Sprayers
Aaerial application technology

Ground spraying is by far the most popular application method for crop protection
products, but aerial application brings advantages such as fast application
during times of difficult weather conditions, covering 160 acres in less than
one hour. Also, aircraft design has changed significantly during the past 25
years, and with those changes comes a better understanding of application technology.
As a result, farmers need to understand how these advances affect their custom
application success.

Pilots have to balance all the factors to get the best efficacy while maintaining operation efficiency. Photo Courtesy Of Air Tractor/Bonnyville Air Services, Bonnyville, Alberta.

"We're seeing a fairly sophisticated machine today with a lot of options,
so it is important to know what to ask your aerial applicator when discussing
your spraying needs," says Tom Wolf, a research scientist with Agriculture
and Agri-Food Canada at the Saskatoon Research Centre.

Compared to older aircraft, Wolf says today's modern units are turbine-powered,
which makes them quieter and more fuel efficient, and they can fly faster, at
about 140mph compared to about 110mph with the older planes. Larger hopper capacities
of 400 to 600 US gallons make these planes more efficient than older planes
with capacities of 200 to 300 gallons. Larger payloads are also combined with
wider swaths of 70 to 80 feet versus narrower 50 to 60 foot swaths obtained
with older models. The planes also fly higher, at 10 to 15 feet off the ground,
rather than eight to 10 feet. With lower, more aerodynamic booms, spray pattern
is also improved.

Most aircraft now use GPS for swath guidance and rate control: no longer are
flaggers required in the field. Adjustable nozzles that allow the operator to
quickly change application volume or droplet size are also common. As a result
of all these improvements, Wolf says that overall accuracy and capacity are
higher, and there is more opportunity for drift management or for higher water
volumes when conditions require them for drift control or coverage.

However, these improvements add a bewildering array of factors to consider
for a farmer who is assessing different aerial application systems.

Application factors to consider
Water volumes used in aerial application are generally one to two US gallons
per acre for insecticides, two gallons per acre for herbicides and four gallons
per acre for fungicides. The basic relationship between water volume, droplet
size and coverage is the same for ground application as it is for aerial application.
Wolf explains that at any given droplet size, less water results in fewer droplets
per unit of area (coverage) and more water increases coverage. He says the best
strategy for optimum spray performance is to maximize application volume, within
practical limits, and to use coarser sprays to control spray deposition and
drift, while retaining adequate droplet density across the swath.

Higher boom heights, finer sprays associated with lower volumes, and wing-tip
turbulence generate conditions where aircraft may be more prone to spray drift
than ground application methods. Wolf says the best way to manage drift is to
spray under good environmental conditions, such as low and consistent wind speeds,
away from sensitive areas, sunny conditions that encourage thermal turbulence,
and higher humidity that prevents droplet evaporation.

Other ways to reduce drift are to use coarser sprays and higher water volumes,
limit boom widths to 65 percent of the wingspan to avoid wing-tip vortices and
to maintain reasonably low spray release heights.

Droplet breakup and the resulting droplet size is influenced by air shear with
aerial application, as opposed to the hydraulic shear used in ground application.
Air shear refers to the process in which a jet of liquid exiting the nozzle
is broken up by the force of air hitting it.

Shear force can be controlled by air speed and deflection angle. Faster speeds
and greater deflection angle into the air stream usually create finer sprays.
Spray pressure also has some effect. One of the most common atomizers, the CP
nozzle, allows the operator to choose from three deflection angles and four
flow rates, which allow the operator to customize the nozzle to fine-tune droplet
size.

Wolf explains that since aircraft travel at high speeds, aerodynamics play
a large role in droplet movement after spray release from the nozzles. Factors
such as propeller wash, landing gear and wing-tip vortices can affect how the
spray redistributes under the boom. Since air pressure is higher under a boom
than above it, air pushes out past the wing tips and swirls up into vortices.
Fine droplets in these air movements are carried along, generating a swath that
is wider than the wingspan and tapering at the edges.

"Managing aerodynamics, pattern uniformity and swath width are still the
largest challenges for aerial applicators," says Wolf.

Air movement around the propeller and the landing gear also disturbs spray
deposits, and the effects may depend on nozzle placement, crosswinds and droplet
size. As a result, Wolf says that uniform patterns are quite difficult to achieve
and require experimentation. Such experimentation includes booms lower from
the wings, narrower than the wings and customized nozzle spacings near the fuselage
to optimize pattern uniformity.

To help assess spray patterns and fine-tune pattern uniformity across the swath
width, many aerial applicators take part in the Canadian Aerial Applicators
Association Calibration Clinics. At the clinics, each aircraft is calibrated
and the spray pattern is analyzed with spaced water-sensitive cards that measure
deposit amount and uniformity. Adjustments are made on site and the aircraft
then make several passes over the equipment. Following the clinic, each pilot
is provided with a computer printout of the aircraft's performance and suggestions
on how to improve the spray pattern. Calibrated aircraft receive a certificate
and windshield sticker indicating an expiry date of 20 months.

Crosswind distortion is an on-going challenge for aerial applicators. A crosswind
from the left, for example, would reduce the outward flow of air from under
the left wing significantly, decreasing spray travel and deposition in the upwind
direction. A significant portion of the deposition that otherwise would be deposited
out past the wing tip would cause a high spot somewhere under the upwind wing.
As a result, an uneven spray pattern across the swath would occur. Wolf says
that solving this distortion is an on-going subject of research for aerial applicators.

Canopy penetration is also much debated. Does air turbulence from aircraft
wings assist in canopy penetration? Wolf says that while it is true that compression
of air under the wings creates a downward airflow, the released spray is not
affected, since the spray enters the crop canopy several seconds after the aircraft
has passed and the downdraft has dissipated by then. The majority of the droplets
are deposited by gravity and wind.

Wolf says that when considering aerial application, the most important part
of the sprayer is the aerial applicator/operator. Pilots have to balance all
the factors to get the best efficacy while maintaining operation efficiency.
And they have to use good agronomic knowledge.

"A well done application at the wrong time isn't that valuable. But an
average application done at the correct time is valuable," says Wolf.

Wolf does not let clients off the hook, either. He says they should be prepared
to discuss their specific application goals with the applicator, including water
volume, spray quality and timing, while remaining aware of the pilot's practical
constraints. A follow-up field inspection after spraying to assess product performance
is also critical.