38 BULLETIN L204, Q. B. DEPARTMENT OF AGRICULTURE. 
irregular clouds must be used, thus requiring more or less overlap- 
ping, the development of the dust would naturally tend toward a 
diluted form which would be comparatively inexpensive and could 
be used at a higher poundage per acre and to make sure of absolutely 
complete covering of all spots. 
CHARACTERISTICS OF AIRPLANES USED. 
The dusting planes used in these tests were those commonly 
termed the "Curtis H," or, more properly, Curtis JN6H. They are 
commonly used for training observers and will safely carry about 
350 pounds besides the pilot. This probably means that the maxi- 
mum dust capacity of one of these planes, with the hopper con- 
structed in the plane, would not exceed 250 pounds. These planes 
had the Hispano, 150-horsepower, 8-cylinder, V-type motor. The 
ordinary number of revolutions per minute at flying speed is about 
1,400 with the heavy propeller, while it is about 1,500 with the 
lighter propeller, commonly known as the "toothpick." The mini- 
mum safe ground speed of this plane is about 50 miles an hour, and 
its maximum speed depends of course on the wind direction and 
velocity, but is at best little over 90 miles an hour. 
This plane has a main tank capacity of 21 to 22 gallons of gaso- 
line, with an emergency tank holding 7 gallons. Normal consump- 
tion usually approximates 10 gallons an hour, so that the safe fly- 
ing time on one filling is not more than 2 hours in the air, and when 
frequent landings are necessary, the small amount which would be 
saved while filling the hopper is compensated for by the extra gas 
used for taking off. Gassing and oiling the plane will require about 
15 minutes, and in figuring on the operation of the plane this time 
should be allowed after every 2 hours. The motor holds 16 quarts 
of oil and will require about 6 quarts per hour of operation. All oil 
must be drained from the motor every 5 flying hours and replaced. 
The gasoline used should be special aviation gasoline, testing from 
68 to 74, and the oil should be a special quality of extra heavy oil. 
Both of these items are thus more expensive than for ordinary 
motors. 
In commercial operation, the length of life of a plane will be a 
very important item. The following figures are probably fair aver- 
ages. The life of the plane itself will be about 200 flying hours, at 
the end of which time it must be overhauled. This overhauling will 
possibly add 150 flying hours to its life. These planes deteriorate 
in storage, and their period of life would probably not vary greatly, 
whether or not they were subjected to maximum use. The motor 
life depends largely on the human element, but the average time 
will be about 80 to 100 hours. It must be then completely over- 
hauled, and at the end of two overhaulings practically a new motor 
has been constructed; thus a total motor life in a plane will be 
from 240 to 300 hours. Such flying as is done in dusting is un- 
usually hard on the motor. It is necessary for the pilot to main- 
tain an excessive speed for safety at the low altitude, and in ad- 
dition the air temperature is so high near the ground that the motor 
heats more rapidly than when flying higher. 
Another type of plane available for study was the " De Haviland 
4B." or, as it is commonly nicknamed, the " D. H." This plane is 
equipped with a 420-horsepower Liberty motor, a 12-cylinder V- 
