isfactory arrangements with the company for its compensation
for the use of its patents. At such licensed meet any machine of any
make may participate freely without securing any further license or
permit. The details and terms of all meets will be arranged by the
committee having in charge the interests of both organizations.

CHAPTER XXIV. HINTS ON PROPELLER CONSTRUCTION.

Every professional aviator has his own ideas as to the design of
the propeller, one of the most important features of flying-machine
construction. While in many instances the propeller, at a casual glance,
may appear to be identical, close inspection will develop the fact
that in nearly every case some individual idea of the designer has been
incorporated. Thus, two propellers of the two-bladed variety, while of
the same general size as to length and width of blade, will vary greatly
as to pitch and "twist" or curvature.

What the Designers Seek.

Every designer is seeking for the same result--the securing of the
greatest possible thrust, or air displacement, with the least possible
energy.

The angles of any screw propeller blade having a uniform or true pitch
change gradually for every increased diameter. In order to give a
reasonably clear explanation, it will be well to review in a primary way
some of the definitions or terms used in connection with and applied to
screw propellers.

Terms in General Use.

Pitch.--The term "pitch," as applied to a screw propeller, is the
theoretical distance through which it would travel without slip in one
revolution, and as applied to a propeller blade it is the angle at
which the blades are set so as to enable them to travel in a spiral path
through a fixed distance theoretically without slip in one revolution.

Pitch speed.--The term "pitch speed" of a screw propeller is the speed
in feet multiplied by the number of revolutions it is caused to make
in one minute of time. If a screw propeller is revolved 600 times
per minute, and if its pitch is 7 ft., then the pitch speed of such a
propeller would be 7x600 revolutions, or 4200 ft. per minute.

Uniform pitch.--A true pitch screw propeller is one having its blades
formed in such a manner as to enable all of its useful portions, from
the portion nearest the hub to its outer portion, to travel at a uniform
pitch speed. Or, in other words, the pitch is uniform when the projected
area of the blade is parallel along its full length and at the same time