Control and prediction of droplet size. It is possible to predict the average drop size 
which will be produced by several nozzles used in agricultural work. Such formulas have 
been developed from dimensional analysis and empirical experiments, and appear to be 
accurate over the range of variables which are encountered in ground agricultural spray- 
ing equipment. For example, the following formula applies to fan nozzles in still air. It is 
not known whether it applies to aircraft spraying: 
1/3 
D = (4.4 + o.15)( 54) I fe ps p 1/2 
where D= mass median diameter, Q, = nozzle discharge, 9= fan angle, ¥ = surface 
tension, p=liquid density, and p = liquid pressure. Similar formulas have been de- 
veloped for prediction of average drop size from hollow cone nozzles, for example: 
D = (2.120) 3 )’? (@ sin B) ° ep” p ? where B= the cone half angle. 
A great deal of work has beendoneon predicting drop size from air atomizers with a 
corresponding variety of formulas available. The following formula, one of the earliest 
developed by Nukiyama and Tanasawa, still seems to fit as well as any: 
3 Q 
D3> - 585 oA ete ba ao Gila fae ee (too0gt 
v2 Py V¥P; a 
rel 
where D3, =surface mean diameter, Vye] = relative velocity of liquid and air, Qa =air 
discharge, and #) =liquid viscosity. However, the formula is completely empirical, has 
little rational basis, and is not even dimensionally consistent. 
Formation of droplets from spinning disks at low flow rates is well understood. 
Under these conditions spinning disks yield nearly homogeneous sprays and may be 
usable in the field, although capacities are very limited. At high capacities we get hetero- 
geneous sprays and the mechanism of droplet formation, though not completely understood, 
is probably similar to that of hydraulic nozzles. 
From the previous discussion it appears we can do a fair job of predicting average 
drop size from various nozzles under certain conditions. However, we do not know 
whether the standard deviation of the drop size distribution is related to any variables, 
and generally the standard deviation has been believed to be uncontrollable, except for 
the fact that extreme upper and lower limits on the drop size usually exist. 
Fan nozzles have been developed for agricultural use with both tapered and uniform 
distribution patterns for use side by side or alone respectively. Field studies have de- 
termined the nozzle configurations which give results which are reasonably satisfactory 
at present for spraying nonrow crops as well as preemergence surface spray, pre- 
emergence incorporated spray, directed herbicidal spray, and directed insecticidal sprays. 
Characteristics of airblast sprayers. In airblast sprayers most spray is formed by 
conventional hydraulic nozzles. The principle of most high-volume airblast sprayers is 
not to produce spray drops by air atomization, but rather to provide a large air-jet to 
transport the spray with enough momentum that the turbulence will not dissipate it until 
it reaches a certain distance from the spraying machine. 
In addition to increased capacity airblast machines provide aturbulent air stream 
which exposes covered foliage and increases the momentum of medium size drops to 
improve their deposition by inertial impaction. 
Characteristics of aircraft application. Research today indicates that spray dis- 
charged along the lateral axis of an airplane will follow the air currents set in motion 
by its passage through the air, until these forces will not longer support the weight of the 
spray droplet. Three principal airstreams, which affect spray distribution, are generated 
2) 
