PROPELLERS, MOTORS, GEARS, AND WINDING DEVICES.
There are four kinds of propellers:
1. Propellers carved out of solid and laminated blocks.
2. Metal propellers with curved or twisted surfaces.
3. Built up propellers. In this type a small block is used as a hub,
and the wood or metal fans are projected out from this. The fans are
attached on the diagonal.
4. Propellers made of pressed wood veneer. These are excellent, but
require more skill and more apparatus to make.
[Illustration: FIGS. 244-251.]
_The carved out propeller_ is the one most generally used and is
not only a good exercise in modeling with a knife, but is a good
serviceable kind. There are a number of types of propellers, named
mostly by men who have designed them. For simplicity in laying out and
carving, I like the Langley type. A rectangle is made of wood, say
¾” × 1½” × 6”. Fig. 244. Draw the diagonals, as in Fig. 245. With a
radius of ¼”, and center at the intersection of the diagonals, draw
a half-inch circle. Connect the diagonal lines and the circumference
of the circle, as in Fig. 246, and cut down to the outline as it now
appears. The blank is now as shown in Fig. 247. We will now take off
two big slices, not all in one cut, but in several. Fig. 248 has the
dotted lines showing the depth to be cut, and Fig. 249 shows these same
parts cut away. Now cut away _x_ and _x_ until the blade is curved back
to edges _z_ and _z_. The cut away portions will be as in Fig. 250. Cut
the opposite side the same way, and cut away the back corners a little,
giving the result as shown in Fig. 251. Sandpaper well and shellac.
Drill hole carefully for the propeller shaft.
[Illustration: FIG. 252.]
[Illustration: FIG. 253.]
[Illustration: FIG. 254.]
The principal objection to the metal propeller is the bending that
is liable to occur when the model lights, unless there are lighting
devices underneath, and they all add weight. The hub propellers may
have metal or hardwood veneers for fans. The hub may be round or
square; see Figs. 252 and 253. Very good propellers may be made in this
way. Extra curvature of the outer ends of the fans is possible, Fig.
254.
=The veneer propeller= must be steamed and pressed. This is by far the
most difficult to make. The 1/16” hardwood veneer is the best. The
propeller is not reinforced to make up for the extra thickness of the
carved propeller, but is of uniform thickness thruout. The veneer is
first cut to shape in outline and then is steamed and twisted to shape.
Fig. 255 shows a pattern for a propeller blade. The veneer should be
steamed or soaked in hot water until the wood is very pliable and
soft. A form should be ready so as to get both wings with equal twist
from the central portion. I will suggest one, others can be devised.
A clamp is necessary for the center, which may be made as follows:
take a one-inch piece of wood about 1¼” wide and any length. Set it
up edgewise. Make a cut 1/16” deep and one inch long across the upper
edge, Fig. 256, and screw a small piece over top as a clamp. After
thoroly steaming the propeller blade until it is very pliable, insert
it into the clamp at the center and twist from the straight side, one
fan up, the other down. It is not easy to get the two sides just alike,
so I recommend the bending of one side at a time, and when that is dry,
remove, and reverse the ends, being sure to keep the straight edge
to the front, or the same as before. To be accurate, there must be a
guide block to bend to. For a nine-inch propeller, a block like Fig.
257 would be about right. Place guide in position, press blank down to
the oblique surface and secure there until dry. Repeat for the other
end. It will be seen that the guide block and clamp are both secured
to a board for a base; they may be secured to a table or bench-top. A
clamp might be devised also that would hold the propeller in a vertical
position with guide blocks on either side of center so as to bend and
secure both ends at the same time.
[Illustration: FIGS. 255, 256, 257, 258.]
[Illustration: FIG. 259.]
=Another way to bend propellers= is to clamp the center of the blank
in a vertical position, and with two clamps made by sawing into the
ends of two pieces of wood, Fig. 258, a cut wide enough to take the
thickness of the blank, and deep enough for the width, bend one blade
forward and the other back, Fig. 259. The small clamps on the propeller
blades should be placed at equal distances from the center, and should
be given an equal amount of twist. The small clamps on the blades will
not be forced over until they touch the base, so blocks of equal size
should be used as stops in the operation of twisting. The clamps should
be secured in the last position by means of cord to the base until the
propeller is dry.
[Illustration: FIG. 260.]
[Illustration: FIG. 261.]
Still another way to attain the twist in veneer propellers, would be
to have two blocks gouged out to the proper shape, one just fitting
in the other. After the propeller is shaped in outline and steamed,
it is placed between the two blocks, which are in turn clamped firmly
together until propeller is dry.
=Motors.= Quite a number of devices have been tried, but the rubber
string is by far the most efficient power yet discovered. Rubber has a
great deal more power than an equal weight of steel in all ways that
steel has been tried. The power of the rubber motor is dependent on
the unwinding of the strands of rubber after having been wound up.
The longer this unwinding may be delayed the further the little air
craft may be propelled, providing there is force enough expended at
any time to give the necessary momentum to the propeller. A long motor
has more revolutions than a short one. Hence some advocate a long
spined frame to the model aeroplane. Others prefer to cut the frame a
little shorter and give great pitch to the propeller which demands more
energy at a given time, hence heavier strands, or more strands of the
smaller rubber string. With the greater pitch propellers, the model is
propelled faster and so may cover as great or greater distance than
one with a motor that gives more revolutions in unwinding, but it is
possible to revolve so fast as not to propel at all. Many use the 1/16”
square rubber string, others the ⅛” while many use ribbon rubber, say
1/32” to 1/16” thick by 3/16”. ¼” and 5/16” wide. For small models,
rubber bands can be looped together.
=Gears.= Small, light weight gears can be made or bought. They are
attached to propeller shafts and are geared back different pitches.
Some one to two, others one to three, while some gears are one to one.
It might seem that one to one is added friction and no gain in winding,
but a hook is attached to each gear wheel which allows two rubber
motors instead of one, and allows longer unwinding. If the rubber
strands are divided they wind up many more turns than when combined in
one bunch. If eight strands are twisted together you cannot turn as
many times as with four strands. Fig. 260 shows a one to one gear and
its connections, and Fig. 261 a gear with a greater ratio.
A gear of one to one might be placed at the opposite end of the
framework from the propeller, thus extending the number of revolutions
in that manner, the second rope or motor extending back and below the
first, Fig. 262.
[Illustration: FIG. 262.]
=Winding Devices.= It is tiresome to wind up the rubber motors by hand,
so mechanical winding devices have been made. A drill with a hook in
the place of a drill-bit is quite satisfactory. It is best to have a
ring on the rubber motor where it is attached to the anchoring hook.
This ring can be hooked on the hook of the drill. This winds but one at
a time, so they should be changed about in the winding process, first
on one and then on the other back and forth, until tight enough. A very
good winder can be made of a revolving egg-beater. The egg-beating part
is cut off, leaving two shafts instead of one. If there is a hook on
each, both motors can be wound at once and as they should be wound in
opposite directions, the device works all right.