Practical Construction: Carving Air-screws
One of the most important units of an aeroplane, whether full-size
or model, is the screw, since excellence of design with regard to
the other portions of the machine are rendered void if the means of
converting the power of the engine into work are inefficient.
The action of an air-screw may be likened to a bolt turning in a nut
(the screw being the bolt and the air the nut), the difference being
that whereas one turn of a bolt with, say, a Whitworth pitch of 14
threads per inch in a nut is bound to advance a distance equal to
the pitch = ¹/₁₄ in., an air-screw may only advance 75 per cent. of
its theoretical pitch, owing to the yielding nature of the air. This
loss in efficiency is called “slip,” and is usually expressed as a
percentage of the theoretical pitch. Thus a screw with a theoretical
pitch of 4 ft., which possesses 75 per cent. efficiency, has an
effective pitch of 3 ft. That is to say, each turn of the screw will
take the aeroplane forward 3 ft. If, however, the screw were working
in a solid, it would advance its theoretical pitch = 4 ft. A greater
efficiency is obtainable with screws working in water, owing to the
difference in density of the two media, namely, air is to water as
800: 1. Probably no air-screw has yet exceeded 80 per cent. efficiency,
70 per cent. being a fair average.
It may, perhaps, not be amiss to outline some of the factors involved
in the design of an air-screw. Having decided on the diameter of it,
the proportions of the block from which the screw is to be carved
are required. It is a very good rule to make the pitch from one and
a half to twice the diameter for single-screw machines, and from two
and a half to three times the diameter for twin-screw machines. It is
possible to use much longer-pitched screws with twin-screw machines
(it being understood that the screws revolve in opposite directions),
since the torque, or tendency of the screw to capsize the machine in
the opposite direction to which it revolves, will be balanced. For the
purposes of this chapter, however, it is presupposed that a screw is
required for a single-screw machine, and a diameter of 12 in. has been
decided on. One and a half times 12 in. gives 18 in. as the pitch.
Remembering the formula for pitch,
(thickness of block)
P = 3¹/₇ × D × --------------------,
(width of block)
where P = pitch, D = diameter of screw, and using a ratio of width of
blade to diameter of screw of 6: 1 (which gives 2 in. as the width of
block) gives
22 12 (thickness of block)
18 = ---- × ---- × ------------------,
7 1 2
61
from which thickness of block = ·954 = ---- approx.
64
[Illustration: Fig. 39]
[Illustration: Fig. 40]
[Illustration: Fig. 41]
[Illustration: Fig. 42]
[Illustration: Fig. 43]
[Illustration: Fig. 44]
[Illustration: Fig. 45]
[Illustration: Fig. 46]
[Illustration: Fig. 47]
[Illustration: Fig. 48
Figs. 39 to 48.—Carving Air-screws.]
The block may now be prepared from these dimensions. American
whitewood, silver spruce, mahogany, or walnut are the most suitable
woods to use. The block should be planed up true and square, and a hole
drilled axially through its geometrical centre. The first operation
is to rough the block out to the shape shown by Fig. 39, which shows
the Chauvière type. Of course, other shapes may be used as desired,
but the method of manufacture is the same. Now, with a flat chisel or
woodworker’s knife pare the wood away (see Fig. 40) until the hollow or
concave side of the blade is formed (see Fig. 41). The obverse side of
the other blade is then similarly treated (see Fig. 42), which clearly
shows how the blade is hollowed out.
Fig. 43 shows the method of forming the boss of the screw, and Fig.
44 how the reverse or convex side of the blade is shaped. Fig. 45
shows the screw roughed out, and Fig. 46 indicates the glass-papering
operation.
At this stage the screw has to be balanced. This is of great
importance, since the screw that is unbalanced loses a great amount
of efficiency owing to the consequent vibration when it rotates. In
full-size practice it would be highly dangerous to use a screw that is
not balanced.
A piece of wire is passed through the hole previously drilled, and the
heavier blade carefully glasspapered down (with No. 00 glasspaper to
finish) until the screw poises in a horizontal plane. Fig. 47 shows the
sort of brush to use for polishing, and Fig. 48 the finished screw.
For models that require a good finish an excellent form of
construction (incidentally it may be remarked that full-size screws
are made in this way) is that shown by Fig. 49, the laminated type.
These laminated screws are exceedingly strong, as the grain, by virtue
of the splayed blanks, follows the blade. Screws carved from the solid
block are a trifle weak near the boss owing to cross grain. The laminæ
could be alternate layers of whitewood and mahogany, which give a
pleasing finish to the screw. =A= is an end view of a carved screw.
[Illustration: Fig. 49.—Laminated Air-screw]
The method of obtaining the pitch angles at various points along a
screw-blade is shown diagrammatically in Fig. 50. It will be obvious
that the pitch of a screw should be constant along the whole length
of blade, so that the air is deflected or driven back at a constant
velocity. An efficient screw will deliver a solid cylinder of air,
whereas an inefficient one delivers a tube of air.
If, for instance, the pitch at the propeller tip is 30 in., whilst at,
say, 3 in. from the centre it is only 25 in., obviously the tip of the
screw will be imparting a higher velocity to the air than the portion
approaching the boss, and thus this latter would be acting as a drag
upon the other portion.
[Illustration: Fig. 50.—Setting Out Pitch Angles]
The method is to lay off a distance, equal to the pitch, to some
convenient scale, and to erect another line vertically and to the
same scale equivalent to the circumference of the disc swept by the
propeller, which may be called the peripheral line. Subdividing this
line into a convenient number of equidistant parts (three or four are
sufficient for screws up to 14 in. in diameter), and connecting up
the points so obtained to the right-hand end of the base line, gives
the pitch angles at the corresponding points of the blade. It is the
subtended angles which are required, as indicated by the arrows.
Templates should be cut to these angles (which, of course, are the
angles made with the axis) with which to check the angles along the
blade during construction. This checking is more necessary with
bentwood screws than with carved ones.