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Aviation History
1934
1934 - 0135.PDF
FLIGHT, FEBRUARY 8> 1934 engines is easily ascertainable—the proportion of waste heatcarried off by exhaust gases, by the lubricating oil, and the remainder to be dissipated mainly by the cylinders isnot known, and there is evidence that the proportion of total 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 coolers and 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 radiator showed 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., and if the whole of the waste heat rejected through the cylin-der walls was required to be dissipated by the radiator during 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, and tend to become increasingly excessive as the performanceof aircraft increases, but increasing speed is normally attended 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 latent heat of evaporation of the same fluid permits of the dissi-pation of heat at a greatly increased rate accompanied by a relatively small loss of water by evaporation. Thusthere is an effective heat ballast system which permits a temporary rate of heat rejection in excess of the rate ofdissipation from the cooling system without temperatures rising to beyond permitted limits. Cooling systems are normally arranged in the airscrewslipstream, whose velocity is more nearly constant than the air speed of the machine itself, and the effective cool-ing velocity does not vary over such a wide range as the machine speed. The effect of airscrew characteristics oncooling problems is considerable. Geared airscrews of large diameter reduce the assistance to cooling given by slip-stream. Behind the boss, and for some distance outward therefrom, the added velocity due to the airscrew has anegative value—which may extend over 0.2 of the whole diameter, and the increase in airscrew diameter followingthe use of reduction gears, with the consequent increase in the area shielded by the centre of the airscrew, is a factorof considerable importance as regards cooling of radial engines in particular. The nature of the permissible limits of temperature of anengine installation enters into the question. There is an upper limit for cylinder heads at which mechanical failureoccurs after a very short period. Below, there is a whole range of operating temperatures, each corresponding tosome life between overhauls of the engine. Temperature limits 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 temperatures for a few minutes during climb, followed by a period ofcruising flight at substantially lower temperatures, is found to 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 which is needed for climb and for high-speed level flight to some-thing considerably less than would appear to be necessary on first consideration. It is generally true that the cooling which has to beprovided on an engine installation is determined by the cooling required on climb, that this cooling system pro-vides excessive cooling capacity and consequently excessive drag at higher speeds. The importance of drag of coolingsystems increases very rapidly with increase in the ratio of climbing to maximum level air speeds, since power ex-pended on such drag varies as the cube of the speed and is 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. The cylinders 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 in which 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 in which 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 radial installation 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 by other means. The figures do not represent the effect, onotherwise identical aircraft, of a change in cowling, as the change to low drag cowling will cause an increase in maxi-mum speed, but indicate directly the change in power spent in cooling on aircraft designed for equal top speed,and therefore of equal total drag, the saving due to low drag 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 actually installed would operate still further to reduce the drag, andconsequently still larger economies in power expended would become possible. If the maximum speed of an air-craft with the higher drag cowling of the figure be 175 m.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 aircraft with low drag cowling is operated at 175 m.p.h., the cool-ing drag falls to 20 per cent, of the total for the original machine 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 reliability need only be mentioned. Means whereby Cooling Resistance may be Reduced Ideal cooling conditions assumed by Pye in the estimateof power necessarily absorbed in cooling, are not generally obtainable. 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 field of utility. The increase in the mean temperature at whichheat is dissipated to actual boiling point reduces the total surface required, and permits the use of the leading edge ofthe wing alone, and the low density of steam—as compared to water—greatly reduces the weight of cooling fluid in 135 :->~-v - r,\.. "-•
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