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
representing a true sector of a circle.
All screw propellers having a pitch equal to their diameters have the
same angle for their blades at their largest diameter.
When Pitch Is Not Uniform.
A screw propeller not having a uniform pitch, but having the same angle
for all portions of its blades, or some arbitrary angle not a true
pitch, is distinguished from one having a true pitch in the variation of
the pitch speeds that the various portions of its blades are forced to
travel through while traveling at its maximum pitch speed.
On this subject Mr. R. W. Jamieson says in Aeronautics:
"Take for example an 8-foot screw propeller having an 8-foot pitch at
its largest diameter. If the angle is the same throughout its entire
blade length, then all the porions of its blades approaching the hub
from its outer portion would have a gradually decreasing pitch. The
2-foot portion would have a 2-foot pitch; the 3-foot portion a 3-foot
pitch, and so on to the 8-foot portion which would have an 8-foot pitch.
When this form of propeller is caused to revolve, say 500 r.p.m., the
8-foot portion would have a calculated pitch speed of 8 feet by 500
revolutions, or 4,000 feet per min.; while the 2-foot portion would have
a calculated pitch speed of 500 revolutions by 2 feet, or 1,000 feet per
minute.
Effect of Non-Uniformity.
"Now, as all of the portions of this type of screw propeller must travel
at some pitch speed, which must have for its maximum a pitch speed
in feet below the calculated pitch speed of the largest diameter, it
follows that some portions of its blades would perform useful work
while the action of the other portions would be negative--resisting
the forward motion of the portions having a greater pitch speed. The
portions having a pitch speed below that at which the screw is traveling
cease to perform useful work after their pitch speed has been exceeded
by the portions having a larger diameter and a greater pitch speed.
"We might compare the larger and smaller diameter portions of this form
of screw propeller, to two power-driven vessels connected with a line,
one capable of traveling 20 miles per hour, the other 10 miles per hour.
It can be readily understood that the boat capable of traveling 10 miles
per hour would have no useful effect to help the one traveling 20 miles
per hour, as its action would be such as to impose a dead load upon the
latter's progress."
The term "slip," as applied to a screw 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 pitch 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.