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Aviation History
1944
1944 - 2011.PDF
SEPTEMBER 28TH, 1944 FLIGHT AIR-COOLED ENGINES to resist bending loads, but to be reasonably light it must also have a large bore. Such a pin is therefore lacking in resistance to ovalling and may fail from this cause, quite apart from the bad effect on the piston. British designers prefer to use a short, stiff gudgeon pin of com paratively small diameter. •A sleeve-valve engine piston is at some slight disad vantage in that cooling through the cylinder is less effective because of the double oil film and the sleeve. This must not, however, be exaggerated and the sleeve is, in fact, surprisingly transfusive of heat. Because of the inherently lower lateral heat flow on a sleeve-valve engine the increase in the severity of the cooling problem with increase in bore is not so great as on a poppet-valve type. Oil jets play on the piston-pin bosses, and as these are a highly stressed part of the piston this is quite a good feature. However, with large bores more direct cooling iMf the crown may be found necessary, and vve must expect Nfcrilled connecting rods feeding a jet of oil on to the centre of the piston if bores exceed, say, 6J or 7 inches. Salt-cooled pistons have been proposed. There are prac tical difficulties with corrosion and erosion, but in any case, as I see it, they are fundamentally bad as they increase the temperature of both piston skirt and rings which are already hot enough. The right solution for big pistons is TABLE I—MAXIMUM B.H.P. PER SQ. INCH PISTON AREA Current Engines using 100 Octane Fuel Engine Sabre Merlin Griffon Wright Tornado P. & W. R2800 .. Wright R3350 .. Hercules Bore (in.) 5.00 5-4° 6.00 4-25 5-75 6.125 5-75 Total Piston Area (sq. in.) . 47i 275 34° 596 467 531 364 B.H.P. 2,200 1,400 — 2,350 2,000 2,300 1,700 B.H.P. per sq. in. Piston Area 4.67 5-oy — 3-94 4.28 - 4 33 4.67 not to pass excessive quantities of heat through the piston skirt, but to rely on direct oil-cooling of the crown. The junk head involves very similar problems to the piston, and recent development has been towards improving the direct air-cooling with less reliance on conduction radially through the sleeve and barrel. Modern liquid-cooled installations have extremely low cooling drag; they may even give a small thrust under certain conditions of flight in temperate climates. Some further aerodynamic improvements allied with neater and less vulnerable installations will, no doubt, be introduced for present-day engines. Possibly, however, the greatest reduction in the cooling drag of liquid-cooled engines is likely to come from the use of higher coolant temperatures. An attempt will undoubtedly be made to achieve these, with the smallest possible increase in cylinder temperatures, by increased coolant velocities and perhaps some local finning or secondary surface on the head. Radial Engine Drag Modern air-cooled engine installations also have ex tremely low cooling drags, the best at the present time approaching the condition of zero cooling drag under I.C.A.N. conditions. In practice, they give less trouble with ground-cooling in the tropics and, at a sacrifice in overhaul life, have a large capacity for overload. On the other hand, a liquid-cooled engine's limitations are far less elas^, and radiator boiling sets a definite limit. However, to return to the matter of drag. The air- cooled engine can expect to gain as much from improve ments in cowling and rather more from the use of higher temperatures. In addition, there is a very large potential Fig. 6. Cooling drag at varying maximum cylinder tem peratures.. Percentage b.h.p. required for cooling a high- power, air-cooled radial engine installed in a high-speed aircraft. improvement possible by better finning. Fig. 6 shows how drag is reduced by higher operating temperatures on a given cylinder. The reduction in drag is due almost entirely to the higher fin-temperature, and the redaction in heat dissipated is negligible. In this case the higher fin-temperature is obtained by increasing the mean operating temperature of the head. There are three other ways of raising the average fin-temperature. (1) Using fins of higher conductivity. (2) Reducing the depth of cool fins on those parts where the cooling is greater than need be. (3) There is a tendency to neglect (2) and to concentrate on improving the cooling of the hot parts of the head. It this can be done (and in practice it always can!) the temperature of the over-cooled portions can be raised by a reduction of the air-pressure drop across the engine. The cooling of an engine can, as is well known, produce a useful thrust. The principle is to take in air, compress it, heat it and then allow it to expand and eject it rear wards. Advantage of Air-cooling at High Speeds The compression is caused by the speed of (light, possibly assisted by a Ian If an air-cooled engine is carefully baffled and finned, so that nc air gets by without doing its job and no parts of the cylinder are unnecessarily cooled, the air-cooled engine can produce a greater air-temperature rise. Further, since the difference between the metal and air temperature is higher, the unavoidable friction asso ciated with the transfer of heat is less. These factors give the air-cooled engine a theoretical advantage at high speeds where cooling thrust is not at all negligible. It is a mattet of design to see that this basic advantage is not squandered in excessive duct losses, etc.. The importance of keeping TABLE H.—POSSIBLE CYLINDER ARRANGEMENTS For engines having Piston Area of 800 sq. in. to give 4,000 B.H.P. with present-day fuels and 5,500-6,000 B.H.P, in seven years' time Number of Cylinders 9 12 14 16 18 2r 22 24 28 32 42 48 Cylinder Bore (in.) 10.6 9.2 8-5 8.0 7-5 7.0 6.8 6.5 6.0 5-6 4.8 4.6 Possible Cylinder Arrangements Kadial. V, V or flat. Radial. H, V, X or 2-crank radial. 2-crank radial or V. 3-crank radial. 2-crank radial. H, X, W or Y. 4-crank radial. H,* X,* W* or 4-crank radial.* 6-crank radial. $ " Double W " 4-crankshaits.* * No engine with this arrangement has been built.
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