Research on Aerial Spraying

J. S. Yuill, D. A. Isler, George D. Childress.

Some day it may be said that the air age in insect control arrived with the discovery of the unusual values of DDT during the Second World War.

In the two preceding decades, the application of insecticides by airplane had been tried against various pests, but the method was not used extensively except over cotton fields. Its advantages were recognized large acreages could be covered quickly with no mechanical damage to the soil or to plants, and forest and swamps and other inaccessible places could be reached by air. There was one great limitation though: The large quantity of insecticides that had to be applied for satisfactory control made the cost too high, even when the poisons were applied as undiluted dry powders. Liquid sprays, being less concentrated, required even greater quantities.

DDT changed that situation. In their search for better insecticides for combating malaria-carrying mosquitoes in the Pacific and other war theaters, entomologists found that DDT in an oil-solution spray gave good results when as little as one-fifth to one-fourth pound per acre was used. Engineers developed spraying apparatus for several types of military planes, and before long entire islands were being sprayed as a routine protective measure against mosquitoes and flies.

The end of the war brought a great demand for adapting aerial spraying to a variety of civil needs. Stimulating influences were the publicity given wartime developments, the availability of war-surplus airplanes at low prices and former military pilots who wanted peacetime occupation in aviation, the discovery of other insecticides, and increasing labor costs. Farmers, owners of timberlands, public health authorities, and we all became air-minded about insect control. Hopes were so high, in fact, that some people got the idea that airplanes and the new insecticides would quickly end all insect problems.

But we soon learned that man's war with insects was not yet over. More was needed than a mere abundance of planes, pilots, and DDT. Much of the wartime development had been made in haste to meet specific military requirements, to get a job done, regardless of cost; in peacetime the idea is to do a job but to do it effectively and economically. After the war, therefore, it was necessary to do a great deal of research to reconvert wartime developments to peacetime uses.

Several Federal and State agencies and many commercial operators conducted the research or assisted by furnishing equipment for making experimental control tests. The investigations have centered on the development of more efficient distributing apparatus and more effective insecticide formulations and the improvement of aircraft for insect-control operations.

Their methods have included generalized observations or appraisals in the field, trial-and-error experiments of limited scope, and broader studies of the principles governing the dispersal and deposition of insecticides from aircraft. Their objective has been to develop wider uses for aerial application of insecticides and to apply them better, faster, and cheaper. Because sprays are less affected by wind and adhere better to foliage, the greater emphasis has been placed on spraying equipment. In the first year or two after the war a great variety of spray equipment was being used. Experimentation and experience, however, have gradually narrowed the field to three main types, boom and nozzles, rotary devices, and exhaust sprayers.

The boom and nozzle sprayer was most commonly in use in 1952. Originally developed for light planes, it has been adapted to large transports. The sprayer usually consists of a spray tank carried inside the plane from which the spray liquid flows to a wind-driven pump. The pump forces the liquids into a tubular boom mounted beneath the wing (beneath the lower wing of biplanes), from which it is discharged as a spray through atomizing nozzles. The spray is turned on or off by a quick-opening gate valve, and a constant pressure in the spray lines is maintained by the use of an adjustable pressure regulator installed in the line between the pump and gate valve.

The chief advantages of this sprayer over others are the simplicity of its installation and maintenance, the ease with which the degree of atomization of the spray or its rate of application can be changed (it is necessary only to changes the size or the number of the nozzles); the use of a pressure regulator, thereby insuring a constant pressure in the system and a uniform discharge rate; and the fact that any excess flow of liquid from the pump is returned to the tank through the bypass from the pressure regulator, thus providing agitation or stirring action in the tank.

VARIATIONS in design have been made for specialized jobs. For example, the boom has been placed inside the wing, and nozzles have been attached to it by short pipe connections that extend vertically beneath the lower surface of the wing. In areas where uniformity of spray coverage has not been a prime requirement, other modifications have been to place the nozzles in Clusters near the wing tips, on the rear edge of the wing, and on the tail assembly. Those installations improve the flight performance of the plane by reducing the air resistance, but they do not allow rapid adjustments of flow rate and atomization, which may be necessary for controlling different pests.

Considerable work has been done on adapting standard pumps and developing special pumps for the sprayers.

Both centrifugal and positive-displacement types have given satisfactory performance. The latter develop higher pressures but often are subject to excessive wear when they are used with certain wettable powders that contain abrasive materials.

Other developments include devices for driving pumps directly from the airplane engine, by hydraulic systems, or by electric motors; the substitution of aluminum for brass or iron to reduce weight; special pump bearings; and pump packing and rubber parts resistant to the solvent action of sprays. Some attempts have been made to eliminate the pump entirely and to depend on gravity flow of the spray liquid from the tank to the boom. It has been found, however, that gravity systems do not deliver at a uniform rate unless some means is provided to compensate for the decreasing hydrostatic pressure as the tank empties.

Many atomizing devices, such as nozzles, jets, slotted orifices, and small venturi tubes, have been tested with varying success. None has been developed which will break up the liquid into drops of a uniform size. In general, though, nozzles that discharge the spray either in a hollow-cone pattern (similar to that of the common sprinkling nozzle) or in a flat fan-like sheet have been the most satisfactory and are the ones most commonly used.

Rotary sprayers were originally developed for applying oils and concentrated slurries of the older type of insecticides, which were too thick to go through pumps. Later they were used for other materials. Some mechanical improvements have been made in postwar models for distributing the newer insecticides, but those sprayers have been less popular than the boom and nozzle type.

The distinctive part of a rotary sprayer is the atomizing unit. It has a shaft with suitable housing and bearings; in front is a small, wind-driven propeller, and on the other end is a series of concave disks or circular wire brushes. The units may be placed on the wings or on outriggers on the sides of the fuselage. Either way, the shaft is parallel to the fuselage. In flight the liquid flows by gravity from the tank to the center of the disks or brushes. It is then thrown outward by centrifugal force to the periphery of the rotating units, where the passing air shears the liquid into drops. One can change the output by regulating the rate of flow of the material to the units. The speed of rotation, the number and spacing of disks or brushes, and, in the latter, the size of the individual bristles govern the atomization.

Exhaust sprayers, first made for mosquito control, were designed to produce a cloud of spray like the mist sometimes applied inside buildings. The spray liquid is atomized by injecting it into the exhaust of the airplane engine. Usually the exhaust pipe is extended somewhat beyond the engine, and the liquid is introduced into the throat of a venturi or into a special atomizing head on the end of the pipe. The apparatus must be carefully designed for each engine because any restriction in the flow of the exhaust gases may create a dangerous back pressure against the engine. Since the war a few exhaust sprayers have been used in combating some species of mosquitoes and other biting pests, but they have not come into general use for two reasons: The application rate is too low to kill many kinds of insects, and the spray is so fine that much of it may be carried away by wind or may evaporate before reaching the ground.

Work with dusting equipment has been directed mainly toward getting a wider and more uniform distribution of the materials beneath the plane. The materials usually are discharged from a spreader, like a venturi, on the under side of the fuselage. Consequently a heavy deposit frequently forms along the flight lines and the lateral spread is limited. Efforts have been made to correct the condition, chiefly by changing the design of the spreaders. Some redesigned spreaders have wide openings at the discharge ends or longitudinal deflecting vanes so arranged that the dust is thrown outward on a diagonal. Other spreaders are bifurcated, each branch being curved outward for the same effect. In the project for development of an agricultural airplane, described later, plans have been made to try building streamlined dusting units into the wings.

One advance in a special field is the development of equipment to distribute grasshopper bait from a multi-engine plane. Such poisoned baits have been applied by small aircraft, but the planes cover only limited areas. In order to combat extensive outbreaks, therefore, a bait spreader was designed for a C 47 transport plane. A large hopper holding 8,000 pounds of dry bait was built into the cargo space. A large air duct on each side of the fuselage extends from an opening near the leading edge of the wing, along the floor of the cargo space, and opens to the outside again near the rear of the fuselage. In operation, vaned rollers feed the bait from the hopper into the ducts. The flow of air carries it to the outside. Such a plane can treat 10,000 acres in a day, compared to 1,000 acres for the biplanes commonly used in crop work. The equipment has been modified to permit its use for applying sprays as well as baits by installing removable tanks inside the bait hopper.