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Aviation History
1951
1951 - 1659.PDF
246 FLIGHT, 3i August x95l THE ICING PROBLEM —from the Airframe Designer's Angle: How Ice Forms: Means of Prevention and Dispersal By S. S. SCHAETZEL, D.I.C., B.Sc, D.L.C., Grad.RAe.S. BEFORE going last year to Australia, where he is in the ProjectDesign Group of the Government Aircraft Factories, Melbourne, the author was on the design staff of the British Aeroplane Co.,he was engaged on the arrangement of the de-icing systems of the Brabazon and the Bristol 175. Among interesting facts whichhe mentions here is one of highly topical importance that will not, perhaps, have been generally recognized by the non-techni-cal—namely, that no anti-icing measures are necessary on air- craft designed to fly at over 500 m.p.h., since kinetic heating of theaircraft skin can be relied upon to do the job automatically. ICING occurs on the forward-facing surface of any objectmoving through an atmosphere in which there is a cer-tain concentration of supercooled water droplets and where the wet surface temperature is below o deg. C. It fol- lows, therefore, that ice will form on the leading edge of the aerofoil surfaces of an aircraft, on the nose of the fuselage, at the lips of the intakes, on whip aerials, in fact on all "profiles," as soon as the relevant ambient conditions occur. It has been estimated by the Canadian airline companies that their aircraft spend about 5 per cent of their total flying time in icing conditions. Some form of anti-icing protection must therefore be provided in order both to safeguard the airworthiness of the air- craft and to allow operation to continue during that quite appreciable amount of time. In the case of aircraft operating in other climates, e.g., in Australia, the percentage of flying time spent in icing con- ditions is smaller, but, on the other hand, very severe icing is often encountered in tropical cloud formations. It is advisable to differentiate at this stage between the terms anti-icing and de-icing. Anti-icing aims at preventing ice forming at all whereas de-icing attempts to remove (usually periodically) the ice already formed. A de-icing system is usually lighter than a comparable anti-icing installation. In this article it is proposed to deal only with airframe icing, and not with that of engines and airscrews. Nor will windscreen icing be discussed; though it concerns the airframe design, it is a some- what detached and specialized aspect of the main problem. Precautions against icing have to be envisaged at an early stage in the aircraft design, since no system incorporated as an after- thought is likely to prove wholly satisfactory. Meteorological Factors.—Variables determining the rate of ice ' accretion on any forward-facing surface are as follow :— (a) ambient temperature. (b) liquid water content per unit volume of cloud (m) (measured in g/m3). (c) diameter of water droplets present, (d) (d) true air speed of section subject to icing. (V) («) nose radius of the section. (D) (/) thickness/chord ratio of the body concerned. (g) altitude effect (i.e., the variation of p) is also important. In a design, however, one critical altitude is considered— usually the upper limit for continuous maximum icing, i.e., 20,000ft. Because of the great number of variables and the difficulty of standard measurements there is still lack of reliable and generalized flight data, although a great number of flight measurements have been made, mostly in the United States. Ref. 1 provides what is considered to the be most reliable data on the subject. Although the icing conditions have been classified under several headings (such as instantaneous max., continuous max., etc.) and each of them subdivided according to the ambient temperature, yet it is impossible to obtain from these data any "Standard Icing Condi- tions" comparable with the Standard Atmospheres. It is also probable that the cloud formations, in which the measurements were made, were typical of the American Continent and that in other parts of the world different icing conditions might be en- countered. It is well known, for instance, that icing encountered in the tops of cumulus clouds of tropical thunderstorms surpasses in severity anything met in temperate climates. Some authorities are of the opinion that there is no limit to the severity of instantaneous icing. This, however (as will be seen later), is of less importance to the airframe designer than it is to the engine designer. In general, continuous icing does not occur above 20,000ft, but instantaneous icing has been known to occurvery much higher. In spite of all this, it is the opinion of the author that, whensufficient data are available (data based on the frequency of occurrence) some sort of "Standard Icing Conditions" should beevolved. After all, the Standard Atmospheres represent nothing else but the mean conditions occurring throughout the year and notthe worst high or low temperature and pressure cases in any climate. The danger of the severity of icing exceeding the maximumis, if anything, less than, for instance, the danger of insufficient power for take-off at a tropical airfield, when the temperatureexceeds by several degrees the tropical maximum conditions defined in the Standard Atmospheres. Yet this does not seem toworry the designers, who for many years now have designed their aircraft against what are, after all, arbitrary conditions. The Mechanism of Icing.—When an object is moving at a sub-sonic speed air is deflected a certain distance in front of its leading edge. Depending on the size and the thickness/chord ratio of theobject this disturbance is felt a longer or shorter distance ahead. By reason of then- inertia however, the water droplets suspended inair are not deflected with the streamlines but, depending on their size, the free stream velocity and the relative abruptness of changeof direction of the streamlines, assume a less curved path. In con- sequence, a certain proportion of the droplets strikes the forward-facing surface of the moving object (Fig. 1). The proportion of water striking the surface divided by thetotal quantity of water present in the path of the frontal area of the moving object is called the efficiency of catch (rj). It is clear thatthe efficiency of catch will be less for large, thick sections, since the disturbance is felt farther ahead than in the case of small, thinobjects and, consequently, more droplets can be deflected and thus avoid striking the body. Several forms of icing are possible, depending on the ambienttemperature and on the liquid water content of the cloud. The most common forms are glaze ice and rime ice. Rime ice formswhen the ambient temperature is low and the l.w.c. is low as well. It usually follows the shape of the section, is porous, has a mattsurface and presents little danger to airworthiness. There is usually no run-back, i.e., all water striking the body freezes immediatelyafter impact. Glaze ice occurs when the ambient temperature is high (i.e.,just below o deg C) and the l.w.c. is high. It begins to form at a very high rate, in the characteristic "cauliflower" shape and witha very rugged outline. Owing to the release of the latent heat of fusion at this relatively high temperature all the water does notfreeze immediately, and there is a considerable amount of run- STREAMUNES PATH OF WATER DROPLETS Fig. I. Deflection of water droplets in airstream past a profile. Depending on certain factors, a larger or smaller proportion of the droplets is caught.
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