by an airplane in flight. These are the right and left wing-tip vortices and the propeller 
vortex. These vortices, plus the effect of gravity, determine the path of the spray dis- 
charge and the resultant deposit pattern. 
Air within the propeller vortex, or slipstream, aside from being driven backward 
with considerable force, is rotated by the directional effect of the propeller blade 
causing the column of air drivenaftto veer off towards the side until overcome by the ad- 
jacent airstreams, and then to parallel the direction of flight. The wing-tip vortices are 
caused by the flow of air from the high-pressure area beneath the wing diagonally around 
the wing tips where it spills over into the temporary low-pressure area above the wings. 
Essentially the same forces act for high level flight as for low, except that the spray 
bearing air currents are not so quickly or greatly compressed by contact with the ground. 
It is obviously impossible to eliminate propeller and wing tip vortices, and doubtful 
that these vortices can be sufficiently altered to improve the spray pattern appreciably, 
although this possibility requires further research before it is completely dropped. 
A normal spray pattern from an even array of nozzles along a full wing span boom 
may be substantially altered by spacing the nozzles, and thus obtain reasonably uniform 
spray distribution. Nevertheless aerodynamic forces are such that deposit rates vary 
considerably from foot to foot across and along the swath. 
Inversion and lapse temperature conditions have not been found to influence mass 
median size of deposited drops but do affect drift, especially at high level applications. 
Since hydraulic nozzles are used on aircraft, drop size distributions have characteristics 
similar to those of ground equipment. 
There has not been much testing of helicopters for spraying and dusting work but 
tests conducted so far indicate, that other things being equal, the deposit pattern from 
helicopters is more uniform than from fixed wing aircraft. 
FUNDAMENTAL PROBLEMS OF PESTICIDE APPLICATION 
The first thing that must be known either accurately or approximately is how much 
chemical is required and where and how it is to be applied. 
It is useful to assume that a pesticide has a definable toxicity.’ A requisite quantity 
of available insecticide to cause the death of an insect may then also be defined. The 
quantity is not constant but may be considered approximately so. In actual practice we 
apply many times as much insecticide per acre as would be necessary to kill the insects 
there if it could be distributed directly on the insects and providing that each insect re- 
ceived his ‘‘fair share.’’ It is interesting to calculate the number of insects that could 
theoretically be killed by 1 pound of insecticide if it could be ideally distributed on the 
insects. For example, USDA entomologists tell us that an ‘‘average’’ boll weevil may 
weigh approximately 17 mg. and that one part per million of a good phosphate poison is 
required to kill him. Then: 
Approximate weight 17 mg. 
Poison required = 1 p.p.m.= approximately 2 mg. 
100,000 
Then 1 1b. would kill approximately 23,000,000,000 weevils. Assuming a plant 
population of 23,000 plants per acre, we could kill 1,000 boll weevils per plant 
on 1,000 acres. 
Even much more startling calculations could be made for much smaller insects such as 
the mosquito and the aphid but such figures only lead to wrong impressions since no field 
application procedure will ever even approach these figures. 
3Courshee, R. J, Some Aspects of the Application of Insecticides, Annual Review of Entomolozy (English) Vol. 5, 1960. 
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