tch in feet.
Then draw a line from the point representing the desired pitch in feet
to the beginning of the circumference line. For example:

If the propeller to be laid out is 7 feet in diameter, and is to have
a 7-foot pitch, the circumference will be 21.99 feet. Draw a diagram
representing the circumference line and pitch in feet. If this diagram
is wrapped around a cylinder the angle line will represent a true thread
7 feet in diameter and 7 feet long, and the angle of the thread will be
17 3/4 degrees.

Relation of Diameter to Circumference.

Since the areas of circles decrease as the diameter lessens, it follows
that if a propeller is to travel at a uniform pitch speed, the volume of
its blade displacement should decrease as its diameter becomes less, so
as to occupy a corresponding relation to the circumferences of larger
diameters, and at the same time the projected area of the blade must be
parallel along its full length and should represent a true sector of a
circle.

Let us suppose a 7-foot circle to be divided into 20 sectors, one of
which represents a propeller blade. If the pitch is to be 7 feet, then
the greatest depth of the angle would be 1/20 part of the pitch, or 4
2/10 inch. If the line representing the greatest depth of the angle is
kept the same width as it approaches the hub, the pitch will be uniform.
If the blade is set at an angle so its projected area is 1/20 part of
the pitch, and if it is moved through 20 divisions for one revolution,
it would have a travel of 7 feet.

CHAPTER XXV. NEW MOTORS AND DEVICES.

Since the first edition of this book was printed, early in 1910, there
has been a remarkable advance in the construction of aeroplane motors,
which has resulted in a wonderful decrease in the amount of surface area
from that formerly required. Marked gain in lightness and speed of
the motor has enabled aviators to get along, in some instances, with
one-quarter of the plane supporting area previously used. The first
Wright biplane, propelled by a motor of 25 h.p., productive of a fair
average speed of 30 miles an hour, had a plane surface of 538 square
feet. Now, by using a specially designed motor of 65 h. p., capable
of developing a speed of from 70 to 80 miles an hour, the Wrights are
enabled to successfully navigate a machine the plane area of which is
about 130 square feet. This apparatus is intended to carry only one
person (the operator). At Belmont Park, N. Y., the Wright