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Technical Bulletin No. 1110

U. S. DEPARTMENT OF AGRICULTURE

Studies of

AIRPLANE SPRAY-DEPOSIT PATTERNS at Low Flight Levels

By Joseph C. Chamberlir Charles W. Getzendaner Harold H. Hessig, and V. D. Young.

Technical Bulletin No. 1110, May 1955

U. S. DEPARTMENT OF AGRICULTURE

Contents Page

Introduction 1 Review of literature 2 Spray equipment 2 Methods 3

Determination of spray-deposit rates 3

Photographic analysis 6 Analyses of sprays by use of pea

aphids 7 General characteristics of airplane

sprays 7 revolution of the spray curtain_ _ 7 Gross swath widths 10 The effective swath in prelimi-

nary studies with DDT and oil 12

Effect of boom position 15 Analysis of the spray-deposit pattern. 16

Patterns from 1-foot boom segments 16

Page Analysis of the spray-deposit pat-

tern—Continued Theoretical patterns 23 Patterns from a tail boom 24 Basic variability of the spray-de-

posit pattern 25 Patterns from variations in noz-

zle placement 29 Patterns from different types of

nozzles 29 Special nozzle arrangements 32

Combinations of fine- and coarse-spray nozzles 32

Other arrangements 34 Effect of skid fin, squared wingtips,

and skid plates on spray patterns— 40 Discussion 41 Summary _ , 44 Literature cited 45

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Studies of Airplane Spray-Deposit Patterns at Low Flight Levels^

By JOSEPH C. CHAMBERLIN, CHARLES W. GETZENDANER, entomologists, and HAROLD H. HESSIG,^ airplane pilot, Entomology Research Branch, and V. D. YOUNG,^ agricultural engineer, Agricultural Engineering Research Branch, Agricultural Research Service

Introduction Eesearch on the development and

improvement of methods of con- trolling insects on vegetables and related truck crops with insecticides applied by aircraft was initiated in 1947. These studies were made jointly by the Bureau of Entomol- ogy and Plant Quarantine and the Bureau of Plant Industry, Soils, and Agricultural Engineering. The headquarters were at Forest Grove, Oreg., but some of the field experiments were conducted in the Blue Mountain pea-growing areas of eastern Oregon and Washington. Preliminary tests showed sprays to be more effective than dusts against the pea aphid {Macrosiphumi pisi (Kit.)). Subsequent studies have been concerned chiefly with the improvement of equipment and spray patterns for the control of this insect.

The primary objectives were (1) to determine for low flight levels the characteristics of a spray deposit from a symmetrical disposi- tion of nozzles along a full-span underwing boom, (2) to chart the spray-deposit pattern at the 2- and 10-foot flight levels for each 1-foot segment of the usual underwing boom as well as for a short tail boom, (3) to study the effect of spray atomization on the uniformity of deposit rates, and (4) from these data to ascertain the length of

boom and arrangement of nozzles required to obtain the most even and economical distribution pattern for these flight levels.

Accordingly, the spray-deposit patterns were analyzed in relation to the various aerodynamic forces, to flight levels, to swath widths, to nozzle type and placement, and to the buildup of low-deposit areas within the treated swath wnth a corresponding decrease of high-deposit areas. These objectives have not been completely realized, and the tentative conclusions presented are subject to revision as further knowledge is obtained. However, it is be- lieved that a substantial beginning has been made.

Among the limitations of the study is the fact that most of the data have been obtained with a single type of airplane. Other types or even the same type of airplane with higher or lower powered engines will no doubt yield results that differ, in detail at least, from those here presented. No attempt has been made to study characteristics of sprays at flights above 50 feet ; emphasis was placed upon ap- lications made at vine-top and up to 15-foot flight levels.

Each test was conducted under cool and nearly windless conditions (less than 1 mile per hour). Even with this limitation, wind drift was an important factor. Most of the tests were conducted with nozzles

* Submitted for publication September 3,1954. ^ Resigned on November 7, 1953. ^Tlie authors of this joint contribution are shown in alphabetical order, neither

seniority nor precedence being implied. Lee Stevens and W^alter Wilson, of Forest Grove, Oreg., and W. C. Cook, of Walla Walla, Wash., contributed substantially to various phases of this work.

TECHNICAL BULLETIN 1110, U. S. DEPT. OF AGRICULTURE

delivering a coarse, medium, or medium-fine spray. More extensive tests, especially with very fine sprays, may show some differences, although it is doubtful that the fun- damental concepts arrived at would be significantly changed.

Review of Literature During the 30 to 35 years since

airplanes were first tested as a means of applying insecticide sprays and dusts, widely scattered reports have been published on the subject, most of which are of ex- ploratory work. A bibliography covering the period 1919 to 1945 has been compiled by Hawes and Eisen- berg (^),* and the most important references up to 1948 have been re- viewed by Brown (1). Yuill, Ea- ton, and Isler (S) have summarized available information on aircraft spraying for the control of forest insects.

The literature shows that extensive rule-of-thumb knowledge has been accumulated on both the ad- vantages and the disadvantages of the aerial method. It also shows a qualitative realization of many of the factors that affect the distribution of sprays and dusts, of the ad- verse effects of winds and thermal currents, and of the importance of flight levels^ as related to swath widths.

Much of the work published thus far has dealt with the control of insect vectors of disease (e. g., mos- quitoes) and of forest insects. In the control of these insects low dosages of the insecticide in maximum possible swath widths are applied at flight levels mostly above 50 feet. Airplane applications of insecticides and herbicides on croplands at high dosages and low flight levels—1 to 15 or 20 feet—^have also

been reported. On the other hand, little quantitative information has been published on the distribution patterns of sprays and dusts applied at the low flight levels, or on the effect on these patterns of the air currents set in motion by the airplane itself. The outstanding concepts along these lines seem to have been based mostly on scanty and inadequate data. The literature has not been searched thoroughly for findings similar to those presented here, nor has an attempt been made to deny or disprove contradictory concepts.

Spray Equipment Most of the tests reported herein

were conducted with a Stearman biplane (fig. 1) equipped with a 300-hp. engine and a constant- speed propeller. The spray system consisted of an interior supply tank; a 1-inch centrifugal pump mounted below the fuselage equidis- tant between the landing-gear struts and powered by a 26-inch, wind-driven, 4-bladed wooden pro- I)eller (fig. 2, A); a diaphragm pressure-regulating valve; a shut- off valve; and a 30-foot steel boom suspended beneath tlie lower wing at a point midway of the wing chord and horizontal to the ground (no dihedral), with nozzle outlets spaced every 4 inches for the full length. The spray system had a capacity sufficient to provide a maximum discharge in excess of 80 gallons per minute. For certain tests a 5-foot tail boom was mounted below the aft section of the fuselage near the tail assembly. It was provided with four nozzle outlets at 9-inch intervals on both sides of the center line of the fuselage (fig. 2, B).

* ItaUc numbers in parentheses refer to Literature Cited, p. 45. ^ FUght level as employed herein means the vertical distance between the landing

wheels and the ground or vegetation.

STUDIES OF AIRPLANE SPRAY-DEPOSIT PATTERNS

PioiiRK 1.—A Stearman l)lplane applying spray at low level for control of the pea aphid (111 (¡uinirig peas near Walla Walla, Wash., June Í), li)")!.

The nozzles were of four principal types^—(1) the poi)pet nozzle, il sprin<>;-loaded popjjet-valve nozzle producing a hollow-cone spray in which the orifice size and discharge I'ate are governed by pressure (the greater the pressure, the greater the effective opening) ; (2) the cap nozzle, a hollow-cone nozzle having a turbulence chamber and interchangeable orifice caps (orifice sizes used were Nos. 2, 3, 5, 8, and 10, denoting orifice diameters of 0.078, 0.094, 0.125, 0.156, and 0.187 inch, respectively) ; (3) the jet nozzle, a diaphragm check valve provided with interchangeable cap orifices of various sizes, from whicli the spray is discharged as a solid stream (orifice sizes as for the cap nozzle) ; and (4) the disk nozzle, a high-pressure, hollow-cone nozzle containing a whirl plate and an interchangeable orifice disk (Nos. 6 and 8 orifices, measuring 0.094 and 0.125 inch).

The poppet nozzle functions as both a nozzle and a check valve. The cap and jet nozzles were used with individual diaphragm check valves, and the disk nozzle with a ball check valve.

Methods

Determination of Spray-Deposit Rates

The spray emjiloyed in these studies was water to which a carmine dye had been added as a tracer at the rate of 1 pound to 50 gallons. Samples of the discharged spray were collected on thin, stainless- steel plates measuring 3 by 6 inches. Quarter-inch rivets, one soldered at each corner, served as legs to fa- cilitate handling the plates and as spacers to minimize contamination. Sampling plates were laid across the potential swath at intervals of 1 or rarely 2 feet (ñg. 3). Im- mediately after the applications the plates were collected m light-tight carrying cases to jjrevent fading of the dye and taken to the laboratory for analysis.

The analytical procedure was to dissolve the dye from each sampling plate in 50 ml. of water and measure the color intensity of the solution by means of a logarithmically scaled photoelectric colorimeter. The si)ray dejyosit was computed, in gallons per acre, for each sample by comparing the individual sample

TECHNICAL BULLETIN 1110, U. S. DEPT. OF AGRICULTURE

FIGURE 2.—A. Wind-flriven pump assembly pmployed in airplane spruv tests durin 1949-52 ; B, experimental tail boom installation on a Stearman biplane. 195Ü.


FIGURE 3.—A, Light-tight carrying cases used in storing and transporting spray- deposit sampling plates ; some of the plates are slunvn racked in the open case. B, Laying out a grid of sampling plates on an airport runway for studies of spray- deposit patterns. Hillsboro, Oreg. 1952.

6 TECHNICAL BULLETIN 1110, U. S. DEPT. OF AGRICULTURE

values with the color intensity of a standard dilution of the stock solution (1 ml. of the original spray stock to 399 ml. of water).

The equation employed in these calculations is

R= O

where R is the rate of spray deposit per acre, B is the colorimeter reading of the standard dilution of the stock solution, G is the colorimeter reading of a standard dilution of the dye deposit recovered from an individual sampling plate, and K is a constant.

The numerical value of K is determined by the size of the sampling plate, the dye concentration, and the units of measurement employed (for example, pounds or gallons per acre). By the standard procedure here employed, the numerical value of K for gallons of spray per acre is 11.508.

In some phases of this study, the amount of spray recovered at any point or within any zone is ex- pressed as a "percent" of the total recovery from all plates. This is the ratio of the deposit rate at any given point to the sum of all the deposits for the particular transect under consideration. The mean rate is obtained by dividing the sum of the rates for the sampling points by the number of points in the swath. Kate and percent plot- ting differ only in the scale employed. However, the percent and gallon-per-acre equivalent will vary between deposit curves unless the sums of the total rate values for each curve are equal.

Photographic Analysis

Considerable use was made of photographic techniques in the study of the behavior of the spray as it leaves the nozzle and as the aerodynamic forces act on it before it finally settles to the ground.

By one procedure color transpar- encies were taken at speeds of 1/500 and 1/1000 second during the spray application (especially the colored spray) to obtain a visual record of the spray curtain as it developed in the air relative to the position of the airplane and to the ground and sampling stations.

By another procedure two 16- mm. motion-picture cameras were used for taking both black-and- white and colored movies. One was a small electrically operated camera with a 1%-inch focal-length lens and having film speeds of 16, 32, and 64 frames per second (normal and two degrees of slow motion, respectively). The other was the usual 1-inch spring-wound camera with five film-drive speeds ranging from 8 to 72 frames per second and with a revolving mount for quick changing of lenses (1, 2, and 71/2 inch).

An approaching airplane flying at 80 m. p. h. and photographed with the 1-inch-lens movie camera produces an image about one-fourth the frame width when approximately 400 feet away. In 3 seconds the airplane has traveled 352 feet and the image has filled the frame; in another second it has passed the camera. On the other hand, with a 71/2-inch lens the image of the airplane is one-fourth the width of the frame when 2,500 feet away; it takes 20 seconds for the airplane to approach close enough (about 600 feet) to fill the frame, and another 5 seconds to pass over the camera, so that a sevenfold increase in time was allowed for pho- tographing the spray action. By this procedure an application on a 600-foot plot may be filmed from a safe distance with an image change from three-fourths to full frame in 5 seconds, which, if filmed in slow motion (72 frames per second), may be projected to run for 88 seconds and thus allow ample time for a


study of the development and behavior of the spray curtain.

Analyses of Sprays by Use of Pea Aphids

In 1948 extensive single-swath applications with DDT in light oil for control of pea aphids on canning peas were made to ascertain the maximum effective swath widths at low to medium flight levels. The mean population counts gave a fairly accurate outline of the swaths. DDT was employed as the test insecticide because of its contact effectiveness against the pea aphid. With parathion and similar insecticides the boundaries are much less distinct.

General Characteristics of Airplane Sprays

Evolution of the Spray Curtain

With ground equipment operat- ing at a very slow speed (less than 5 or 6 miles per hour), the discharge pattern from an evenly spaced series of spray nozzles depends largely on the amount of overlap from the discharge cones of the individual nozzles. A series of nozzles can be set up to give a fairly uniform deposit except for the effect of surface winds and possible differences between nozzles. On the other hand, with aerial equipment oper- ating at 60 to 150 miles per hour, violent air currents are set up which modify the initial symmetry.

These air currents result primarily from the slipstream (vortex) of the rotating propeller and from the vortices generated at either wingtip by the outward and upward flow of air in its spiraling rush to nil the temporary vacuum created by the moving wing. As with the wake produced in water by the passage of a ship, these vortices, plus other effects such as parasite

327380°—55 2

drag components, constitute the elements of an expanding three-di- mensional aerial wake in which the spray droplets are suspended until gravitational pull predominates and they settle out. Obviously, the smaller and more buoyant the droplets the longer they are carried in the wake. The borders of the swath, therefore, are indefinite and merge gradually into the adjacent areas.

The evolution of the spray curtain from an airplane in low-level flight as seen from a, slight angle under dead calm is shown by the motion picture sequence in figures 4 and 5. The development of the wingtip vortices is particularly well shown,, as is the persistence of the finer spray droplets in the still air after the completion of the run. The spray curtain from such an application superficially shows excellent coverage, not only across the wingspan of the airplane but substantially beyond each wingtip.

The uniformity of spray deposition within a treated swath cannot be adequately estimated from a sequence of photographs such as is shown in figures 4 and 5. In this sequence the spray curtain was produced from the uneven nozzle arrangement visible in figure 1. From an even nozzle arrangement, and at a similar flight level, the development of the spray curtain would appear the same. However, in both cases the actual deposition of spray across the treated swath varies greatly from point to point. This variation in spray deposits from an even arrangement of No. 10 cap nozzles is shown in figure 6. Deter- minations were made at 1-foot intervals both across a 50-foot swath and along the swath for 8 feet. The maximum, minimum, and mean of the eight deposit values were determined for each distance from the center of the swath. The high peak


FIGURE 4.—Airplane spray application at low level showing the development of the spray curtain and the wlngtip vortices. Photographs were taken at intervals of about 0.3 second. Walla Walla, Wash., June 9,1951.

STUDIES OF AIRPLANE SPRAY-DEPOSIT PATTERNS 9

FiGuEE 5.—Continuation of spray-application sequence in figure 4, showing the spray curtain at the end of the run and after 0.2,1, and 2 seconds.


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25 20 15 10 5 FEET LEFT OF CENTER

5 Í0 15 20 FEET RIGHT OF CENTER

FIGURF: 6.—Variation in spray deposit from foot to foot along and across the treated swath in a typical application from uniformly spaced nozzles on a high-lift Stear- man biplane flying at a 1-foot level. October 2,1952.

left of center, the trough right of center,^ and subsidiary peaks near either wingtip are typical, although for this particular application the wingtip peaks are not so far outboard as usual.

In low-level applications (figs. 4 and 5) the spray curtain is momen- tarily compressed against the ground. This causes the spray re- leased within the outer wing sections to be squeezed or forced beyond the wingtips into a flat-bot- tomed spiral. At higher flight levels this ground-cushion effect is not apparent and the wingtip vortices develop normally. Owing to the low-angle perspective, these vortices, which actually extend further outboard before settling than at lower flight levels, are masked by the descending spray curtain until it has pretty well settled. Figure 7 shows four successive scenes from a 10-foot application in still air and

figures 4 and 5, scenes from the low- level application.

Gross Swath Widths

It is evident from the preceding discussion that an airplane spray swath is not sharply defined and that its width varies. To obtain data on swath widths, colored sprays were applied at various flight levels to wheat stubble so that deposits could be studied visually and photographed for permanent rec- ords. Applications were made with an asymmetrical nozzle arrangement at flight levels ranging from 2 to 100 feet. Height of application was gaged by the use of helium- filled, captive marker balloons. Markers were placed on the ground at intervals from the center of each plot, so that swath widths could be measured from aerial photographs. These swath widths were determined to be 48 to 50 feet for the

"Thronghont this paper the terms "right" or "left of center" refer to lateral position relative to the direction of the line of flight.


1* rouKE 7.—Airplane spray application at a 10-foot fliglit level, showing the spray curtain, the settling of the curtain, and the wingtlp vortices. Walla Walla Wash June 9, li)51. ' '


2-foot, 65 feet for the 10-foot, and 80 feet for the 20-foot flight level. Swath widths were not clearly out- lined for the 50- and 100-foot flight levels, owing to excessive drift from a cross wind of less than 1 m. p. h.

Aerial photographs clearly showed some of the spray-pattern characteristics of airplane applications, such as the lighter zone of deposit slightly to the right of center, the heavier deposit along the outside edges, the scalloped boundary effect from the wingtip vortices, and the effect of the very light cross wind. The cross wind compressed and narrowed the swath on the windward side but attenuated and widened it on the leeward side. Consequently, the swath boundaries were most clearly visible on the windward side.

Preliminary tests were conducted in which colored sprays were applied on snow as a possible tech- nique for studying swath widths and spray patterns. The snow held the color of the spray exceptionally well, especially where a water-solu- ble dye was employed. The method should prove of value in areas where snow cover can be counted on for such tests.

While gross measurements are of value, it is clear that swath widths so determined do not necessarily reflect effective swath coverage.

The Effective Swath in Preliminary Studies With DDT and Oil

An effective swath is the zone over which spray is deposited at a rate sufficient for the purpose intended. It varies with the equipment employed, the height of flight, the weather (particularly the wind) at the time of application, the character of the plant canopy, the purpose of the application, and, in insecticide applications, the suscepti- bility of the insect to be controlled.

Studies on effective swath w^idth undertaken in 1948 were restricted

largely to single-swath applications for control of the pea aphid on canning peas. Applications with 4.5 percent of DDT in light oil at 2 to 3 gallons per acre were made on plots 100 feet wide and 500 to 1,000 feet long. Forty-four disk nozzles were evenly spaced on a 30-foot boom. Duplicate counts of aphid populations were made at 5-foot intervals along each of five replicate transects across the line of flight. Data for certain of these tests at three flight levels are summarized in table 1.

The effective swath width was about 50 feet for flight levels of 1-3 and 3-5 feet. For a 20- to 25-foot flight level it was approximately 75 feet. These data are in substantial agreement with the 50- and 80-foot swaths observed at the 2- and 20- foot flight levels in applications made to wheat stubble.

The degree of pea aphid control varied with the flight level of application. The two low-level applications gave 83-92 and 64-79 percent control, respectively. For the highest flight the mean control was even lower (49 percent), but this was probably due to a lower rate of application resulting from the wider sw^ath covered.

The effective swath consists of an underwing zone where control is definitely the best—averaging 81 percent for the 11 plots—and a wingtip-vortex zone extending approximately 10 feet both sides of the underwing zone for the low- level flights and more than 20 feet for the highest flight, with an aver- age control of 71 percent. A drift zone is also recognized, the width of which varies with the cross wind at the time of application. There is no obvious correlation between the amount of this drift and the mean observed velocity of the wind at the time of application, probably because of lack of sufficiently accurate measurements. Pea aphid control in the drift zone, while evident, is


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significantly inferior to that obtained in the wingtip vortex, averaging 46 percent. Finally, there is a check, or untreated, zone lying beyond the drift zone on both sides of the swath. In this area insect populations are unaffected by drift to any measurable extent and these values are emploj^ed in computing control.

The reason for the observed difference in pea aphid control between the 1- to 3- and the 3- to 5- foot levels is a puzzling one. No significant difference in effective swath width is apparent from the data in table 1, and the conditions of application were about the same ; nevertheless, substantially better results were obtained at the lower flight level. Experiments conducted since 1948 have failed to clarify this point, and further work is needed.

Mean population data for selected tests from the series sum-

marized in table 1 and for a repre- sentative check test were plotted for each collection point for ready visual comparison. The numbers of aphids were computed to a common base of 100 aphids per tip.

Figure 8 shows the basic distribution pattern of mean aphid populations in an untreated plot (test 14), and in plots sprayed at various flight levels (tests 12, 13, and 18) — plotted at 5-foot intervals across the swath.

There was considerable irregu- larity in the population counts taken in the untreated area over a swath 110 feet wide, w^hich reflects the uneven nature of the original infestation. Nevertheless, the counts were higher than in the sprayed plots. The populations in the treated zones were significantly highest where the application was made at 20-25 feet and lowest where made at 2 feet. Undoubtedly the decreased effectiveness of the ap-

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15 25 35 45 FEET RIGHT OF CENTER

55

FIGURE 8.—Effect of flight level of application on effective swjith width as measured by pea aphid populations. Weston, Oreg., July 18-26, 1948.


plication at the hio^hest level was due in part to the lower mean application rate. In all three tests at different flight levels populations were reduced the most in the 30-foot zone lying 15 feet on either side of the line of flight and corresponding to the area actually trav- ersed by the wings of the plane. Beyond (outboard of) the underwing zone on both sides the aphid populations were also greatly reduced, although not quite so much so as in the underwing zone. The effective aphid control showed that the wingtip zones averaged about 10 feet in width and were not oblit- erated at low flight levels even on the windward side of the plot when cross winds were less than 4 m.p.h.

The excellent inverse relation between pea aphid populations and the observed mean spray-deposit rates for the same airplane and nozzle arrangements is clearly shown for a low-level application in figure 9. Aphid-control and spray- distribution tests were conducted

with 44 disk nozzles 8 inches apart, but with No. 8 and No. 10 disks, respectively. Data for both tests show a 50-foot effective swath.

Effect of Boom Position

During July 1948 experiments were conducted near Weston, Oreg., to ascertain whether variations in the fore and aft positions of the spray boom below the wing had any detectable influence on pea aphid control with a spray containing 4.5 percent of DDT in a light kerosene.

Single-swath applications were made at flight levels of 2-3 feet with the booms mounted at the follow- ing positions: (1) In a forward position directly below the leading edge of the wing, (2) directly below the center of the wing chord (the normal or customary position), and (3) directly below the trailing edge of the wing. No differences were noted in pea aphid control for the three boom positions.

Since changes in the boom position fore and aft did not apirear to

40

CENTER

FIGURE 9.—Relation between a spray-distribution curve for application at a 2-foot fliglit level (dotted line) and pea aphid control (solid line). Aphid control plot was treated at Weston, Oreg., July 18-26, 1048. Distribution curve (mean of two tests) was determined at Forest Grove, Oreg., May 5,1949.

327380°—55 3


influence the effectiveness of the spray, the distribution patterns were not determined. The fore- going results and aerodynamic con- siderations made it seem improba- ble that such changes could play a significant role.

Analysis of the Spray-Deposit Pattern

Preliminary tests with 24 to 44 disk nozzles evenly spaced along a 30-foot underwing boom showed wide fluctuations from foot to foot in the amount of spray deposited across a swath laid down by low- level applications. Such deposit patterns are bimodal or irregularly tri modal in character, with a low- deposit zone at the right of center, a correspondingly high-deposit zone at the left of center, and weaker peak-deposit zones near or just beyond the wingtip positions, as has already been noted (fig. 6).

Further tests with different nozzle arrangements were conducted in an attempt to lower the peak deposits and simultaneously to raise the zones of low deposit. By a tri al- and-error method regular nozzles wêre grouped into various asymmetrical arrangements, sometimes in combination with jet nozzles. These tests showed that the spray pattern could be modified, and even sliglitly improved, by variations in nozzle arrangements, but that the problem was too complex to be solved by trial and error. It became obvious that further progress would require a more analytical approach.

Patterns From 1-Foot Boom Segments

Studies were initiated in 1950 to determine spray-deposit patterns for each 1-foot segment of a 30-foot boom. Three nozzles wêre employed for each segment in tests made at flight levels of 1-2 and 10 feet. The first low-flight applica-

tions were made with poppet nozzles, which produce a coarse spray. The applications wêre repeated in 1951 with No. 8 cap nozzles, which produce a medium spray. Tests at the 10-foot flight level in both years were made with the poppet nozzles only. All tests were conducted with the wind not exceed- ing 1 m. p. h. Even so, lateral drift often affected the deposit patterns, especially that portion laid down by the outboard boom segments (9 to 15 feet both sides of center). Sprays being applied from diff'erent boom segments at the two flight levels are sliow^n in figrire 10. ""

The spray-deposit curves for applications from each segment are shown in figures 11 and 12. Left and right deposit curves are shown below and above the base lines, respectively.

For both flight levels the deposit from any segment has a lateral spread ranging from about 15 to more than 40 feet. For any one of these segments the curve is much flattened, almost invariably skewed inboard toward the center of the line of flight. The curves for the lO-foot flight level are much more flattened and the overall spread is greater than for the lower flight level.

The peaks of deposit for the individual segments (figs. 11 and 12) tend to move progressively outboard with corresponding shifts in nozzle placement. This trend be- comes less distinct as the curves be- come more flattened, but is still apparent from the 12th to the 15tli segments for the low-level application. For the 10-foot flight level, on the other hand, there is no clear indication that spray discharged from the 10th to the 15th segments is carried farther outboard than from the 9th foot. This outboard trend for low-level applications is shown in figure 13, Ä. The


FIGURE 10.—Sprays being applied from 1-foot boom segments : A, At the lO-foot flight level from the eighth foi>t both sides of centei- immediately after the start of the spray discharge and before it has been influenced by the wingtlp vortices ; B, at the 2-foot flight level, showing that some of the spray discharged from the seventh foot right of center has been drawn out toward and beyond the wingtlp and into the wingtlp vortex. li)."i() and l!)."il.


40 90 20 FEET LEFT OF CENTER

FIGURE 11.—Spray-deposit curves for applications at a 1- to 2-foot flight level with poppet (dotted lines) and No. 8 cap nozzles (solid lines) for successive 1-foot boom segments from wingtips to center. Arrows indicate the direction and velocity of the wind ( m. p. h. ). 1950-51.


FEET LEFT OF CENTER ¿0 30 40 FEET RIGHT OF CENTER

FIGURE 12.—Spray-deposit curves for applications at a 10-foot flight level with poppet nozzles for successive 1-foot boom segments from wingtips to center. Arrows indicate the direction and velocity of the wind (m. p. h.). 1950.


25 20 15 10 5 " C 5 10 15 20 25 FEET LEFT OF CENTER FEET RIGHT OF CENTER

MEDIAN POSITION OF SPRAY DEPOSIT FOR EACH BOOM SEGMENT LDW FLIGHT (1-3 FTj: POPPET (COARSE SPRAV) NOZZLES: -^—^—2—r

CAP 8 (MEDIUM SPRAY) NOZZLES: -i"-—;^—-S—;.

IS

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/ 30 25 20 15 10 5 C 5 10 15 20 25

FEET LEFT OF CENTER FEET RIGHT OF CENTER MEDIAN POSITION OF SPRAY DEPOSIT FOR EACH BOOM SEGMENT

LOW FLIGHT 0-3 FT)-'^—-:^—-*—---^" MEDIUM FLIGHT (10 Ft) ° a COARSE SPRAY ONLY (POPPET NOZZLES)- CENTER VALUES (C) ARE INTERPOLATED

FiGUKp] 18.—Median points of individual spray-deposit curves foi- each 1-foot boom segment from wingtip to center : A, For applications of coarse and medium sprays at 1- to 3-foot flight level; B, for applications of coarse sprays at 10- and 1- to 3-foot levels. 1950-51.


smoothed curves (based on P>-point moving averages) for the two types of nozzles show close agreement, although the median-deposit points for the coarse spray originating from the first and second segments right of center are shifted farther to the left than are the corresponding deposits of the finer spray.

With coarse sprays at the 10-f oot flight level (fig. 13,7^) there is little or no further outboard movement between the 9th and 12th or 18th segments. The 14th and 15th segments show a distinct trend inboard, the median point for the last segment lying about the same distance outboard as that from the 8th or 9th segment. Figure 13, B shows an odd feature that may or may not prove to be significant. At the 10- foot flight level the rate of change in the outboard trend for the median points is greatly reduced between the 4th and 7th segments as compared with that between the 1st and 4th or between the 7th and 9th segments.

Spray discharged from the central boom segments is influenced mostly by the counter-clockwise rotation of the propeller, especially at low-level applications (fig. 6). This rotation strongly shifts the spray from right to left, resulting in a high-deposit zone left of center and in a corresponding low-deposit zone right of center (figs. 11 and 12). At low flight levels the expanding vortex of the propeller strikes the ground almost immediately, so that the spray is deposited before circulation has been completed and consequently distribution is very erratic. At application levels of 8 to 10 feet, the spray has a greater opportunity to circulate Avithin the propeller slipstream before it is deposited, and hence distribution is more uniform.

Data for the individual l-foot boom segments at a flight level of 1-3 feet show three distinct but

overla])ping zones: (1) A center section, including the deposits from segments 1-3 and possibly 4 feet both sides of center and lying within the propeller slipstream; (2) a midwing section, including deposits from boom segments 4-7 feet both sides of center; and (3) a wingtip section, comprising deposits from segments 8-15 feet both sides of center. Three No. 10 cap nozzles were employed per foot for the center and midwing zones and one poppet nozzle per foot for the wingtip zones. The spray pattern in each zone is shown in figure 14. Mean and extreme deposit values are shown for 16 replicate transects for the center and 8 each for the midwing and the wingtip zones.

Spray originating from the center boom section is deposited in a comparatively narrow zone with the peak at the left of center. The great variation from point to point across this zone is notew^orthy.

Spray from the midwing section is deposited outboard of the points of origin, although still largely within the wingspan. The peak of deposit lies farther outboard on the right than on the left side.

Spray from the wingtip sections is affected mostly by the wingtip vortices, and the deposit curves are flattened and only obscurely peaked. Most of this spray is deposited in a zone extending from the wingtips outboard more than 20 feet. Finally, as with the peak of the midwing deposits, the spray is carried farther outboard on the right side than on the left. This difference between the right and left wingtip vortices is show^n in figures 4 and 5. That this difference is not due to the angle from which the photographs were taken is clear from a comparison of figure 1 with the final frame shown in figure 4. Figure 1 was taken from a ]:)Osition only 3 or 4 feet to the right of the center of the line of flight, and yet


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10 15 20 25 30 35 FEET RIGHT OF CENTER

FIGURE 14.—Spray-deposit ciu-ves for applications at 1- to ^-foot flight level from the three boom sections of a high-lift Stearman biplane : A, Center ; B, midwing : and C, wingtip. 1952.


shows the difference even more dis- tinctly than does figure 4, which was taken from a position several feet farther off center.

Theoretical Patterns

To verify the accuracy of the preceding analysis the deposit data for i he poppet nozzles on the 30 individual 1-foot boom segments for the low-level flight (fig. 11) were com- bined into the theoretical spray pat-

tern that would be expected from an even nozzle distribution along the full 30-foot boom. For comparison a comparable test application was made in 1952 with a full boom. The corresponding expected and observed deposit curves are shown in figure 15, ^. Similar curves for the ISTo. 8 cap nozzles are shown in figure 15, B,

The agreement between the expected and observed patterns is at least fair, although some discrepan-

14

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40 30 20 FEET LEFT OF CENTER FEET RIGHT OF CENTER

FIGURE 15.—Theoretical spray-deposit curves (dotted Unes) for applications at 2 to 3 feet: A, With poppet nozzles ; and B, with No. 8 cap nozzles, each in comparison with the curve for an observed application (solid line). 1950-52.


cies are evident. The reason for the most obvious of these discrepan- cies will be apparent when the data on pattern variations in single flights are considered.

Two theoretical deposit curves are shown in figure 16. They were computed from the 1-foot segment data for applications at flight levels of 2-3 and 10 feet with 30 evenly spaced poppet nozzles. These data show the effect of height of application on swath width and uniformity of distribution. Unfortunately, no verification tests were conducted at the 10-foot level with an even nozzle spacing. As previously shown, the theoretical patterns agree substantially with patterns of comparable actual tests. The spray from the 10-foot level is carried farther beyond the wingtips than that from the low level and produces a

substantially wider overall swath— 65 feet compared with 50 feet at the 3-gallon per acre level. Further- more, the curve for the 10-foot flight level is flatter and more uniform.

Patterns From a Tail Boom

Tests with a short tail boom and poppet nozzles (fig. 2,^) were undertaken to determine whether spray discharged from this position on the airplane would be affected by the propeller slipstream in the same manner as was spray discharged from the center section of the underwing boom. The results for the segments situated 1 and 2 feet both sides of center for 1- to 2-foot and 10-foot flight levels are shown in figure 17. Deposit curves left and right of center are plotted below and above the base lines, respec-

14

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FFFT LEFT OF CENTER FEET RIGHT OF CENTER

FIGURE 16.—Theoretical spray-deposit curves for two flight levels—based on the same mean rate—obtained by summation of deposit data of individual boom segments. 1950.


50 40 30 20 10 C

MEDIUM FLIGHT ELEVATION OO FEET)


20 30 FEET RIGHT OF CENTER

LOW FLIGHT ELEVATION (1-2 FEET)

FIGURE 17 —Spray-deposit curves for segments 1 and 2 feet both sides of center of a supplementary tail boom (see fig. %B) for low and medium flight levels. Arrows indicate the direction and velocity of the wind (m.p.h. ). 1950.

tively. Two poppet nozzles were used per segment. These curves show few differences from corresponding ones in figures 11 and 12, except that the individual curves for the low-level application are nar- rower and more highly peaked for the tail boom than for the corresponding segments of the underwing boom. For the 10-foot level there seems to be no significant difference. A supplementary nozzle or two located in the tail area may possibly help to secure a more even overall spray distribution for low- level flights, but considerable addi- tional testing must be done before a definite conclusion can be drawn.

Basic Variability of the Spray- Deposit Pattern

It was recognized early in these studies that it w^as extremely difficult to obtain identical deposit patterns with the same nozzle arrangements. This was especially true of the central zone of the swath—i.e., within the section most affected by the propeller slipstream.

A preliminary experiment was therefore conducted to check the consistency of deposits from point to point along the swath from a single application, where conditions would be as nearly identical as possible. The spray was discharged


from three No. 8 cap nozzles mounted on the 3d 1-foot boom seo^ment right of center, and the deposit was sampled by means of 12 parallel transects placed at irregular intervals across the line of flight. This test was repeated with the nozzles mounted on the 12th segment right of center. The deposits from both segments are shown in fii^ure 18.

The deposit curves for the 3d segment varied widely along the line of flight. At 2 feet right of center, for example, the rate of deposit ranged from less than 1 to more than 3 gallons per acre. The peaks of the curves varied in position between 1 and 6 feet right of center. In other words, within the median-deposit zones, rates of deposit vary greatly along as well as across the line of flight.

Some variation also occurred for the 12th segment, although the individual curves tend to parallel each other, to reach a peak at about the same point, and to show a maximum difference in deposit rates of less than 1 gallon per acre.

In 1952 grid tests were conducted at 1- to 3-foot flight levels to determine with greater precision the longitudinal as well as the lateral variations in deposit patterns for different nozzles, combinations of nozzles, and nozzle placement. Grids comprised 8 to 32 parallel transects with sampling points 1 foot apart in both directions, thus covering an area from 400 to 800 square feet. Mean and extreme deposit rates were determined for each foot along 16 parallel transects 1 foot apart for a low-level application from 29 evenly spaced No. 8

4 \ 1

1 3 12

Q: 3 05

1

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4 6 8 10 12 FEET FROM CENTER

14 16 18 20

FIGURE 18.—Variation in spray deposits along the Une of flight at a level of 1-2 feet for spray originating from the 3d and 12th 1-foot boom segments right of center September 21, 1951.


cap nozzles on a Stearman biplane ( fig. 19, ^ ). The same data plotted on an area basis and representing, in essence, a contour diagram of the sprayed area are shown in fiirure 19, B.

That such variations in deposit rates are not restricted to biplanes is shown by the comparable patterns obtained from a 2-foot flight level with a light high-wing monoplane (90-hp. engine) as shown in fig-

25 20 15 10 FEET LEFT OF CENTER


25

\J-I.0 GPA 1 /./ -2.0 G PA \2.l -J.06PA \3J-4.0GPA Í4.t- SÛGPA %5.l -6.0GPA 16./ 7.0 GPA \7.l -aOQPA \9.I-I0.0GPA

FiGUKE 19.—Spray distribution at a 3-foot flight level from evenly spaced nozzles on a Stearman biplane: A, Spray-deposit curve; B, contour diagram, showing variations in spray deposit from foot to foot along and across the treated swath. 1952.


ure 20. In this test No. 8 cap nozzles were evenly spaced 1 foot apart, except none were placed directly beneath the fuselage. The nozzle from the left wingtip ( J) was inad- vertently omitted during this test, resulting in an abnormally high rate of discharge from this point.

These examples clearly demonstrate the variations within the treated swath and emphasize the

fact that multiple transect sampling is required to give a constant mean, even under identical conditions (i. e., in a single flight).

The contour diagrams in particular show the irregular variations in areas of equal deposit for the midwing and wingtip zones. They also show widely differing rates within the propeller-vortex area where most evidences of zonation are lost,


C 5 10 15 20 FEET RIGHT OF CENTER

25

NO WIND

FIGURE 20.—Spray distribution at a 2-foot flight level from evenly spaced nozzles on a light high-wing monoplane : A, Spray-deposit curve ; B, contour diagram, showing variations in spray deposit from foot to foot along and across the treated sw^ath. September 11,1952.


or nearly so, in the biplane pattern (fig. 19, B), The still apparent zonation in the center section for the deposit from the highwing monoplane (fig. 20, B) is probably due to the absence of nozzles from the center section of the boom and from the weaker propeller vortex. Asso- ciated with these pattern irregulari- ties is the wide variation in the mean rates of deposit for individual transects along the line of flight, ranging from 2.9 to 3.3 gallons per acre for the biplane (fig. 19,^) and from 2.8 to 3.4 gallons per acre for the monoplane (fig. 20, B)^ depending upon the particular transect selected.

Zones of equal spray deposit, as already noted, are most clearly defined in the mid wing deposit zone and outboard to the edge of the swath. These zones vary greatly in lateral extent and oscillate widely along the sampled swath. This ex- plains the scalloping so frequently seen near the swath boundary, particularly where spray has been deposited on grain stubble or snow. The erratic zoning in areas of equal deposit for the wingtip boom section is shown in figure 21. Deposit rates are shown for a distance of 32 feet along the line of flight.

Patterns From Variations in Nozzle Placement

Variation in nozzle placement is effective in modifying swath widths and in changing mean deposit rates in both low- and high-deposit zones. This is clearly shown by a comparison of the deposit diagrams for uneven and even nozzle arrangements. Mean and extreme values for an uneven arrangement of 26 No. 8 cap nozzles were determined at 1- foot intervals for 16 parallel transects 1 foot apart as shown in figure 22. In comparison with the diagram for evenly spaced nozzles (fig. 19), the swath for the uneven arrangement is somewhat narrowed

for comparable deposit zones and the mean rate in the low-deposit area right of center has been substantially raised. It is also apparent that the erratically variable deposit rates in the center section are still as great for the uneven as for the original even nozzle spacing (e. g., samples showing a deposit rate of 12 to 13 gallons per acre occur within 1 or 2 feet of another sample showing a deposit rate of only 3 to 4 gallons per acre).

These findings demonstrate why it is so difficult to determine the best possible nozzle placement from single-transect data such as were obtained for the individual 1-foot boom segments (figs. 11, 12). Such data do not show the great longitudinal or lateral variation known to occur, and hence are only roughly indicative of the actual spray pattern to be expected from a specific nozzle arrangement based upon their values. Furthermore, even if they did represent the actual mean values, any addition or deletion of nozzles as a means of altering the pattern for any particular zone would also affect neighboring zones often in an undesirable way. For example, it might be considered necessary to place a nozzle at a selected point to increase the spray deposit in a zone which may be only 3 or 4 feet wide. Since the spray discharge from any one nozzle cannot be restricted to such narrow limits, a large part of the discharge would settle where it is unwanted or un- necessary.

Patterns From Different Types of Nozzles

In 1951 tests were conducted with nozzles of various types and orifice sizes to determine the specific effects of atomization on spray-deposit patterns. Included were Nos. 3, 5, and 10 jet nozzles, poppet nozzles, and Nos. 3, 5, 8, and 10 cap nozzles, which gave atomizations ranging


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C 5 10 15 20 25 FEET RIGHT OF CENTER

FIGURE 22.—Spray distribution at a 1-foot flight level from the nozzle arrangement shown in the spacing diagram : A, Spray-deposit curve ; B, contour diagram, showing variations in spray deposit from foot to foot along and across the treated swath. 1952.


from fine to coarse. All tests were conducted at the eighth-foot segment (fig. 10, Ä) both sides of center. The tests with the Nos. 3 and 10 cap nozzles and the poppet nozzles w^ere replicated once.

All flights were made at a 2- to 3- foot level and closely parallel to the wind direction at times when velocities averaged less than 1 m.p.h. The data are shown in figure 23 and in table 2.

The degree of atomization is re- flected by the peaking and lateral dispersion of the pattern. In general, the spray from all nozzles was deposited within the same approxi- mate zone relative to the center of the line of flight, but showed dis- tinctly sharper peaking and nar- rower swath widths for the jets and coarser spray nozzles than for the finer spray nozzles. The highest and narrowest peak rate of deposit was obtained with the largest orifice jet and the largest total deposit with the poppet nozzle. The greatest spray dispersion was obtained with the cap nozzle having the smallest

orifice. The data show that the concentration and dispersion of the spray from the midwing section is dependent upon both the nozzle discharge rate and the degree to which the spray is atomized.

The swath widths as measured at the 2-percent rate level tend to be narrowest for the jet nozzles, slightly wider for the poppet and the three larger orifice cap nozzles, and widest for the fine-spray No. 3 cap nozzle. Peak percentage rates are highest for the jets, intermediate for the poppet and larger orifice sizes of the cap nozzles, and least for the fine spray No. 3 cap nozzles. Finally, considering all patterns collectively, the swath width tends to be about 1 foot wider to the right of center than to the left, while the peak value is correspondingly about 2 percent lower.

As a followup to the preceding tests, applications at the 2-foot level over duplicate transects were made early in 1952 to compare medium- and coarse-spray applications. The distribution rates were first plotted

TABLE 2.—Effect of nozzle type and orifice size on spray deposits from the eighth foot hoth sides of center for a Stearman hiplane at a 2- to 3-foot fight level, Outhoard deposits of less than the 2-percent rate level were disregarded^ in determining swath widths. Based on data shown figure 23.

zn

Nozzle and orifice size

Swath width Peak rate of deposit

Mean rate of application per acre

Left Right Left Right Of swath At peak

Jet: No. 3

Feet 15 15 10 12. 5

14 11 10 15. 5

Feet 10 10 14 13. 5

18. 5 12 15 19

Percent 16. 1 12. 9 20. 1 13. 8

16.2 15. 2 17.5 13. 8

Percent 16. 9 18.5 18. 8 11.9

10. 6 13. 6 8.5 8. 8

Gallons 2. 6 3. 1 4.4 5.4

1. 4 1.2 1. 5 .8

Gallons 6. 2

No. 5 -- 6. 4 No. 10

Poppet 11. 2 10. 4

Cap: No. 10_ 3. 6 No. 8 2. 2 No. 5_ 3. 0 No. 3 1.7

Mean (all) 12.9 14. 0 15. 7 13. 5



10 20 30 FEET RIGHT OF CENTER

FIGURE 23.—Deposit curves of various nozzle types and orifice sizes plotted on equivalent discharge basis. Spray was emitted from the eighth 1-foot boom segment both sides of center of a Stearman biplane at a flight level of 2 to 3 feet. 1951.


on a percentage basis as shown in figure 24.

The same data plotted on an observed rate-per-acre basis are shown in figure 25.

Tlie mean rates of deposit for the two applications (fig. 25) were markedly different, averaging about 4.4 gallons per acre for the poppet nozzles and 2.3 gallons for the No. 8 cap nozzles. In figure 24, where variation due to the absolute rate of deposit has been eliminated, the deposit curves are quite different. Fluctiiations are more extreme, with the right-of-center low much more pronounced for the coarse than for the medium spray. The swath width for the coarse spray is also wider than that for the medium spray—55 and 47 feet at the 1-percent rate level. However, when plotted on a rate-per-acre basis (fig. 25) the observed mean rate in the characteristic, right-of-center low is nearly the same, although the mean overall rate of deposit of the coarse spray is nearly double that of the medium spray. This is a strong indication that a better spray pattern should be possible with a combination of coarse-spray nozzles outboard to obtain maximum swath width and fine-spray nozzles inboard for a better mean deposit rate in the right-of-center low.

Special Nozzle Arrangements Combinations of Fine- and Coarse-

Spray Nozzles The fact that fine sprays from

low-level applications are deposited more uniformly than coarse sprays, especially in the turbulent wake of the propeller slipstream, also sug- gested that overall distribution patterns might be substantially improved by combinations of fine- spray nozzles inboard and coarse- spray nozzles outboard. This idea was first tested by employing the asymmetrical nozzle spacing for medium-spray No. 8 cap nozzles

shown in figure 22, except that 8 coarse-spray No. 10 cap nozzles were used outboard in combination with 18 fine-spray No. 3 cap nozzles inboard. Mean and extreme values were determined at 1-foot intervals for 16 parallel transects 1 foot apart. The resulting curve for a 2-foot flight level is shown in figure 26, A. The mean rate of deposit for the 5()-foot swath was reduced by approximately % of a gallon per acre. In addition, variations in the rate within the propeller-vortex zone were reduced and the overall swath at the mean-rate level was increased by about 7 feet. However, the rate of discharge for the No. 8 cap nozzles, which contributed to the deposit from the center of the line of flight to 5 or 6 feet outboard, was inadequate to fill in the typical low to the right of center.

Subsequent tests were conducted in which progressive compensatory improvements were made, finally resulting in a nozzle combination and spacing which at a 4-foot flight level gave the excellent pattern shown in figure 26, B. The right- of-center low was largely eliminated by the judicious spacing of the fine-spray nozzles inboard, while the overall swath width was maintained by the spacing of the coarse nozzles outboard. For this nozzle combination, overall apparent swath widtlis were found to be 55 feet for vine-top flight levels, 58-60 feet for the 4-foot level, and 65-68 feet for the 10-foot level. These findings were confirmed by experiments conducted in 1953 for control of the pea aphid on canning peas, when effective swaths were 55 feet at vine-top to 4-foot flight levels and 65 feet at the 10-foot level.

Other Arrangements

Spray patterns to fit specific requirements may often be desirable


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FIGURE 24.—Spray-distribution curves for applications at a 2-foot flight level from 30 evenly spaced nozzles on a Stearman biplane : A, Coarse spray from poppet nozzles ; B, medium spray from No. 8 cap nozzles. 1952.


!Pi?i'iVi'i'i'i^'i'i'i'i'i'i'i'i'i'i'i'«'i'i'i'i'i'i4'



FIGURE 25.—Distribution curves for coarse- and medium-spray applications plotted on a rate-per-acre basis. Same data as for figure 24.

in insect-control work. Although investigations have not progressed to a point where requirements are met in all cases, a beginning has been made through variations in nozzle arrangement and in the degree of atomization.

As an example, an attempt was made to compute the arrangement of 20 coarse-spray poppet nozzles that, theoretically, would give the best spray pattern for a 1- to 3-foot flight application for use against such exposed infestations as pea aphids on peas. The arrangement was made on the basis of data from the spray patterns shown in figure 11. The theoretical nozzle arrangement so deduced and the distribution curves to be expected from its use and from tests verifying the data are shown in figure 27. The validity of the theoretical curve, as well as the general excellence of the nozzle arrangement, is borne out by actual verification tests made with a stock Stearman on June 1,1950 and with a high-lift Stearman (mean values for eight parallel transects) on October 3,1952. Both tests were

made with winds of less than 1 m. p. h. This is an excellent arrangement for coarse-spray applications where penetration and circulation of spray within a deep vine canopy is not required.

Another example is shown by an experiment conducted during the summer of 1952 for control of the two-spotted spider mite {Tetrany- chiis himaculatus Harvey) on hops. Since the hop vines were on trel- lises 12 feet or more tall in rows 8 feet apart and since contact of the spray with the mite is required for control, it was considered that best results would be obtained with a very fine spray applied at a relatively high rate—about 7 or 8 gallons per acre. Such a spray should penetrate and circulate through the vines so that a portion, at least, should be deposited on the under- surf ace of the leaves. Furthermore, to reduce the shadowing effect of one row of vines on the next, it appeared that best results would be obtained by flying a narrow, 32-foot swath. Therefore, 60 No. 2 cap nozzles were distributed on the median


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LEFT Of 0 / - CENTER

0 c / 0 2 FEET RIG

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4 Ü 50

FIGURE 26.—Spray-distribution curves for asymmetrical arrangements of nozzles: A A preliminary arrangement of 8 coarse-spray No. 10 cap nozzles (circles m spacing diagram) outboard and 18 fine-spray No. 3 cap nozzles inboard with application at 2-foot flight level, 1952; B, An almost fully compensated arrangement of 15 coarse-spray nozzles outboard and 11 ñne-spray nozzles inboard with application at 4-foot level, 1953.


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FiGUKE 27.—Theoretical and observed spray curves for applications at a 1- to 3-foot flight level with poppet nozzles asymmetrically spaced (as diagramed) on a Stearman biplane. The theoretical curve was computed from the deposit data for the individual 1-foot boom segments (see fig. 11). 1950-52.

22 feet of the underwing boom to minimize the amount of spray carried into the wingtip vortices. Mean and extreme values were determined for multiple transects 1 foot apart. The spray-distribution curve and rates secured with this arrangement are shown for different wind velocities in figure 28.

Apparently the initial requirements were fairly well met; the overall pattern shows a narrow swath—about 30 feet at the 4-gallon dosage. Penetration of the spray through the vine canopy and to the lower leaves was fair to excellent as shown by direct observation and mite kill. However, the difference in the wind did affect the spray pattern. Under a wind velocity of 0.5 m. p. h. (fig. 28, B) there are distinct peaks at and left of center, with a weak indication of the right- of-center low. At a slight diagonal upwind (drift component from right to left) when the velocity averaged 5.8 m. p. h. (fig. 28, J.) the overall pattern shows about the same swath width, but at the right of center it has been crowded in-

board against the spiraling propeller vortex. The right-of-center low has been eliminated and the rate drops off rapidly at 10, instead of 15, feet to the right of center. The center peaks have been reduced, while the spray left of center has been carried 20, instead of 15, feet before the rate drops below the mean.

Only limited attempts have been made to work out specialized nozzle spacings for monoplane aircraft. However, in 1952 one such attempt was made to improve the spray pattern for a light, high-wing monoplane (90-hp. engine) by using a shortened effective boom and an asymmetrical nozzle spacing. A low-level application was made wîth 30 No. 8 cap nozzles, which were moved inboard from the five terminal boom segments as shown in figure 29. Mean and extreme values were determined for eight parallel transects 1 foot apart. The resulting spray curve shows that the pattern was adversely altered rather than improved. The sharp peaking just within the wingtips



5 10 15 20 25 FEET RIGHT OF CENTER

FIGURE 28.—Spray-distribution curves for applications at 1- to 2-foot flight levels of very fine spray from No. 2 cap nozzles (spaced as in diagram) : A, Under relatively strong but nearly downwind conditions (14 transects) ; B, under a low wind velocity (16 transects). August 1952.


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FIGURE 29.—Spray-distribution curve for an application at a 1- to 2-foot flight level from an uneven arrangement of nozzles (as diagramed) on a light, high-wing monoplane. No nozzles were placed in the center boom section. Compare this curve with that for an even nozzle spacing with the same airplane as shown in figure 20, A, Moving the wingtip-section nozzles inboard narrows the swath and accentuates the peaking. September 11,1952.

seems to indicate that their vortices were too weak to pull the spray sufficiently beyond the wingtips to justify the use of the shortened boom. Apparently the center low in both patterns is caused by an in- sufficient number of nozzles in the central portion of the boom.

Effect of Skid Fins, Squared Wingtips, and Skid Plates on Spray Patterns The use of deflectors of one type

or another (skid fíns and skid plates) and wingtip modifications (squared wingtips) have not proved of much value for improving spray patterns. Thus, early in these studies an attempt was made to prevent or reduce the right-to-left crossover of spray in the propeller vortex of

a Stearman biplane by means of a skid fin. This fin wa*s 6 feet long and 21/^ feet maximum vertical depth. It was attached parallel to the longitudinal axis of the airplane under the left wing root hinge point and vertical to the wing chord, and was streamlined toward the trailing edge of the wing. The intended purpose was to deflect the propeller slipstream and thus, in theory, to increase the amount of spray deposited 1 to 3 feet to the right of center. The skid fin caused no ab- normal flight characteristics, but neither did it alter the spray deposit pattern.

Late in 1952 special tests were conducted with an experimental high-lift Stearman biplane which had been modified at the Aircraft and Special Equipment Center of


the Bureau of Entomology and Plant Quarantine in cooperation with engineers of the Civil Aero- nautics Administration. The wings were rebuilt to conform to a medium, high-lift airfoil (modified NACA 4412) with squared wingtips and ailerons, and terminal wingtip skid plates. It was hoped that, in addition to improving the overall performance of the airplane, the squared wingtips and skid plates might also improve the spray-distribution pattern through resultant changes in the air flow, especially for low-level applications. The results of some of these tests have already been shown in figure 14.

In experiments conducted with and without the wingtip skid plates there was no apparent difference m the spray patterns. However, there was some evidence that a greater overall swath was obtainable with the modified Stearman, presumably because the squared wings had affected the wingtip vortices. This is shown particularly well in figure 27 and is also evident in figure 14. With identical nozzle arrangements, spray-deposit levels were maintained at a high rate over a swath at least 5 to 8 feet wider for the squared wingtip airplane than for the stock Stearman. During flight the pilot observed that the wingtip vortices appeared to be somewhat flattened and that the spray seemed to be carried farther laterally and not so high as with the stock Stearman.

Discussion From the evidence presented it

is clear that spray discharged along the lateral axis of an airplane will follow the air currents set in motion by its passage through the air until these forces will no longer support the weight of the spray droplets.

Three principal airstreams, from the standpoint of spray distribution, are generated by an airplane in flight—the right and the left wingtip vortices and the propeller slipstream. They determine the path of the spray discharge and the resultant deposit pattern.

Three principal spray-boom sections may be recognized :

(1) The center section, comprising that portion of the boom approximately the propeller radius both sides of center and contributing spray to the slipstream.

(2) The midwing section, comprising that portion of the boom lying outboard of the effects of the propeller slipstream and inboard of the main effects of the wingtip vortex (about 4-7 feet both sides of center with the Stearman biplane). The spray discharged from this section does not contribute significantly to either the wingtip or the propeller vortices, although it is given a distinct outboard impetus by the same forces that produce the former.

(3) The wingtip section, comprising the outboard portions of the boom (8-15 feet both sides of center with the Stearman biplane) and contributing spray to the wingtip vortices. The bulk of such spray is deposited well beyond the wingtips even at the lowest flight levels.

Air currents within the propeller slipstream, aside from being driven aft with considerable force, are ro- tated counterclockwise by the direc- tional effect of the propeller blades. The propeller rotation causes the column of air driven aft to veer off toward the right until overcome by the adjacent airstreams and thence to parallel the direction of flight. The corkscrew effect is strongest immediately aft of the propeller, and the main vortex crosses beneath the fuselage from right to left—up on the left side and back over the fuselage and tail section to the right


side. This movement has a tendency to force the wingtip vortex outboard on the right side and to pull in the left vortex toward the center—a phenomenon readily visible to the pilot and also in the motion-picture sequences shown in figure 4 and more especially in figure 5.

The wingtip vortices, which comprise the most visible and often spectacular part of the usual spray curtain (figs. 4, 5, and 7), are caused by the flow of air from the high- pressure area beneath the wings diagonally around the wingtips where it spills over into the temporary low-pressure area above the wings as described by Jones (S),

The effect of aerodynamic forces on the distribution of spray from an airplane in low-level flight is shown in figures 4 and 5. While these views lack sharpness and detail (they are enlargements from a 16 mm. movie sequence), they show various stages in the development of the spray curtain, especially the way in which the spray is carried diagonally out and beyond the wingtips into the wingtip vortices. This diagonal flow of the spray toward the wingtips is especially weU shown in figure 1, which is essentially a duplicate of the last picture of the sequence shown in figure 4 (except for superior sharpness). The greater distinctness of the right wingtip vortex and its tendency to veer outboard as compared with the left are clearly shown. The left wingtip vortex tends to be pulled inboard, where it contacts the propeller vortex—represented by the central cloud of spray surrounding the fuselage in the final sequences of figures 1 and 4.

Essentially the same forces obtain for the higher flight levels as for the low, except that the spray- bearing air currents are not so quickly or greatly compressed by contact with the ground. The gross

appearance of the spray curtain during and immediately after an application from a 10-foot flight level is shown in figure 7. The development of the wingtip vortices appears to be delayed when compared with low-level applications. Actually this is not true. From a suitable point of view, such as the vantage point of the pilot, the vortices are seen to develop normally and even more symmetrically than at low flight levels, owing to less- ened ground friction. From a low- angle head-on view this fact is ob- scured by both the intervening spray curtain and the perspective.

Burble generated by the parasite components, such as the landing gear, exterior wind-driven pump, and underwing spray booms, has shown little gross effect on spray patterns. Such burble is mostly overcome by the higher velocity airstreams created by the propeller and airfoil sections of the airplane.

It is obviously impossible to elim- inate the propeller and wingtip vortices. It also appears doubtful, on the basis of present evidence, that these vortices can be sufficiently altered to improve the spray pattern appreciably. Reduction in engine- power requirements and the use of a smaller propeller might minimize midzone disturbances and possibly give a more even midzone deposit if such a development were feasible from a practical standpoint. Squaring the wingtip gives an apparently wider swath at low flight levels, at least as far as the Stear- man biplane is concerned.

The normal spray pattern from an even array of nozzles along a fuU-wingspan boom may, of course, be substantially altered by the spacing of nozzles (see figs. 22, 26, 27, and ^28). At low flight levels with a biplane, to minimize the mean peak deposit to the left of center the nozzles should be transferred from the first 3 feet of boom segment left


of center to the first 3 feet right of center. This tends to build up the low-deposit area to the right of center and somewhat reduce the amount of spray deposited to the left of center. This nozzle arrangement does not prevent erratic extremes of deposit in the wake of the propeller slipstream (see figs. 14, 18, and 19), but it does give a better mean for this problem area. Sim- ilarly, if the maximum effective swath width is to be achieved, more nozzles should be placed in the wingtip than in the midwing zone boom segments, since they necessarily supply a considerably wider section of the overall swath.

With an increase in the height of application there is a corresponding decrease in the force with which the f »ropeller slipstream contacts the deposit area. For higher flights the suspended spray can circulate and thus be deposited in a more even and consistent pattern. The spray from applications made above 10 to 15 feet was not markedly distorted by the propeller slipstream. The capa- bility of the slipstream to support spray depends upon droplet size and specific gravity. A fine spray will remain suspended longer in the slipstream before reaching the deposit area than will a coarse spray.

Since the distribution of spray within the propeller vortex im- proves with increased elevation of flight, any other method that would permit the retention of spray droplets within the vortex for a more ex- tended period of time would also improve the distribution pattern. This can be accomplished for low- level flights by discharging fine to very fine sprays from the center boom sections and coarser sprays outboard (fig. 26).

For applications at 2 to 6 feet, it may be difficult to obtain satisfac- tory deposit patterns over a maximum effective swath by employing fewer than 20 nozzles. Neverthe-

less, an excellent pattern has been obtained with as few as 20 nozzles, as has been shown in figure 27. Sat- isfactory deposit patterns are dependent upon the nozzle types and spacing, the rate and height of application, the degree of atomization, and last, but not least, pump and line capacity sufficient to supply; the outboard nozzles with an adequate amount of spray.

For low-level applications with a Stearman biplane it appears that a boom of 25 feet or more is required to obtain maximum swath widths. At flight levels of 10 to 20 feet sat- isfactory patterns are obtainable with boom lengths of 20 to 24 feet.

Swath widths vary with the flight level of application. With the Stearman biplane the maximum effective swaths as measured by the control of pea aphids on canning peas were found to be approximately 50-55 feet for 1- to 3-foot, 63-65 feet for 10-foot, and 75-80 feet for 20- to 25-foot flight levels. Optimum flagging intervals are those that allow for an overlap sufficient to maintain a fairly uniform application rate throughout. Im- proper overlapping increases or accentuates high- and low-deposit zones and may even result in skips (see Sanders 4). For a low-application level with a single-swath pattern, such as shown in figures 26 and 27, the optimum flagging interval would be 55 feet. Similarly for a 10-foot flight level (fig. 16) the optimum would be 70 feet.

The problem of applying sprays from aircraft under cross-wind conditions is a complex one. At low- application levels the effect of cross drift is minimized but, unless suitable nozzles and nozzle arrangements are employed, marked streak- ing may result. Conversely, at application levels of 8 to 10 feet a uniform pattern may be more easily achieved, but the effects of cross winds are much more critical. Such


winds pile up the spray on the windward side, often resulting in over- application. Within moderate limits, at least, the greater the wind velocity and the finer the spray the greater the pileup.

Summary The spray patterns developed by

airplanes flying at low levels were studied jointly by the Bureau of Entomology and Plant Quarantine and the Bureau of Plant Industry, Soils, and Agricultural Engineer- ing at Forest Grove, Oreg., and in the pea-growing areas of eastern Oregon and Washington. Most of the spraying was done with a Stear- man biplane equipped with a 300- horsepower engine and a constant- speed propeller.

The primary purpose of these studies was to ascertain and improve aerial spray-deposit patterns at flight levels up to 25 feet. To achieve this objective, studies were made of the characteristics of the pattern from an even distribution of nozzles along a full-span underwing boom; the patterns of spray from individual l-foot segments of underwing and tail booms with re- spect to the aerodynamic forces that affect them; the effect of spray atomization on the consistency of deposit rates, especially in the zone affected by the propeller vortex ; and the arrangement of nozzles, both with regard to atomization and spacing, required for optimum pattern and swath width. A carmine dye was used as a tracer in the sprays. The spray deposits were collected on stainless-steel plates and measured by colorimetric analyses. Movies were taken during application to show the development of the spray curtain as it was affected by air currents generated by the airplane flight. Effective swath widths were determined by practical field tests on insect control.

With aerial spray applications aerodynamic forces primarily determine the pattern and swath width covered. Even though the spray is discharged in equal amounts from evenly spaced nozzles along an underwing boom, these forces greatly influence the resultant deposits from foot-to-foot both across and along the line of flight. Consequently, some zones of the treated swath receive an excess of spray, while others receive minimal or even subminimal amounts. Moreover, the spray at low flight levels is spi'ead laterally over a swath from 20 to 50 feet wider than the boom, depending upon the height of flight and the fineness of the spray.

Aerial applications are especially subject to drift from surface winds. The effect of such air movements increases with the height of the flight as well as with the fineness of the spray.

The air currents generated by the airplane in flight constitute an expanding wake, which is comprised of three whirling airstreams (or vortices) —one produced by the pro- ])eller slipstream and the other two by the wingtips. These latter currents carry the spray out well beyond the wingtips. The resultant deposit pattern within the propeller slipstream zone is extremely erratic, areas of high and low deposits varying greatly and unpredictably both across and along the line of flight. A second effect of the propeller vortex is to shift much of the spray from that part of the boom lying from 1-3 or 4 feet right of center to a deposit zone left of center. In addition, this spray is deposited more rapidly and forcibly than the spray affected by the wingtip vortices. Beyond the zone affected by the propeller vortex and out to the margin of the swath, spray deposits tend to be zoned, but with areas of


equal deposit varying in width and tions made at moderate rather than oscillating irregularly parallel to low flight levels. Nevertheless, the line of flight. aerodynamic forces are such that

Considerable improvement i n deposit rates will vary considerably mean spray-deposit rates across a from foot-to-foot across and along treated swath may be obtained from the treated swath. Applications at asymmetrical nozzle arrangements, low flight levels should be keyed to with a massing of nozzles immedi- areas of minimal rather than mean ately to the right of center and cor- deposit rates. Such applications responding reductions to the left of will necessarily give deposits in ex- center, the use of ñner sprays in- cess of requirements over portions board than outboard, and applica- of the effective swath.

Literature Cited

(1) BROWN, A. W. A. 1951. THE APPLICATION OF INSECTICIDES FROM AIRCRAFT. HÍS IllSect Control

by Chemicals. Chapter 6, pp. 414-466. New York. (Includes a bibliography of 102 papers relating to the use of aircraft in insect control.)

(2) HAWES, INA L., and EISENRERG, ROSE.

1947. BIRLIOGRAPHY OF AVIATION AND ECONOMIC ENTOMOLOGY. U. S. Dcpt. Agr. Bibliog. Bui. 8, 186 pp. (Publications for 1919-1945 are included. )

(3) JONES, BRADLEY. 1940. AERODYNAMICS FOR PILOTS. U. S. Dept. Com. Civ. Aeronaut. Admin.

Bui. 26,158 pp., illus. (4) SANDERS, GEORGE E.

1953. EQUIPMENT AND PROCEDURES FOR THE MEASUREMENT OF DEPOSITS OF AERIALLY APPLIED MATERIALS. OhlO Agr. Expt. Sta. RCS. Bul. 727, 30 pp., illus.

(5) YuiLL, J. S., EATON, C. B., and ISLER, D. A. 1951. AIRPLANE SPRAYING FOR FOREST PEST CONTROL. U. S. Dcpt. Agr., Bur.

Ent. and Plant Quar. E-823, 21 pp., illus.

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