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Aviation History
1941
1941 - 0633.PDF
MARCH 13TH, 1941. 209 AIR-COOLED v. LIQUID-COOLED AIRCRAFT (Continued) certain amount of wide open running, at which time the liquid-cooled engine has the better economy. Weights ' In studying the comparative weights of air-cooled and liquid-cooled installations, the actual weights of a consider- able group of engines and their installations were investi- gated. This resulted in a mass of data, much of it confi- dential, but in all events too extensive to be presented in this paper. In addition, many of the units could not be ^assigned permanent weight values. The propeller size and weight, for instance, depends upon the speed and type of the machine, the engine gear ratio, etc. Consequently, the weight data have been summarised in Table I, giving the limits within which the unit weights were varied throughout this study. Whenever possible, actual weight values were used. It will be seen that some of the weight values varied over rather wide limits. The liquid-cooled engine weights, for instance, have a range of about 15 per cent. This is because some engines included two-stage gear-driven superchargers TABLE I Comparison of Engine Installation Weights Engine Weight : Air-cooled Liquid-cooled Engine dry weight—Ib./h.p.V.. — • l.'ifi —1.85 1.10—1.2(5 Engine power—h.p./cu. in.* ... ..". O.f>6 —0.75 0.70—0.81 Power Plant Installation Weight:Propellers and controls 0.302—0.320 0.302—0.320 Starting system (less batteries) 0.020—0.045 0.020—0.045Engine mount .'. 0.045—0.080 0.045—0.080 Cowling ... -0.003—0.100 0.06S—0.100Oil system (inc. tanks) ... 0.040—0.060 0.040—0.060 Fuel system (less tanks) ... ... 0.020—0.040 0.020—0.040Exhaust system • 0.027—0.040 0.027—0.040 Radiators and coolant — — 0.275—0.300Miscellaneous** 0.015—0.040 0.015—0.040 Total normal P.P. last. Wt.—lb./h.p 0.532—0.725 0.807—1.025 Add'tiona! Power Plant Installation Items :Intercoolers and ducts 0.090—0.095 0.090—0.095 Turbo-supercharger install. ... 0.100—0.132 0.109—0.132Gearing shafting— twin prop.*" 0.344—0.354 0.344—11.354 Gearing, shafting—single prop.*** 0.072—0.086 0.072—0.080 Notes:* These values used when manufacturer's ratings were not available. Engine power is take-off rating. •• Includes diaphragms, carburettor air intake ducts, power plant controls, etc. •** Twin-propeller installation is for tractor airplane with right angle drives. Single-propeller installation is for pusher airplane with extension shaft. while others relied upon turbo installations. In all cases, the individual weights were picked with due consideration and with every effort to make them strictly comparable. As an off-hand comparison between the weight of air- ^•Cooled and liquid-cooled installations, it is interesting to compare the average values from Table I. All figures are expressed in pounds per take-off horse-power rating. The average air-cooled engine weight is 1.31 lb./h.p. and its installation weight is 0.63 ib./h.p., making an average in- stalled weight of 1.94 lb./h.p. The corresponding figures for the liquid-cooled engine are 1.18 lb. /h.p. for the engine dry, and 0.91 lb./h.p. for the installation, making an aver- age installed weight of 2.09. This is some 7.5 per cent, heavier than the air-cooled installation. Comparison with Present Practice The evidence presented thus far indicates that the cool- ing power and the fuel consumption of the air-cooled and liquid-cooled engines may be taken as substantially equal, but that the liquid-cooled installation has a certain advan- tage in drag and the air-cooled has a corresponding advantage in weight. It is not very satisfactory to try to evaluate the one advantage in terms of the other. • For example, weight saved in the engine installation permits a weight saving in the structure ; both savings permit a smaller airplane, which has less drag, and hence requires less fuel; this in turn permits another weight saving, and so on. The only practical method is to postulate a group of comparable airplanes, each designed from the ground up, in accordance with the proper area, the proper fuel load, etc. This is a tedious method, involving a consider- able amount of "cut and-try," but it is the only method TABLE II Drag Coefficients of Conventional Modem Pursuit Airplanes Airplane Air-cooled Pursuits Drag Coef. Virtual Drag Coei.* A .mini • .021a B .02311 .0258 C .<<2ifl .023!) D .0224 .0210 E .0173 .0164 Average Air-cooled values ... — .0217 Liquid-cooled PursuitsF .0173 .0176 G .0107 .0178H ; .(UH8 .021! I .0224 .02(12K ... .0224 .0220 Average liquid-cooled values — .0197 * " Virtual drag coefficient " represents the normal drag coefficient adjusted to acommon value of wing-loading of 27.0 lb./stj. ft. which includes all the interacting factors, and is the method used in this study. Before going on to this generalisation, it is advisable to check the values of weight, drag, etc., which have been presented, against actual present-day performance. This has been done for the pursuit type airplane. In Table II the drag coefficients of five conventional modern air-cooled pursuit planes and five conventional modern liquid-cooled pursuit planes have been tabulated. These figures have been obtained from flight test, and are "all-up" figures, including the drag ol full military equipment and the thrust of exhaust jets, when present. The engine power was known, and only the propeller efficiency required estima- tion. For obvious military reasons it is impossible to iden- tify these airplanes, or to quote the particular speeds and powers involved. Some are foreign and some domestic It is believed that the figures are reliable. The drag figures are presented as coefficients based upon wing area, in the manner of recent N.A.C.A. reports. How- ever, since the pursuits all had different wing loading, these coefficients are not strictly comparable. Airplanes with high wing-loading tend to show abnormally high drag coefficients. Accordingly, the speeds of all airplanes were corrected to a common wing-loading of 27.0 lb./sq. ft., the average of the group, and the drag coefficients were recalculated, based upon the new wing area and speed. These are the values quoted in Table II as " Virtual Drag Coefficients," and are strictly comparable, so that they can TABLE III Weight and Drag Analysis 0? Hypothetical Pursuit Airplanes . . Air-cooled Liquid-cooled Engine rated power, b.h.p. at 10,000ft. ; - 1,250 1,150 Gross weight, lb ... 7,000 7,000 Wing loading, Ib./sq. ft ... 27.0 27.0 Wing area, sq.ft. (1) 25,!) 250Tail area, sq. ft. (25°,, wing nrea) «"> 05 Max. speed at 20,500ft., m.p.h 363 !iO4Fixed weight, 1b. (2) ],r*0 1,500 Structural weight, Ib. (8.0 Ik'sq. ft.) (3) 2.070 2,070Power plant installation weight, lb. (4) XIO '1,000 Fuel and oil weight, lb. (.70 lb./h.p.) (5) S70 -811) Engine bare weight, dry, 1b. (0) 1,750 1,5:50 Gross weight, ib .* ... 7,000 7,000 Power loading, Ib./h.p 5.0 (i.1 Average dragcoefficient, CD 0217 .011)7Equivalent drag area, f = Ci,S, sq. ft. 5.02 5.10 Max; frontal area, ihclud. cockpit windshield, sq. ft. 10.3 12.8 Item : Equivalent drag area, I SfccioT InCHHog roughness, gaps, etc jjj»» »•• Fuselage (C,, — .0(15) ... • 1.540 l.SifiMiscellaneous .7fi0 .700 Inter-cooler .. ..- ... ... .-. ... ,175 • —Oilccoler ... ' .025 .025 Cooling at 3% ... ... 104 .140 Total drag area, f 0.62 5.10 (1! Aspect ratio 5.70. (2) Fixed weight, includes pilot, armour, 4 machine gnus, ammunition, controls,radio, furnishings, and mAtrimiei.t^. (3) Structural weight include- \viiii;> tail fuselage, landing gear, and rontrol system. (4) Power plant installation weight .fij Ib./h.p. for air-cooled and .115 Ib./li.p.for liquid-cooled. Includes engine mount, inter-cooHrs. cowling, exhaust system, engine accessories, ducts, bullet-proof tankage, fuel and oil systems, a.ul coolant andradiators on liquid-cooled type. (5) Fuel and oil based on .040 lb./h.p.-hr. oil consumption and ..r>'> ib. '!i.p.-hr.fuel consumption at 50% rated power for 2.0 hr. (6) Engine bare weights based on typical two-stage air-coo'cd and two-speedliquid-cooled engines of 1.40 lb./h.p. and 1.33 Ib./h.p. respeel ively.
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