FLIGHT, FEBRUARY 8, 1934

and " zone control in force." Incidentally, the zone con-trol proved a popular topic for conversation.

MAT. MEALING, in proposing the toast of " All Firms andOperating Companies," said that he often wondered who ownsCroydon Aerodrome—Imperial Airways or the Air Ministry ?He complimented the organisers of the dinner, and said itwas an occasion where people could tell each other whatthey thought and get to know each other. Maj. Mealingasked the operating firms at Croydon to keep the AirMinistry informed of their troubles. On this remark therewas an immediate show of hands by several operators.Operating Companies, said Maj. Mealing, were welcomed toCroydon, even if the charges were high.

MR. EMILE BOUDERIE, aerodrome manager for Air-Franceat Croydon, responding to the toast, proposed by Maj.Mealing, said that the • Press kept everyone informed ofwhat was happening in aviation. He recalled the old daysat Hounslow and spoke of the machines which carried oneor two passengers and perhaps two parcels, comparingthem with such machines as those which operate servicesto Batavia and India. Australia may soon be included inthe list, and perhaps this year, or perhaps next, SouthAmerica as well.

MR. " JOE " CHAMBERLAIN, in proposing the toast of theladies, said he was not a lady's man. A speech, he said,should be like a lady's dress—short enough to be enter-taining and long enough to cover the subject.

MRS. LEVERTON recalled the Croydon of thirteen or four-teen years ago, and expressed her pleasure at seeing at thedinner many friends of the early days.

MR. L. PACE proposed the toast of the visitors, and ex-pressed his pleasure at the presence of many from Bristoland Lympne.

COM. DEACON, who was at one time second in commandat Croydon, and is now in charge of Lympne aerodrome,responded on behalf of the visitors. He did not, he said,

feel like a visitor with so many familiar faces round him.All the traffic staff of Lympne who were off duty werepresent at the dinner.

MR. " CY." HOLMES, another old Croydon pilot, now atBristol, referred to Capt. O. P. Jones as an old pupil ofhis, whom he had taught cross-country flying. Mr. Holmessaid he had not been to Croydon for 10£ years and, onarriving there by air, was surprised to see what he calleda lot of " red things " through the fog. In the old days,he said, pilots arrived by Grace of God. He referredto the way in which Croydon will persist in butting in onthe Heston wireless weather reports. He admitted thatCroydon to-day is marvellous ; ten years ago, he said, itwas wonderful.

MR. G. W. THOMPSON proposed the toast of the chair-man, including in the toast the name of Mrs. Richard.As a member of Maj. Richard's staff, it seemed that hewas compelled to say " nice things." Having had thehonour of serving under Maj. Richard for many years, hewas in a position to judge him. Maj. Richard's positionwas not to be envied, for he was in charge of a governmentdepartment which.worked alongside private enterprise.

MAJ. L. F. RICHARD, in replying, said that the kindthings which Mr. Thompson had said applied to the staffin general. He was a little disappointed, he confessed,that there were not many more present at the dinner. Heexpected an increase in the number of passengers, andthought that a figure of 100,000 passengers a month wasnot beyond hope. In referring to the new zone controlsystem, Maj. Richard complimented the pilots for their co-operation in the scheme. If they had failed, he said, thewhole scheme would have dropped to the ground. He con-gratulated also the Duty Officers and the Wireless Staff.The system was not yet complete, as it was too much toexpect that such a scheme could be finished at the firstattempt.

ENGINE COWLINGWith Special Reference to the Air-Cooled Engine

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

Lecture, abridged, delivered before the Royal Aeronauti-cal Society on Thursday, February 1, 1934.

IN the earliest stages of the development of the aero-plane successful flight depended on the achievement,rather of minimum weight, minimum wing loadingand maximum engine power, than on minimum

parasite resistance. At a very early date the rotary Gnomeengine achieved a predominant position amongst aeroplanepower plants. In the early Farman and similar machineswith engines at the rear, even oil guards were often notused, but as the tractor became normal, cowling wasadopted. Such cowling consisted of a sheet metal hood,which either completely surrounded the engine or whichextended some 270 deg. round the engine, this hood beingcontinuous with the fuselage surface at least over theupper part of that surface, and permitting the escape ofexhaust gases, of cooling air, and of oil, only towards thelower part of the fuselage.

Apart from rotary engines, the only air-cooled engineof pre-war days which used cowling was the eight-cylinderVee Renault. The cowling, provided by the makers, wasessentially a system of air ducts through which cooling airwas drawn by a fan, and the cowling was essentially acooling device, not a resistance reducer. It is interestingthat neither in the rotary nor in the Renault did coolingdepend to more than a limited extent on the speed offlight of the aircraft, the rotation of the cylinders in theone case, and the fan in the other, providing the relativemotion of air and cylinders required.

It is also interesting to note the high proportion ofengine power absorbed in rotating the cylinders of therotary engine and thus ensuring cooling. Speaking frommemory, the reaction torque of the nominal 50-h.p. Gnomecorresponded to the 50-h.p. rating, the h.p. available wasabout 37 h.p., some 13 h.p., or 26 per cent, of the powerdeveloped in the engine, being used up in rotating theengine.

In early water-cooled engine installations, it was the

custom to leave the cylinders exposed, and to use radiatorsmounted clear of the engine, not influencing the form ofsuch cowling as was used. At a later date radiatorsmounted ahead of the engine, forming the front end of acowling, as, for example, in the S.E.5, or larger radiatorsbehind the engine, as in the R.E.7, came into fashion.The practice of leaving water-cooled cylinders exposed tothe air persisted on the assumption that some direct heatdissipation from the jacket walls was obtainable. Modernwater-cooled engines are normally cowled completely, andradiators retractable within the fuselage or more or lessshielded from the free air stream, are usually employed.The cowling has become merely a case designed to causethe minimum resistance.

Cowling of Air-Cooled EnginesAlthough the cowling of air-cooled engines, even to so

late a date as the end of the war, was dictated normallyby considerations other than that of reducing resistance;the question of resistance was not entirely neglected. Alow resistance installation is the Paulhan-Tatin monoplane(Fig 1), designed circa 1913, in which a rotary air-cooledengine was enclosed in the body, driving an airscrew atthe rear of the fuselage by a shaft.

In 1913 M. Jean Armand Deperdussin proposed meansof cowling rotary engines* with a view to ensuringminimum resistance. The engine was mounted onthe front bulkhead of a fuselage of circular section, andenclosed in a casing which, in conjunction with the body,formed a continuous streamline form. At the front of thecowling an aperture admitted cooling air to the engine.At the rear, louvres allowed the air to emerge.

The standard cowling used by all nations during thewar for rotary engines was influenced by these patents.The cowl formed a continuation of, or was faired into thefuselage lines. The fore end was curved inwards to forma fair entry to the body lines. But it was not found

133• Deperdussin. French Patents 444619, 447788 (1912).

FLIGHT, FEBRUARY 8, 1934

practicable t<> enclose the engine completely, and a free exitfor air, exhaust, and oil was provided by leaving a largegap between cowl and fuselage.

Towards the middle of the war the Salmson company, inFrance, introduced a cowling similar to the Deperdussin,for their radial Canton-Unne engines. The engines, thoughradial, were water-cooled, and fitted with annular radia-tors, mounted within the cowling. A similar type of cowl-ling was later applied to Salmson air-cooled radials andby the Caudron company to Anzani radial air-cooledengines. Essentially the method of reducing drag consistedin all these cases in shielding the engine from the free airstream and in cooling by a stream of velocity consider-ably reduced by the cowling. The engines were all oflow power and relatively large dimensions, and henceshould have been easily cooled.

In a cowling with rearwardly facing air outlets sur-rounding a rotary engine, the engine may act as a rotaryblower, inducing a rearwardly directed flow of air at highvelocity, producing a measurable thrust, and effectivelydecreasing engine drag. The existence of this has beenverified in the wind tunnel.

The demand for engines of increasing power, and thedilficulties of producing stationary air-cooled cylinders ofhigh output led towards the end of the war to an increas-ing use of water-cooled engines and to a partial eclipse ofair-cooled type. Work on the air-cooled type continued, how-ever, and in the ten years or so following the war, the high-powered radial air-cooled engine established itself, as beingthe most widely used type for both military and commer-cial service throughout the world. During this periodradial engines were used either with no cowling or withcowling which had little effect in reducing drag. It wasrealised that the power wasted in driving radial enginesthrough the air was worth saving, and as speeds and per-formance increased, this waste of power became important.

Fig. 2 shows an arrangement in which cowling is con- •fined to a casing surrounding the crankcase, of a form toblend into an airscrew spinner forward, and the fuselagebehind, the engine cylinders projecting out into the open.A tail is added to the cowling behind each cylinder, withthe object of partly streamlining the projection. Unfor-tunately, the efficacy of these arrangements as drag re-ducers was small.

' Fig. 3 shows the cowling fitted in 1923 to the Gourdou-Leseurre type C.I. Each cylinder of the " Jupiter "engine is enclosed in a separate streamline chest, with alimited aperture at the front for admission of cooling air,and an exit at the rear so that the emerging air may blendwith the general air stream. The principle is similar tothat of the single streamline casing enclosing the wholeengine. This cowling is capable of reducing resistancesubstantially, but has a serious effect on the cooling ofcylinders,* and has not been found practicable except pos-sibly for very high speed aircraft.

The growing ascendancy of the air-cooled radial was fora time seriously threatened by the development in America—:by the Curtiss company—of the combination of a twelve-

, • • N.A.C.A. report No. 313.

Fig. 1 : Paulhan-Tatin " Torpille Aerienne " of 1912. Theengine was fully enclosed in the centre of the fuselage and

drove an airscrew in the tail by a shaft.

cylinder Vee engine of very small frontal area, completeenclosing of that engine in a cowling faired to the fuse-lage lines and the adoption of wing radiators.

It is well to note that " low frontal area," often appliedto streamline bodies, is a misleading phrase, and thatfrontal area is only a measure of resistance in the case ofgeometrically similar solids of revolution. It was enclosingthe Curtiss engine in a body of good shape and of smallsurface area, rather than of small cross-section which wasof importance.

The last five years have seen the development in Englandof the Townend Ring, and in America of the original formof N.A.C.A. cowling which, when properly applied, are ableto reduce the drag of a radial engine to values only afraction of those previously attainable! without prohibitiveinterference with cooling. There can be little doubt thatto the introduction of satisfactory forms of low drag cowl-ing, the radial air-cooled engine owes a renewed lease oflife.

Engine CoolingIt will be clear from the historical outline given that the

factor limiting the use of low drag cowlings is cooling.To enclose the power plant in a streamline casing was theobvious method of obtaining low resistance, and wouldhave been adopted at a very early stage were it not for thenecessity of cooling engines. The theoretical aspects ofcooling have already been ably dealt with by Pye.*

By the use of the wing surface, or of some other neces-sary part as a cooling surface, it is possible to secure cool-ing without increase in the resistance of the aircraft—except in so far as the skin friction coefficient may beaffected by change in temperature of the air flowing overthe surface. But it has not become practicable generallyto use wing or body surfaces for engine cooling, and theadded resistance of special cooling surfaces must betolerated. For any given form of cooling surface the rateof heat dissipation depends on the temperature differencebetween the surface and the air and on the relative velocityof air and cooling surface. It is nearly directly propor-tional to the temperature difference and varies as somepower of the relative velocity depending on the charac-teristics of the cooling arrangements which is rather lessthan unity.

In the problem of engine cooling, as a first approxi-mation, the temperature difference between the heat dissi-pating surface and the air may be regarded as having aconstant value, fixed by the permissible upper limit of theboiling point of water in water-cooled engines and of per-missible operating cylinder temperatures in the air-cooledcase. The critical cooling condition must correspond tofull throttle operation of the engine.

In service the engine will normally be opened up to fullthrottle for take-off with the aeroplane stationary andremain at full throttle during acceleration on the ground,during climb to operating height, and may remain at fullthrottle during full-speed flight.

At the start, the temperatures are substantially belowtheir steady normal operating level, and the effective heatreservoir afforded by the power plant as a whole preventsexcessive temperatures being reached during the initialperiod of a flight in which the air speed of the machine isvery low. The condition which will determine the areaof cooling surface is the heat dissipation obtainable at theminimum speed of flight which can be maintained at fullthrottle for any appreciable period. For practical purposesthis condition is generally that of a continued climb atmaximum climbing speed.

Pye's estimate of the power expended on cooling a500-h.p. engine on an aircraft flving at 150 m.p.h. isfi.8 h.p., or about 1.5 per cent, of the engine output.

Change in fin proportions, particularly as regards pitch/height ratio have a considerable effect on heat dissipationand—apparently—appreciably affect the ratio between re-sistance and heat dissipation. The temperature of air-cooled cylinders varies over a wide range, for differentpositions on any cylinder, and it is quite impossible toestimate from available data as to maximum safe tem-peratures at specified positions of current types of suchcylinders, whether the assumed mean temperature differ-ences represent conditions which could be tolerated. Afurther uncertainty, particularly in air-cooled engines, isthe proportion of total heat of combustion rejected in-various ways. The overall thermal efficiency of such

* Pye, The Principle of Air Cooling, "Aircraft Engineering," Feb. ,March", April, 1933. „„.,

134

FLIGHT, FEBRUARY 8> 1934

engines is easily ascertainable—the proportion of waste heatcarried off by exhaust gases, by the lubricating oil, andthe remainder to be dissipated mainly by the cylinders isnot known, and there is evidence that the proportion oftotal waste heat carried off in these various ways differswidely for different types of engine.

The estimates of power required relate to heat dissipatedfrom cylinders alone, but the resistance both of oil coolersand exhaust systems may be regarded as part of the cool-ing resistance, and allowance should be made for these.

Pye shows that the difference in performance of an aero-plane with a water-cooled engine and retractable radiatorshowed a difference in resistance, radiator exposed andradiator completely retracted, of 9 per cent, of the remain-ing resistance of the aircraft, the top speed of the machinebeing about 150 m.p.h. Presumably the radiator in ques-tion gave sufficient cooling for the engine on the climb,and if the climbing speed of machine was 75 m.p.h., andif the whole of the waste heat rejected through the cylin-der walls was required to be dissipated by the radiatorduring climb, it should be capable of dissipating nearlytwice the amount of heat required for cooling at top speed.Cooling systems sufficient to cope with climbing conditionsare of excessive capacity for flight at high speeds, andtend to become increasingly excessive as the performanceof aircraft increases, but increasing speed is normallyattended by increased rate of climb and decreased timeto reach operating height. The capacity of the heat reser-voir afforded by the engine and lubricating oil, duringthe rise in temperature from take-off conditions to maxi-mum permitted temperature, can therefore absorb heat ata rate inversely proportional to the time taken in climbing.

With water-cooled engines the high specific heat of waterprovides a very large heat reservoir, and the high latentheat of evaporation of the same fluid permits of the dissi-pation of heat at a greatly increased rate accompanied bya relatively small loss of water by evaporation. Thusthere is an effective heat ballast system which permits atemporary rate of heat rejection in excess of the rate ofdissipation from the cooling system without temperaturesrising to beyond permitted limits.

Cooling systems are normally arranged in the airscrewslipstream, whose velocity is more nearly constant thanthe air speed of the machine itself, and the effective cool-ing velocity does not vary over such a wide range as themachine speed. The effect of airscrew characteristics oncooling problems is considerable. Geared airscrews of largediameter reduce the assistance to cooling given by slip-stream. Behind the boss, and for some distance outwardtherefrom, the added velocity due to the airscrew has anegative value—which may extend over 0.2 of the wholediameter, and the increase in airscrew diameter followingthe use of reduction gears, with the consequent increase inthe area shielded by the centre of the airscrew, is a factorof considerable importance as regards cooling of radialengines in particular.

The nature of the permissible limits of temperature of anengine installation enters into the question. There is anupper limit for cylinder heads at which mechanical failureoccurs after a very short period. Below, there is a wholerange of operating temperatures, each corresponding tosome life between overhauls of the engine. Temperaturelimits are commonly determined as those which continu-ously maintained are consistent with a particular life be-tween overhauls—generally 100 hours. In regular service,the attainment of these maximum permissible temperaturesfor a few minutes during climb, followed by a period ofcruising flight at substantially lower temperatures, is foundto give a service life between overhauls two to three timesas long as would result from continuous operation at maxi-mum temperature. All these factors reduce the disparitybetween the amount of fully-exposed cooling surface whichis needed for climb and for high-speed level flight to some-thing considerably less than would appear to be necessaryon first consideration.

It is generally true that the cooling which has to beprovided on an engine installation is determined by thecooling required on climb, that this cooling system pro-vides excessive cooling capacity and consequently excessivedrag at higher speeds. The importance of drag of coolingsystems increases very rapidly with increase in the ratioof climbing to maximum level air speeds, since power ex-pended on such drag varies as the cube of the speed andis graphically indicated by Fig. 4, which shows the varia-tion in the proportion of the total drag of an aircraft

Fig. 2 : Normal cowling for fixed radial air-cooled engines.The spinner, crankcase, cowl and body lines blend. Thecylinders project from the cowling and have tails behind

them to assist streamlining.

represented by fixed cooling systems with variation inmaximum speed. Two different cases are shown, one inwhich the drag of the cooling system at 100 ft. /sec. is1 lb. per 20 b.h.p. (25 lb. for 500 b.h.p.j, and the other inwhich it is 1 lb. per 7.14 b.h.p. (70 lb. for 500 b.h.p.).The figures correspond closely to a good ring-cowled radialinstallation and to a good radial installation with crank-case cowling only, using current types of air-cooled engines,and do not indicate the limits of possible reduction in cool-ing drag possible as a result of engine development or byother means. The figures do not represent the effect, onotherwise identical aircraft, of a change in cowling, as thechange to low drag cowling will cause an increase in maxi-mum speed, but indicate directly the change in powerspent in cooling on aircraft designed for equal top speed,and therefore of equal total drag, the saving due to lowdrag cowling being used, for example, for increasing usefulload or fuel.

They do indicate directly the reduction in engine powerrequired for a given speed on such otherwise identical air-craft caused by the assumed change of power plant drag,but such reduction in the power of the engine actuallyinstalled would operate still further to reduce the drag, andconsequently still larger economies in power expendedwould become possible. If the maximum speed of an air-craft with the higher drag cowling of the figure be 175m.p.h., the cooling drag and h.p. will be 55 per cent, of thetotal. If the lower drag cowling be fitted, the engine maxi-mum power being not altered, the maximum speed willincrease to about 200 m.p.h. If, however, the aircraftwith low drag cowling is operated at 175 m.p.h., the cool-ing drag falls to 20 per cent, of the total for the originalmachine at the same speed, and the power absorbed andthe fuel consumption at 175 m.p.h. falls to 65 per cent,of that of the original machine. The effect on range forequal fuel, on load for equal range, and in engine reliabilityneed only be mentioned.

Means whereby Cooling Resistance may be ReducedIdeal cooling conditions assumed by Pye in the estimate

of power necessarily absorbed in cooling, are not generallyobtainable. The wing radiator for water-cooled engines israrely practically possible. Steam cooling of the kind pro-posed by Wing Com. T. R. Cave-Brown-Cave many yearsago, has, however, given wing surface cooling a new fieldof utility. The increase in the mean temperature at whichheat is dissipated to actual boiling point reduces the totalsurface required, and permits the use of the leading edge ofthe wing alone, and the low density of steam—as comparedto water—greatly reduces the weight of cooling fluid in

135 :->~-v - r , \ . . "-•

FLIGHT, FEBRUARY 8, 1934

Fig. 3 : Gourdou-Leseurre fully helmeted cowling of 1923.

Circulation. For the steam-cooled case, cooling, for little orno added drag, now appears to be a possibility so far asrelates to the dissipation of heat from cylinder walls alone.Oil, and exhaust cooling, and the drag of air intakesremain and assume an added importance in consequence.

In the case of the air-cooled engine, the form of coolingsurface is characteristic of the engine itself, and the air-craft designer has no such control over that form as isavailable in the liquid-cooled case, and has there resultedin the development of wing nose condensers for steam-cooled engines. As the drag of any given cooling surfaceincreases as the square of the velocity, while heat dissipa-tion varies at a lesser rate, cooling drag can be reduced byincreasing cooling surface and so shielding the surface fromthe air that the air speed over the cooling surface isreduced. It should therefore pay the designer of the air-cooled engine to increase the cooling surface output ratioto enable cooling to be effected at reduced air speeds whichwould permit of decreasing the necessary exposure of theengine to the air by means of cowling. This has beenrecognised by the engine designers, who in producing newtype engines have invariably aimed at providing as largea margin of cooling surface as was practicable. Develop-ment of such an engine then proceeds in the direction ofprogressively increasing the output per ex. of cylindercapacity, and, with output,, the heat dissipation rateneeded for cooling—until finally the cooling margin origin-ally provided disappears and redesign for improved coolingonce more becomes necessary.

The fact that the net thrust h.p. available, after deduct-ing engine drag, rather than the b.h.p. developed is ameasure of the useful output of the engine, and that thecapacity to cool with a low air velocity, may lead to aconsiderable economy both in the ratio net t.h.p. : b.h.p.and in the effective fuel economy of the engine, deservesmore attention than has been given to it.

The principle of using lowered air velocities and increasedcooling surfaces to reduce cooling drag is well known.Junkers has proposed radiators enclosed in casings, opento the air stream, with internal passages expanding fromopening to radiator position and thereafter contracting.*The radiator is thus at a region of reduced air flow, and,for given cooling, involves lowered drag. The casing wasto be given an external form conducive to low drag, andthe method is undoubtedly capable of useful results. Alarge saving in drag by this method involves increase indimensions and weight of radiator of substantial amount.

The helmet cowling (Fig. 3), and the enclosure of air-cooled engines in streamline cases (Deperdussin, etc., andthe original N.A.C.A. cowlings), depend for their practic-ability on the engines possessing some excess cooling capa-city, and on cooling by the use of a flow of cooling air at a

• Junkers British Patents, Nos. 147003, 148889.

velocity less than that of the free air stream. Radiatorsfitted with adjustable shutters or other screening deviceshave been used to reduce cooling drag at high air speeds.Such arrangements may operate either by reducing theair velocity over the cooling surface, or by reducing thearea of the cooling surface exposed to the free air streamduring high-speed flight, or by a combination of the two.In modern low drag installations of liquid-cooled engines,the cowling is purely an engine fairing, and cooling iseffected by radiators (or condensers) which are not part ofthe cowling. Nevertheless, the development of such lowresistance cowling has been made worth while only by thedevelopment of low drag cooling organs.

In the air-cooled engine, cowling and cooling are indis-solubly associated, and it is with such engines than cowlingdesign is a matter of outstanding importance. The finnedcylinder, which serves in such engines as the main coolingorgan, is a body of very bad aerodynamic shape whose highresistance is mainly due to eddy making. When suchcylinders protrude from a reasonably good streamline shape,the eddying wake behind the cylinders causes a generalbreakaway of the flow from the body behind, and thisbreakaway of flow is the cause of the high resistance ofinstallations of this type. The streamline " tails " behindcylinders of Fig. 2 were an effort to suppress the turbulentwake, and hence to prevent the breakaway of flow, butproved to have but a negligible effect (vide N.A.C.A.Report No. 313). Complete " helmeting " on the lines ofFig. 3 are shown in the same report to be exceedingly effec-tive as reducers of drag—but to interfere prohibitively withcooling.

It was only in 1928 that the nearly simultaneous pub-lication of the results obtained by Mr. H. C. H. Townendat the National Physical Laboratory, with the TownendRing, and of the N.C.A. tests at Langley Field on theoriginal complete N.A.C.A. cowling that any really impor-tant advance towards low drag installation of radial air-cooled engines was made.

Ring Cowlings GenerallyThere still appears to be a considerable amount of mis-

apprehension as to the relation between Mr. Townend'soriginal work with the Townend Ring and the tests madeby the N.A.C.A. on cowlings in 1928. Mr. Townend'sinvention is clearly described in his patent specification.*The Townend Ring is an annular aerofoil surroundingradial projections from a streamline body, the aerofoilsection being arranged to have a positive angle of attackto the local air flow. Mr. Townend found that the in-wardly directed downwash from this aerofoil suppressedthe breakaway of flow which would otherwise have beenproduced by the projections, and consequently greatly re-duced the resistance of the whole combination.

The earliest account of the N.A.C.A. complete cowlingto be published is contained in Technical Note No. 301,which is dated October 13, 1928, some months later thanthe date of Townend's patent application. This noterecords the results of tests on a variety of cowlings forradial engines, including types in current^use, the " hel-met " type, and the " complete cowling " since particu-larly identified as the " N.A.C.A." type.

This complete cowling may well be described in termsadopted by the N.A.C.A. themselves: —

" Cowling No. 9 completely covered the engine. Theair was taken in at the nose and allowed to flow past theengine, which was entirely uncowled, and out of anannular slot, similar in section to some wing slots whichhave been tested. This type of nose and slot were de-signed to offer as little resistance to the flow of air overthe fuselage as possible, separating air for cooling theengine from the general flow and then feeding it backsmoothly through the slot " (N.A.C.A. Tech. NoteNo. 301).

" When the slot was originally designed for the com-plete cowling (Nos. 9 and 10) it was hoped that it wouldtend to decrease the drag because of its effect oh theboundary layer. A test made on the nacelle with theslot covered, however, showed that the drag is 10 lb. lessat 100 m.p.h. without the slot. . . . It is likely thatthe cooling air could be collected after it has passedthe cylinders and directed out through one or two open-ings at the bottom or sides with no increase in drag overthat with the annular slot. The annular slot is, how-ever, a very convenient means for getting the used

• Townend British Patent No. 320131.

136

FLIGHT, FEBRUARY 8, 1934

cooling air back to the general outside flow." (N.A.C.A.Report No. 314, pp. 20 and 21.)

" The now widespread use of air-cooled engine cowlingshas brought forth many modifications of the originalN.A.C.A. type. In fact the cowling which was intendedto form a smooth streamline shape of the average fuselagewith its projecting cylinders has in some cases been re-duced to a small continuous ring above the cylinderheads. . . . A cowling of the latter form, commonlyknown as the Townend Ring, has been developed else-where." (N.A.C.A. Tech. Note No. 335, p. 1, datedNovember 1, 1930).It may be noted that the No. 10 cowling mentioned in

the second quotation above differs from No. 9 mentionedin the first quotation by the provision of internal cowlingshielding the crankcase and thus increasing the air flowover the cylinders and by the use of intercylinder baffles.The statements quoted indicate the similarity of theoriginal N.A.C.A. cowling and the early arrangements ofDeperdussin. The annular slot arrangement—impossible toDeperdussin on account of the oil-throwing propensity ofthe rotary engine—is shown to be of no advantage otherthan convenience. The special arrangements for improv-ing cylinder cooling in the final form (No. 10) are notnovel in principle, but only in conjunction with the specifictype of cowling. This comment on the lack of essentialnovelty in the principle of the N.A.C.A. cowl does not inany way detract from the value of the investigation madeat Langley Field on cowlings generally. The discoverythat a device proposed before the time was ripe for itsgeneral adoption could, with improvements and develop-ment and under the changed conditions of increased speedsof flight and more effectively cooled engines, be of greatpractical utility deserves the fullest possible recognition.

It is clear that the ideas underlying the design of theoriginal N.A.C.A. " complete cowling " involved no suchnovel and broad principle as suppression of the breakawayof flow behind the body by the downwash from an annularaerofoil—a conception entirely novel and one enunciatingan aerodynamic principle of first-class importance. Thefigures in Townend's specification show no approach to acowling, which, with the body, forms a streamline wholeenclosing the engine. The ring is something entirely out-side of, and divorced from, the general streamline of thebody, and it is extremely doubtful if anyone with a reason-able knowledge of aerodynamics, confronted with thesefigures for the first time, would recognise them as showingforms likely to have a low resistance. I confess freelythat I myself at first sight of the Townend Ring concludedthat it looked far more likely to increase than to reducedrag.

Particular attention is drawn to the date of the lastquoted of the three extracts from N.A.C.A. reports, whichrefers to " narrow rings." By November, 1930, the resultsoi Mr. Townend's work were widely known in aeronauticalcircles throughout the world, and the suggestion that such" narrow rings " were merely developments of the originalN.A.C.A. cowl is obviously without foundation. The re-sults obtained with the N.A.C.A. complete cowlingattracted extraordinary interest. The great reduction indrag shown was measured on a real engine, driving anairscrew, mounted on a full-scale body at a speed of 100m.p.h. The tests included cooling tests on the enginewhen running, and the cooling of the engine was pro-nounced to be satisfactory. The absence of any doubt asto possible scale effect and the evidence as to cooling wereproperly regarded as of fundamental importance—the latterparticularly so in view of the repeated failures of early lowdrag cowlings to provide this essential.

The early results published by Townend using the ringshowed striking reductions in drag, but not such specta-cular figures as those of the N.A.C.A. This difference isnot surprising, since Townend was investigating a newgeneral principle and his models were of small scale andconventionalised form, whereas N.A.C.A. were essentiallyengaged in discovering how far the development of a well-known principle could be carried in practice. It may benoted here that by the time the N.A.C.A. Report No. 314giving drag tests on a streamline nacelle " Whirlwind "engine, and complete cowling was available in this country,tests made in the Boulton & Paul wind channel on a nacelle

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Fig. 4 : Percentage of max. t.h.p. available absorbed inovercoming power plant drag for variation in max. speed.(A) Power plant drag = 1 lb./lOO ft./sec. per 7.14 b.h.p.(B) Power plant drag = 1 lb./100 ft./gee. per 20 b.h.p.

" Jupiter " engine, and Townend Ring, had shown dragswhich corrected for the difference in diameter of the twoejigines, were practically identical with those obtained byN.A.C.A. The British tests were made on ^-scale models,fully representative, and hence in themselves might be sub-ject to " scale effect," but full-scale evidence that theTownend Ring can equal the N.A.C.A. type cowling as adrag reducer is now fairly conclusive.

The broad principle underlying the Townend Ring givesthe device a number of obvious advantages over theN.A.C.A. cowling. In the latter, cowl, engine and bodyimmediately behind engine, must all be of substantiallyequal diameter. The cowl must curve inwardly forwardto a considerable degree to give a " streamline entry " tothe whole, and hence restrict the entrance for cooling air.These considerations impose restrictions on body form, whichaffect field of vision for the crew, field of fire for forwardfiring guns, and on cooling which are not present to thesame extent when a Townend Ring is used. The restric-tion on the entry of cooling air in the N.A.C.A. type cowl-ing is of considerable importance, despite the reassuringnature of the original N.A.C.A. tests of cooling. Thesewere made with an engine having a direct drive airscrew.There is conclusive evidence that, with such engines,N.A.C.A. complete cowling, with carefully-designed in-ternal passages and proper cylinder baffles, does give low-drag and satisfactory cooling. In this country, efforts touse N.A.C.A.-type cowling on geared engines have beendisappointing, and with geared and supercharged enginesgenerally unsatisfactory.* Even in the U.S.A. difficulty isgenerally admitted to occur with geared and/or super-charged engines fitted with N.A.C.A. cowls. This diffi-culty of cooling in the case of geared engines arises un-doubtedly from the increased diameter of the region ofreduced airscrew outflow with increased airscrew diameter.

With the Townend Ring, the ring alone must be of adiameter substantially equal to the engine, the bodydiameter behind the ring may be, for best results, of theorder of 0.75 of engine diameter. The pronounced inwardcurve of the part of the N.A C.A. cowl is not necessary.Body shape and size are consequently less strictly limited,and view and gun fire less restricted. A less restrictedentry and exit for cooling air are provided, and engineswhich will not cool in an N.A C.A. complete cowl, coolsatisfactorily with a properly designed Townend Ring.

•Fedden, Research on Low Drag Cowling, Aeronautical Engineering,

(The concluding portion of this paper will, later, be dealt with in" The Aircraft Engineer " for February 22 next.)

137

FLIGHT, FEBRUARY 16, 1934

ENGINE COWLINGAT the meeting of the Royal Aeronautical Society on

February 1 at which Mr. J. D. North read apaper on Engine Cowling, with Special Refer-ence to the Air-Cooled Engine," Mr. C. R Fairev

WaS,, in, u C fr ' I n i n t r o ducing the lecturer, Mr. Fairevrecalled that they had listened recently to the story of thedevelopment of the air-cooled engine, told by Mr FeddenIt was generally conceded that a great part of that develop-ment had been due to improvements in cowling and Mr-North was to tell them that evening the story of improve-ments in cowling.

Mr. North's paper was a long and interesting one. Inlast week s issue of FLIGHT we published a summary of thefirst part of the paper. The second, and rather moretechnical, part will be summarised in THE AIRCRAFTENGINEER (Monthly Technical Supplement to FLIGHT) onFebruary 22. Extracts from the discussion which followedthe reading of Mr. North's paper are given below.

THE DISCUSSIONMR. A. H. R. FEDDEN, who congratulated Mr. North

upon the thorough manner in which he had reviewed theimportant and interesting subject of engine cowling, saidhe had seen the paper for the first time only that morning,and had not yet had an opportunity to consider it reallycarefully. Mr. North was one of the first people in thiscountry to appreciate the possibilities of drag reductionby ring cowling on air-cooled radial engines, and had com-pleted much valuable work in this direction.

Mr. Fedden understood from Capt. Barnwell's windtunnel investigations that there was no engineering differ-ence between the forms of external cowlings known asN.A.C.A. and Townend Rings, and that all our modeltests had shown that any reduction in drag due to thefitting of an external cowling round a radial engine wasentirely due to a forward component of air force on thiscowling, and was invariably accompanied by an increaseof drag on the rest of the model. Assuming that these windtunnel tests were correct, he felt that the N.A.C.A. cowl-ing, with suitably proportioned baffles to the cylinders, wasan interesting combination in certain types of machines.

Perhaps Mr. North's remarks on the N.A.C.A. cowlingwere somewhat severe when he had said that cooling diffi-culties arose with geared and supercharged engines. Thelatest Douglas high-speed passenger machine was poweredby two geared and supercharged 700-h.p. " Cyclone "engines, and it was maintained that without the N.A.C.A.cowling it would be impossible to cool adequately underthe particularly arduous test conditions which the Depart-ment of Commerce tests called for. This machine had totake off from an aerodrome at an altitude of 4,943 ft. withone engine out, and to climb over 8,000 ft. with manifoldpressures rising as high as 3 lb. per sq. in.

What had not been fully realised in this country was theimportance of correctly designed inter-cylinder baffles, andthe great effect of fuselage shape.

The great importance of the fuselage had recently beendemonstrated most convincingly by a test in which agiven N.A.C.A. cowled engine cooled adequately on amachine with nicely faired nacelles ; but when the sameengine and cowling were installed in another single-enginedmachine with an unsuitable body shape and humps, theengine overheated very seriously. The cylinder tempera-tures were about 60 deg. higher than on the twin-enginenacelles.

Another point of interest was that velocity measure-ments in the neighbourhood of the cylinder barrels mightprove unreliable, and in the United States it was usualto use the difference in total head in front of and behindthe inter-cylinder baffle as a measure of cooling air speed.In the case mentioned, the differential head was 4 in. ofwater during climb at 100 m.p.h. with the correct fuse-lage, and only 1£ in. of water at the same speed on thesecond machine. In level flight cruising, the differentialhead was 12 in. of water on the first machine. It wasinteresting to note Mr. North's statement that struts,cylinder baffles, etc., within a Townend Ring had a neg-ligible effect on the total drag, and further informationon this point would be appreciated. Mr. Fedden wouldbe glad to feel that this was so ; but he suggested thatthe statement was contrary to the results of practical full-scale tests made in this country and America, where it hadbeen found that inter-cylinder baffles improved cooling, but

increased drag with a Townend Ring, but that they im-proved cooling and did not increase drag with an N.A.C.A.cowl in front of a properly proportioned body.

Whilst fully appreciating the value of the Townend Ring,and acknowledging the force of Mr. North's remarks re-garding field of vision, field of fire for forward guns, etc.,it was worthy of note that, whereas Townend Rings wereused to some extent in the United States, more generallyon single-engined pursuit machines, the tendency with thelatest high-speed machines was to have complete N.A.C.A.cowling, especially where there were two engines mountedin wing nacelles. It was felt that the N.A.C.A. cowlingwas more valuable than the Townend Ring from the pointof view of drag, and that, provided the fuselage was cor-rectly designed and suitable inter-cylinder baffles were in-stalled, the engine could be cooled more satisfactorily.

With reference to Mr. North's remarks on the effect ofring cowling on carburation, it was agreed that any formof cowling round a radial engine altered the configurationof the air stream, and that it was necessary to have a .design of carburetter in which the pressure balance effectswere not altered ; this matter had been dealt with onthe carburetters of recent type Bristol engines.

Mr. Fedden was fully alive to, and in agreement with,Mr. North's view in regard to the importance of suitablyring cowling air-cooled radial engines, but he was hopingthat the aircraft designer would give as much attention tothe body immediately behind the cowling as to the cowl-ing itself. For the future, he suggested that controllablecowling would be absolutely necessary, and it was interest-ing to note, in connection with the patent applicationsgiven in the paper, that Patent Specification No. 11315was granted to Mr. Granville Bradshaw in 1917, whichprovided for a cowling practically identical with theN.A.C.A. type, and with controllable flaps to meet climb-ing conditions, combined with a really close cowling withinter-cylinder baffles.

MR. H. C. H. TOWNEND, discussing the Townend Ring,and the rough rule given by Mr. North in the paper forestimating what ring angle was likely to give success, putforward another point of view, which supported Mr.North's statements. He illustrated it by means of slides.

The first slide showed a model tested in the windtunnel. The model was made in two dimensions, i.e..the body was represented by a thick symmetrical aero-foil, with a row of knobs to represent the cylinders and astraight aerofoil to represent the ring.

The second slide indicated the air flow around the bodyin the absence of both the cylinders and the ring. Thecylinders were dotted in in the position which they wouldeventually occupy, and the ring was also shown in theposition which gave minimum drag. He pointed out thatthe section was lying with its no-lift line practically alongthe local streamlines, so that if the engine only were re-moved, the ring would not be expected to have muchinfluence on the flow around the body. When the enginewas introduced, it immediately put the ring up to a highlift coefficient ; and it was possible that if the streamlinescould be obtained when the model was mounted in thewind tunnel without the engine on it, a very good ideacould be obtained of the angle at which the ring ought tobe set when the engine was eventually put in its properplace.

With regard to the point made in the paper that thebulbous nose was a good thing, he asked if Mr. Northcould state his views as to the reason for that.

Discussing the statement that sometimes the effect ofthe ordinary crankcase cowling was to warm up the crank-case and reduce the cooling of it, Mr. Townend asked if itdid not also probably make the cylinder heads somewhatcooler.

MR. HOFMAN, after assuring Mr. North that the in-formation contained in the paper was welcomed and wouldbe studied very carefully, suggested that Mr. North hadbeen rather hard on the N.A.C.A. cowling, particularlyin his remarks as to the obvious advantages of the Town-end Ring over the N.A.C.A. cowling. The difficulty aboutthe latter interfering with gunfire had been overcome inseveral cases by the slight readjustment of the position ofthe gun, and a large number of military planes had beenbuilt in the United States with N.A.C.A. cowls fitted, therebeing no interference with vision.

With regard to cooling, Mr. Hofman referred to some157

FLIGHT, FEBRUARY ;lfS, JJ&S4

tests on similar installations (with supercharged andgeared engine), the one having no ring cowl and no baffles,whilst the other had N.A.C.A. cowl and baffles. In thelatter case there was a reduction of nearly 50 deg. F. inthe maximum cylinder head temperature and an increaseof speed of 15 m.p.h., under conditions of constant powerand r.p.m., constant fuel consumption and constant den-sity altitude.

MR. HOLLIS WILLIAMS said there was one interestingpoint in the paper which rather revised his ideas of howrings and the N.A.C.A. cowling worked. It was interestingto see working out in full scale a physical phenomenonwhich one knew must be true, i.e., that if a ring cowlingwere working properly, symptoms of vibration, wind-screenchatter, and so on, would disappear, and there would belaminar flow. One could feel the warm air down the sidesof the fuselage, and that was one of the best methods oftelling whether a cowling was working properly. It hadalways seemed to him that to secure the peculiar pheno-menon of laminar flow it was necessary to rely on thesluicing of the air at the back of the ring. There was ahigh-velocity air stream rejected from the back of thering, and that tended to pack the air down on to the bodyand produce the laminar flow.

I t had been stated by Mr. North that in tests carriedout in America, where the slots had been filled up behindthe N.A.C.A. cowling, the drag was reduced. That upsetthe ideas which he (Mr. Hollis Williams) had had ; itmeant that the war-time cowlings were as good as anythingnow produced, and he was not sure that this was not afact. The point was whether the slot was absolutely neces-sary for the high-speed sluicing which gave the laminarflow.

MR. NORTH, replying to the discussion, suggested thatpart of the difficulty which had arisen was due to theunavoidable doubt as to what was an N.A.C.A. cowling.He had quoted extracts from the statements of theN.A.C.A. at the time they had produced their cowling, inorder to try to show what was in fact in their mindsprior to the publication of Mr. Townend's results, andwhat they had produced. The line of the cowling was a

.'•continuation of the shape of the fuselage. It was, there-'fore, not surprising under those circumstances that the•filling up of the slot, which was a horizontal and not a.vertical slot, had the effect of reducing resistance. But ifone filled up the slot of the Townend Ring the drag in-creased tremendously. One could regard that as being thefundamental difference between the N.A.C.A. cowling andthe Townend Ring.

Then Mr. Fedden has referred to the Douglas fittedwith the N.A.C.A. cowling. Mr. North did not knowwhether it fell within the definition which the N.A.C.A.themselves had laid down in their reports. When he hadsaid that the N.A.C.A. cowling caused other troubles as

Death of Fit. Lt. R. E. H. AllenIT is with the greatest regret that we have to record

the untimely death of Fit. Lt. Ralph Eric Herbert Allen,'A.M.I.A.E., M.I.Ae.E., R.A.F.O. After being knockeddown by a motor car in Whitehall on Friday, February 9,he died in Westminster Hospital on the following Sunday.Allen was a well-known and cheerful person in aviationcircles. He kept his own " Bluebird " at Han worth andflew consistently for his own pleasure and often, it is under-stood, for the Metropolitan Police. He was employed atScotland Yard as an assistant engineer where the extensiveknowledge he gained in the Royal Air Force of groundtransport vehicles made him very valuable. Born in 1892,he joined the R.N.A.S. in 1914, and served throughout thewar in France and on Coastal Defence duties. From 1919to 1925 he was an engineer officer and instructor in theR.A.F., and subsequently worked at the Air Ministry until1928. He also designed and patented many specialisedvehicles in connection with aviation. His loss will be feltkeenly both by his employers and his many friends ; tothem we offer our sincerest sympathy.

Mr. Shackleton's illnessHis many friends will be sorry to hear that Mr.

W. S. Shackleton recently had to undergo an operation.He is at present in the London Clinic, 20, DevonshirePlace, London, W.I, where he is, at the time of writing,progressing favourably, and there is, we are informed,no cause to anticipate complications. For the benefit of

well as heating, he was quoting Mr. Fedden. There again,the importance of that was that Mr. Fedden's figures,where he had shown that there had been overheating withthe N.A.C.A. cowling, arose from direct comparisons withsome installations of the Townend Ring, and certainly,from what he himself had said on the matter, he had foundthat the temperatures were considerably greater with theN.A.C.A. cowling than with the Townend Ring. But inthese matters one was not dealing with things that wereso precise that one could be compared with another.

No doubt there was a very large number of installations-which conformed to the original definition of the N.A.C.A.cowling, and which functioned well, particularly at thevery high speeds at which the machines were working.When Mr. Fedden had said that there was no engineer-ing difference between the Townend Ring and the N.A.C.A.cowling, because both had a forward component, Mr. Northwas unable to understand it. The mere fact that bothhad an upward component was only a sort of manifesta-tion of the pressure distribution and something which wasinevitable from the position in which the ring was placed.He did not think the fact that the Townend Ring gave alarge upward force and also gave a drag had any moresignificance than the fact that if one took a slice from astreamline body, the nose might have an upward com-ponent, although the back part had a drag ; it was aquestion of cause' and effect.

The interesting suggestions made by Mr. Townend withregard to methods of experiment would be very helpful.As a matter of fact, one did not find very much difficultyin hitting off these things in the wind tunnel now ; it wasmuch easier intuitively to know how to set about the jobthan to explain how to do it. He could give some figureswith regard to the bulbous nose which would surprise andinterest Mr. Townend. Some wings were tested by them-selves in free air. With the one-tenth scale polygonalring, using the bulbous nose, the drag was 18.26 lb., andwith the one-fifth scale ring it was 17.72 lb., showingpractically no scale effect. When the bulbous nose wasremoved, the drag was 31 on the one-tenth scale and 22on the one-fifth scale. So that there was increased dragwhen the bulbous nose was taken off and a very large scaleeffect. But he could not give an explanation ; this wasfar more Mr. Townend's province than his own.

He agreed with Mr. Hollis Williams' remarks with re-gard to the general steadying up of the flow when the ringwas functioning satisfactorily.

Finally, commenting on Mr. Fedden's emphasis of theimportance of the shape of the after body, Mr. North saidthere was a great deal in the paper which he had not hadtime to read at the meeting, and he, too, had referred tothe importance of the shape and size of body. He agreedwith Mr. Fedden as to the sort of effects produced whenthere was a badly shaped body behind a ring.

s sany of our readers who might like to make inquiries, thetelephone number of the London Clinic is Welbeck 4444.

Major Mealing's accidentMAJOR MEALING'S marriage to Miss Cournede has un-

fortunately had to be postponed owing to an accident,Major Mealing having fallen from a ladder and hurt hisspine. We hope that his injuries will prove to be lessserious than they seem, and that he will soon be ableto leave the Nightingale Nursing Home, Twickenham.

U.S.A. air mail contracts cancelledON February 9, President Roosevelt, through the U.S.

Postmaster General, issued an order cancelling all domesticair mail contracts on the ground that sufficient evidenceof collusion and fraud in securing them was believed toexist. The Army Flying Corps was placed at the disposalof the Postmaster General for the carrying on of the airmail services. Col. Lindbergh sent a telegram to thePiesident, in the course of which he said: "Your ordercancelling all air mail contracts condemns the largest por-tion of our commercial aviation without a just trial." I t" does not discriminate between the innocent and theguilty, and places no premium on honest business."According to Col. Lindbergh, America has been leadingthe world in commercial aircraft, engines, equipment, andorganisation of air lines, and the greatest part of this pro-gress has, he says, been brought about through the airmail, subsidised by the Government.

158

February 22, 1934 Supplement to FLIGHT

FLIGHTENGINEERING SECTION

Edited by C. M. POULSEN

No. 97 (V°£™ e2

I X ) 9th Year February 22, 1934

CONTENTSPage

Engine Cowling. By J. D. North, F.R.Ae.S., M.I.Ae.E 9Ethyl. By F. E. Backs. O.B.E.. F.K.Ae.s.. M.I.A.E.. M.Inst.P.T.,

M.S.A.E 14Technical Literature—

Summaries of Aeronautical Research Committee Report s... ... 16

ENGINE COWLING

By 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 interests

in the patents covering the Townend Ring, and havedevoted much attention to the development of thisdevice, 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 SectionsThe section of a Townend Ring is an aerofoil section

in 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 plainplate type, presumably because the mean curvature is

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 ChordThe best angle between the chord line of the ring

section 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,

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FIG. 9

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174 a

SUPPLEMENT TOFLIGHT

FEBRUARY 22, 1934

THE AIRCRAFT ENGINEER

useful 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 that there is a range ofchord angle from —2 deg. to —6 deg. over which thedrag is practically constant, and that at —8 deg. thedrag starts 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. With 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 TownendRing relatively to the body. The results relate only toone particular type of ring used on a particular form ofbody, and even for that case do not extend sufficientlyfar to prove that position 1 is the best possible. Experi-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 degree

of 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 whichhave 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 three different rings, (a) is a ring havinga 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-haust 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. With 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). I t may be remarked that 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 engine. Tested on a streamlinenacelle, the measured drag using the polygonal ring was

FEBRUARY 22, 193411

THE AIRCRAFT ENGINEERSUPPLEMENT TO

FLIGHT

FIG. 12.SECTION OHMTUW UNg f T P f t ^ - I

found to be slightly less than that with the circularring. For a nine-cylinder engine 54 in. in overalldiameter, the equivalent 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 advantage. Tesits made with theairscrew running show that slipstream does not adverselyaffect the superiority of the polygon.

Body ShapesSo many practical considerations govern the design

of 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 Ring. 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 rapidity. What the body form further 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 aresolids 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. 13

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FEBRUARY 22, 1934

THE 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 InterferenceThe effect of the Townend Ring on the drag of an

engine 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 where, 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 that 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 backthat the leading edge must be mutilated to clear theengine itself.

174

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 CoolingComparison of a typical Townend Ring with any other

form 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 ringwhich this implies, involves some reduction 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 ̂ has shown that the TownendRing does give cooling superior to that 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

FEBRUARY 22, 1934 U

THE AIRCRAFT ENGINEERTO

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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 tunnel cooling arrangement. Two curves, whichare very nearly identical, relate to the temperatureswith and without a Townend Ring. The absence of anyappreciable difference in temperature under the twoconditions may be explained by the artificial coolingconditions. Attention 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.

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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 appreciably. 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 aircraft, haveshown that this irregularity, though always present,varied from flight to flight, and it was quite usual tofind that a cylinder which had developed an abnormallyhigh 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 unsatisfactory.Tests were accordingly made with inter-cylindeir bafflesaround cylinder No. 3, which also half encircled cylinders

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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 that No. 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 third test, 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. Whereverthe 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

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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 the Townend Ring rarely arise, except in the rapidly-diminishing number of cases in which the cooling marginof 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 be concluded.)

ETHYL

BY P. R. BANKS, O.B.E., F .R .AE.S . , M.I.A.E.,M.INST.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 dye which 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, any leakBwhich 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 ornot. 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 any water 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 the modern 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

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MARC H 29, 1934

THE AIRCRAFT ENGINEER

ENGINE COWLING

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

(Concluded from page 14)

Townend Rings on Pusher EnginesA Townend Ring will effectively reduce the resistance

caused by an engine fitted at the rear of an aircraftbody. For use under these conditions the ring chordangle requires to converge fairly rapidly to the rear,whereas for the more usual type of installation the con-vergence is almost invariably in the opposite sense. 4ring used on such an installation is subject to a largftdown-wind force, whereas on the normal installationthere is a large up-wind force. A reduction in drag inthis case must be associated with large increases in thepressure on the rear end of the body within the ring, butthe essential characteristic of the Townend Ring—thatof producing a downwash which prevents the flow frombreaking away—remains unaltered.

In tandem installations Townend Rings have beenused on both front and rear engines. It appears that inthese cases the reduction in drag which is possible' ismainly that due to the front ring. In certain cases theuse of a Townend Ring on the front engine of a tandempair is stated to have very considerably improved tl.iecooling of the rear engine—a point of very considerableimportance. In view of the effect of the ring in pre-venting a breakaway of flow behind the engine, thiseffect is not altogether surprising.

Model Tests and Full-Scale Results on TownendjRingsThe Townend Ring owes its origin to investigations

carried on in the wind channel with small-scale models.There is inevitably some doubt as to the direct applic-ability of the results of such tests to the prediction ofperformance for the full-scale aeroplane.

Full-scale tests of Townend Rings have now been madein sufficient numbers to place it beyond doubt that theeffect shown by small-scale models also occur in the full-size aeroplane, and there is enough evidence to showthat the magnitude of the saving in resistance shown bymodel tests is of the same order as that indicated byfull-scale performance tests.

Models used for Townend Ring investigations requireto be a fairly large scale. Our own experience indicatesthat a scale of one-fifth with a wind channel speed of60 ft./sec. is sufficient for most purposes, but that it isdangerous to go to a much smaller scale.

I t is very common practice to represent the engine onsmall-scale models by a conventionalised dummy enginewhich is arranged to give the known or assumed dragof the real engine. Such conventionalised models cannotsafely be used with a Townend Ring on any scale, be-cause the change in drag caused by a Townend Ringdepends on the exact pattern of the air flow caused bythe engine, and not by the engine's absolute resistance.The representation of engine installations by conven-tionalised models is usually extended to cover cowlingdetails, and air passages through cowlings are either notrepresented at all or are replaced by a few passagesdrilled through the body block. Tests which hare beencarried out comparing the resistance of a representa-tive model of a complete engine with the normal typeof cowling accurately represented, invariably shows verymuch higher drags for the normally-installed enginethan are given by the usual t3pe of conventional repre-sentation.

Many comparisons between the results of model testsmade by my firm and of full-scale tests showing theeffect of Townend Rings are available. Where the full-scale tests show directly the change in performance dueto the fitting of the Townend Ring, the correspondencebetween model result and full-scale test has generallybeen very satisfactory. Where estimates of the effectof fitting Townend Rings have had to be made from the

estimated drag of the unringed machine, no check onthe real drag being available from a full-scale testwith the ring omitted, the maximum speed obtained hasfrequently fallen short of the designer's hopes, but thischaracteristic is not confined to aircraft with TownendRings, and should not be taken as evidence of theirfailure to produce, full scale, the saving in drag shownby model tests.

In a figure shown earlier, the model results of varyingthe chord angle of incidence of the Townend Ring wereshown. In order to obtain if possible full-scale con-firmation of these results, a variable angle TownendRang, precisely similar to the model, was tested fullscale.

Fig. 20 shows the estimated variation in performancewith ring angle, and that measured full scale. I tshould be noted that the full-scale results plotted arethe mean of six speed readings for each ring chordangle. The total estimated change in speed between noTownend Ring and Townend Ring at its most effectivesetting is 15.5 m.p.h. The variation between the vari-ous measured full-scale speeds at each individual ringangle vary between 2 and 7 m.p.h. I do not regardthese particular tests as showing any abnormal degreeof variability.

Total Power Plant Drag with the Townend RingIt may be interesting to compare estimates of the

drag necessary for cooling with results which haveactually been obtained with the Townend Ring. Theestimate made by Pye, which has been earlier men-tioned, that at 150 m.p.h.. about 1.5 per cent, of thetotal engine output must necessarily be used in pro-viding engine cooling when the temperature differenceavailable for heat transfer is 180 deg. C , relates only toideal conditions which we can scarcely hope to approachin the air-cooled engine, even if the suggestion whichhas once been made of using streamline sections insteadof circular ones for engine cylinders were found to bepracticable.

McKinnon Wood, in a paper to which reference hasalso been made, has suggested a method of cooling whichis certainly practicable, involving the enclosure of thecylinders in ducts which confine the cooling air flow toa path closely following the cylinder contour and em-bodying a fan so that the cooling is not dependent onthe air speed of the aircraft. The use of the fan isindicated as preferable on account of the claimed self-regulating characteristics of this cooling system, and ifaircraft of much higher speeds than are at presentcommon are to be attained with engines having charac-teristics not greatly different from those of existing

306/

MARCH 29, 1934 23

THE AIRCRAFT ENGINEERSUPPLEMENT TO

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type*, fan cooling may become necessary and does notappear to be inconsistent with the use of ring cowlineof the Townend type.

Assuming a mean temperature difference of 250 deg.C. between cylinders and cooling air, McKinnon Woodestimates that, allowing for fan losses, 4 pea- cent, ofthe b.h.p. will be used in providing cylinder coolingalone. A mean temperature of 250 deg. C. betweencylinders and air is certainly not available in themajority of engines to-day in service. The maximumpermitted temperature at a position on the cylinderheads, which is certainly above the mean temperatureof the cylinders as a whole, is normally between 215 deg.and 235 deg. C. For a mean temperature difference of200 deg. C, McKinnon Wood gives the power absorbedin cooling cylinders as 10 per cent, of the b.h.p. Even200 deg. C. is almost certainly higher than the meandifference which can be permitted with engines of exist-ing type, but we may take this figure of 10 per cent, asrepresenting about the figure which might be achievedin practice with a cooling system of the type suggested.

The best result within my knowledge which has so farbeen obtained, using a Townend Ring, is that given by54-dn. diameter nine-cylinder radial engine mounted ona streamline nacelle of 42-in. maximum diameter, fittedwith a polygonal Townend Ring, and having a totaldrag at 100 ft./sec. of 27 lb. This result has alreadybeen referred to (Fig. 12).

With an engine developing the maximum b.h.p. of480 at 4,000 ft., the speed of the aircraft fitted withthis installation was 140 m.p.h. true (132 i.a.s. at4,000 ft.), and the power absorbed by a resistance of27 lb. at 100 ft./sec. at this speed is 43.5 h.p., or 9 percent, of the engine b.h.p. When an engine of thesame overall dimensions and type, but supercharged togive 600 h.p. at 5,500 ft., was fitted in place of thelower-powered engine, the speed of the aircraft in-creased to 160 m.p.h. true (148 m.p.h. indicated at5,500 ft.), the engine installation drag being unaltered,and the power absorbed in overcoming engine dragbecame 54 h.p.—still 9 per cent, of the total.

Allowing for airscrew efficiency, the drag of theengine installation in this case at top speed is about11.5 per cent, of the total drag of the aircraft.

I understand that Messrs. Armstrong Siddeley Motors,Ltd., have measured a total drag of 35 lb. at 100 ft./sec. for the fuselage, engine and Townend Ring of amilitary machine with open cockpit, wind screen andpilot. The engine in this case was a " J a g u a r "capable of developing about 500 h.p. at sea level. Thisis 8 lb. in excess of the nacelle figure above quoted, andit is certainly not to be expected that the resistanceof the fuselage alone, engine removed and a faired nosesubstituted, could reach so small a figure as this. Thedrag to be attributed to the engine in this case musttherefore have been less than in the examples which Ihave considered.

EVAPORATIVE COOLING

By R. HALEY*

EVAPORATIVE cooling in aircraft engines has undoubtedlymade great progress in the past five years, and theattempt to improve the efficiency of the air-cooledengines by the addition of the Townend Ring andothers has no doubt hastened the efforts of the water-cooled engine manufacturers to improve their system ofcooling.

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Fig. 1 : Professor Gibson's Experiment.

A practical method has been developed for increasingthe capacity of a cooling fluid for heat dissipation byusing the latent heat of vaporisation.

The greatest difficulty to be overcome on any typeof I.C. engine is to keep the working temperature level.Temperatures should be kept high, but within the limitsof effective lubrication. The most desirable conditionis a uniform high temperature, in all parts of theengine. That is to say, an important feature of thecooling system is to keep the engine hot.

In an aircraft installation, fitted with evaporativecooling, the control of temperature, external to thesystem, is of course impossible, due to varying condi-tions of flight, but some form of control over the cool-ing system fitted is necessary, and the method adoptedwill be described later.

In this country the cooling system is vented to theatmosphere, but in America they are using successfullyan unvented system, i.e., a pressure gauge is fitted, setto 5 lb. per sq. in.

Before describing the layout of an evaporative cooledsystem it will be interesting to note the results ofcertain experiments carried out by Pirof. Gibson in 1910and reported to the Institute of Engineers and Ship-builders.

The tests consisted of measuring the surface tempera-ture of an iron vessel in which water was heated by agas flame. The water was vigorously stirred during theexperiment, and it was found that the temperature ofthe surface actually decreased as boiling point wasreached.

Fig. 1 shows graphically, on a base of water tem-peratures, the results he obtained. As the tempera-ture of the water increased, that of the vessel increased

Mr. Haley is on the Technical Staff of the Gloster Aircraft Co., Ltd. Fig. 2 : Diagram of the Antoinette cooling system.

306 g