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Aviation History
1951
1951 - 1663.PDF
FLIGHT THE ICING PROBLEM . . . discussed later, together with the remaining thermal systems. We may next consider thermal methods using hot gas. With the advent of the gas turbine as a prime mover in passenger aircraft there is a readily available source of heat, either in a direct supply of hot gas or from a surface heat exchanger. For empennage protection combustion heaters are usually employed, since it is inadvisable to have long ducts passing along the fuselage. The usual arrangement with the wing system is to lead hot gas in a spanwise duct along the wing and then discharge it into some form of small chordwise ducts, formed by double skin or corruga- tions. (Fig. 2). The use of undiluted exhaust gases originating from kerosene combustion does not create corrosion problems. A series of tests has been made to that effect by the Tiltman-Langley Laboratories, with positive results. The internal soot deposit does decrease the gas-to-skin heat transfer after a certain time, but at least ioo hours' exposure is necessary before any difference can be detected. The systems using hot gas or air can offer an excellent degree o protection, but require very careful design-study. They also have several drawbacks, the principle one of which is the fact that they can be designed economically, as continuously operating anti- icing protection, only for continuous maximum icing. In any instantaneous maximum icing condition there will be a certain amount of run-back. The second difficulty is that, although the heat required for distribution can be quite accurately determined, the actual heat supplied is dependent on the gas-flow distribution achieved, which in turn depends on pressure drops in the various components of the system. Pressure-drop is very difficult to determine accurately otherwise than by experiment and is even Fig. 2. Typical arrangement of a hot gas anti-icing system for a wing. difficult to reproduce accurately on various aircraft of the same series. The leading edge double skinning or corrugations can easily be designed as stress-carrying components of the wing, and a properly designed structure need not be heavier than an orthodox unheated wing. The surface of such a wing can be kept very smooth indeed, a fact of great importance in these days of high cruising speeds. Use of the Redux bonding process must be mentioned in this connection as providing a very satisfactory solution of the problem. Its use, however, requires a careful temperature-control of the hot gases, since any overheating of the structure above 90 deg C may greatly reduce the strength of the bonding. Relative Merits of the Two Types of Thermal System:— Chief advantages of the electrical system are as follow:— (a) Ability to deal with any icing condition, no matter how severe. It is immaterial to the system how much ice is accumulated on the heating pads; the same small quantity of ice has only to be melted, from underneath, to ensure the breakaway of the deposit. (b) No run-back is formed, even in the most severe icing. (c) In aircraft relying on electricity for most of the auxiliarv services there is no need for separate generators, since all the power required can be supplied by the existing generators, because icing occurs in flight, i.e., during the time when the main auxiliarv services are not required. (<f) Possibility of ensuring accurately the required rate of heat- ing. (e) Lightness of complete installation. (/) Ease of ground checking and fault finding. The disadvantages are:—(a) Increased drag due to the external heating pads. (b) Further slight increase in drag in icing conditions, since acertain amount of ice will always be present for most of the icing time, due to the periodicity of removal. (c) In aircraft which are not "all electric," it may be necessaryto install special de-icing alternators. The main advantages of the hot-gas systems are:— (a) Very clean aerofoil surfaces are possible and, if the doubleskin is designed to carry stresses, little or no increase in total weight of wings is possible. (b) All the water caught can be evaporated in continuous icing,which means that no increase in drag will occur in icing conditions. (c) The heat carried by the air in the boundary layer preventssome freak forms of icing farther along the chord, past the usual protected zone. (d) The spent gases can be used to ensure complete freedomfrom icing in control and flap gaps. The most serious disadvantages are:— (a) The system can economically be designed for continuousmaximum icing. In instantaneous maximum conditions, which usually last a few seconds only, some run-back must invariablyoccur. If by any misfortune it occurs at the beginning of the cruise, the ice formed behind the protected zone will persist until thefinal descent, thus increasing the drag and cutting the range. (b) Necessity of evaluating very carefully the pressure drops inthe system, many of which will have to be determined empirically. Any manufacturing inaccuracies will alter the designed mass flow. (c) Necessity of providing separate heaters for empennage pro-tection. id) In comparison with the electrical system, the weight penaltyis slightly higher. (e) Relatively long calculations are required to establish thecorrect heat requirements, pressure drops and mass flow distribu- tion (about 1,000 man-hours are required on a large size airliner)as compared with those for the standard power requirements for the electrical system. It might be useful to mention that it is sometimes advantageousto adopt a mixed system, where the wings are heated by hot gas and the empennage by electricity. Conclusions The following conclusions summarize the main points of the foregoing review:— (i) The methods of computation available are more accurate than the meteorological data against which the system has to be designed. (ii) The effect of ice is more pronounced on smaller aircraft. (iii) No protection is required for airframes of aircraft cruising above 500 m.p.h. T.A.S. The pessimum speed (at which the heat requirements are largest), occurs at about 370 m.p.h. T.A.S. (iv) Thermal systems of protection seem to provide the most satisfactory way of eliminating the danger of icing. (v) Hot-gas thermal anti-icing systems can be designed with a high degree of efficiency; they require, however, a precise know- ledge of the prevailing continuous icing conditions on the intended routes of the aircraft. (vi) For turboprop or turbojet long-range aircraft freedom front run-back is essential and the method of protection adopted (a continuous) should ensure complete evaporation of water caught/ (vii) With recently developed materials used on aircraft already having powerful generators for auxiliary services, the electrical de-icing is the lightest and the most efficient form of icing protec- tion. References (1) Jones A., Lewis, W.: "Recommended Values of MeteorologicalFactors to be Considered in the design of Aircraft Ice-Prevention Equipment." N.A.C.A.TN 1855 (1949). (2) Hardy, J. K.: "Protection of Aircraft Against Ice," S.M.E.3380,R.A.E. (1946). (3) Neel, Carr B., Jr.: "The Calculation of Heat Required for WingThermal Ice Prevention in Specified Icing Conditions." Pre-print of paper for presentation at the S.A.E. National Aeronautical Meet-ing, October, 1947. . (4) Guibert, Janssen and Robins: "Determination of Rate, Area andDistribution of Impingement of Waterdrops on Aerofoils from Trajectories obtained on the Differential Analyzer." N.A.C.A. RM9AO5 (1948). (5) Bergrun, N. R., "A Method for Numerically Calculating the Areaand Distribution of Water Impingement on the Leading Edge of an Aerofoil in a Cloud." N.A.C.A. TN 1397, (1947). (6) Glauert, Muriel: "A Method of Constructing the Paths of Raindropsof Different Diameters Moving In the Neighbourhood of... etc. R. and M. 2025, A.R.C., (1940). (8) Schaetzel, S. S.: "A Rapid Method ofIcing," Aircraft Engineering, July, 1950.
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