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Aviation History
1941
1941 - 0629.PDF
MARCH 13TH, 1941. AIR-COOLED v. LIQUID-COOLED AIRCRAFT (Continued) and it now seems almost impossible to design an engine which may be wholly housed within a wing and which will have sufficient power to be atti active. If the power of these "fiat" engines is not really large, higher per- formance can be obtained in an airplane using larger engines of orthodox type. The tests in Fig. 4 show, how- ever, that considerable improvement can be obtained by mounting '' flat'' engines horizontally in front of the wing, or by housing radial engines partially within the wing Summarising the material presented so far, it is apparent that .increasing the diameter of a fuselage or nacelle of given length results in an increase in drag, due principally to the increased skin friction area and to the interference effects upon the wing. This increase in drag is very much smaller than the increase in area, but is sufficient to count in favour of a compact iiquid-cooled engine. Cooling Meredith* and others have shown that if air is admitted to a passage at high speed, expanded through a diffuser to a condition of low velocity and high pressure, and then heated, the heat energy imparted to it may be recovered as thrust, if the air is discharged rearwardly through a nozzle or gill. This is the familiar "ducted" radiator of the liquid-cooled engine It is also the air-cooled engine itself. It makes no difference what the source of heat is, whether it be radiator, engine, or open flame, the same principles apply. To quote Meredith* specifically: "By correct design of low velocity cooling systems, in which the surface (whether in the form of a honeycomb radiator or of the fins on the cylinder heads and barrels) is exposed in an internal duct, the power expended on cooling does not increase with the speed of flight but ... on the con- trary, it should diminish to the vanishing point at a prac- ticable speed, beyond which the cool- ing system contributes to the pro- pulsion." The only difference between the radiator and the air-cooled engine is that the radiator can be fitted- with duct work without modification of the remainder of the cowl, and hence engineers found this a relatively simple change to make. It was made first. Meredith's principles have been re- cently applied to the air-cooled engine, and the problems appear not to be particularly difficult. As the speed of airplanes increases, the effectiveness of this method of energy recovery im- • F. W. Meredith: "Cooling of Aircraft Engines." ^— R. & M. 1683, August, 1935. Fig. 4. Nacelle dra? and drag coefficients(no cooling airflow—approximate Reynolds Number 3 x 106). Fig. 5. (Below) Variation of nacelle dragcoefficient with wing thickness. 1.2 V) u°,.o V ARE A NT DRA G s3 O .04 j / ! A 1 • y V MINIMU M ARE A \ FO R PILO T < .— • > so* 2 1 \ »— f «** tI• O LZUi JO *-— .»•—' 1 32 26uj 22 S 16 12 ^ a s 1 FRONTAL AREA (S)-SQ FT. Fig. 3. Drag of total nacelle and fuselage. proves. The important point is not whether the air-cooled or the liquid-cooled engine has the lower cooling drag, but that the cooling drag of both is being rapidly reduced, and may ultimately become zero. This will occur when the heat energy recovered equals the duct loss. Any system of heat energy recovery from the engine requires heat transfer from the engine, ultimately, to the cooling air. Heat transfer, to be efficient, requires that the heat be available at the highest possible temperature. Hence, the high temperature air-cooled cylinders are intrinsically more effective for heat energy recovery than the lower temperature coolant radiators. It should be noted also that only a minimum amount of cooling air should be wasted. This is where the "ducted" radiator has had MINIMUM C. DRAC-LB. *"'»• .034 .034 9.36 0 .089 .045 .049 .024 .013 I—10 34--^ CQTR .057 MIT AERO REPORT-4ir AIRFOIL-N A C A OOI5' 41.8 MPH AT StALEVEL 75 FT WIND TUNNEL cos = 03 I 2 <z CURVC PERFO OF Vt RUAN tLU» ;c CA / USED IN.CULATI0N3 • r"" ^ y% • V - t •ROM no, 4) • 0 0.2 04 0 6 0 8 10 12 14 16 18 i.0 2 2 2.4 2.6 2.8 NACELLE DIAMETER/ WING THICKNESS an advantage in the past. The baffles of the air-cooled engine have not been good enough. Improvement in tho internal cowling and baffling of air-cooled engines is under way. In connection with the general problem of heat energy recovery, the smoke-flow photographs taken by Mr. Roger Griswold, II, and shown in Fig. 6, may be of interest. The flow shown in these pictures is two-dimensional outside of the model, but three-dimensional inside, with the direc- tion of flow from top to bottom. Two conditions of flow around a typical engine nacelle are shown ; that on the left represents the condition where no air is passing through the engine, and the flow around the exit gills produces a considerable suction. That on the right shows the same arrangement where air is admitted to the nacelle under
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