w propeller, is the distance
between its calculated pitch speed and the actual distance it travels
through under load, depending upon the efficiency and proportion of its
blades and the amount of load it has to carry.

The action of a screw propeller while performing useful work might be
compared to a nut traveling on a threaded bolt; little resistance is
offered to its forward motion while it spins freely without load, but
give it a load to carry; then it will take more power to keep up its
speed; if too great a load is applied the thread will strip, and so
it is with a screw propeller gliding spirally on the air. A propeller
traveling without load on to new air might be compared to the nut
traveling freely on the bolt. It would consume but little power and it
would travel at nearly its calculated pitch speed, but give it work to
do and then it will take power to drive it.

There is a reaction caused from the propeller projecting air backward
when it slips, which, together with the supporting effect of the blades,
combine to produce useful work or pull on the object to be carried.

A screw propeller working under load approaches more closely to its
maximum efficiency as it carries its load with a minimum amount of slip,
or nearing its calculated pitch speed.

Why Blades Are Curved.

It has been pointed out by experiment that certain forms of curved
surfaces as applied to aeroplanes will lift more per horse power, per
unit of square foot, while on the other hand it has been shown that a
flat surface will lift more per horse power, but requires more area of
surface to do it.

As a true pitch screw propeller is virtually a rotating aeroplane, a
curved surface may be advantageously employed when the limit of size
prevents using large plane surfaces for the blades.

Care should be exercised in keeping the chord of any curve to be used
for the blades at the proper pitch angle, and in all cases propeller
blades should be made rigid so as to preserve the true angle and not be
distorted by centrifugal force or from any other cause, as flexibility
will seriously affect their pitch speed and otherwise affect their
efficiency.

How to Determine Angle.

To find the angle for the proper pitch at any point in the diameter of
a propeller, determine the circumference by multiplying the diameter by
3.1416, which represent by drawing a line to scale in feet. At the end
of this line draw another line to represent the desired pi