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Aviation History
1956
1956 - 0336.PDF
336 FLIGHT, 23 March 1956 EFFICIENCY IN CROP SPRAYING . . . give (1) reduction in droplet surface area and so reduce losses by evaporation; (2) reduction in loss by drift. The chemical treat- ment involved may also place limitations upon the droplet size. Experience with the Tiger Moths in the Sudan has shown that a spray composed of droplets as small as 46 /* can be successfully applied on crops from aircraft even in high winds. Rotary Atomization. The working principle of all rotaryatomizers is that liquid fed on to a spinning disc will be thrown off the periphery in droplets of uniform size—provided the liquidhas attained the disc's full circumferential speed. The droplet size is controlled by the diameter of the disc and its r.p.m.Practical application of the spinning disc for agricultural spraying is severely limited by the low feed of liquid possible before "slip"takes place and rotary atomization breaks down. Mutliple spinning discs and brushes have been tried;, but, dueto slip, true rotary atomization with this equipment is not possible at the rate of feed usually required in commercial spraying work.The surface of the rotating cage of the Micron atomizer main- tains rotary atomization up to a feed of several gallons per minute.Atomizing capacity is a direct function of the surface area of the cylinder, a typical example being 4in in diameter and 14in long.The atomizers are driven by windmills and the blades are adjust- able to give the desired r.p.m. and droplet size. Normal atomizerspeed is 10,000-15,000 r.p.m. Effect of Wing Vortices on Swath. On a light aircraft only twoof die atomizers need be fitted. These are placed at the wing tips, so that the tip vortices spread the spray stream into awide swath. With a Tiger Moth operating in still air, swath width proved to be about 60 yd, e.g., six wing-spans. This remarkablywide swath develops because the small droplets have negligible inertia and are therefore readily carried by the disturbed air ofthe vortex. For crop spraying a flight spacing of 20-30 yd proved desirable, so that the area treated was sprayed at least twice andan even cover was guaranteed. A very important effect resulting from the use of only smalldroplets is die excellent under-leaf cover attained, the spray being worked right into the crop by the aircraft turbulence. Drift Spraying. With conventional equipment, the unevendroplet spectrum emitted is often said to be an advantage when using drift spraying or cross-wind techniques. The larger dropletsland almost beneath the aircraft, whilst the smaller droplets fall more slowly and carry farmer down-wind. This effect gives thevery wide swath desired. In fact, however, efficiency suffers be- cause of the unevenness of the cover achieved. At the up-windend of the swath cover is poor, as it is composed of limited numbers of large droplets; down-wind, density of cover improves until apoint is reached beyond which the large numbers of small drop- lets no longer contain sufficient chemical to be effective. However, with the Micron equipment a much more even coveris obtained over a wider effective swath-width. This is brought about by the vortex effect, which spreads the spray stream inthe vertical sense. The droplets in the lower part of the vortices land immediately beneath the aircraft, while those above arecarried progressively farther down-wind proportionately to the height to which they were originally carried by the vortices. Theeliptical cross-sectional shape of the swath formed by the tip vortices results in a very even "fall out" of droplets. Since the droplets are exposed to evaporation in the air fora varied period, a carrier liquid having a low volatility is desirable. Space Spraying. Space spraying is the term applied to opera-tions such as spraying insecticide upon or within swarms of flying locusts. The object is to fill the maximum air space with a sprayor mist of lethal droplets. Here again uniform droplet spectrum is desirable to ensure production of the largest number of lethal The "Micronair" fan-driven rotary atomizer. Liquid fed on to the rotating cylinder (4in diameter and 14in length in this example) is thrown oft the periphery in droplets of uniform size. droplets from the available spray liquid. If 50 /* droplets will dothe job it is exceedingly wasteful to use oversize droplets. The dispersion of the spray stream emitted from the atomizers mountedin the wing vortex substantially increases the amount of lethal air space on each spraying pass. Performance in the Field. Field trials upon cotton crops in theSudan, using Tiger Moths fitted with Micron rotary atomizers set to produce 80 t* droplets, showed that about 90 per cent ofthe liquid sprayed was in droplets of plus or minus 10 /* of the desired size. The test was made with microscope slides placedat ground level, half way up the plants and towards the tops of the plants. Samples of the spray were also taken upon hand-held slides as it was leaving the wing-tipj these confirmed the droplet samples taken within the crop. With die atomizers set to produce 46 p droplets, control wasless precise because of the minute size of the droplets, and the varying length of time for which they were exposed to evapora-tion whilst in the air. It was found that 70 per cent of the liquid sprayed was in droplets of plus or minus 10 // of the desired size.The dense cover achieved was remarkable when less than a quart of liquid (0.2 gal) per acre was sprayed in droplets of this size.An average deposit of 1,800 droplets/sq in was recovered through- out the crop. Underleaf cover was 30 per cent. This figure forunderleaf cover is the highest yet recorded for spray applications from fixed-wing aircraft and equals the best recorded figure forhelicopter spraying. This particular treatment gave successful insect control. In service the equipment was completely trouble-free. Duringthe first few days of operation the atomizers were stripped and cleaned at the end of each day's work, and ball and roller raceswere repacked widi grease. As experience was gained, these daily inspections were discontinued and, for the last few weeks ofspraying, the operations of stripping, cleaning and greasing were abandoned altogether. No blockages were experienced in the spray rig, although thechemical used was pumped straight into the aircraft from drums, unfiltered. The only filter used in the plumbing of the aircraftis a coarse-mesh "spanner catcher" filter in the filler neck of the chemical tank. The reason for the lack of blockages of a kindso often experienced with aircraft spraying systems is the absence of any small constrictions in the atomizers and plumbing. Installation in the Tiger Moth. In the Tiger Moth equipment,the spray liquid is fed by gravity to a windmill-driven centrifugal pump placed beneath the aircraft. From the pump the liquidunder pressure is released to the wing tips via an on-off cock and a "T" connection, or recirculated to the chemical tank through agate valve. Pressure in the system is adjusted by varying the flow through the gate valve, which is controllable by the pilot.A pressure gauge is fitted in the cockpit. Spring-loaded pressure shut-off valves are positioned at the atomizers and are set to closewhen the pressure at the wing tips falls below lOlb/sq in, thus ensuring a positive, drip-free shut-off. The pump and valves are standard commercial components.The tank fitted in the aircraft is a synthetic rubber bag clipped into the fertilizer hopper of a standard topdressing conversionof the Tiger Moth as supplied to New Zealand. A levered-float- type contents gauge calibrated from 0 to 65 gal is fitted. Spraying with the Tiger Moth. The Tiger Moth sprayersshowed up well in Sudan operating conditions. Although a fine- pitch airscrew was fitted there was no evidence of engine over-heating. Thirty different pump schemes (privately owned cotton- growing projects) along the Blue and White Nile were sprayed,and no special landing strips were required. The low-pressure tyres enabled landings and take-offs to be made safely from un-prepared open ground close to the cotton field. Experience proved that pilots soon acquired a technique forjudging the necessary drift allowances for satisfactory spraying in winds up to 20 m.p.h. The aircraft were flown at a wheel heightof about 5ft over the crop, although results proved that small variations in height had no apparent effect upon cover. Whenthe wing-tip installations were inadvertently flown through tall grasses, bushes, etc., during spray runs, no damage to rotatingcomponents occurred. Two points emerged from the experience gained in the Sudan.Firstly, effective crop spraying can be carried out with much finer droplet sprays than are conventionally used; with the exception offogging apparatus, conventional methods of application in current use have not been capable of such fine atomization. Secondly,the use of small, uniformed-sized droplets opens the way to substantial reductions in the volume of carrier liquid necessaryfor a successful chemical treatment. In practice a reduction in the application rate from two gallons to half a gallon per acre morethan doubles the daily work output of an aircraft.
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