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load slinging, except you use engine
driven devices like buckets to spread
solutions over forests. Unlike crop
spraying, it can be done in strongish
wind conditions, but, otherwise, it's
characterised by always being in, or
very near, the avoid curve and many
other situations that you're taught to
avoid normally. You can tell with
forests that have been sprayed in the
early stages of their growth as to
whether the pilots were successful or
not—you very often see trees
shorter than others, which is where
they missed. Helicopters are
particularly useful when the leaves
need to be sprayed from underneath,
because of the downwash.
In fact, your downwash (or rotor
wake) can be used to good effect. At
low speed, you can spray a small area
underneath the flight path. At higher
speeds, the wake helps spread the
load behind and to either side (the
term rotor wake means all the air
displaced by the helicopter, as
opposed to just the downwash).
Knowledge of wake management
will therefore help you become a
better spray pilot.
The Rotor Wake
This changes within three distinct
speed ranges:
· Up to 20 mph, the air moves
primarily downwards, most of it
descending from the outer
edges of the blades, so you get a
relatively calm area around the
fuselage (in other words, you
Special Use Of Aircraft 195
are in the middle of a ring, like a
doughnut - you can see this by
hovering over water). The force
in the outer ring can agitate the
foliage so it collects chemical
above and below, in a fog over a
relatively large area. Slow
speeds, however, cost money.
· At 18-22 mph (on a Bell), the
annular ring shortens in the
direction of movement to
become an ellipse, coinciding
with translation. Above 20 mph,
the annular ring disappears, and
a large amount of separate,
small airflows coalesce to
provide an area of ill-defined
airflow with a general
downward direction.
· Above 35 mph, two distinct
rotating vortices are formed
from directly behind the
machine to a long way behind,
assuming no outside influence
(they are fully developed about
1 rotor diameter behind the
mast, and can be sustained for
up to 2500 feet). Each vortex
starts from where the annular
ring would be in the hover, and
is relatively calm in the center
(in fact, the centre to centre
distance between them is just
under the rotor diameter, and
slightly displaced from the
centre towards the retreating
blade). Regard them as large
funnels extending rearward and
downward, getting bigger as
they go. The point is, they can
be used to direct chemical into
the foliage. Ground cushion,
however, can cause them to
separate, because there is
nowhere else for them to go.
There is still a downward flow
as well as the vortices.
Particle size at low heights should be
larger to prevent them being sucked
into the vortex areas, where they will
not be effective – low level
operations should plan to use the
lower parts of the generated airflow
(say for economical application of
pesticides, or where penetration is
not required). You would spray at
higher levels if you want to spray the
underside of foliage, as with an
orchard, and make full use of the
vortices. In this case, particle size
would be smaller, but large enough
to fall out of the air stream. The
denser the foliage, the higher the air
stream needs to be.
It has been found that, between 40-
80 mph, the swath width can be
relatively constant, meaning that you
can get the same physical coverage
while slowing down to concentrate
on more heavily infested areas.
Application is inversely proportional
to airspeed, so 2 gallons an acre at 60
mph translates to 3 gallons at 40 or
1.5 at 80.
Note that there is a different
optimum particle size according to
the foliage - a good reference book
for your library in this respect is
Concentrated Spray Equipment, Mixtures
and Application Methods, by S F Potts
(Dorland Books, NJ).
The Procedure
Crop-spraying, like slinging, is very
satisfying when you get a good
rhythm and an efficient team that
keeps you in the air as much as
possible. Unlike it, however, you will
be operating a heavy machine with
unwieldy spray booms (actually just
like bouncy missiles, if you ever did
196 Operational Flying
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