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Aviation History
1941
1941 - 0630.PDF
2O8 MARCH 13TH, 1941. Fig. 6. Two types of flow. On the left, no air is passingthrough the engine. On the right, air is admitted under pressure. pressure through the back wall of the tunnel and. is allowed to flow out through the exit gills. Thus, the left-hand flow represents the limiting condition where the air stream is giving up energy to do work in sucking air through an engine, while the right-hand flow represents the condition where energy is supplied to the cooling air by means of heat or by a blower, or any other device, so that the air issuing from the exit gill has substantially the same velocity as the main stream. As might be expected, the left-hand flow results in a considerable disturbance, while the right- hand flow is smooth. The condition on the inside of the cowl differs in the two pictures; that on the left contains smoke, that on the right air. This fact has no significance in the comparison. Photographs of this sort cannot be used for any quantitative calculation, but they do indicate that as we learn to handle the flow of cooling air through an air-cooled engine, the power required to accomplish cooling will ultimately be reduced to a very small figure indeed, with no harmful effect on the external drag of the remainder of the airplane. Another of Mr. Griswold's smoke-flow photographs in Fig. 7 shows the air entering the model more clearly than in Fig 6. Notice in particular how the streams approaching the model become wider, which indicates that they are slowing down. From this photograph it appears that the velocity of flow has been reduced about 50 per cent, before it enters the N.A.C.A. cowl. Thus, we have obtained a virtually perfect diffuser, without providing either space or weight. This model also was fitted with internal smoke jets to define the internal flow ; it is not believed that these jets are of sufficient power to affect the external flow. Throughout the remainder of this paper the cooling power for both air-cooled and liquid-cooled engines has been arbitrarily taken as 3 per cent, of the engine take-off rating. This value represents a conservative average from Pratt and Whitney two-row radials. The later engines have I 38 8 37 AIR-COOLEDvs. LIQUID-COOLED UNDER IDENTICAL CONDITIONS u. . y .36 34 — .33 A 1 R-CO DLEO-* Llqu M —DET | X K ONAT / M ON LI 7 tn-' / 130 MO 160 180 200INDICATED MEAN EFFECTIVE PRESSURE LB/SQJN 220 AIR-COOLED v. LIQUID-COOLED AIRCRAFT (Continued) Fig. 8. Best economy fuel consumption tests (P and W 1830single-cylinder engine). Fig. 7. The streams approaching the model become wider;indicating that they are slowing down. The velocity has been reduced about 50 per cent. values of approximately 2 per cent. Furthermore, this assumption does not make use of Meredith's heat energy recovery, which, as has been shown, will reduce the cooling drag of both types. ; . Fuel Consumption In any discussion ol aiT-cooled and liquid-cooled engines, the question of the relative fuel consumption always comes up. It is recognised by all that the fuel consumption under full throttle conditions of the air-cooled engine is materially poorer than for the liquid-cooled engine. However, no? engine, whether military or commercial, flies at wide open throttle for more than a very small fraction of its total time. Even the pursuit plane must spend the majority of its time looking for its enemy, or getting from its base to the probable scene of operations. The significant fuel consumption, then, is the cruising fuel consumption. Much conflicting data has been presented on this sub- ject. Since the compression ratio, supercharging, mechani- cal efficiency, and several other factors influence the fuel consumption, it is almost impossible to compare a group of liquid-cooled engines with a group of air-cooled engines and obtain any generalised result. In an attempt to eliminate these extraneous factors, Fig. 8 may be of inter- est. Fig. 8 shows the indicated specific fuel consumption of two single-cylinder engines run under identical condi- tions ; in fact, they are really the same engine. In the lower curve the engine is air-cooled. In the upper curve the fins were machined oft the cylinder, a liquid-cooled jacket substituted, and the same identical cylinder retested as a liquid-cooled engine. The tests were run at the same speed, valve setting, spark advance, and all other control- lable conditions. It is apparent that the air-cooled cylinder is definitely more economical than the liquid-cooled,.irrespective of the mean effective pressure. Note that the fuel consumptions are based on indicated horse-power, that is, brake-horse- power plus friction-horse-power. If the fuel consumptions are divided by the mechanical efficiency of the engine, which may be of the order of 85 per cent., specific fuel con- sumptions will be obtained close to the usually quoted values. It may also be noted that these tests were run two years ago, and considerable improvement has since been made in the detonation limit of air-cooled engines. It may be appropriate to remark that the United Aircraft Corporation has developed liquid-cooled experimental engines continuously over a considerable period of years. Consequently, it is justifiable to state that the data shown in Fig. 8, plus fuel consumption tests on a number of complete engines, both by United Aircraft and by others, lead inevitably to the conclusion that, under cruising con- ditions, the air-cooled engine is definitely more economical than the liquid-cooled. However, in making this study, the same fuel consumption figures have been used for both types, which is probably justified, since it will permit a
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