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tubing instead of solid rods； and also by the paring away
of material wherever it can be done without sacrificing
strength。 This plan； with the exclusive use of the best
grades of steel； regardless of cost； makes possible a
marked reduction in weight。
Multiplicity of Cylinders。
Strange as it may seem； multiplicity of cylinders does
not always add proportionate weight。 Because a 4…
cylinder motor weighs say 100 pounds； it does not necessarily
follow that an 8…cylinder equipment will weigh
200 pounds。 The reason of this will be plain when it
is understood that many of the parts essential to a 4…
cylinder motor will fill the requirements of an 8…cylinder
motor without enlargement or addition。
Neither does multiplying the cylinders always increase
the horsepower proportionately。 If a 4…cylinder
motor is rated at 25 horsepower it is not safe to take
it for granted that double the number of cylinders will
give 50 horsepower。 Generally speaking； eight cylinders；
the bore； stroke and speed being the same； will give
double the power that can be obtained from four； but
this does not always hold good。 Just why this exception
should occur is not explainable by any accepted rule。
Horse Power and Speed。
Speed is an important requisite in a flying…machine
motor； as the velocity of the aeroplane is a vital factor
in flotation。 At first thought； the propeller and similar
adjuncts being equal； the inexperienced mind would
naturally argue that a 50…horsepower engine should
produce just double the speed of one of 25…horsepower。
That this is a fallacy is shown by actual performances。
The Wrights； using a 25…horsepower motor； have made
44 miles an hour； while Bleriot； with a 50…horsepower
motor； has a record of a short…distance flight at the rate
of 52 miles an hour。 The fact is that； so far as speed
is concerned； much depends upon the velocity of the
wind； the size and shape of the aeroplane itself； and the
size； shape and gearing of the propeller。 The stronger
the wind is blowing the easier it will be for the aeroplane
to ascend； but at the same time the more difficult
it will be to make headway against the wind in a horizontal
direction。 With a strong head wind； and proper
engine force； your machine will progress to a certain
extent； but it will be at an angle。 If the aviator desired
to keep on going upward this would be all right； but
there is a limit to the altitude which it is desirable to
reachfrom 100 to 500 feet for expertsand after that
it becomes a question of going straight ahead。
Great Waste of Power。
One thing is certaineven in the most efficient of
modern aerial motors there is a great loss of power between
the two points of production and effect。 The
Wright outfit； which is admittedly one of the most effective
in use； takes one horsepower of force for the raising
and propulsion of each 50 pounds of weight。 This；
for a 25…horsepower engine； would give a maximum lifting
capacity of 1250 pounds。 It is doubtful if any of the
higher rated motors have greater efficiency。 As an 8…
cylinder motor requires more fuel to operate than a 4…
cylinder； it naturally follows that it is more expensive
to run than the smaller motor； and a normal increase in
capacity； taking actual performances as a criterion； is
lacking。 In other words； what is the sense of using an
8…cylinder motor when one of 4 cylinders is sufficient？
What the Propeller Does。
Much of the efficiency of the motor is due to the form
and gearing of the propeller。 Here again； as in other
vital parts of flying…machine mechanism； we have a wide
divergence of opinion as to the best form。 A fish makes
progress through the water by using its fins and tail；
a bird makes its way through the air in a similar manner
by the use of its wings and tail。 In both instances the
motive power comes from the body of the fish or bird。
In place of fins or wings the flying machine is equipped
with a propeller； the action of which is furnished by the
engine。 Fins and wings have been tried； but they don't
work。
While operating on the same general principle； aerial
propellers are much larger than those used on boats。
This is because the boat propeller has a denser； more
substantial medium to work in （water）； and consequently
can get a better 〃hold；〃 and produce more propulsive
force than one of the same size revolving in the air。
This necessitates the aerial propellers being much larger
than those employed for marine purposes。 Up to this
point all aviators agree； but as to the best form most of
them differ。
Kinds of Propellers Used。
One of the most simple is that used by Curtiss。 It
consists of two pear…shaped blades of laminated wood；
each blade being 5 inches wide at its extreme point；
tapering slightly to the shaft connection。 These blades
are joined at the engine shaft； in a direct line。 The propeller
has a pitch of 5 feet； and weighs； complete； less
than 10 pounds。 The length from end to end of the two
blades is 6 1/2 feet。
Wright uses two wooden propellers； in the rear of his
biplane； revolving in opposite directions。 Each propeller
is two…bladed。
Bleriot also uses a two…blade wooden propeller； but
it is placed in front of his machine。 The blades are each
about 3 1/2 feet long and have an acute 〃twist。〃
Santos…Dumont uses a two…blade wooden propeller；
strikingly similar to the Bleriot。
On the Antoinette monoplane； with which good records
have been made； the propeller consists of two spoon…
shaped pieces of metal； joined at the engine shaft in
front； and with the concave surfaces facing the machine。
The propeller on the Voisin biplane is also of metal；
consisting of two aluminum blades connected by a forged
steel arm。
Maximum thrust； or stressexercise of the greatest
air…displacing forceis the object sought。 This； according
to experts； is best obtained with a large propeller
diameter and reasonably low speed。 The diameter is the
distance from end to end of the blades； which on the
largest propellers ranges from 6 to 8 feet。 The larger
the blade surface the greater will be the volume of air
displaced； and； following this； the greater will be the
impulse which forces the aeroplane ahead。 In all centrifugal
motion there is more or less tendency to disintegration
in the form of 〃flying off〃 from the center； and
the larger the revolving object is the stronger is this
tendency。 This is illustrated in the many instances in
which big grindstones and fly…wheels have burst from
being revolved too fast。 To have a propeller break
apart in the air would jeopardize the life of the aviator；
and to guard against this it has been found best to make
its revolving action comparatively slow。 Besides this
the slow motion （it is only comparatively slow） gives
the atmosphere a chance to refill the area disturbed by
one propeller blade； and thus have a new surface for