The ground recovery and drop spectra of sprays dispersed from two types of aircraft

Abstract

Field trials were conducted to determine the ground recovery and drop spectra of the spray dispersed from aerial spraying apparatus attached to a Beaufort and to a Dakota aircraft, lent and operated by the Royal Australian Air Force. Samples of the spray deposit were taken over a 200-yard swathe on slides coated with magnesium oxide. The spray drops were sized and counted by means of a microscope fitted with an eyepiece graticule. The mean diameter of the drops recovered from the Beaufort over a swathe of 200 yards was 151 microns and the number per cm.² ranged from 0.5 to 80.7. From the Dakota the mean diameter was 133 microns and the number per cm2 from 1.8 to 43.7. The ground recovery of spray was estimated at 63 per cent. of the volume dispersed from the Beaufort and 72.8 per cent. of that from the Dakota. The drop spectrum of the recovered spray was found for each aircraft, and the percentage number and volume of each size group and their distriibution pattern revealed sources of wastage. The HU product (product of aircraft height and wind speed) used for the Beaufort appeared to be too low, but that for the Dakota was suitable. The effect on the dosage level of making successive runs is illustrated for each aircraft. The distribution of the drops to be expected on the basis of their terminal velocities and the HU product is shown, and possible reasons for variations of the actual from the expected distribution are discussed. The distribution from the Beaufort approximated to that calculated but from the Dakota the actual distribution was inferior to that expected. Observations on drop behaviour were made and, in conjunction with the distribution data, led to certain conclusions on the optimum size of drop. The effect on distribution of removing certain fractions of the spray was estimated. It was concluded from the results of the experiments that for general aerial spraying purposes the Dakota modification is more satisfactory than the Beaufort but that further improvements were both desirable and practicable. The principles on which further improvements should be based are discussed.