Higher altitude above the crop for better coverage and visibility

Legendary crop dusters are known for their ability to spin the wheels on

cabbage furrows without damaging the crop; the emphasis in flying has

usually been to get the airplane as close to the crop as possible, without

actually harvesting it. Both farmers and operators have held the belief

that the lower the aircraft flew, the better the application - especially

as regards pesticides.

The USDA and FAA ag aircraft study has disproved this theory and actually

has shown that because of the influence of ground effect, very low flying

airplanes have a less effective distribution than those flown somewhat

higher. Spray discharged from the central boom segments is influenced

mostly by the clockwise, as the pilot sees it, rotation of the propeller,

especially at low-level applications. This rotation strongly shifts the

spray from right to left resulting in a high-deposit zone left of center

with a corresponding low-deposit zone right of center. At low flight levels

the expanding vortex of the propeller strikes the ground almost

immediately, so that the spray is deposited before circulation has been

completed and consequently distribution is very erratic. At application

levels of 8 to 10 feet, the spray has a greater opportunity to circulate

within the propeller slip-stream before it is deposited, therefore, the

distribution is more uniform.

Most ag pilots are familiar with ground effect - the interference with the

swirling flow of air over the wingtip or aircraft (wingtip, vortices) when

the plane is very close to the ground. Vortices tend to help spread the

application over a wide swath, since the direction flow is outward from the

wingtip. Ground effect, however, breaks up vortices and interferes with

their distribution of ag materials.

The study determined that the best general application height for

agricultural aircraft was approximately one-half of the wing length above

the ground (15 to 20 ft.). The height for maximum swath should be slightly

lower. These distances are given above the ground; for crops with a very

thick canopy the height should be varied accordingly. The uniformity of the

distribution is greatly improved by the additional height because of better

mixing of chemicals in the airflow.

Flying somewhat higher over the ground, also relieves the stress on the

pilot, who is in a better position to see and clear field obstacles. Of

course, when operating higher over the crop it is necessary to recalibrate

the distributing equipment from the standpoint of swath width which thereby

affects the area covered.

Another interesting discovery turned up in the FAA tests was that cost, in

terms of horsepower, of the "free" or wind-driven dispersal system used to

spread liquid materials was not free, because of the excess drag involved

in the wind-driven system. It was found that most windmill systems have

such a loss efficiency that approximately 20 horsepower must be taken from

the aircraft distributor pump.

SHORT FIELD TAKEOFFS - Pilots have long argued the question as to which is

the most efficient way of taking off in a tailwheel aircraft; keep the tail

on the ground until lift-off, or raise the tail shortly after moving down

the runway. Ag test pilots have shown clearly that the technique of raising

the tail before taking off is best. On a standard day, the average off roll

using the three-point takeoff was about 1000 feet, as compared to 900 feet

for a tail-up takeoff. Furthermore, in order to clear a 50 foot obstacle,

the distance required for a three-point takeoff was 1,550 feet compared to

only 1,350 feet for a tail-up takeoff.

It was noted, however, that with a tail-up takeoff the tail must not be

raised to soon or the tailwheel will bounce along the ground, causing

impaired performance and steering problems.