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Classic Aviation Ads: The Everest Bristol Pegasus S.3 Engine 1933

 

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THE EVEREST FLIGHT " BRISTOL" PEGASUS 5.3 ENGINE

THE EVEREST FLIGHT " BRISTOL" PEGASUS S.3 ENGINE

In the selection of engines the most important points to be considered were the power available at great altitudes and for taking off and climbing with a heavy load, the weight per effective horsepower under various conditions, reliability, and fuel consumption. The type of engine selected had also to be one suitable for the aeroplane obtainable, or such that the installation could be arranged with a minimum of structural alterations. As already seen the engine selected was the Pegasus S.3, manufactured by the Bristol Aeroplane Company.

Of the air-cooled, radial type, the Pegasus engine has nine cylinders of 5i-inch bore and 7rinch stroke, giving a total swept volume of 1,753 cubic inches or just over 28 litres. It is fully supercharged to a rated altitude of 11,000 feet at its normal speed of 2,000 revolutions per minute, the rated power then being 525 brake horsepower. At its maximum permissible speed of 2,300 revolutions per minute the power is 580 brake horsepower at 13,500 feet, while for taking off at sea-level it is 550 brake horsepower. The airscrew is driven at half crankshaft speed thro ugh a reduction gear, and therefore gives high efficiency for taking off and climbing, as well as when flying level.

In regard to the principles of supercharging, it may be explained that the cylinders of an un-supercharged or naturally aspirated engine cannot be filled with combustible mixture at a pressure exceeding that of the surrounding atmosphere, while in actual practice the induction pressure is usually somewhat less than this. Such an engine will give its normal full power at sea-level, but as the aeroplane climbs the power drops off in proportion to the density of the surrounding atmosphere, until at a height equal to that of Mount Everest the power will be only about 27 per cent. of that at sea-level. Obviously, then, an aeroplane fitted with a naturally aspirated engine will have but a limited ceiling and is quite unsuitable for really high altitude work.

To increase the power at altitude it is necessary to force the mixture into the cylinders at a pressure exceeding that of the atmosphere in which the engine is operating, when the power will be increased in proportion to the difference in induction pressure. T his difference is generally called the "boost," and for convenience is reckoned from the standard atmospheric pressure at sea-level.

Thus, an engine supercharged to plus 1 lb. boost is one in which the pressure of the charge in the induction system is I lb. per sq. in. above the normal pressure of 14.7 lb. per sq. in. at sea level; while "zero boost" is just 14.7 lb. absolute pressure. When the engine is running throttled down the boost is, of course, negative, say minus 4 lb. or minus 2 lb. A supercharged engine, then, is one provided with some form of air compressor, fan, or blower, by means of which the cylinder charge can be increased in pressure, and the rated altitude of such an engine is the height at which the induction pressure can be maintained at the rated boost, usually zero. The Pegasus 5.3 engine has a rotary fan, driven from the crankshaft by a train of gearing, drawing the mixture from the carburettor and forcing it into the cylinders so effectively that the rated boost of zero (14 .7 lb. per sq. in. absolute) is maintained up to an altitude of I 1,00 0 feet at normal speed. Above this height the power falls off as in the case of the naturally aspirated engine, but not so rapidly; at the height of Mount Everest the power is just half that for taking off at sea level, or nearly twice that of the naturally aspirated engine.


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