is compared (a) with the result of assuming the number ofblades to be infinite (Vortex theory) and (b) with the result of determiningthe performance of each blade element separately on the assumption thatthe rest of the wake has the special distribution (minimum energy loss) for

which the n umerical solution was given bv Goldstein (approxim ate Goldsteinmethod of R. & M. 1377). Num erical comparison s are confined to the caseof an airscrew of constant pitch with " square tipped " blades.A formula for an approximate overall correction, given by Prandtl, toresults calculated by m ethod (a) is discussed and it is shown that If the formulaiB applied to calculations for an infinite number of blades with square tips (asin Glauert 's " Airscrews for high speed aeroplanes "*) the corrected valueswill apply approximately to two- and four-bladers of normal plan form.Alternative formulae are given which can be used to convert from an infinitenumber of blades of normal plan form to two- and four-bladers of the sameplan form. E. &M. 1342.

ABSTRACT. '" ''*'- ''""

THE FLOW PAST CIRCULAR CYLINDERS AT LOW SPEEDS.By A. Thorn, D .Sc , Ph.D . R. & M. No. 1539. (2 pagesand 3 diagrams.) June, 1932. Price 3d. net.Abstract only of paper published in Royal Society Proe.

ABSTRACT.THE CONVECTION OF HEAT FROM ISOLATED PLATES AND

CYLINDERS IN AN INVISCID STREAM. By N. A. V. Piercy,D.Sc , an d H. F . Winny, Ph.D. R. & M. No. 1540.(2 pages.) September 22, 1932. Price 2d. net.Abstract only of paper published in the Phil. Mag

TH E RADIALLY-BRACED A I R S H I P R I N G . By Prof. L.Bairfltow, C.B.E., F.R .S . Communicated by D.S.R.,

Air Ministry. R . & M. No. 1551.12 diagrams.) March 22, 1933.The investigation represents an attempt to reduce the considerable labourinvol ved in computing the stresses in the rings of r igid airships. The analysisis developed in terms of the radially-braced ring, but gives the essentialsfor an unbraced ring. The account is divisible into three sections:Section 1.It is shown th at ca lculations of stresses, etc., in a sy mmetricalr ing, loaded in any possible way In its own plane may be made to dependon th e a ddition of results of calculations for two simple standard -types.

The considerable reduction of labour which follows for all calculations savethe first is independent of the particular system of stress analysis adopted.Section 2.An approximate solution to reduce the labour connectedwith the standard ty pe s; i t is probable tha t the approximation could becarried further, if desired.Section 3.An illustration of the use of Sections 1 and 2 in the case ofan airship ring. The illustration deals \*1th the problems which arise whenwires become slack under load.S UMMARY OF THE P RES ENT S TATE OF KNOWLEDGE

REGARD ING SHEET-M ETAL CONSTRUCTION. By H. L. Cox,B.A. R. & M. No. 1553. (20 pages.) August 3, 1933.Price Is . net.Investigation into the strength of constructions in thin sheet metal dividesitself naturally into two parts, f irstly, consideration of the phenomenon ofbuckling and the determ ination of buckling load ; secondly, the investiga-tion of conditions after buckling has commenced. In the majority of air-craft constructions, buckling will normally occur at so low a load that theoccurrence of buckling will not in itself be of great importance. Neverthe-less there are impo rtant exceptions to this state ment, of which built-up sparsand constructions in corrugated sheet may be cited.In th is summa ry atte ntion is mainly confined to the effect of buckling undershear or under uniaxia l compressive loading. The problem of buckling undercombined shear and compressive loading has not been considered by anyinvestigator.

THE EF F EC T OF AI LERONS ON THE S P I NNI NG OF ABRI S TOL F I GH TER AEROP LANE. By A . V . Stephens, B.A.Communicated by the Director of Scientific R e s e a r c h ,A ir M i n i s t r y . R . & M. N o. 1555. (7 pages and 5 dia-grams.) June 10, 1933. Price 6d. ne t .

It has long been accepted* tha t ordinary ailerons, although of li t t le value asa control for recovering from spins, may exert a large influence upon thecharacter of the steady spin. Conventional ailerons have been found tomaintain their power of producing a rolling moment (chord axes) up to largeangles of incidence, but they also tend to induce a yawing couple of theopposite sign. Moreover, i t has been shown theoreticallyt th at the effects ofapplying a pure rolling couple against a spin, whether dynamically or other-wise, are not in general such as to accelerate recove ry; and also that even asmall applied yawing couple may cause a radical change in the nature of thespin. Accordingly, the influence of ailerons is in general governed by theyawing moment (chord axes) due to them.Complete measurements of a series of spine, covering the available range ofaileron movement were obtained (a) with the aeroplane loaded normally;(6) with 40 lb. of shot in each wing tip . The effect of applying aileron in eitherdirection during the recovery from a normal spin was also investigated.Ailerons were found to exert a large influence upon the character of thesteady spin ; crossing the ailerons rendered the spin flatter, whereas settingthem " with " the spin had th e opposite effect. The total range of incidenceobtainable in this way was from 33 to 60 for the aircraft und er normal load. " The Spinning of Aeroplanes." Gates and B ryant, October, 1926.H, & M. 1001.t " Note on Recovery from a Spin." Brya nt and Jones , October, 1930.R . & M. 1426.HEAT TRANSMISSION THROUGH CIRCULAR, SQUAREAND RECTANGULAR P I P E S . By A. Bailey, M . S c ,Assoc.M.Inst.C.E., and W. F. Cope, B.A. Work per-formed for the Department of Scientific and IndustrialResearch. R. & M. No. 1560. (11 pages and 12diagrams.) May 24, 1933.

The present state of our knowledge of heat transmission has been summarisedby M. Fishenden and O. A. Saunders,* and a study of the chapter of theirbook dealing with convective flow in pipes shows that although the resultsof many ad hoc researches have been published, there have been few systematicinvestiga tions of the problem. In particular, l i t t le work appears to havebeen done on pipes of other than circular cross section. At the suggestionOf the Aerodynamics Sub-Committee Of the A eronautical Research Committeethe present investigation was undertaken at the N.P.L. using an apparatuswhich would enable the friction and heat transmission to a stream of fluidflowing through a pipe of given cross section to be measured sim ultaneouslyover the same portion of the pipe. For simplicity, i t was decided to sta rtwith water as the working fluid and to use drawn copper pipes, as thesewere easily obtainable in a variety of cross sections.A technique has been worked out and apparatus constructed for thesimultaneous measurement of the fluid friction and the heat transfer fromeither tube to fluid or fluid to tube over the central portion of a copper pipeof circular or rectangular section. Tests have been carried out on pipeshaving internal dimensions 1 71 cm. diameter, 1 40 cm. square, 1 90 cm.x 0-90 cm., 2-22 cm. x 0-63 cm., a nd 2-53 cm. X 0-32 cm . rectanglesover a range of Reynolds num bers from 3,000 to 25,000.The observations taken of both friction and heat transmission have beenreduced by the method of dimensional analysis.The following conclusions have beea drawn :(1) That the hydraulic diameter is the correct length parameter to usein correlating heat-transmission results.(2) That an increase in surface friction does not necessarily lead to acorresponding increase in heat transmission,(3) Tha t ths he at transmission of a narrow rectangular pipe throug hwhich water is f lowing and being heated is greater than that of a circularpipe under the same circumstances. If the water is being cooled, the he attransmission of the flat pipe is less than that of the circular pipe.It is proposed to extend the range of the observations with water and tocarry out similar tests w ith other f luids.

' The Calculation of Hea t Transm ission." H.M.S.O. 1932.174 h


17/94

March 29, 1934 Supplement to FLIGHT

liliFLIGHT"ENGINEERING SECTION

Edited by C. M. POULSENNo. 98 Year March 39, 1934

CONTENTSSome Developments in Aircraft ConstructionEngine CowlingTechnical LiteratureSummaries of Aeronautical Research Comm ittee Reports ...

Page1722

SOME DEVELOPMENTS IN AIRCRAFT CONSTRUCTIONTHE paper under above title written by Mr. H. J.Pollard, of the Bristol Aeroplane Co., Ltd., and of whichhe gave a spoken summary at a meeting of the R.Ae.S.on March 15, was of particular interest to aircraftengineers. In FLIGHT last week we published a summaryof the paper, and some of the photographs which illus-trated it. For the benefit of our more technically-minded readers we give below the appendices in whichMr. Pollard gives details of various formulae, etc., fromcertain publications to which he referred, but which arenot readily available to British aircraft engineers.Before giving Mr. Pollard's appendices, there is asection of his paper which had to be dealt with some-what sketchily in FLIGHT last week, but which shouldbe mentioned here. That is the section on multi-sparwings. Concerning the problems of economic use of

materials in monoplane and biplane wings, Mr. Pollardsaid : " As to structure weights all tha t can be said atpresent is that when we have chosen the most favour-able material and placed it most favourably for givingstrength and stiffness, then we have done all tha t can bedone to obtain the minimum possible weight. Theprinciples I have outlined certainly show the way. Anidea of what is to be gained as regards spar weights isshown in Fig. 11, the two aircraft concerned being verynearly the same weight and designed to the same loadfactors. The variations in sectional area along thelength of a monoplane front spar are shown in curve o,while the stresses along the boom for one condition offlight are shown in curve b. Curve c shows the variationin stress along the length of the top front spar of abiplane wing, while curve d gives the average sectionalarea of the front top and bottom wing spars takentogether as a single spar. The variation of sectionalarea of this virtual combination of spars cannot beshown conveniently in the diagram, but the variations inarea of the top front spar in regions of high stress canbe clearly distinguished at points z, z, etc., on thestress curve b. Due to the fixity at the point p, wherethe centre section and outer wing spars join, together

Fig 11. Sectional area andstress curves for monoplaneand biplane spars, plottedagainst span, a, area ofmonoplane front spar. b.stress in monoplane frontspar, c, stress in biplanetop front spar, d, averagearea of all spar sections infront truss of biplane.


18/94

SfJPPLRUZNT TOPLIGHTIS

MARCH 29, 1934THE AIRCRAFT ENGINEERFig. 14. The mechanicalproperties of 4S weregiven in a table last week.The relationship betweenfatigue shear stress andendurance for variousmaterials is shown here-with. The consistency ofthe 4S alloy is mostnotable.

Nt2ji

8izII&*

OOOC

KaOO Vs.

\= 1 =

- .1

o* -

o i 2 3 * 3 * y q to I I X I O 6

with the lamination, the stress distribution shown isgood for a single bay biplane wing spar, but even ifimprovement could be obtained by modification to thedetail design, the construction of a biplane spar memberhaving the uniformity of stress indicated in curve bis not a feasible proposition." Only in really large biplane constructions would suchrefinements in design be possible, whereas no difficultyexists in getting the uniformity of stress shown in curveb, in monoplane constructions as described. At themoment we cannot proceed further than a simple mono-plane wing; the word simple is perhaps a misnomerhere, the calculations may, in a casual manner, beregarded as simple in comparison with what is requiredfor a biplane structure. In the case of the monoplane,however, flutter and allied investigations become efprime importance and the additional calculations con-nected therewith make the matter more involved andcomplicated than it might at first sight appear." Another form of wing construction in which thewhole skin or a large part of it is reinforced againstconsiderable longitudinal forces is worth attention; inthis the bending is resisted by a reinforcing combinationan d not by the booms of separate and distinct spars.Corrugated internal plate attached to a plain outerskin would appear to be the best arrangement, gradua-tion in area against the variation in load could well beexecuted through the use of lamination in the flat skin.

There are certain manufacturing difficulties, however,which so far have precluded an experimental investiga-tion of this construction. The main difficulty, which hasbeen excessive rivetin g, is, however, likely to be overcomein the near future."

Appendix ICritical Stresses of Thin Curved and Flat PlatesThe formula derived by Redshaw for the critical bucklingstress for a curved p anel axially loaded and simply supportedat axial and circumfe

the aircraft engineer 1934

Documents