Simple Twin-Screw Monoplane
The accompanying illustrations show as simple a type of model aeroplane
as it is well possible to make, excluding the now obsolete single-stick
hand-launched 1—1—P1. It is thus a suitable model for beginners,
flights of well over a quarter of a mile being easily obtainable.
The main spar (see Fig. 66) is cut from straight-grained birch, to the
dimensions given, each end of it being tapered down to ³/₁₆ in. square.
The propeller bar is of silver spruce, ⅜ in. by ⅛ in. in cross-section.
The end of the main spar is slotted to receive the propeller bar, this
latter being pinned and glued into position. The propeller bar support
is similarly slotted to take the bar, a pin being driven through the
two and clinched over on the under-side. Fig. 67 clearly shows both
joints. At 6 in. from one end the main spar is mortised to receive two
tenons which are cut on the ends of the bar supports. These tenons
should be so cut that they butt to one another in the centre of the
mortise. An idea of the shape of the tenon will be gathered from Fig.
68, a view of the joint assembled being given.
[Illustration: Fig. 66]
[Illustration: Fig. 67]
[Illustration: Fig. 68]
[Illustration: Fig. 69]
[Illustration: Fig. 70
Figs. 66 to 70.—Details of Simple Monoplane]
Two brass propeller bearings will now be required. They should be cut
from No. 20 gauge brass, a hole being drilled in each to allow the No.
18 gauge propeller shafts to rotate freely. Each bearing is bound on
with three-cord carpet thread, a portion of each being left overhanging
the bar to provide clearance for the revolution of the shaft. These
projections should be bent at an angle of 90° to the skeins of rubber,
so that the bearing faces present true surfaces for the screws to
revolve on. Details of the bearings are given by Fig. 69. Two hooks
bent from one continuous length of wire are bound to the nose of the
machine, to embrace the skeins of rubber. All bindings on the machine
should be smeared with weak glue.
In Fig. 70 details are given of the spar bracing outrigger. The
binding, for the sake of clearness, is omitted. A piece of wire
(hard-drawn brass is suited to the purpose) is passed through the spar,
a portion being bent to align with each side of this. It is then bent
outwards, the ends being pulled round a piece of No. 20 gauge wire
secured in the vice, to form eyes, through which the bracing passes.
The outrigger arm is 2 in. long, the cranked portion of it being bound
to the spar. The bracing is attached to small No. 20 S.W.G. hooks bound
to each end of the spar at the points shown. Care should be taken to
apply equal tension on each wire, or the spar will become warped.
The elevator is built from No. 18 S.W.G. piano wire. All joints are
bound with fine wire and soldered. The centre rib continues over the
leading edge, being bent downwards and backwards as at =A= (Fig. 66).
This projection fits into a hole drilled in the nose of the model;
and, being bent at an angle, the trailing edge binds on the spar with
sufficient friction to retain it in place, but yet permitting it to
swivel should it strike any object when flying.
[Illustration: Fig. 71.—Plane Fastening]
[Illustration: Fig. 73.—Elevation of Main Plane]
[Illustration: Fig. 74.—Finished Screws]
[Illustration: Fig. 72.—Detail of Plane Bracing]
The main planes are built from birch ¼ in. by ¹/₁₆ in. in cross
section, the trailing spar being bent in a jet of steam, so that its
ends sweep forward, as shown in the plan view. The ribs are pinned
and glued to the spars, the pins being clinched on the under-side.
The centre rib is left overhanging the spars, as shown in Fig. 71, to
enable the tin straps (lapped and soldered together as at =A=) to slide
over them and secure the wing to the spar. A section of the joint is
given by =B=. By removing these straps it is thus possible to alter
the disposition of the main surface, when it is desired to adjust the
elevation of the complete model. Fig. 71 is a perspective view of the
centre rib, the strap being shown black. To move the main plane each
clip is forced off the extensions of the centre rib and thus releases
the wing. Each clip is cut from tinfoil to the dimensions given at =A=
(Fig. 71), being bent to a rectangular shape, and soldered up. B is a
section of the joint. Fig. 72 indicates the method of attaching the
diagonal wing bracing, which imparts a dihedral angle of 1½ in. to the
plane. A 1½-in. dihedral means that each wing tip is 1½ in. above the
level of the spar. An elevation of the main plane is given by Fig. 73.
Two propellers, of right-handed and left-handed pitch (for the reason,
see p. 36), must be bent from birchwood 12 in. dia. × 1½ in. wide, by
¹/₁₆ in. thick; a finished view of the two screws is given by Fig. 74.
For more comprehensive details of air-screw construction see chapters
IV and V.
[Illustration: Fig. 75.—Plan of Model]
[Illustration: Fig. 76.—Outrigger Details]
[Illustration: Fig. 77.—View of Elevator
Figs. 75 to 77.—Another Simple Monoplane Design]
Some details are also given of a similar design, the difference being
that the former machine is built for distance, whilst the latter (see
Fig. 75) is built for duration. The main spar is of spruce ⅜ in. ×
¼ in. in cross-section and of the length shown in Fig. 75. The main
plane, as with the previous machine dealt with, is adjusted by means of
the tin clips and extending centre-rib; a plan of the outrigger is also
given in Fig. 76. The main plane has a dihedral angle of 1½ in. =A= is
a side elevation of the elevator, showing its angle of incidence in
relation to the spar, and Fig. 77 a perspective sketch of the elevator.
An idea of a model of this type flying can be gathered from Fig. 78.
[Illustration: Fig. 78.—Finished Model]