Practical Construction: Bending Air-screws
Great diversity of practice exists with regard to the construction
of model air-screws, some aero-modellists favouring small diameter
with long pitch, others long diameter and short pitch, and still
others who adhere to either bentwood or carved screws in either of
the above forms. Generally speaking, a screw with a large diameter
in proportion to short span has a short pitch, say one and a quarter
times the diameter, while those having a short diameter in relation
to span should have a fairly long pitch, from one and a half to twice
the diameter. It is a useful rule to make the diameter approximately
one-third the span of the machine for either single-screw or twin-screw
machines. This relation seems to give a very small effect on lateral
stability, whereas when the diameter is made larger, the machine has a
tendency to capsize laterally in the opposite direction to which the
screw revolves. This force is known as torque.
It can, however, be fairly claimed that, for a given torque or turning
power, better results are usually obtained with carved screws, whether
short or large ones are used. The writer personally prefers a large
diameter and short-pitched screw, because, as the screw thrust is equal
to the weight of air displaced, the larger the screw the greater is the
proportion of air driven back in proportion to diameter. That is to
say, double the diameter and four times the volume of air is displaced
for only a double expenditure of power.
[Illustration: Fig. 51.—Bentwood Screws]
[Illustration: Fig. 52]
[Illustration: Fig. 53
Figs. 52 and 53.—Standard Types of Bentwood Screws]
It is difficult to speak positively on the question of the best speed
at which a screw should rotate, as the loading per square foot of
surface enters into the proposition. If a model has 1 sq. ft. of
surface for every ounce of its weight, there is a speed at which the
main surface will give a maximum of lift for a minimum of power, and
a screw must be fitted whose pitch, multiplied by its revolutions per
minute, equals the distance per minute the model should fly. If a screw
that is too fast is fitted the model will show a tendency to “stall,”
or ascend nose first, and if too slow a one is used the model will
appear to be under-powered.
The writer has outlined these points to emphasise the fact that no
definite rules, but only approximations, can be laid down, owing to the
large number of unknown quantities which would have to be taken into
consideration. As the aero-modellist, however, becomes accustomed to
puzzling out the many little problems connected with model aeroplaning,
he speedily diagnoses the complaint of a refractory machine, and
applies a remedy accordingly.
[Illustration: Fig. 54.—Bentwood Shaft Attachment]
[Illustration: Fig. 56.—Safety Hook]
[Illustration: Fig. 55.—Carved Screw Shaft]
[Illustration: Fig. 57.—Proportions of Camm Air-Screw]
The accompanying illustrations (see page 43) show the method of
making bentwood and screws. Fig. 51 is a view of a finished pair of
propellers. To the left of this illustration is given the method of
setting out the blank in terms of pitch and diameter relations. The
maximum blade width should be located one-third of the radius from the
screw tip, and should be about one-eighth the diameter. This latter,
in turn, should be two-thirds of the pitch. Inversely, therefore, the
pitch should be one and a half timed the diameter. With twin-screw
machines this may be extended to twice the diameter, or even more, but
should never exceed three times the diameter.
Fig. 52 is a view of the Camm type of bentwood screw, which has a high
thrust to power ratio. Birch should be used for bentwood screws, as
this bends easily and yet has a tenacity which is lacking in other
woods. Ash or hickory may be used as an alternative, but neither of
these is as satisfactory as birch. Before bending, the blanks should
be filled with gold size to keep the blade as rigid as possible, and
prevent it from going back or flattening out after bending.
Fig. 53 shows the Twining type of screw, which has long, narrow
tapering blades and fine pitch. Under test this has given extremely
satisfactory results, and can be recommended.
Fig. 54 shows the method of attaching spindles to bentwood screws, a
strap of tin being wrapped round the blank centre to which the shaft
is soldered. Care should be taken to ensure that the shaft is quite
central sectionally and diametrically.
A method of securing carved screw shafts is shown by Fig. 55, and
is self-explanatory. When the elastic skein is in tension it has a
tendency to pull the hooks out straight, so releasing the skein, with
sometimes painful consequences to the hand. The safety hook shown by
Fig. 56 has a brass-tube collar which slides over the end. All the
hooks should be covered with valve tubing, to prevent the elastic
cutting through.
Fig. 57 gives the proportions of the Camm bentwood blank, and will
require no explanation beyond the fact that it is bent along the dotted
lines.