The Airplane in Forest-Pest Control 



475 



ardized. A wide variety of distributing 

 devices is used, such as rotating disks 

 and brushes, Venturis, nozzles, jets, and 

 combinations of them. 



For general forest spraying, the 

 type of spray equipment shown in the 

 diagram has been fairly satisfactory 

 for light planes it is simple to make, 

 and the quantity of liquid applied and 

 the atomization can be varied by 

 changing the size and number of noz- 

 zles on the boom. 



On the biplanes the tank is placed 

 in the front cockpit, the pump is 

 mounted on the landing-gear assembly, 

 and the nozzle boom is suspended on 

 brackets beneath the lower wing. The 

 same type of apparatus can be adapted 

 for larger planes, but even simpler 

 equipment often has given fairly good 

 performance, because the higher speed 

 of large planes makes it possible to ob- 

 tain adequate atomization of the liquid 

 when it is discharged through straight 

 pipes or horizontal tubes placed in 

 the air stream beneath the fuselage. 



Several types of spray apparatus 

 have been reasonably satisfactory, but 

 a great deal of improvement is still 

 needed for more efficient distribution. 

 Much remains to be learned about the 

 type of outlets and their placement on 

 the aircraft to obtain wider and more 

 uniform deposit of the spray under dif- 

 ferent forest conditions. The spray ap- 

 paratus in use today is patterned after 

 ground equipment, but as our knowl- 

 edge of the aerodynamics involved 

 in spray dispersal increases, radically 

 different sprayers may be developed. 



THE PROCEDURES in applying aerial 

 sprays over forests are necessarily dif- 

 ferent from those employed in treating 

 agricultural crops because of the 

 larger areas involved, the irregular ter- 

 rain, and uneven heights of trees. 



Ordinarily, the area to be treated is 

 first divided into blocks, using water- 

 courses, ridges, and other features of 

 the terrain as boundaries. If the ground 

 is relatively flat, the pilot flies a grid 

 pattern back and forth across the area, 

 spacing the flight lines at a width pre- 



viously determined to give satisfactory 

 coverage of the area. This spacing, or 

 swath width, ranges from about 60 to 

 400 feet, depending on the type of air- 

 craft and the spray apparatus; for the 

 biplane trainers the effective swath is 

 100 to 150 feet. Where the terrain is 

 steep and irregular, as in many of the 

 western forests, a grid flight pattern 

 is unsafe, so the pilot must fly along 

 the contours or down slope. 



One of the most serious difficulties 

 in forest spraying is to maintain the 

 proper spacing of the flight lines. It is 

 impossible for even the most expert 

 pilot to estimate accurately the distance 

 of successive swaths from the air, 

 especially when spraying mountainous 

 areas. Therefore small captive bal- 

 loons, small wind socks, flags, or other 

 markers often are placed in the tree- 

 tops at intervals along the boundaries 

 of the treated blocks to aid the pilot in 

 maintaining an even pattern. 



But where the area to be treated is 

 large or inaccessible, the placing of 

 markers by ground crews has been too 

 time-consuming to be practical. At- 

 tempts have been made to drop mark- 

 ers from the air and to incorporate dyes 

 or other materials in the spray liquid 

 in order to make the spray deposit 

 visible to the pilot. None of these de- 

 vices have been successful, however, 

 and considerably more developmental 

 work will be needed to improve this 

 phase of the operation. 



In contrast to crop spraying, which 

 is done 5 to 10 feet above the fields, the 

 minimum safe altitude for forest spray- 

 ing is 50 feet above the treetops. Over 

 rough terrain or with the larger, less 

 maneuverable planes, the altitude must 

 be increased. 



Wind and convection currents the 

 warm air rising from the ground 

 often carry the spray away from the 

 area being treated or keep the spray 

 cloud suspended above the treetops. 

 For that reason spraying is usually con- 

 fined to the early morning and evening 

 hours, when air movement is at a min- 

 imum. Generally no spraying is done 

 when the wind velocity is more than 10 



