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Aviation History
1968
1968 - 0236.PDF
230 FLIGHT International, IS Februo'y I96j FLIGHT SYSTEMS DOWN TO EARTH The Compact Micronair is the latest of a series of rotary atomisers produced by Britten Norman Ltd over a period ., some IS years. On the right is seen the port half of a six-unit installation on an Ag Commander S-2D (formerly Snow) 1.3gal/acre) to achieve the same coverage. With 400 micron droplets, nearly 200 times as much liquid would be needed. With the simple types of nozzle mentioned earlier, the droplets are far from being a constant size, and may range over a wider spectrum than those shown in the table. In the theoretical case under discussion, if only one of the droplets in each sq cm were of 600 microns diameter, these droplets alone would account for 11.31 litres per hectare of liquid; this would be wasted, since it would contribute little to the coverage of the surface. To avoid a broad spectrum of droplets, atomising devices have been developed which are capable of generating spray composed of reasonably evenly sized droplets. The best known of these, the Britten-Norman Micronair, utilises the principle of a spinning gauze cage, into the centre of which the spray liquid is released. The liquid is atomised as it is flung through the cage. In most cases the cage is driven by a windmill placed in the slipstream. Four or six atomisers are normally mounted on an aeroplane. A number of miniature rotating atomisers, similar in principle to the Micronair, have appeared in the USA during the last two or three years; these are designed to be mounted in place of nozzles along a conventional spray boom. A more recent atomiser, which is claimed to produce a very narrow spectrum of droplet sizes indeed, is the Turbaero. This uses the principle of a number of concentric spinning discs, driven at about 10-12,000 r.p.m. by an electric motor; spray liquid is released at the centres of the discs, and atomised as it is flung off the edges by centrifugal force. The speed of rotation of atomisers has an important effect on droplet size; the use of an electric drive obviates unwanted fluctuations in speed arising from variations in the speed of aircraft, and raises the possibility of the direct control of droplet size by the pilot during flight. It does not follow that by closely controlling, and reducing, the size of droplets, the application rate per acre can be lowered without deterioration of coverage. Apart from the effect of such factors as humidity and wind, which can cause the smaller droplets to be lost altogether, the increasing con- centration of the chemical may actually prove harmful to the plant, or it may become so viscous that the atomiser will not handle it. (It is important to note that reductions in application rate can as a rule be achieved only by reducing the quantity of carrier—usually water or oil—and not the quantity of active chemical.) There are, however, one or two chemicals of which the properties are such that they can be sprayed undiluted at rates as low as 12 fluid oz/acre with a suitable type of atomiser. The success of this "ultra-low-volume" spraying is dependent upon the use of materials which are almost completely non- volatile, since the droplets must be small—around 70 microns- to achieve adequate cover. Experimental work is now proceeding to discover additional chemicals that can be used in this manner. The foregoing has been mainly concerned with the produc- tion of droplets which are small enough to provide a go<» An indication of the possibilities of reducing spray application rate with- out sacrificing coverage is shown in this table. The performances of two hypothetical atomisers are compared with an ideal, and the theoretical amount of liquid per hectare to pro- duce 100 droplets per sq cm is shown at the foot of the table (the effects of evaporation, drift, etc are ignored) Droplet diameter (micron*) 20 30 40 50 60 70 80 90 100 110 120 200 400 600 800 Liquid needed to cover 1 hectare with 100 droplets per sq cm (litres) 0.042 0.141 0.335 0.654 1.131 1.796 2.681 3.817 5.236 6.969 9.048 41.888 335.103 1,130.973 2,680.826 Totals Example of ideal droplet spectrum (no of droplets) 1 1 1 1 IS I 1 1 1 1 1 1 1 1 100 Liquid needed per hectare (litres) 1.796 1.796 ' Example of narrow spectrum (no of droplets) 9 14 II 6 6 8 20 19 4 2 1 100 Liquid j needed per hectare (litres) 0.004 0.020 0.037 0.039 0.068 0.144 0.536 0.725 0.209 0.139 0.090 2.011 Example : . .. of wide Liquid spectrum ne.ed!f-(no of Per *f ?P droplets) ^JWW_ 14 24 43 16 2 1 100 0.0« 0.034 0772 67« 6.702 11.310 25.526
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