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Aviation History
1951
1951 - 1662.PDF
f 'LIGHT, 31 August 1951 247 XHE ICING PROBLEM . . . back. Secondary icing centres appear on all excrescences, such asrivet heads, skin joints and access panels. This form of icing is extremely dangerous. The shape of the aerofoil section is com-pletely spoiled; and the smaller the section the higher the relative rate of icing and the subsequent alteration of shape. Sometimes freak forms of icing occur. In one of the T.C.A.reports there is mention of a very severe case, where ice started to form on the lower surface of the wings. Spikes of ice, over 2in inlength, were formed on the rivets and were so hard that it was impossible to snap them with the lingers alone. The loss of liftwas very noticeable and a rapid loss of height occurred, even with fiill power on. Measuring Severity of Icing. Methods of Calculating Catch onAerofoils.—There are several methods of measuring the severity of icing as represented by the three "atmospheric" variables, i.e.,temperature, liquid water content and droplet size (d). To measure the temperature some form of sheltered thermo-meter is used, giving a reading for the ambient temperature. The methods used to determine the two remaining variables can bedivided, broadly, into mechanical and optical methods. The mechanical ways of determining the l.w.c. and d are two-fold: the multiple-cylinder method and the rotating disc method. The former is not a direct-reading methoH, but a comparison ofthe rate of catch on cylinders of dirferent radii makes it possible to evaluate the l.w.c. and the drop diameter. The rotating-discmethod depends on the use of a thin rotating disc, edge-on to the airstream. The thinness of the disc renders the efficiency of catchvirtually 100 per cent, and automatic devices are usually used to transmit the thickness of accumulated ice to a dial graduateddirectly in arbitrary units. The reading, after correction for the T.A.S.J represents the liquid water content only, since the effectof the drop size shows itself only in the efficiency of catch. Optical methods may consist of direct photographs of suitablymagnified droplets, eitner caught on some special surface or else photographed in motion by compensating their movement by amoving arrangement. By utilizing the absorption of light passing through a certain distance in cloud it is possible to determine thel.w.c. with the aid of a direct-reading photo-cell. Thanks to the pioneer work of M. Glauert (Ref. 6), J. K. Hardy(Ref. 2) and many others, it is possible, knowing the variables men- tioned under "Meteorological Factors", to determine with areasonable degree of accuracy the rate of catch and the heat re- quired to prevent icing. Once the rate of catch is known, it ispossible to calculate (in the case of thermal anti-icing), the heat required for complete ice prevention. Determining the Worst Case It has to be stressed here that the main uncertainty is the icingcase against which to design. Using the method suggested by the author in Ref. 8, the com-putations are shortened and, in case of thermal anti-icing, it is possible to determine relatively quickly, the worst icing conditionlor a given aircraft. Even then, it will be necessary to decide which will be the worst design-case for a given aircraft. For instance, willthe climbing case be worst than the descent on reduced power (in the case of exhaust-heat exchangers) ? Or will the aircraft have tocruise in icing conditions for any length of time ? Very often the one-engine-off cruise is the worst case, owing to the reduced heator power output and reduced height. It is to be noted that no protection is required in case of aircraftflying at speeds above 500 m.p.h. T.A.S., since the kinetic heating is then sufficient to prevent accretion in most icing conditions. It has to be stressed that, if a continuously acting thermal systemis used, it is essential to evaporate all the water caught, since any run-back water will eventually encounter an unheated portion ofthe aerofoil and solidify there. Ice thus formed will then persist, (luring the remainder of the flight, increasing, often quite appreci-ably, the drag of the aircraft. This fact is especially important in the case of turboprop or pure-jet aircraft, where the relationbetween drag and range is extremely critical and where any increase of drag means a drastic cut in range. Methods of Protection.—The methods of protection can bedivided into three classes: (a) mechanical; (b) chemical; and (c) thermal. The mechanical methods of protection are mainly based on theuse of inflatable rubber bags located along the leading edge of the aerofoil. The best known is the Goodrich system, widely used ontoe Constellations and the Douglas series of post-war airliners. This is essentially a de-icing system and ice is removed periodicallyby inflating the bags. The central bag is inflated first, which breaks the ice roughly along the leading edge; the remaining two are thenoperated and the accumulated ice breaks away and is removed by the airstream. The system has its drawbacks, mainly arising from™eresulting drag-rise, limited chordwise extent, inability to deal wtii all types of icing, and vulnerability. It is, however, a working system and has rendered good service in the past. The chemical method depends on the use cf various substances,in liquid or paste forms, which lower the melting point of ice and thus decrease the adhesion between the surface of the aerofoil andthe layer of ice. It can be used as a de-icing or anti-icing form of protection. The liquid usually has a proportion of alcohol orglycol and is exuded through a porous leading edge or is guided (in the case of airscrews) along spanwise rubber grooves. The maindifficulty consists in ensuring a uniform distribution of the fluid and in preserving the porosity of the leading-edge material in spiteof the dust and animal particles encountered in low-altitude flight. Also, the liquid carried to ensure protection during pro-longed periods of icing represents a dead load which, to counter an expected one to two hours of icing, can run into 300 to 500 lb on alarge airliner. Though systems of this kind now seem to be less favoured by designers, one fluid method—using the new Porosintporous leading-edge material—has attracted attention; a series of tests were made on a Viking aircraft and good results claimed. The thermal systems can be applied as anti-icing or de-icingarrangements. Since icing is essentially a thermal phenomenon, thermal methods seem to be the logical way of preventing it. Theycan be broadly divided into systems using electricity as a source of heat and those using warmed gas, such as hot air, air mixed withexhaust gases, or exhaust gases alone. Electrical Heating Systems Heating of the aerofoil surfaces by electric current was firstdeveloped in Germany during the last war. This method and the new materials developed recently were subjected to an extensiveseries of tests in England last year, with very satisfactory results. The Canadian Air Research Council has also embarked on anextensive test programme with their specially equipped DC-4M, the Rockcliffe Ice Wagon, which toured Britain in September, 1950. It is considered that the electrical methods now being developedwill provide the lightest and most effective way of protecting air- craft against the hazards of icing. The electrical system is essentially a de-icing form of protection.Aerofoil surfaces are divided into a certain number of sections to which current is applied periodically. Continuous heating is notpracticable, since a prohibitive current supply would be required. Periodic heating, and the fact that ice is allowed to form in local-zed areas, has other advantages, which will be discussed later. The heating elements, embedded in thin rubber pads, areapplied externally and form a certain number of independent cir- cuits covering the leading edge of the aerofoil surfaces. At thestagnation line and at the junctions of the various sections thin strips are provided which are heated continuously and at a slightlyhigher rate than the remainder of the system. For the equipment to function properly there must be a certain amount of ice accumu-lated. The current is then switched on, ice begins to melt from the inside, the cohesion between accretion and aerofoil is destroyed andthe ice is removed by the airstream. There are two regions, the primary shedding zone and thesecondary shedding zone. The role of the secondary zone, which is cycled at a much slower rate than the primary one, is to removeany ice which might in some icing conditions accumulate past the normal area of catch covered by the primary zone. Its duty is alsoto get rid of any (usually negligible) run-back which may occur from the primary zone. The usual dimensions and rates of power supply are as follow:— (1) Stagnation Line Parting Strip: Width, iin; ContinuousHeating, 12 watts/sq in. (2) Primary Shedding Zone: Width, 6in on both sides of (1);heating, 8 to 8.5 watts/sq in; usual cycling, 1: 8 (severe icing, 20 sec on, 140 sec off). (3) Secondary Shedding Zone: Width, 6 to 8in on both sides of(2); heating, 5.5 to 6.5 watts/sq in; cycling, once every 10 cycles of the primary zone. Rate of Cycling The cycling can be varied to suit the icing conditions, and veryoften it is sufficient to cycle at a rate 1: 16. There is a critical relation between the heating and the width of the parting strips.It has been established that the thinner the leading-edge parting strip the shorter the shedding time. The best results have beenobtained with an inch-wide strip in the cruising stagnation-point position. The main problem with electrical de-icing is the choice ofsuitable heating pads and the provision of satisfactory bonding between the aerofoil and the pad surface. It has been found thatpads with rubber forming the external surface stand up to abrasion much better than more resilient plastics (this is specially importantin the case of airscrews). The application of heating pads inside the wing, although givinga cleaner external surface, is less efficient, owing to the fact that the thermal capacity of the skin destroys some of the advantages ofperiodic heating. The merits and disadvantages of the electrical methods will be
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