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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。
CHAPTER XXV。
NEW MOTORS AND DEVICES。
Since the first edition of this book was printed； early in 1910；
there has been a remarkable advance in the construction of
aeroplane motors； which has resulted in a wonderful decrease
in the amount of surface area from that formerly required。
Marked gain in lightness and speed of the motor has enabled
aviators to get along； in some instances； with one…quarter of
the plane supporting area previously used。 The first Wright
biplane； propelled by a motor of 25 h。p。； productive of a fair
average speed of 30 miles an hour； had a plane surface of 538
square feet。 Now； by using a specially designed motor of 65
h。 p。； capable of developing a speed of from 70 to 80 miles an
hour； the Wrights are enabled to successfully navigate a machine
the plane area of which is about 130 square feet。 This
apparatus is intended to carry only one person （the operator）。
At Belmont Park； N。 Y。； the Wrights demonstrated that the
small…surfaced biplane is much faster； easier to manage in the
hands of a skilled manipulator； and a better altitude climber
than the large and cumbersome machines with 538 square feet
of surface heretofore used by them。
In this may be found a practical illustration of the principle
that increased speed permits of a reduction in plane area in
mathematical ratio to the gain in speed。 The faster any object
can be made to move through the air； the less will be the
supporting
surface required to sustain a given weight。 But； there
is a limit beyond which the plane surface cannot be reduced
with safety。 Regard must always be had to the securing of
an ample sustaining surface so that in case of motor stoppage
there will be sufficient buoyancy to enable the operator to
descend safely。
The baby Wright used at the Belmont Park （N。 Y。） aviation
meet in the fall of 1910； had a plane length of 19 feet 6 inches；
and an extreme breadth of 21 feet 6 inches； with a total surface
area of 146 square feet。 It was equipped with a new Wright
8…cylinder motor of 60 h。 p。； and two Wright propellers of 8
feet 6 inches diameter and 500 r。 p。 m。 It was easily the fastest
machine at the meet。 After the tests； Wilbur Wright said：
〃It is our intention to put together a machine with specially
designed propellers； specially designed gears and a motor which
will give us 65 horsepower at least。 We will then be able；
after some experimental work we are doing now； to send forth
a machine that will make a new speed record。〃
In the new Wright machines the front elevating planes for
up…and…down control have been eliminated； and the movements
of the apparatus are now regulated solely by the rear； or
〃tail〃
control。
A Powerful Light Motor。
Another successful American aviation motor is the aeromotor；
manufactured by the Detroit Aeronautic Construction。
Aeromotors are made in four models as follows：
Model 1。4…cylinder； 30…40 h。 p。； weight 200 pounds。
Model 2。4…cylinder； （larger stroke and bore） 40…50 h。 p。；
weight 225 pounds。
Model 3。6…cylinder。 50…60 h。 p。； weight 210 pounds。
Model 4。6…cylinder； 60…75 h。 p。； weight 275 pounds。
This motor is of the 4…cycle； vertical； water…cooled type。
Roberts Aviation Motor。
One of the successful aviation motors of American make； is
that produced by the Roberts Motor Co。； of Sandusky； Ohio。
It is designed by E。 W。 Roberts； M。 E。； who was formerly
chief assistant and designer for Sir Hiram Maxim； when the
latter was making his celebrated aeronautical experiments in
England in 1894…95。 This motor is made in both the 4… and
6…cylinder forms。