CROP P R O T E C T I O N (1986) 5 (6), 417-421

Agricultural aircraft spray performance: calibration for commercial operations

NICHOLAS WOODS

Blanch's Aerial Agriculture, PO Box 219, Ingham, 4850, Queensland, Australia*

ABSTRACT. A system for testing agricultural aircraft spray systems alongside a commercial operator in Australia is described. Factors affecting the uniformity of deposit achieved on the ground are discussed, together with the need to make suitable adjustments to airborne spraying and spreading equipment. Measurements from a single flight-line are used to determine the optimum swath width for a particular aircraft configuration. The role of a microcomputer in storing, presenting and evaluating spray distribution data in routine commercial operations is demonstrated.

Introduction

Aerial application of chemicals requires the transfer of a product from an aircraft to the biological target with the maximum efficiency and minimum loss en route. For an aerial operator seeking to achieve a specified biological objective, the requirements for each particular task are unique and in practical terms this means that an aircraft must be set up carefully and calibrated to perform a specific task. However, the spray distribution pattern or spray cloud released from an aircraft is dependent on many factors including the meteorological conditions, the crop, target site, pesticide formulation and droplet size, as well as the geometry of the aircraft, its operating speed and its height. The aerial operator therefore needs a system for carefully calibrating and setting up his agricultural aircraft before treating a crop, particularly where the cropping situation is diverse and local weather conditions are extreme, as in the tropics.

The initial calibration of an aircraft is ideally carried out under standard test conditions. By sampling the ground deposit pattern beneath an aircraft, any mal- distribution caused by incorrectly adjusted application equipment or the aircraft's aerodynamic effects upon a spray cloud can be identified and corrected. Such data can be used to simulate the coverage across a whole field (as long as droplet capture is occurring primarily by sedimentation) and thereby to identify the best flight-lane separation or swath width for the aircraft. Such information can be used to predict poor distribution which could result in bad striping and uneven coverage in a crop. In commercial operations it is important to be able to store, process and present

* Present address: Pesticide Application Centre, Department of Plant Protection, Queensland Agricultural College, Lawes, 4343, Queensland, Australia.

0261-2194/86/06/0417-05 $03.00 (~) 1986 Butterworth & Co (Publishers) Ltd

such information routinely and quickly. This paper describes the techniques used, including data pro- cessing by microcomputer, and presents the results obtained in monitoring a fleet of agricultural aircraft in commercial operations in northern tropical Australia.

Testing method and procedures

Several techniques are available for measuring the ground deposit achieved from agricultural aircraft and these have been reviewed by Parkin and Wyatt (1982). For simplicity, dictated by the need to repeat tests well away from sophisticated equipment, a simple droplet stain counting technique was used in order to assess the density of material arriving at sample points, beneath and at right angles to the line of flight of the aircraft under test. Using artificial surfaces in con- junction with sensitive materials such as water- sensitive paper, the amount of material reaching a sampling point was recorded. For the data presented in this paper, a series of flat plates (100x 100mm) were used, placed at equidistant sampling points beneath the path of the aircraft and at a height of 0.4 m above the ground. In most cases, the targets were laid out across a mown and level field and orientated so that the line of flight of the aircraft was directly into the wind, particularly where a ground pattern was needed to assess equipment settings.

An aircraft spray system was first set up according to manufacturers' instructions or from previous experience. The aircraft was then flown over the target array along a prefixed centre-line. In order to keep

418 Aerial spray calibration

certain important parameters constant, such as flow rate and air speed, spraying was started well before reaching the target area and wings were maintained level both before and after passing the test area. For low volume (LV) herbicide calibration, an aircraft was flown at normal crop-emission heights (2-3 m). After spraying, the number of droplets at each sampling point was counted using a hand magnifying lens.

When measuring spray deposition in the field, natural targets should be used wherever possible, as the shape, texture and size of a target (e.g. a plant) greatly affect the amount of material that is caught (Uk, 1977). Results from artificial surfaces must be treated with caution, because different quantities of droplets are captured depending on the shape and location of the collectors. The choice of artificial target alone can change the aircraft ground-deposit pattern obtained (Parkin, Wyatt and Courshee, 1983). Large spray droplets are caught primarily by sedimentation as they fall at relatively high sedimentation velocities: thus, artificial targets such as flat plates become more appropriate when the natural target is large and hori- zontal and droplet size is increased. Flat plates may not always be suitable collectors as small droplets are subject to eddy displacement and inertial impaction is the predominant mechanism for capture, particularly under high wind conditions.

During aircraft testing, the local wind direction and speed in relation to the target array was recorded, as well as other parameters such as the temperature and relative humidity. For the assessment of crosswind (large droplet) herbicide application, low wind speeds (0-2m/s) were used in order to catch droplets by sedimentation within the target area and thus avoid the need for an extended up-wind emission to ensure a complete fall-out of the small droplet component of the spray. Only very low (less than 0" 5 m/s) or zero wind- speeds were used for determining nozzle positions and

settings. Droplet density data obtained from the sampling sites were fed into a Canon AS100 micro- computer for storage and processing. An electronic spreadsheet (Canon Canobrain) was used for display- ing data graphically, additional programmes being set up for the calculation of optimum swath widths.

Results

The ground pattern obtained from a Piper Pawnee Brave 400 equipped with four Micronair AU3000 nozzles (Figure 1) illustrates many of the aerodynamic problems associated with aerial spraying (Table 1 gives aircraft calibration details). At the time of this test, a slight crosswind (of 0-5 m/s) occurred from the star- board side of the aircraft which shifted the distribu- tion, giving rise to a downwind tail of smaller droplets, accentuated by the downwind wing-tip vortex. Con- versely, the interaction of the upwind tip vortex and crosswind produced a sharp cut-off on the upwind side. In addition, the effect of the induced flow formed by the propeller (in this case moving anticlockwise) tended to concentrate the deposit beneath the port wing, a phenomenon which is often visible to an observer on the ground. Clearly, a uniform pattern in

to

"13

120

I00

80

60

(stbd.) I I I w/tip(port)

2O

0 i I 0 8 6 4 2 0 8 6 4 2 C 2 4 6 8 0 2 4 6 8 0 2 4 6 8

Distance across spray pottern (m)

FIGURE 1. Piper Pawnee Brave 400 ground distribution pattern. Nozzles: Micronair AU3000 (4 units). Blade angles set at 45 degrees. All VRUs set at position 13.

TABLE 1. Application and equipment installation details for aircraft ground-deposit density graphs

Equipment distances along boom (m)

Figure Aircraft type Engine type Equipment type Equipment settings Starboard Port

1 Piper Pa36-400 10-720 AIB Micronair AU3000 BA =45 degrees 4 units VRUs= 13

2 Piper Pa36-400 IO-720 AIB Micronair AU3000 Pawnee Brave 6 units

3 Piper Pa36-400 IO-720 AIB Micronair AU3000 Pawnee Brave 6 units

4 Piper Pa36-285 6-285-C2 Spraying systems Pawnee Brave Continental Tiara 6515 Flat Fan.

28 nozzles 6 Piper Pa36-375 10-720 D1CD Transland stainless steel

Pawnee Brave spreader

BA =45 degrees VRUs = 13 BA =55 degrees V R U s = l l , 13, 13-11, 13, 11 Streamline boom.

180 deg. to airflow

Modified: Lower lip removed to meet dump requirements

(as measured from quick disconnects inboard)

3.5, 1 . 3 2 - 1 . 2 8 , 3.45 3.45, 2-14, 0 . 8 5 - 0 - 8 8 , 2-18, 3.48

3"45, 2-14, 0 . 9 9 - 0 - 8 8 , 2.18, 3"48

Nozzle spacing = 300 mm. 14 port and starboard, no centre boom nozzles

Figure Boom pressure Aircraft speed Height (kPa) (knots) (m)

Wind direction (dog. to flightline)

Wind speed (m/s) Application rate

1 310 100 2"5 30 2 241 98 2"5 330 3 241 98 3 90 4 138 93 2 90 6 -- 90 15 360

0 .5 28 l/ha at 18m 0 .5 28 1/ha at 18m 0 .5 28 l/ha at 18m 0-5 19 l]ha at 18m 0" 5 80 kg/ha at 11 m

NICHOLAS WOODS

terms of droplet density was not achieved over the ground during these test conditions. As well as some material being moved by propeller and wing-tip effects, peaks in deposit due to individual nozzles were indicated.

When a Piper Pawnee Brave equipped with six Micronair AU3000 units was used with all the variable restrictor units (VRUs) set at position 13 and blade angles to 45 degrees, movement of small droplets due to wing-tip effects occurred (Figure 2) and the ground pattern was also influenced by a wind 30 degrees away from the centre-line. When the volume flowing through the two outboard and port inboard nozzles was reduced by changing the VRUs from position 13 to 11 and all the blade angles were increased to 55 degrees, there was a reduction in the amount of material being entrained into induced air- flows (Figure 3). The overall density was reduced, partly because of an increase in droplet size.

By using this technique, irregularities in the ground distribution can be monitored and then nozzle arrange- ments corrected accordingly. In commercial opera- tions with centrifugal-energy nozzles, changes in ground distribution patterns were altered in a similar way by:

1. Adjusting the positions of units along a wing, e.g. the movement of outermost nozzles inboard to reduce the effect of vortices and inboard units outwards;

2. Reducing the volume of material through indi- vidual nozzles;

3. Increasing the blade angle of units to reduce the droplet size of material emitted. Small droplets with

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Eo

" o

120

I00

80

60

(s tbd) I I Iw / t i p (port)

2O

0 I I I J = I I l ~

8 6 4 2 0 8 6 4 2 0 8 6 4 2 C 2 4 6 8 0 2 4 6 8 0

D i s t a n c e a c r o s s s p r a y pattern (m)

FIGURE 2. Piper Pawnee Brave 400 ground distribution pattern. Nozzles: Micronair AU3000 (6 units). Blade angles set at 45 degrees. All VRUs set at position 13.

IOOi E u "-- 8 0

¢' 60

-~ 4 0

-~ 2 0 g a 0

(stbd.) I I I w/tip (port)

2 0 8 6 4 2 0 8 6 4 2 C 2 4 6 8 0 2 4 6 8 0 2 4 6

D i s t a n c e a c r o s s s p r a y p a t t e r n (rn)

FIGURE 3. Piper Pawnee Brave 400 ground distribution pattern. Nozzles: Micronair AU3000 (6 units). Blade angles set at 55 degrees. VRU settings 11, 13, 13 - (starboard), - 11, 13, 11 (port).

419

very low sedimentation velocities and stop distances are much more likely to be caught up by swirling vortices induced by an aircraft.

The result of this approach was the setting up of Micronair units differently on the same aircraft in order to (a) offset the aerodynamic effects of aircraft on the spray distribution and (b) set up dual droplet spectra for the coverage of complex target profiles.

Similar correction ofmaldistribution with hydraulic nozzle systems is possible. Principally, correction is obtained by removing and relocating nozzles across a boom (Kuhlman, 1981; Johnston and Matthews, 1965). The pattern obtained for a Piper Pawnee Brave 285 fitted with spraying systems 6515 flat-fan nozzles angled at 180 degrees to the airflow (Figure 4) was developed for the application of phenoxyacetic acid herbicides in sugar-cane. The deposition due to wing- tip vortices was minimized by correctly positioned nozzles and some grading of the peripheral areas was achieved, even with a slight crosswind drift of about 0" 5 m/s at 90 degrees to the flight-line. The downwind movement of smaller droplets was also minimized.

When a satisfactory pattern has been achieved, a simulation of the total deposit across a field can be made. The mean density (droplets per sq. cm) and coefficient of variation across a field at various swath widths (Figure 5) were calculated using a micro-

~-" I 0 0 E o " 8 0

0. 6 0 o

~ 4 0

~ 20 E

g~ o

(s tbd) I I I w/ t i p (po r t )

4 2 0 8 6 4 2 C 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 Distonce ocross spray pottern (m)

FIGURE 4. Piper Pawnee Brave 285 ground distribution pattern. Nozzles: 6515 flat fan hydraulic nozzles angled at 180 degrees to airflow.

E 90

,. 80

g. 7o -~ 6 0

~ 5 0

~ 40 § 3o E 2 0

o IO

~ 0 >

(For Piper Pawnee Brove 285)

\

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I I I I I I I I I I I I I I 8 I0 12 14 16 18 20 22 24 26 28 30 32 34

F l ight - lone seporotion (m)

FIGURE 5. CV % ( ) and mean droplet density (-- -- --) vs lane separation. Coefficient of variation (CV) is given by:

100 ( ~ (X i - g m ) 2 / ( n - 1))v2

Xm

where n = number of samples in overlap pattern X i = overlapped deposit

X m = mean of overlapping deposits.

420 Aerial spray calibration

computer, from the single flight-line deposit shown in Figure 4. The coefficient of variation gives a measure of the irregularity of the distribution so a high degree of uniformity across a field was obtained with a 14 m swath. However, the aircraft could be recommended to fly in a racetrack pattern at 18 m swath width. At this lane separation, the coefficient of variation is 21% and the slope of the line is shallow, indicating a fair degree of tolerance either side of the optimum. The droplet density could be increased above 35 droplets/cm 2 by angling the nozzles at 90-135 degrees to the airflow, but a lower VMD would be obtained. A larger droplet size is favourable for this type of work, given the need for penetration through sugar-cane on to broad-leaved weeds and to ensure a reduction in drift.

Data on the uniformity of the overall ground distribution at different swath widths can be used to display total ground cover that could be obtained across a field (Figures 6 and 7). The deposit achieved on the ground from a Piper Pawnee Brave 375, set up to apply granules at about 80 kg/ha through a modified 'Transland' stainless steel spreader (Figure 6) shows that much of the material was deposited beneath the port wing, due to the propeller slipstream, and an incorrectly adjusted spreader. Using a microcomputer it can be shown that a reasonably uniform deposit would be placed across a crop flying a racetrack pattern at 11 m swath width. However, because this asym- metric pattern was influenced primarily by the aircraft

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03 O~

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7:

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-w/tip (stbd.) I I I w/tip(port)

2 1 0 9 8 7 6 5 4 3 2 I C I E 3 4 5 6 7 8 9 0 I E3 Distance across spray pattern (m)

FIGURE 6. Pa 36-375 ground deposit pattern. Granules applied at 80 kg/ha; deposit collected in containers of diameter 0.26 m. Mean of three aircraft runs displayed.

0 . 5

Reciprocal flight pattern ~ 0.4 ~ _

0 . 3 o

"~ 0 .2 .=z

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, , , , O'C 0 2 3 4 5 6 7 8 9 0 6 7 8 9 0

Distonce ocross sproy pottern (m)

FIGURE 7. Pa 36-375 total deposit overlap pattern at 11 metres.

and application equipment, and not caused by a cross- wind, severe unevenness could result by flying the same aircraft on reciprocal headings. At the same swath width of 11 m the coefficient of variation increases from 33% to over 70% on reciprocal headings. This type of maldistribution could lead to bad striping in a crop.

Discuss ion

Despite some inherent weaknesses in assessing spray coverage from aircraft by counting droplet stains, the method is suitable for aiding the setting up of agri- cultural aircraft in commercial operations. The tech- nique is simple enough to be used by operators in the field for experimentally determining the spray con- figurations of agricultural aircraft and thus improving the quality of application. By assessing the number of droplets reaching the ground from an aircraft, the movement of small droplets can be detected, especially when their distribution is affected by aircraft-induced airflows and they are carried downwind from an aircraft spray line. In commercial operations, where sophisticated methods often are not available for routinely assessing herbicide and granule application, the technique allows industry personnel to understand more clearly the way in which material is deposited according to droplet size. The method also demon- strates the increase in droplet numbers obtained when smaller droplets are formed from a given spray volume and provides an indication of the performance of a chemical whose activity is closely linked to the density of coverage on a target.

However, results obtained must always be inter- preted with care. Peaks in the ground deposit can be emphasized and the usefulness of a wide swath exaggerated by the entrainment of small droplets into aircraft-induced airflows. Similarly, the number of droplets recorded per unit area on a target cannot be directly related to the volume of material recovered, as a high proportion of spray can be contained in rela- tively few large droplets or a considerable number of small droplets. Droplet stain counting, however, will tend to parallel the volume distribution of a spray more where droplets are produced from equipment with a narrower droplet spectrum, e.g. centrifugal energy nozzles. A closer approximation to volume analysis can be obtained, if required, by ignoring the collection of small droplets (Parkin and Wyatt, 1982).

The system requires aircraft to be assessed under a standard set of conditions. Accuracy is improved by taking the mean of several aircraft runs; however, for good reproducibility of data it is more important to maintain consistent meteorological parameters such as wind direction and humidity, than to obtain many multiple readings. In summer under tropical con- ditions the window for accurate spray system evaluation can be as little as 30 minutes, shortly after day-break.

NICHOLAS WOODS 421

Conclusions

This technique has been used successfully in commercial operations to help to eliminate errors due to poor aircraft calibration and thus to improve the quality of application and pest control available to the grower. The relatively slow process of counting droplet stains was speeded up by the introduction of a microcomputer for the storage, evaluation and rapid presentation of pattern data into a format of immediate use to the pilot, the visual display unit of a computer being excellent for conveying aircraft spray per- formance details to both pilots and ground crew. The technique is not designed to evaluate droplet deposition in a crop canopy: secondary data from the actual crop canopy is needed if the deposition of a chemical product is to be understood more fully. Repeated checking using this technique enabled aircraft to be 'fine-tuned' for specific tasks: thus, when certain fungicides were applied with 100-200/am droplets, the method was used to set up particular ground-deposit patterns by 'finger-printing' the spray- system configuration under very low windspeed con- ditions. Where the placement spraying of herbicides or application of granules was assessed, ground deposit data were useful in determining optimum swath widths and overall distribution patterns across a field.

Acknowledgements

I would like to thank Mr E. G. Blanch of Blanch's Aerial Agriculture, Ingham, Queensland, Australia, with whose aircraft the examples in this paper were obtained.

References

JOHNSTONE, D. R. AND MATTHEWS, G. A. (1965). Evaluation of swath pattern and spray droplet size provided by a boom and nozzle installation fitted to a Hiller UH-12E helicopter. Agricultural Aviation 7, 46-50, 52.

KUHLMAN, D. K. (1981). Fly-in technology for agricultural aircraft. World of Agricultural A viation 8(1), 12-17.

PARKIN, C. S. AND WYATT, J. C. (1982). The determination of flight-lane separations for the aerial application of herbicides. Crop Protection 1, 309-321.

PARKIN, C. S., WYATT, J. AND COURSHEE, R. J. (1983). Economic evaluation of the aerial application of cereal herbicides. EPPO Bulletin 13(3), 405-411.

UK, S. (1977). Tracing insecticide spray droplets by sizes on natural surfaces--the state of the art and its value. Pesticide Science 8, 501-509.

Accepted 28 April 1986