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Aviation History
1944
1944 - 1894.PDF
FLIGHT SEPTEMBER 14TH, 1944 THE SUPERMARINE SPITFIRE XIV 4 blower of the same type as that used on the Merlin XX. It is, therefore, logical to assume that the Griffon in the Spitfire XIV has a two-stage supercharger similar to that of the Merlin LXI, which has proved so successful in the Spitfire IX with an output of 1,650 h.p. and a speed at its operational height of more than 400 m.p.h. With an engine of greater capacity (the Griffon is of 36.7 litres against the Merlin's 27) it can safely be assumed that the Spitfire XIV is even faster than the IX. Engine Details With a bore of 6in. and a stroke of 6.6in. the twelve cylinders give, as already mentioned, a capacity of 36.7 litres. The compression ratio is 6:1 and there is a choice of airscrew reduction-gear ratios of 0.51:1 and 0.45:1 respectively. In.spite of having a two-stage supercharger, the Griffon XLV is of relatively short length. This-is partly due to the placing of the drives for the camshaft and ignition at the front of the engine. Dual ignition in one magneto unit achieves a clean layout of the wiring. Starting can be by either electric motor or cartridge starter. The starter is mounted on the reduction gear casing with transmission to the reduction gear. A drive for a remote gear box for various aircraft services is provided at the rear of the engine. For easy and uniform operation, inter connected engine, airscrew and ignition controls are fitted. In order to prevent use of high supercharger gear at sea level, an automatic gear change comes into opera tion at the height at which maximum performance will be obtained at the prevailing airspeed. At the rear of the cylinder blocks is installed a liquid-cooled intercooler. Oil and fuel systems of the Griffon XLV are unusual. The oil is fed to all crankshaft bearings, thus minimising pres sure loss and improving oil-flow control. The fuel feed control is of an improved type which ensures correct car- • buration during fighting aerobatics. Yet one more feature has been responsible for the amaz ing performance of the Spitfire XIV. There is little use in adding more power and in developing that power by the engine at great heights if it cannot be converted efficiently into thrust. Here Rotols came to the rescue with their five-bladed airscrew. Ideally no doubt the six-bladed, contra-rotating type would give the best solution, but this would introduce not inconsiderable modifications to engine and airframe. That this type will come into quite exten sive use is certain, but the Spitfire XIV was wanted quickly, and so the single-hub, five-bladed airscrew seemed to offer a very effective intermediate step. At the speeds attained by fighter aircraft at altitudes of 30,000ft. and upwards, the four-blader cannot efficiently absorb the power of engines of the 2,000 h.p. class. More over, there is the question of limitations on diameter, partly dictated by airscrew tip speeds and partly by the question of undercarriage height. The problem of the five blader appears relatively simple, but several problems were worrying the designers when they began to examine it. It was known, for example, that with the increase in the number.of blades on a single hub, energy losses due to the rotational swirl of the slipstream would be present. Unfounded Fears It had also been thought that vibration troubles would be encountered due to the effect of reactionless modes of vibration which appear in airscrews of this type. This form of vibration is aerodynamically excited and can occur on four- or five-bladed airscrews, but not in the three-blader. It has the peculiarity that the blades vibrate in such a manner that there is no reaction present at the airscrew shaft, which tends to displace the shaft either laterally or torsionally. Since the vibration is contained entirely in the airscrew itself, no advantage can l>e taken of any inherent damping or energy-dissipation which may be present in the engine system. Hence it had been thought that resonance of this mode of vibration would cause pro hibitively high stresses in the blades. This reasoning was, so far as present airscrew experience is concerned, proved unfounded, probably on account of the extra damp ing which is introduced by the additional number of blade- root fixings. Further, it appears from recent experience that the aerodynamic forces which tend to excite this form of vibration are small compared with other sources of excitation. The use of the five-bladed airscrew gives a pronounced improvement in climb characteristics, at the same time giving very good efficiency at top speed. The necessity for keeping down the weight becomes more apparent than ever with five-bladers, and intensive work on wood blade design is going on to give a performance equal to that of the metal blade by reducing aerofoil section thicknesses and cleaning up the roots to improve the airflow over the spinner. As for the Spitfire XIV itself, the reinforced airframe of the Mark VIII was used as a basis for development, with the result that the new model could be put into production in a very short space of time. The forward lines of the aircraft were slightly modified to accommodate the longer engine, and special attention was given to surface finish, an expedient which has become possible by the sorry state of the Luftwaffe. The result has been that the Spitfire XIV is faster than any other Spitfire in service, as has been proved in opera tions in the north-western European theatre of war. It has also been in action against flying bombs. Operational Experience The Spitfire XIV has brought disappointment to the pilots of the R.A.F. who have flown it deep into enemy territory in recent months. Knowing they were flying a fighter faster than any other Spitfire in service, they were anxious to test its undoubted qualities against the best the Luftwaffe could produce, but, unfortunately, combats have been rare. The qualities the R.A.F. pilots have recognised in the "Spit. XIV" have included an unequalled rate of climb and a high-altitude performance calculated to keep it above any fighter the enemy can put in the air. Pilots have reached over 40,000ft. in this aircraft. "When you open the throttle," said Wing Cdr. R. W. Oxspring, D.F.C. and two Bars, "the new "Spit." leaps off the ground and. at its height, there is nothing to touch it. It is the smoothest aircraft I have ever flown, highly manoeuvrable and entirely without vices. I'm itching to get a chance to let the Hun see what it can do in action." Prior to D-day the Germans had caught fleeting glimpses of the Spitfire XIVs as they flashed over enemy territory on weather and tactical reconnaissance; and particularly when they swooped suddenly down on rail or road trans port. Absence of enemy opposition—they were attacked frequently enough by anti-aircraft guns—and supreme confidence in the speed of their aircraft has made the pilots particularly venturesome. In a recent sweep over the Brussels-Antwerp-Bruges area one of the pilots was so tired of searching in vain for enemy aircraft in the air that he went right down " on the deck " and, as he put it, "Took a peep into the hangars at Antwerp " His further report was: "There didn't seem to be any there either.'' Armament of the Spitfire XIV may be four 20 mm. can non, two 20 mm. cannon and tw;> 6.5m. machine guns, or two 20 mm. cannon and four 0.303 machine guns. In addition, a bomb carrier for either a 250 lb. or a 500 lb. bomb can be fixed under the fuselage in the drop tank fittings. ( Main data of the Spitfire XIV are: Wing span: 36ft. ioin. Length o.a.: 32ft. 7.75m. Wing aiea: 248 5 sq. ft. Loaded weight: 8,000 lb. (approx.).
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