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 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.





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