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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
width and pitch at the two points named； the pull
at the tips will be four times greater than at the
intermediate point。
CHAPTER XIV
EXPERIMENTAL GLIDERS AND MODEL AEROPLANES
AN amusing and very instructive pastime is
afforded by constructing and flying gliding machines；
and operating model aeroplanes； the latter
being equipped with their own power。
Abroad this work has been very successful as
a means of interesting boys； and； indeed； men
who have taken up the science of aviation are
giving this sport serious thought and study。
When a machine of small dimensions is made
the boy wonders why a large machine does not
bear the same relation in weight as a small machine。
This is one of the first lessons to learn。
THE RELATION OF MODELS TO FLYING MACHINES。
A model aeroplane； say two feet in length； which
has； we will assume； 50 square inches of supporting
surface； seems to be a very rigid structure；
in proport