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, w
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.
More
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