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Aviation History
1944
1944 - 1487.PDF
JULY 2OTH, 1944 FLIGHT I •'-• 71 Fighting an Implacable Fpe^ Engine Life Prolonged by Filtration : A Vokes Exhibition DESERT DUST STORM: Pilots in Libya report having flown through dust-laden atmosphere as high as 17,000ft Visibility in a storm such as this is as little as three feet. A N exhibition of the means used to combat one of the r\ most implacable foes our mechanised forces have •*• •*• to fight was recently opened at Messrs. Vokes 'uy Mr. A. T. Lennox-Boyd, M.P., Parliamentary Secretary to the Ministry of Aircraft Production. The exhibition itself is of great interest and not only shows the multiplicity of diverse filtering and silencing problems that have been tackled, but also makes it quite clear that filtration is one of the most important adjuncts to the efficient operation of engines. Another point which is demonstrated is that dust, or minute sand particles, has basically the same characteristics no matter where it is found—and very surprisingly, dust is found in the middle of the Atlantic as well as in North Africa. During our campaigns in the Middle East and the Far East, numerous of our aircraft were fitted with " tropi- calised'' air intakes, and illustrations of various types have been published from time to time in Flight. Not all of our machines, however, were so fitted,, and in cases where no means were possible to combat the amazingly destructive effect of dust, the percentages of aircraft grounded and unserviceable and the flying life of the engines were appalling. It is not difficult to visualise the effect of dust inside an engine when it is realised that before a serious inroad can be made against engine wear, 97 per cent, (by graded size) of foreign matter must be removed from the air, oil and fuel. The remaining 3 per cent, is the most difficult to remove, being composed of matter between one and five microns in size; one micron = 0.00004m., that is, one- hundredth the thickness of the average human hair. The highly critical performance required of filters, if they are to be of any real good, can thus be easily understood. Where some filters fail is that general operating condi tions introduce a variety of densities, velocities and par ticle sizes of dusts, with the result that in any conditions other than those for which the figures claimed the filters are virtually useless. One of the secrets of the success is the ratio of filtering element area to volume and velocity of flow. In Vokes air types the area of the filtering element is 200 times the area of the inlet and outlet ports, and on their oil filters the ratio is 600:1. By utilising so generous a filtering area in relation to the inlet and outlet areas, obviously the speed of the media through the filter element is reduced to a very low figure, and this enables deleterious matter to be arrested with very low back pressure losses. Dust Particles It seems amazing at first sight that dust, with which we are all familiar, can have such a tremendous effect upon the wear of an engine. The dust which does the real damage is as fine as face powder or flour, is sometimes com posed of as much as 85 per cent, quartz, which is harder than steel, about 0.7 the hardness of diamond, and consists largely of angular and sub-angular grains. Dust particles of 5 to 15 microns in size get between bearing surfaces, span the oil film, and cause extreme abrasions. Particles of less than one micron cannot span the oil film and are relatively harmless. As a brief illustration, the wear on bearing surfaces in the average pre-war car was about 0.001 to 0.0015m. per 1,000 miles on English roads over 10-15,000 miles running period. With aircraft engines it can be said, in general, that any make in the world is in serious trouble within 35-50 hours' flying in dusty conditions, and large numbers have been reported as seriously damaged in as low a period as 10 hours. On the 35-50 hours basis the engines are so badly damaged that a serious major overhaul is necessary, and frequently the engine is not worth any attempt at repair. With a really efficient filter fitted, however, reports indicate that engines are giving 180-200 hours service. During the Libyan campaign, for instance, it was noticed by our pilots that large numbers of Italian aircraft could be seen.lined up on the various airfields, as well as'con siderable numbers of tanks and vehicles outnumbering those possessed by our Forces. Yet very few aircraft ever seemed to be flying, nor were the tanks or armoured cars in motion, and it was not until Sidi Barrani and Bardia were taken that it was found that practically the whble of these vehicles and aircraft were completely vinserviceable due to sand dust having ruined their engine*
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