Ever since windmills were first set up, men have been studying the
merits of different propellers. By the time steamships came to be driven
through the water by rotary blades or screws, their modeling had become
a science. The builders of rotary fans in turn contributed still further
to our knowledge on the subject. Drawing largely upon all this
experience, the aviator has learned to build fairly efficient
propellers, although there is probably no department of aeronautics
to-day so little understood.
In a windmill a current or cylinder of air flows, of course, against the
propeller. The blades must be shaped and spaced with this in view.
Reverse the action of the windmill, and the propeller proves
inefficient. The broad blades will stir up a current of air, to be sure,
but a very weak one. A revolving fan solves a very different problem in
detaching a cylinder of air from the atmosphere and propelling it with
the greatest possible momentum. Here, again, the propellers must be
differently modeled and spaced. Neither the reversed windmill propeller
nor the electric fan, however, would serve to drive an aeroplane.
[Illustration: A beautiful monoplane built by R. Mungokee]
[Illustration: Detail of a model built by R. Mungokee]
[Illustration: An ingenious application of the dihedral angle]
The propeller of an aeroplane must cut its way smoothly, pressing the
air backward without splashing. It is only when an aeroplane is held
fast that its propellers kick up such a fuss and blow your hat off. The
aeroplane propeller’s work is much the same as that of a steamship,
although the air through which it travels has many tricks not yet
understood. The density of the air compares to that of water as one to
eight hundred, but the friction encountered by the air propellers is
much greater than 1-800th that of water. It may be laid down as a
general rule, however, that the driving force of an aeroplane propeller
varies as the square of the number of revolutions per minute.
There is at present no standard form of propeller for the man-carrying
or model aeroplane. One school of designers favors a small blade
revolved at high speed, while others claim that a larger propeller
driven more slowly is more efficient. As a general rule it may be laid
down that a model with a span of thirty inches should be driven by twin
propellers eight inches in length or diameter. They should have a speed
of about 1,200 revolutions per minute, or at the rate of some 200 turns
every ten seconds. To test the strength of your motor, give the
propeller 200 or 400 turns, and watch in hand, find how long it takes to
run down.
[Illustration: Diagram Showing How To Make A Propeller From A Wooden
Blank]
There is much difference of opinion as to the proper modeling of the
propeller. It has been worked out by elaborate equations that the blade
should be concave and yet in actual tests it has been found that some
machines are driven faster by a flat blade propeller. By a flat screw we
mean a straight pitch propeller, or one in which the angle does not vary
from the hub to the tip. When Mr. Glenn H. Curtiss made his famous
record flight at Rheims, he used a straight pitch propeller, and when,
later, his machine was equipped with propellers scientifically curved,
his aeroplane lost speed. Evidently the exact relation of propeller
forms to the machine still remains much of a mystery.
[Illustration: Design of Metal Propeller]
A very simple test of the efficiency of propellers of various modeling
may be made by running them in heavy smoke. By burning a piece of oily
cotton waste, you may watch the action of the propellers on the smoke,
while, at the same time, this greasy smoke will leave its mark on the
section of the propeller blade which does the most work. The speed of
the blades near the hub of the propeller is, of course, much less than
at the tips, and consequently the work they perform in sending the
aeroplane forward is small. At the extreme end of the propellers, the
air, of course, tends to slip off.
The most efficient part of the blade, according to these tests, is about
one-third of the radius distant from the center. Less than two-thirds of
the propeller seems to do effective driving work. On the propellers
driven against greasy smoke, the blades near the hub remain
comparatively clean while the portion most soiled extends outward from
this point. The test would seem to indicate that a broad blade narrowing
to the hub would develop the maximum thrust. It would also seem that it
is unnecessary to carry the lines of the blade close to the axle,
thereby possibly weakening the propeller.
To understand the theory of the propeller, bear in mind the principle of
the action of the wings, for the analogy between the two is very close.
The forward, or entering, edge of the propeller is curved so that it
will cut its way smoothly and offer less resistance than a straight
entering edge. The blade of the propeller is made slightly concave for
exactly the same reason that the plane is curved. Like the plane, such a
surface takes advantage of the resistance of the air.
The curve of the propeller blade near the hub is made much higher than
further on because this part travels more slowly, and it is important
that the cylinder of air set in motion by the blade should have the same
velocity throughout its diameter. The blade is made widest at its outer
end, since this is the most effective surface and is expected to do the
greatest amount of work. The other end of the propeller blade is rounded
off in order that the air may slip away, thus avoiding skin friction,
which at this point is naturally high.
[Illustration: A test of high aspect ratio planes]
[Illustration: A modified Bleriot built by Cecil Peoli]
The width of the propeller blade has been the subject of an immense
amount of investigation and discussion. The friends of both the wide and
narrow blade back up their arguments with complicated equations, which
it would only be confusing to repeat. It is argued by some authorities
that since the narrow blade does not stir up the air as long a time as
the wide blade, therefore one blade does not stir up the air enough to
interfere with the action of the second blade.
[Illustration: Langley Propeller Blade]
A small blade may be driven by a much lighter motor, and is, of course,
capable of much higher speed. On the other hand, the wide blade drives
the model much further ahead per turn than the narrow blade, while
making a much greater demand upon the motor.
Briefly a narrow propeller is best for speed and the wide blade
propeller for power. There is an immense amount of difference of opinion
concerning the form and position of the propeller so that it is
impossible to lay down any hard and fast rules. It is argued by several
well-known aviators that a propeller is more effective when driven with
its straight edge forward and there is scarcely a point not in dispute.
One of the most novel propeller designs, the Cowley, is a blade bent in
the form of an arc of a circle, the radius of the curve being equal to
the diameter of the propeller. The propeller is mounted with the convex
surface forward. The theory of this propeller is that it focuses the
air, as it were, which it throws back forming a cylinder of air which
travels at a higher speed than one set in motion by the horizontal
blades.
The tendency for the air to slip off the ends of the propeller blades is
probably reduced. This form of propeller may be made by steaming the
blades and bending them into position. A mould may be prepared and the
steamed blades forced to take their shape and held in position until
they have dried.
A series of experiments have been made in England with boomerangs to
discover the effect of curved surfaces on flight. The Langeley
propeller, which embodies the information gained in this way, has a flat
back while the face is concave, following the general stream line form.
The ends of the propeller blades are practically square. Some of the new
propellers are covered with a thin canvass glued smoothly over the
greater part of the blade. The covering guards somewhat against
splitting and splintering.
In the latest Percy Pierce models, for instance, the blade is carried
out in a semicircle at the end of the propeller, thus practically
doubling its surface. The driving power of this blade is very high. It
is argued for this design that the blade being very thin is forced
slightly backward at the beginning of the flight, while the model is
gathering motion, but later, when the tension is removed, springs back
thus increasing its effective surface and the thrust. The propeller thus
automatically adjusts itself for the work it has to perform.
Since it is so difficult to fix upon the right pitch of a propeller, the
builder of model aeroplanes had best work out this problem for himself.
The propeller blank described later on, with a depth of three-fourths of
one inch to an eight inch diameter, will give you a comparatively
low-pitch propeller. An eight-inch propeller cut from a block one inch
in depth will give you as high a pitch as you are likely to need. As you
increase the pitch, you, of course, increase the power of your
aeroplane, while at the same time you make a greater demand upon your
motor. Try the propellers of different pitch until you find the one
which gives you the greatest stability and the highest speed. It is well
to remember that in increasing the width of your propeller blade you add
to the skin friction.
Some designers carry the curve of the propeller blade to the center of
the axle, while others leave the center blank. It is argued by the
former that the longer the blade the greater is the thrust. Others
believe that the blade exerts little or no thrust near the center and is
weakened by being cut away too much. The builder of model aeroplanes has
one great advantage over the designer of passenger-carrying craft. The
model does not have to carry fuel. After all, the difference in the
power required for the various models is so slight that an extra strand
or two on the motor will probably solve the problem.
Many successful builders of model aeroplanes now carve their propellers
from solid blocks of wood. This method, to be sure, allows the designer
to shape the propeller blades with more freedom than with the ordinary
or built-up propeller, and of course does away with much of the
preliminary work. So great is the demand for the one piece propellers
that the manufacturers of accessories now prepare "propeller blanks" or
pieces of wood in a variety of sizes ready to be carved. The one-piece
propeller is likely to split, but they are easy to make, and this work
is a very fascinating kind of whittling.
[Illustration: A combination of several interesting features]
[Illustration: A skilful adjustment of the front plane and skid built by
Percy Pierce]
Propeller blanks are easily prepared in case you find it inconvenient to
buy them. The following directions refer to a propeller eight inches in
length, but the same proportions hold good for any size. Select a piece
of some straight-grained wood, white pine is best, which will not split
readily, and is easy to work. The original block for an eight-inch
propeller should be eight inches in length, two inches in width, and
three-fourths of one inch thick. Now draw a line lengthwise, exactly
bisecting the block, and mark off the middle of the line, and two points
one inch from either end. With one of these outer points as a center,
describe a quadrant of a circle above the line, and from the
corresponding point, draw a similar circle below the line. From the
center of the blocks draw a complete circle one-half of one inch in
diameter. Draw straight lines from the ends of the arcs to the vertical
diameters of the circle, and saw away the wood to these lines. In
carving your propeller, first cut away the wood from the longer straight
lines of the block on opposite sides. The blades should be slightly
concave. It will be found a good plan to finish one side of the blade
before cutting away the opposite side. Cut away the wood until the blade
is less than one-eighth of an inch thick, and sandpaper away all marks
of the knife or chisel. The wood should then be oiled or covered with a
light coat of varnish. It is very important that the two ends of the
propellers should be uniform both as to their modeling and weight. To
mount the axle, drill a hole at the center just large enough to admit
the wire. The outer end may be bent over and inserted into the hole to
keep it rigid. If the axle does not fit tightly, drive in small wedges
of wood, such as a toothpick, at both sides.
The propeller used by the Wright Brothers on their machines is very
simple to construct. Prepare a propeller blank eight inches in length,
two in width and three-fourths of an inch in depth. Draw two lines
parallel with the longer sides, the first seven-eighths of an inch and
the second one and one-eighth inches back. Now at the upper right-hand
corner mark off a point one and one-half inches from the end, and from
the opposite corner on the lower base the same dimension. Connect these
two points.
[Illustration: Wright Propeller Blade]
The blank is completed by cutting away to these lines, leaving the
blades each one and one-eighth inches in width. The axle should be left
a little full, say three-eighths of an inch across. Round off the outer
corners. In modeling your propeller cut away or bevel the sides formed
by the two intersecting lines, which will form the entering edge of the
propeller. The blade should be cut to a very slight concave, although
some prefer a flat blade. The propeller is mounted by drilling a hole at
the center and mounting in the usual way.
The propellers of a model aeroplane are subject to more wear and tear
than those of a regular passenger-carrying machine. At the end of every
flight, they face a possible catastrophe. In the search for some durable
form of screw, a number of interesting discoveries have been made. One
builder has succeeded in coating a wooden propeller with bronze by
subjecting it to an electroplating process, but this is much too
complicated for the amateur. The lighter metals, aluminum and magnalium,
naturally suggest themselves for the purpose. Such propellers weigh no
more than wood and may be readily bent to the required shape.
Procure a thin sheet of aluminum, or, if this cannot be had, a smooth
piece of tin will do. It must, however, be heavy enough to hold its
shape. The design of the propeller may be laid out on the tin, and the
metal trimmed away. To make an eight-inch propeller, draw a rectangle
eight inches in length and two inches broad, and draw a line joining the
middle of short sides. At the center, draw two vertical lines half an
inch on either side of the center lines, half an inch above and below
the center, forming a small inner rectangle. Now from a point on the
bisecting line, one inch from either end, draw two semicircles. Next,
connect the top of one of these circles with the nearest point of the
inner rectangle and draw another line from the point below to the
corresponding corner of the large rectangle. Repeat the diagram on the
other end of the rectangle, reversing the curve as indicated in Fig. A.
In cutting out the design, allow the straight sections running to the
sides of the larger rectangle to remain. They will be needed to hold the
central piece in position. This consists of a block of wood measuring
one inch by one-half an inch and one-quarter of an inch in thickness.
The strips at the center should be bent tightly over the corners,
overlapped, and nailed firmly down with brads. Next, at the center,
punch a small hole and drill through the block a shaft large enough to
hold the axle of the propeller which is then firmly imbedded in it. One
great advantage of the metal propeller is the fact that you may readily
alter its pitch.
An efficient propeller may be made by mounting metal blades on a wooden
shaft. Procure a stick one quarter of an inch square and three inches in
length, and saw through both ends for a distance of three quarters of an
inch. Prepare your propeller blades by plotting them out on a sheet of
aluminum, as described above, and cut away the middle section. The
blades may then be inserted in the open ends of the stock and nailed
securely in position. The edges of the wood may then be rounded off and
the axle inserted firmly at the center. The metal sheet should be bent
into the proper pitch as in the case of other metal propellers.
FABRIC PROPELLERS
The most nearly indestructible propellers are the fabric screws. They
are also doubtless the lightest form. The blades will, of course, be
perfectly flat, making straight pitch propellers. You will need a small
cylindrical piece of wood one half an inch in diameter, and one half an
inch in height, of some tough, hard wood. The blades may be made of reed
or cane, or, still better, of wire. Aluminum wire being very light is
probably the best for the purpose. Bend the wire into the form of a
triangle two inches in width and four inches in length. Determine at
what angle you wish them to be set, and bore holes in the hub and fix
wires of each frame firmly in them. Cover the frames neatly with cloth
and mount it in the usual way.
[Illustration: An efficient model, showing excellent construction,
designed by John Caresi]
[Illustration: One of the best minimum plane models of 1911]
THE LANGELEY BLANK.
Many model builders still retain the Langeley propeller. It is a very
simple one to build. To prepare a blank secure a block, as before, eight
by two inches and three-fourths of an inch in depth. Connect the four
corners with diagonal lines. Parallel to the longer side draw two lines,
one three-fourths of an inch inside and the second one-half inch below
it. Cut away the block forming a double fan-shaped piece. Some prefer a
wider center and the hub may be made a trifle broader if desired.
In shaping the propeller cut away from opposite sides of the blank. The
original Langeley is a flat blade propeller so that the modeling is very
simple. You may use your own judgement as to the thickness of the blade,
although about one-eighth of an inch is suggested. The Langeley is
mounted in the usual way. To heighten the pitch of your propeller secure
a thicker blank.