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Aviation History
1948
1948 - 1816.PDF
524 FLIGHT OCTOBER 28TH, 19 Cold Weather Operation of Aircraft . scraper rings need to be fitted round the telescopic tubes, pre- fe.rence being given to gaiters. Synthetic tyres behave reasonably well at low temperatures, although flat spots develop when the aircraft has been station- ary for several hours. Treaded tyres are far superior to the smooth type for maintaining grip on snow-packed surfaces, tee-grip tyre- show a slight improvement over the treaded type when used on icy surfaces. • Cabin and Cockpit Heating From experience gained on different types of aircraft, the crew stations, or eabiri temperatures, should be limited to minima of o to —5 deg C, particularly in fighter aircraft where the crew cannot divest themselves of any clothing whilst in flight. The exception to this is the navigator. This member of the crew nfeeds to work his instruments with bare hands and some forth••oi- additional heat is necessary. Further,- the navigator's table requires surface heating. Limited tests have been Conducted using electro-thermal heating for this, and have shown promising results. ,. The main problem concerned with inadequate heating of cockpits and cabins is chiefly at the beginning of the flight. It is found that excessive misting and frosting of windshields, canopies and similar items takes place through the formation of condensate soon after the crew enter the aircraft. A hot-air spray is therefore required which can be directed on to the affected parts, but would not have to rely on ram effect for supply. '• Engine Operation T^HE second part of the lecture was devoted to considerations •*• of engine operation, the first item to be dealt with under this heading being lubrication systems. Normally, aircraft engines are designed to operate primarily on a 100-second viscosity lubricating oil. Under arctic conditions they do so satisfactorily when dilution is used, but many problems arise from the oil system installations, and these are, in fact, the greatest single cause of aircraft and engine unserviceability. Experience has shown that the standard type of oil tank does not usually incorporate a hot-well of adequate size to maintain an ample amount of diluted oil, and to keep it segregated from the make-up oil supply in the tank. Unless there is proper segregation, undiluted oil finds its way into the oil inlet, causing cavitation with possible starvation when the engine is started. Oil-tank shapes vary with each type of aircraft, and the shape of a tank has some bearing on the warm-up rate of the make-up oil, it having been proved that a tank with a narrow cross-section, where the make-up oil does not extend a large distance from the hot-well, warms up more rapidly than one with a larger cross-section. Advantage, too, should be taken to run the oil return inlet pipe in such a manner inter- naBy that the heat from the return oil assists the warming-up rate. With most present types of oil tanks, the warming-up rate is very slow indeed. Oil tank drain fittings are a source of great inconvenience. They should be easily accessible and the drain holes should be large enough to permit rapid draining. The filler, too, needs to be of ample size and diameter. This is particularly necessary if an aircraft has to be replenished with oil from drums. Additionally, the cap should be designed to permit a simple locking and unlocking operation, bearing in mind that the operator will have to wear heavy gloves. Locking wire as a safety measure should be avoided. The layout and run of most oil-tank and engine vent-pipes produce breakdowns because of the trapping of condensate which freezes and blocks up the pipe-lines, causing high pressures which bulge and split the tanks. • Oil coolers, unless fitted with pressure relief valves to take care of sudden surge, fail with unceasing regularity. This type of valve is. in fact, an essential requirement, even when the oil is diluted. Thermostatic by-pass valves are an advan- tage in that they give a closer control of oil temperatures over a wide range of conditions and also prevent by-passing through over-cooling. Even with extensive nse of dilution, oil consumptions, both on liquid- and air-cooled engines, are practically the same as when used under temperate conditions. Although oil dilution has many advantages it does accentuate the aeration already present in the oil system, which builds up in large quantities of emulsified oil, both in the crankcase and tank, and normally shows its presence by venting oil from the breather when the engine is near, or at, lull power. The intensity oi the venting, is associated with the quantity of diluent in the system and din be brought under control only by reducing power. This venting is particularly noticeable on in-line engines and, - although various' types of de-aeraters have been designed, the most successful method so far encountered is that where, the crankcase vent is connected to the forward ends of the . camshaft covers, and the vapour is released from the covers at the rear into a common pipe to atmosphere. The motion of the valves and springs, etc., appears to break up the emulsified oil, which drains back to the sump by way of the camshaft drive casings. •-- • There are some who deprecate any form of oil dilution on the assumption that the engine life is reduced, their argu- ment being: first, that at low viscosity the oil has inadequate -• lubricating properties and finds its way to the bearings; secondly, that the diluent washes the accumulated sludge and ; carbon deposits into the restricted oil-ways, and, thirdly, when T this deposit does find its way to the filters, they beconie ; choked. The lecturer went on to say that, after a number ; of years' joint experience, he and his co-author had yet to encounter this and, in fact, their experience has shown that the engine is left remarkably clean and the overhaul period is lengthened. In fact, one well-knoun airline in North America adopts dilution procedure the whole vea.i round tor this reason. Cold Starting The requirement for starting reciprocating engines cold under winter conditions raises two major problems, (a) provision for obtaining adequate cranking speed, and (b) supplying varjor- ized fuel to the engine in order that combustion will readiify take place. . • - The supply oi electric power for engine starting presents a problem at low temperatures because of the reduced a'ceuran- lator capacity. Although a determination of the minimtte cranking speed at which starts can be obtained is difficult, tests have indicated that 25 r.p.m. is the minimum at "which' starts are normally accomplished. The next and most important factor in starting engines at low temperatures is the priming;. A satisfactory priming system must (a) vaporise the priming fuel sufficiency so that combustion will readily take place, and (b) supjfly sufficient fuel to the cylinders after the initial firing to airoW the engine to pick up and run smoothly on its main fuel system. ... ,' .-•;, Many types of priming jets are available* rrrit the type that has proved most satisfactory is that,developed by the R.A.E. and used extensively on Rolls-Royce engines, Two of the more important factors are the configuration of the induction system and the location of the priming jets in that both<>f these have a great effect on the efficiency of the primffii|[ system. . • >"- Briefly, the present status is that in-line engines can :Be started consistently down to —35 deg C with loo-octane fael1 although, on occasion, they have been started as low as — 50 deg C. Radial engines have not been as satisfactory and it is considered that more development work is necessary on the priming systems before they can be considered satis- factory for arctic operation. Gas-turbine engines are much easier to start at low tempera- tures. This is attributed to the following factors: (a) The lubricants used are less viscous than those for reciprocating engines, (b) There are fewer moving parts and, consequently, much less engine friction, (c) The initial rate of light-up is less critical because of the continuous combustion. Experi- ence with the Derwent and Goblin engines has proved very satisfactory, both engines starting readily below — 45 deg ,C. ' Induction Systems '. ; : ; ' 5 ' :'•; Low outside air temperatures have little effect on the fuoc- tioning of induction systems, although aircraft fitted with inter- coolers have been found to give low charge-temperature ratings at low O.A.T.s with consequent rough running. The intro- duction of a Thrustat oil control valve into the intercooler system operating at 55 deg C and fully open at 75 deg C has produced satisfactory engine operation. .„ A few isolated tests have indicated that improved fuel coft- sumptions may be obtained by the use of hot air during flight. As this was noticed on aircraft fitted with intercoolers which maintained a constant charge temperature, it was attributed to the fact that the warm air gave better vaporization of the fuel in the carburettor. The decrease in fuel consumption
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