We might compare a propeller to an ordinary screw or bolt by likening
the thread of the screw to the two blades of the propeller. If the screw
penetrates wood or metal nut it will advance a certain distance known as
the _pitch_ which is always the same, namely, the distance separating
two consecutive turns of the threads. The revolving blades of the
propeller cut their way through the air in identically the same manner.
But since air is a very thin medium as compared to wood or iron the
propeller slips a little just like a screw going into an unsteady nut
and does not advance the distance it theoretically should considering
the angle of the blades. The distance lost in each revolution is called
the _slip_. Thus a screw having a ten-foot pitch in actual operation
perhaps only advances the aeroplane eight feet.
[Illustration: FIG. 25.]
If a propeller blade had a uniform angle throughout its entire length
the portions of the blade near the centre would not have as great a
pitch as the extreme tips because the diameter of the circle they travel
in one revolution is not as great as that at the tips. For this reason
it is usual to give the blades an increasing angle as they approach the
centre.
[Illustration: FIG. 26. Method of laying out a screw propeller, that is,
determining the angle of the blades at different points.]
Fig 26 shows a diagram illustrating the theoretical pitch of a screw,
the angle of the blade varying inversely as its radial distance from the
centre of the screw.
When a propeller revolves it sets in motion a cylinder of air. If the
angle of the blades is uniform throughout their length the air in the
centre of the cylinder will move much more slowly than that near the
outside as shown by the arrow heads in A of Fig. 27. If the blades are
given an increasing pitch, the air in all parts of the cylinder will
move away from the propeller at the same speed.
From a diagram like this it is very easy to calculate the angle of a
blade at any point to secure a certain pitch. Suppose that the problem
in hand is to design a propeller eight inches in diameter and a pitch of
twelve inches. On a sheet of paper draw a vertical line AM twelve inches
long to represent the pitch. Draw a long horizontal line AN of
indefinite length from the lower end of AM and at right angles to it.
The diameter of the propeller being eight inches, the tips of the blades
must travel in one revolution 8 x 3.1416 (the circumference of an eight
inch circle in inches), a distance of 33.1 inches. Lay off on AN the
distance AB which is 33.1 inches, draw the line MB. The angle MB forms
with AN is the proper angle for the blades at the tips. To find the
angle one inch from the tips lay off the distance AC, which is. 8 - 2 x
3.1416 or 24.8 inches. MC gives the right angle. The angle two inches
from the tip would be shown by MD where AD is 8 - 4 x 3.1416 or 18.8
inches. Any other points can be located in the same manner.
[Illustration: FIG. 27. A propeller of the truly helical type delivers a
cylinder of air in which all parts move at the same speed as at A. A
propeller having blades of the same angle throughout their length throws
the air as in B in which the centre of the cylinder moves more slowly
than the outside.]
[Illustration: FIG. 28. Templets for testing and carving a propeller.]
If desirable, a number of small templets having the proper angle may be
cut out of sheet tin and fastened to a board as shown in Fig 28. When
making the propeller it can be frequently laid on the templets to see if
the proper angle has been secured yet.
There are a great many other ways of making propellers for model
aeroplanes, the simplest and best of which are described below.
*Metal Propellers* have advantages and disadvantages which may be summed
up only to find that as far as efficiency is concerned the advantages
outweigh the disadvantages.
[Illustration: FIG. 29. A simple method of forming a propeller from
sheet metal.]
The simplest method of making a small metal propeller is to cut a piece
of sheet aluminum into the shape shown by A in Fig. 29. Fold along the
dotted lines so that the result is like B in the same illustration. The
shaft may be a small piece of piano wire passed through the hole in the
centre and bent around as shown.
[Illustration: FIG. 30. A built-up metal propeller made of aluminum.]
Another method of making a metal propeller which is more suitable for
large machines than that just described is illustrated in Fig. 30. The
blades are cut out of sheet aluminum to the shape shown and set in the
slots in the end of a small aluminum tube. They are held in position
with aluminum solder. Ordinary solder will not accomplish the work and
cannot be used. The shaft is soldered into a hole in the tube halfway
between the two blades.
[Illustration: FIG. 31. Metal Propeller.]
The propeller shown in Fig. 31 is extensively used by manufacturers of
model aeroplanes because of its simplicity and strength. The propeller
is cut out of sheet aluminum and then bent and folded. The shaft is held
in place by a brass eyelet riveted firmly over on both sides.
[Illustration: FIG. 32. Method of carving a propeller of the truly
helical type.]
*Wooden Screws*. Single piece screws cut out of a block of wood are easy
to make and very efficient. The propeller is laid out on a square or
rectangular strip of wood (according to the pitch), cut to the required
length. A pocket knife or a wood rasp is used to rough the wood down to
the shape shown at B. It is then finished down to the form shown at C.
After rubbing with sand-paper a coat of varnish is applied and allowed
to dry. The varnish is then rubbed down to a smooth surface.
[Illustration: FIG. 33. Methods of fastening propellers to shaft.]
Fig. 33 shows a very good method of mounting the propeller on the shaft.
A piece of stiff brass is soldered to one end of a bicycle spoke and
bent around the propeller. A small nut made by cutting a spoke nipple is
screwed on the end to hold the propeller in position. (The same
illustration shows another very good method of fastening the propeller
to the shaft. The end of the shaft is filed to a sharp point, bent into
the shape shown in the illustration and then driven into the propeller.
A small pin hole should be made in the propeller at the place where the
sharp point is to enter in order to avoid the possibility of splitting.)
[Illustration: FIG. 34. Method of forming sockets for joining struts,
etc., by cutting from sheet metal.]
There are two methods of making propellers by steaming and bending thin
wood. American whitewood and spruce are the best woods for the purpose.
After steaming place one end of the strip in a vise and holding the
other in the fingers twist it into the right shape. Fasten it in
position and allow it to remain so until dry. Then give it a coat of
varnish to prevent the absorption of moisture and consequent warping.
The method of fastening the shaft, which in this case is a piece of
piano wire or a bicycle spoke is illustrated in Fig. 35. Two small
pieces of wood shaped like a half cylinder and having a groove cut on
the curved surface are glued on either side at the centre. The shaft is
then bent around and twisted.
[Illustration: FIG. 35. Bent wood propellers and the methods of
fastening them to the shaft.]
[Illustration: FIG. 36. Propeller blank (top). Carved propeller
(bottom).]
In order to make the second type the wood must first be bent into shape.
It is steamed and bent along the dotted lines. It is attached to the
shaft by means of a piece of sheet brass doubled over the edge and
soldered to the end of a bicycle spoke. The only disadvantage of this
form of propeller is that it is easily broken. It turns very easily with
little expenditure of power.
[Illustration: FIG. 37. Langley type propeller (top). Wright type
propeller (bottom).]
*Size of Propeller.* One bad feature about most of the model aeroplanes
offered for sale in toy shops is the propeller. In almost every case it
is decidedly too small. In order for a model to fly really well the
propeller must usually be out of all proportion to the rest of the
machine. In fact its size will make the machine appear very awkward and
unsightly.
[Illustration: FIG. 38. Quasi-helical propeller.]
The enormous slip of small screw propellers when turning rapidly makes
them very inefficient. The thrust of the propeller is dependent upon the
volume of air sent backwards. A large propeller naturally deflects more
air than a small one and so in order for the latter to equal the work of
a large propeller it must either have an increased pitch or revolve more
rapidly.
[Illustration: FIG. 39. Blanks for racing (top) and chauviere (bottom)
propellers.]
A small pitched propeller is less wasteful of power than one having a
high pitch and so it is of no advantage to make a small screw do the
work of a larger one. It is not only wasteful of energy but also permits
the rubber skein to untwist too rapidly. The advantage therefore lies
with a propeller of low pitch driven slowly.
[Illustration: FIG. 40. The first step in carving a propeller. The
blank. Hollowing the first blade.]
[Illustration: FIG. 41. One blade hollowed. Hollowing the second blade.]
The average propeller should have a pitch of from 2-3 times its
diameter, that is, the blade should have an angle at the tips of
slightly less than 45 degrees.
The propeller diameter (of course this rule is not infallible, but only
a general statement) should be about one-third the spread of the planes.
The edges of the blades should come to a clean edge but not be too
sharp.
[Illustration: FIG. 42 Rounding the back of the first blade. Rounding
the back of the second blade.]
One of the best means of determining the efficiency of a propeller is to
connect it to a small electric motor which will drive it at high speed
and by blowing tobacco smoke around it or holding a piece of burning rag
nearby and test whether or not the air is thrown out from the sides by
centrifugal force. A correctly designed propeller will pull air in from
the sides instead of throwing it out.
[Illustration: FIG. 43. All carving finished. Sandpapering to secure a
smooth surface.]
Calculation in fitting a model with a propeller is almost useless. The
experimental error is so large that the empirical or "cut and try"
method is the only reliable one. It is best to make a number of
propellers of varying pitch and diameter and give to each a thorough
tryout on the machine before making a decision.
*The Single Screw Machine.* A propeller placed in the rear of a machine
is usually more efficient than a "tractor" screw placed in front. A
machine drags along considerable air with it (due to skin friction of
the planes, etc.), and so a screw placed in the rear revolves in air
which is really traveling with the machine itself and so the effect is
somewhat as though it were traveling with the wind. A further advantage
of placing the propeller in the rear of the machine lies in the fact
that there is less likelihood of damage in landing.
[Illustration: FIG. 44. Varnishing. The propeller finished.]
An aeroplane having a single screw always betrays a marked tendency to
turn completely over in a direction opposite to that in which the screw
is rotating. Action and reaction are always equal and opposite in their
effects and so the motor has a tendency to rotate the machine against
the resistance of the screw as well as to rotate the screw against the
resistance of the machine.
One way in overcoming this difficulty is to set the two halves of the
plane at a slight angle to one another or at a _dihedral_ angle as it is
called. Then if the machine tends to twist and turn over the lifting
power of the lower wing becomes greater as it approaches the horizontal
while that of the other wing grows less. Accordingly the machine resists
and tends to turn back to its normal position.
Another method is to keep the weight or centre of gravity as low as
possible so that the machine will automatically right itself as soon as
it begins to turn. The objection to this, however, is that the machine
will fly very unsteadily on a gusty day (and most days are more or less
gusty). The effect of placing the centre of gravity low is shown in Fig.
45. The dotted line represents the centre of pressure acting against a
plane P. The weight of the machine is centred at W. Imagine the machine
in flight. Then the resistance of the plane P acting along the dotted
line will tend to stop the machine while W tends to still go forward
because of its inertia. As a result, the front of the machine tilts
upwards and increases the angle of P, which in turn increases the
resistance. The machine therefore slows down but W tends to still move
forward and tilt the machine further until the thrust of the screw is
unable to support the weight and so W swings back down and beyond the
position shown at B. The angle of P decreases, the machine travels
forward quickly and gathers sufficient speed for W to swing up again.
Thus the performance is repeated and the machine will have a flight path
very much like the dotted line shown in the lower part of the
illustration. The motion is slight but is sufficient to considerably
shorten the length of the flight.
[Illustration: FIG. 45. Accentricity. The effect of placing the center
of gravity too low.]
If the machine meets wind, the motion is somewhat increased. In fact the
author has seen a small biplane turn completely over and actually "loop
the loop." When the machine flies with the wind the effect is largely
reduced. If the wind is of just the right strength and comes from the
rear, the machine will fly quite steadily. If too strong, however, the
model will dive to the ground. A tail somewhat dampens the swing while
an elevator will slightly increase it.
The only other methods of partially mitigating the evils of a single
screw are to ballast the machine, that is, place a weight on one side or
to give one plane an increased sustaining surface. The first may be
dismissed immediately because the weight will cause one side of the
machine to drop as the elastic runs down and the reaction of the
propeller becomes smaller. The last named method is the usual one
employed. The wing on that side of the machine opposite to which the
propeller is revolving is given a larger surface than the other and so
exerts a greater lift on that side. This also has disadvantages,
however, for by giving one wing a greater lifting power the machine is
caused to fly in a long spiral path when the propeller begins to run
down and when it stops completely to glide in the same manner.
[Illustration: FIG. 46. Simplest method of fitting two propellers to a
model aeroplane.]
The propeller should be placed as nearly as possible on a level with the
planes. The _centre of pressure_ on the planes and the centre of gravity
should coincide if true stability is desired. The centre of pressure on
a machine having the planes set at a dihedral angle is halfway between
the lowest point and the highest providing the planes are the same width
all the way along. If they taper towards the ends it is slightly lower
while if they are wider at the extremities it is higher. The rubber
skein and the propeller are usually placed on top of the fusellage of a
dihedral winged machine.
*The Double Propeller Machine.* The best method and the only one which
entirely removes the difficulty is to fit the machine with two
propellers. A machine having two propellers to the author’s mind is the
only one worth much attention.
Fig. 46 illustrates the simplest arrangement for fitting two propellers
to a machine. In the first a second propeller is attached to the other
end of the skein. At first it might seem in the second arrangement that
there would be difficulty in getting the screws to revolve at the same
speed. However, if the propellers are similar and the same number of
rubber strands employed to drive each, the difference will be so small
as to be negligible.
When the first arrangement is employed the pitch of the screw in the
rear must be slightly greater than that in the front because it is
revolving in the slip of the latter.
Placing both propellers on a double shaft on the same axis has the
disadvantage of decreasing the efficiency of the propellers because they
are operating in each other’s draft.
[Illustration: FIG. 47. A method of arranging two propellers on the same
axis.]
The first of these methods is undoubtedly the best construction. It is
then possible to use the same rubber skein to drive both propellers.
Also any possible difference in their speed will not so readily cause
the machine to change its course as if the propellers were alongside of
each other. When two propellers are used in this latter position it is a
very good idea to fit them with two small pulleys and a connecting belt
so that any tendency for a difference in speed between the two will be
immediately equalized.
The power absorbed varies directly with the volume of air acted upon and
the square of the speed with which it moves away. If the pitch of the
propeller or its rate of revolution were doubled, four times the power
previously required would be necessary. Vice versa, decreasing the rate
of revolution or the pitch by one-half will make only one-fourth the
power previously required necessary.
Doubling the speed and doubling the diameter requires eight times more
power. Doubling the diameter, halving the pitch and halving the speed
will give twice the thrust for the same power as in the first case.