FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1934
1934 - 0134.PDF
FLIGHT, FEBRUARY 8, 1934 practicable t<> enclose the engine completely, and a free exitfor air, exhaust, and oil was provided by leaving a large gap 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 and by the Caudron company to Anzani radial air-cooledengines. Essentially the method of reducing drag consisted in 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 hence should have been easily cooled. In a cowling with rearwardly facing air outlets sur-rounding a rotary engine, the engine may act as a rotary blower, inducing a rearwardly directed flow of air at highvelocity, producing a measurable thrust, and effectively decreasing 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 of high output led towards the end of the war to an increas-ing use of water-cooled engines and to a partial eclipse of air-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 with cowling which had little effect in reducing drag. It wasrealised that the power wasted in driving radial engines through 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 to blend 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 to that of the single streamline casing enclosing the wholeengine. This cowling is capable of reducing resistance substantially, 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. The engine 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 that frontal area is only a measure of resistance in the case ofgeometrically similar solids of revolution. It was enclosing the Curtiss engine in a body of good shape and of smallsurface area, rather than of small cross-section which was of importance. The last five years have seen the development in Englandof the Townend Ring, and in America of the original form of N.A.C.A. cowling which, when properly applied, are ableto reduce the drag of a radial engine to values only a fraction of those previously attainable! without prohibitiveinterference with cooling. There can be little doubt that to the introduction of satisfactory forms of low drag cowl-ing, the radial air-cooled engine owes a renewed lease of life. Engine Cooling It will be clear from the historical outline given that thefactor 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 would have been adopted at a very early stage were it not for thenecessity of cooling engines. The theoretical aspects of cooling 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 be affected by change in temperature of the air flowing overthe surface. But it has not become practicable generally to use wing or body surfaces for engine cooling, and theadded resistance of special cooling surfaces must be tolerated. For any given form of cooling surface the rateof heat dissipation depends on the temperature difference between 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 the boiling point of water in water-cooled engines and of per-missible operating cylinder temperatures in the air-cooled case. 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 and remain at full throttle during acceleration on the ground,during climb to operating height, and may remain at full throttle during full-speed flight. At the start, the temperatures are substantially belowtheir steady normal operating level, and the effective heat reservoir afforded by the power plant as a whole preventsexcessive temperatures being reached during the initial period of a flight in which the air speed of the machine isvery low. The condition which will determine the area of cooling surface is the heat dissipation obtainable at theminimum speed of flight which can be maintained at full throttle for any appreciable period. For practical purposesthis condition is generally that of a continued climb at maximum climbing speed. Pye's estimate of the power expended on cooling a500-h.p. engine on an aircraft flving at 150 m.p.h. is fi.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 different positions 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, is the 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
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
