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STATE i-L/AI[%L' LUARD
June 1950 ET4-:
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomologr anad Plant Quarantine
GRAVITY-FLOW EQUIPMENT FOR DISPERSING INSECTICIDES
By Frank S. Faulkner, C. C. Deonier, and A. N. Davis,
Division of Insects Affecting Man and Animals
Because of the uneven output of spray from gravity-flow equipment
previously used in airplanes, such equipment has proved less satis-
factory generally than pump-opersted spray equipment. A gravity-flow
unit has been developed which provides a fairly constant flow of solu-
tion and appears to give about as good results as a pressure unit.
The gravity-flow unit was designed for use on PT-13 or PT-17
planes. It consists of a tank in the front cockpit and under-wing
venturis to increase the breakup of the sprays. The installation is
satisfactory for dispersing oil solutions or stable water emulsions,
but not for suspensions, which require constant agitation to prevent
Description of Equipment
An SO-gallon aluminum tank, designed to fit the front cockpit
without removing the rudder pedals or altering any of the structures
in the plane, rests on a 2-inch angle aluminum platform and is held
in place by a clamp over the top of the tank. Baffles in the tank
prevent the liquid from shifting when the plane is maneuvered and
make the sides of the tank more rigid. A constant flow rate is
obtained by sealing the filler neck with an airtight cap, and as the
liquid is released the displacing air is drawn in through a breather
tube. The breather tube extends from 1/2 inch above the bottom of
the tank, through the tank, to 1/2 inch above the top of the tank.
Tanks of higher capacity can be used when the front cockpit control
does not have to be retained.
Two 7/S-inch (inside diameter) booms, each 110 inches long, are
located under the wings 24 inches below the bottom of the tank. A
1/2-inch, quick-action cut-off valve is located 7 inches inboard from
1/ This work was conducted under funds allotted by the Department
of Defense to the Bureau of Entomology and Plant Quarantine.
the tip of each boom to obtain instant control of the liquid. Inter-
changeable nozzles are inserted into the end of the booms to give the
breakup and rate of flow desired. These nozzles are made from pipes
having inside diameters of 1/4, 5/16, or 1/2 inch, the tips of the
pipes being cut off at a 32 angle. The spray is released into
venturis attached to the ends of the booms (fig. 1).
Figure l.-Valve, venturi, and cut-off valve assembly.
The venturi has an over-all length of 13 inches, a maximum
diameter of 5j inches, and a throat diameter of 3 inches. A sleeve
made of tubing having a slightly larger inside diameter than the out-
cide diameter of the boom is riveted to the throat of the venturi and
extends out from the venturi at a right angle. This sleeve telescopes
over the end of the boom and is secured to the boom by set screws.
The tip of the nozzle extends 2 inches into the throat of the venturi
and the 32-angle cut faces toward the rear.
Studies to determine the delivery rates for the gravity-flow
equipment were made -ith several sizes of nozzles and breather tubes.
The results of these determinations are shown in table 1. Two 1/4-
inch nozzles gave a rate of flow of 2.9 gallons per minute; 5/16-inch
nozzles gave 4.3 gallons; and 1/2-inch nozzles gave 7.4 gallons. Con-
stant rates of flow were obtained with each nozzle. Little or no
difference in rate was caused by varying the size of the breather tube
from 3/16 to 3/8 inch. The greatest differences occurred with the
1/2-inch nozzle. Without a breather tube each nozzle gave a high
initial rate of flow which declined gradually as the volume of liquid
With a single venturi under each wing, the spray was distributed
unevenly, the heaviest deposit being directly under each wing tip.
This pattern apparently was caused by the air-flow characteristic of
the plane, which forced the spray outward to the wing tips regardless
of where the venturis were placed. This action was immediate, under
favorable weather conditions, when the venturi outlets were 4 1/2 to
7 1/2 feet inside the wing tips, but at 9 feet it was somewhat delayed.
A second venturi was placed under each wing in an effort to
improve the performance of the equipment. The two venturis were
attached to opposite ends of an inverted T-pipe that was connected
into the boom just beyond the cut-off valve. They were 3 feet apart
and the outside one was 4 1/2 feet from the wing tip. With 1/4-inch
nozzles the flow rate was increased from 2.9 to 5.4 gallons per minute.
Particle-size determinations for this equipment showed an average
median diameter of 53 microns and an average mass median diameter of
Three flights were made upwind at a height of 20 to 25 feet to
insure a heavy deposit, and thereby increase the accuracy of colori-
meter determinations. The quantity of spray distributed from the dual
venturi tubes was determined from deposits of tracer dye on 6- by 6-
inch plates placed at intervals of 20 feet across the swath.
The results of these tests, given in table 2 and figure 2, show
an average deposit of 0.03 pound per acre, or more, over a swath
width of at least 100 feet. The breakup of the spray was slightly
improved by the addition of a second venturi under each wing, but the
spray pattern was similar to that produced by the single venturi tubes.
In each case the deposits were heavier under the tip of each wing than
directly beneath the plane.
Table 1.-Delivery rate of water from gravity-flow equipment provided with nozzles and breather tubes
of different sizes.
Size Size of
(inch) i (inch)
Quarts delivered in indicated number of minutes
18-20 20-22 1 22-24 24-26
1/ 2 nozzles used for each.
Table 2.--Recovery of DDT at various stations at 20-foot intervals from line
of flight (station 9). Sprays applied with PT-13 airplane provided with
L ght Pounds of DDT per acre
To. N.3 No.4 No.5 No.6 No.7 18
0.004 0.003 0.282 0.044 0.081 0.058
3 .002 .002 .010 .009 .019 .124
t .- .002 .033 .091
Av. .003 .003 .019 .018 .045 .091
at indicated station
.o .9 o1 N o. No.12 No.13 No.14
0.040 0.140 0.060 0.011 0.004
.019 .093 .047 .033 .017 0.001
.075 .054 .065 .056 .008 .003
.045 .096 .058 .034 .010 .002
bO 40 20 0 20 40
Distance from center of flight (feet)
Figure 2.--Graphical representation of average data given in table 2.
UNIVERSITY OF FLORIDA
6 3 1262 09242 9140
Effect of Air Currents )n Sprays
Observations were made on the effects of air current on sprays
released at different points under the wings and under different weather
conditions. The release of a quantity of material at a single point
emphasized the effects of erratic air currents which are not so apparent
when the conventional multiple-nozzle boom is used.
From photographs taken in the afternoon to show the roll given the
spray by the wing tips, it was found that the spray pattern changed com-
pletely as the afternoon temperatures and wind speeds increased, and
that under such conditions the wing-tip air currents did not appreciably
affect the spray released 6 feet inside the wing tips. In cross-wind
flights light breezes prevented the spray from fanning out in a charac-
teristic roll and the result was a heavier deposit under the up-wind wing.
Samples of the dispersed sprays were obtained on slides coated with
magnesium oxide. Each slide was exposed to the spray immediately after
it was released from the plane and before the large and small droplets
became segregated. Under some conditions more large drops were taken
on the outside of the spray-roll off the wing tip and smaller drops were
present on slides exposed inside and close under the plane.
A test of sampling technique was made in which slides were placed.
at 3-foot intervals across a concrete air strip and at right angles to
the line of flight. The spray plane was flown so low that the wheels
touched the runway momentarily as the airplane passed over the line of
slides. At this elevation drops were not impinged on any of the slides
except those at the wing tips and beyond, even though the spray was
released from venturis located 6 feet and 9 feet in from the wing tips.
The point of spray release is highly important and further studies
are necessary on proper locations for nozzles to give the most uniform
swath. Further studies on air currents set up by various types of air-
craft are also needed.