aeroplanes-第24节
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PROPELLER SHAPE。It cannot be possible that
the present shape of the propeller will be its ultimate
form。 It is inconceivable that the propeller
is so inefficient that only one sixty…fifth of the
power of the engine is available。 The improvement
in propeller efficiency is a direction which
calls for experimental work on the part of inventors
everywhere。
The making of a propeller； although it appears
a difficult task； is not as complicated as would appear；
and with the object in view of making the
subject readily understood； an explanation will be
given of the terms 〃Diameter；〃 and 〃Pitch；〃 as
used in the art。
The Diameter has reference to the length of
the propeller； from end to end。 In calculating
propeller pull； the diameter is that which indicates
the speed of travel； and for this reason is
a necessary element。
Thus； for instance； a propeller three feet in
diameter； rotating 500 times a minute； has a tip
speed of 1500 feet； whereas a six foot propeller；
rotating at the same speed； moves 3000 feet at the
tips。
PITCH。This is the term which is most confusing；
and is that which causes the most frequent
trouble in the mind of the novice。 The term will
be made clear by carefully examining the accompanying
illustration and the following description：
In Fig。 76 is shown a side view of a propeller
A； mounted on a shaft B； which is free to move
longitudinally。 Suppose we turn the shaft so the
tip will move along on the line indicated by the
arrow C。
Now the pitch of the blade at D is such that it
will be exactly in line with the spirally…formed
course E； for one complete turn。 As the propeller
shaft has now advanced； along the line E； and
stopped after one turn； at F； the measure between
the points F and G represents the pitch of the propeller。
Another way to express it would be to
call the angle of the blade a five； or six； or a seven
foot pitch； as the pitches are measured in feet。
_Fig。 76。 Describing the Pitch Line。_
In the illustration thus given the propeller shaft；
having advanced six feet； we have what is called
a six foot pitch。
Now； to lay out such a pitch is an easy matter。
Assume； as in Fig。 77； that A represents the end
of the blank from which the propeller is to be cut；
and that the diameter of this blank； or its length
from end to end is seven feet。 The problem now
is to cut the blades at such an angle that we shall
have a six foot pitch。
_Fig。 77。 Laying out the Pitch。_
LAYING OUT THE PITCH。First； we must get the
circumference of the propeller； that is； the distance
the tip of the propeller will travel in making
one complete turn。 This is done by multiplying
7 by 3。1416。 This equals 21。99； or； practically； 22
feet。
A line B is drawn； extending out horizontally
along one side of the blank A； this line being made
on a scale； to represent 22 feet。 Secondly； at the
end of this line drawn a perpendicular line C； 6
feet long。 A perpendicular line is always one
which is at right angles to a base line。 In this
case B is the base line。
Line C is made 6 feet long； because we are trying
to find the angle of a 6 foot pitch。 If； now； a
line D is drawn from the ends of the two lines B；
C； it will represent the pitch which； marked across
the end of the blank A； will indicate the line to cut
the blade。
PITCH RULE。The rule may； therefore； be
stated as follows： Multiply the diameter （in
feet） of the propeller by 3。1416； and draw a line
the length indicated by the product。 At one end
of this line draw a perpendicular line the length
of the pitch requirement （in feet）； and join the
ends of the two lines by a diagonal line； and this
line will represent the pitch angle。
Propellers may be made of wood or metal； the
former being preferred for the reason that this
material makes a lighter article； and is stronger；
in some respects； than any metal yet suggested。
LAMINATED CONSTRUCTION。All propellers
should be laminated；that is； built up of layers
of wood； glued together and thoroughly dried；
from which the propeller is cut。
A product thus made is much more serviceable
than if made of one piece； even though the laminated
parts are of the same wood； because the
different strips used will have their fibers overlapping
each other； and thus greatly augment the
strength of the whole。
Generally the alternate strips are of different
materials； black walnut； mahogany； birch； spruce；
and maple being the most largely used； but mahogany
and birch seem to be mostly favored。
LAYING UP A PROPELLER FORM。The first step
necessary is to prepare thin strips； each； say；
seven feet long； and five inches wide； and three…
eighths of an inch thick。 If seven such pieces are
put together； as in Fig。 78； it will make an assemblage
of two and five…eighth inches high。
_Fig。 78。 A Laminated Blank。_
Bore a hole centrally through the assemblage；
and place therein a pin B。 The contact faces of
these strips should be previously well painted
over with hot glue liberally applied。 When they
are then placed in position and the pin is in place；
the ends of the separate pieces are offset； one beyond
the other； a half inch； as shown； for instance；
in Fig。 79。
This will provide ends which are eight and a
half inches broad； and thus furnish sufficient
material for the blades。 The mass is then subjected
to heavy pressure； and allowed to dry before the
blades are pared down。
_Fig。 79。 Arranging the Strips。_
MAKING WIDE BLADES。If a wider blade is desired；
a greater number of steps may be made by
adding the requisite number of strips； or； the
strips may be made thicker。 In many propellers；
not to exceed four different strips are thus glued
together。 The number is optional with the
maker。
An end view of such an assemblage of strips
is illustrated in Fig。 80。 The next step is to lay
off the pitch； the method of obtaining which has
been explained。
_Fig。 80。 End view of Blank。_
Before starting work the sides； as well as the
ends； should be marked； and care observed to
place a distinctive mark on the front side of the
propeller。
Around the pin B； Fig。 81； make S…shaped
marks C； to indicate where the cuts on the faces
of the blades are to begin。 Then on the ends of
the block； scribe the pitch angle； which is indicated
by the diagonal line D； Fig。 80。
_Fig。 81。 Marking the Side。_
This line is on the rear side of the propeller；
and is perfectly straight。 Along the front of this
line is a bowline E； which indicates the front surface
of the propeller blade。
PROPELLER OUTLINE。While the marks thus
given show the angles； and are designed to indicate
the two faces of the blades； there is still another
important element to be considered； and
that is the final outline of the blades。
_Fig。 82。 Outlining。_
It is obvious that the outline may be varied
so that the entire width at 1； Fig。 82； may be used；
or it may have an outline； as represented by the
line 2； in this figure； so that the widest part will
be at or near the dotted line 3； say two…thirds of
the distance from the center of the blade。
This is the practice with most of the manufacturers
at the present time； and some of them
claim that this form produces the best results。
FOR HIGHER SPEEDS。Fig。 83 shows a propeller
cut from a blank； 4〃 x 6〃 in cross section； not
laminated。
_Fig。 83。 Cut from a 4〃 x 6〃 Single Blank。_
It should be borne in mind that for high speeds
the blades must be narrow。 A propeller seven
feet in diameter with a six foot pitch； turning
950 revolutions per minute； will produce a pull of
350 pounds； if properly made。
Such a propeller can be readily handled by a
forty horse power motor； such as are specially
constructed for flying machine purposes。
INCREASING PROPELLER EFFICIENCY。Some experiments
have been made lately； which； it is
claimed； largely increase the efficiency of propellers。
The improvement is directed to the outline
shape of the blade。
The typical propeller； such as we have illustrated；
is one with the wide part of the blade at
the extremity。 The new type； as suggested； reverses
this； and makes the wide part of the blade
near the hub； so that it gradually tapers down to
a narrow tip。
Such a form of construction is shown in Fig。
84。 This outline has some advantages from one
standpoint； namely； that it utilizes that part of
the blade near the hub； to produce a pull； and
does not relegate all the duty to the extreme ends
or tips。
_Fig。 84。 A Suggested Form。_
To understand this more fully； let us take a
propeller six feet in diameter； and measure the
pull or thrust at the tips； and also at a point half
way between the tip and the hub。
In such a propeller； if the blade is the same
