Simple Twin-screw Biplane
According to the type formula, the machine illustrated on p. 63 is
of the 1-2-p2 type, which signifies that it has two superposed main
supporting surfaces and twin screws, and that it flies with the small
plane leading. The writer, in testing the model from which the drawings
were made, found that 300 yards were easily obtainable at every flight
at an altitude of 40 ft. or so. Although its construction is slightly
more complicated than a monoplane, this is amply compensated for by its
majestic appearance in the air.
The central spar is hollow, and measures 36 in. by ⅜ in. by ⁵/₁₆ in.
Fig. 79 shows a cross-section of it. Spruce was used for this, a groove
⅛ in. by ⁹/₃₂ in. being ploughed in the spruce, and a ³/₃₂-in. strip
being glued over the open side. The spar should be tapered off from a
point 12 in. from the front end to ¼ in. by ¼ in., to give additional
strength.
The propeller bar is mortised into one end of the central spar, and is
stayed from a point 6 in. from the rear end to ½ in. in from each end
of the propeller bar, to which projection the bearings, cut from sheet
brass, are lashed. These latter are shown in Fig. 80. To the front of
the spar are bound two hooks, formed from one continuous length of
wire. These embrace the rubber, and should be covered with valve tubing.
Four birch struts should next be cut, ¼ in. by ¹/₁₆ in. in section and
7 in. long, to support the main aerofoils on the spar. They should fit
over the spar in the position shown, small blocks uniting them top and
bottom, and should be so fixed that their upper ends are 3 in. off the
spar. Notches are to be cut in them 2¼ in. below the spar, to form a
convenient fixing for the lower main plane. Fig. 81 clearly shows the
struts united to one of the blocks; the saw cuts or notches will also
be apparent from this.
A cane skid is bound to the lower blocks, and to the spar at the
position indicated in Fig. 82. A half-section of a round cane is quite
suitable for this purpose.
A plan view of the model is given by Fig. 83, from which the relative
position of ribs, planes, etc., will be seen. Fig. 84 is an end
elevation looking through from the propeller end. It will be observed
that the screws rotate from approximately the centre of resistance. The
measurements given should be carefully followed to see that this is
so in the completed model. The dihedral angle of the planes should be
made 1¼ in., which is ample to provide lateral stability. Longitudinal
stability is provided by an angle of incidence on the elevator (Fig.
85).
[Illustration: Fig. 79]
[Illustration: Fig. 80]
[Illustration: Fig. 81]
[Illustration: Fig. 82]
[Illustration: Fig. 83]
[Illustration: Fig. 84]
[Illustration: Fig. 85]
[Illustration: Figs. 79 to 85.—General Arrangement and Details of
Biplane]
The elevator is constructed from one continuous length of No. 18 gauge
piano wire. It is rectangular in plan, the joint being a soldered one
at the centre of the trailing edge. The centre rib projects downwards
for 1½ in., which projection fits into a hole bored in the nose of the
machine with just sufficient friction to retain it in place. It should
also be bent back at an angle to cause the trailing edge to bind on the
spar sufficiently to allow it to swivel in the event of it striking
any fixed object. From this centre rib the elevation of the complete
machine is adjusted. The main planes are constructed from birch ¼ in.
by ¹/₁₆ in. in cross-section, five ribs connecting the longer spars. No
camber is given to the ribs; they should be cut off 1 in. or no longer
than is necessary, pinned to the spars, so that the latter are 5 in.
apart, and cut off flush after the glue is dry. The top plane, it will
be noticed, has an overhang of 3 in. The planes are covered underneath
to eliminate the undulations which would otherwise be caused by the
ribs. Fabric should be sewn to the elevator frame.
The top plane is lashed to the struts in the manner shown in Fig. 86,
the centre rib resting on the small wooden blocks, while the bottom
plane is sprung into the notches previously referred to. Four No. 20
S.W.G. wire stanchions, with eyes bent in them top and bottom, as shown
in Fig. 87, will next be required to form an anchorage for the wing
bracing, and to maintain the “gap” at the tips of the wings. Brass wire
will do for them, and when made their ends should be forced through the
spars in the position shown in the end elevation, and then clinched
over.
Bracing the wings should next be undertaken, and carpet thread should
be requisitioned for this purpose. It is the easiest matter possible
to warp the wings in this operation, so that too much care cannot be
taken in this respect. It should be understood that the bracing is
fixed to holes in the wooden stanchions, where it must be securely
tied, and not continued to the opposite side without, or the wings will
rock laterally, and so cause instability. Sufficient tension should be
placed on the threads which pass to the wing tips of the bottom plane
to impart a 1¼-in. dihedral angle.
[Illustration: Fig. 86]
[Illustration: Fig. 87]
[Illustration: Fig. 88
Figs. 86 to 88.—Details of Biplane]
The last, and perhaps the most important, unit of the model should be
made—the propellers. Cut a pair of blanks, as shown in Fig. 88, to
shape from ¹/₁₆-in. birch to form the propellers. Strips of tinfoil
are wrapped round their centres, to which the spindles are soldered.
Bend the blades at the dotted lines under a jet of steam from a
kettle, making them to revolve in opposite directions. They rotate on
steel-cupped washers placed on the spindles.
Motive power is supplied from eight strands per side of ¼-in. strip
rubber well lubricated with soft soap emulsified with water. These
skeins will stand 650 turns each, which number should be gradually
worked up to on new rubber, and _not_ applied at the first flight. A
perspective view of the finished model is given in Fig. 89.
=Flying the Model.=—Having selected a large open space clear of trees,
give about 100 turns on the propellers in order to adjust the elevator.
If the model points its nose in the air it is elevated too much. If the
model flies too low it is not elevated enough. In each case it requires
adjustment until the precise position is arrived at. The elevator
should never be at a greater angle than 8° or less than 5°. If the
machine still flies too low with the elevator at 5° the main planes
will have to be moved forward slightly; but the exact position is only
found by experiment. If it flies too high with the elevator at 5° the
planes will have to be put back.
[Illustration: Fig. 89.—The Finished Biplane]
Much depends on the way a model is launched. The proper way is to hold
it by the propellers, with the thumb and forefinger along the main
sticks, taking care not to bend the propeller hooks; then hold at about
the angle shown in the photograph (Fig. 90). and launch as near as
possible with the wind.
[Illustration: Fig. 90.—Launching the Model]
It is necessary when flying in windy weather to launch the machine high
and smartly, as the wind has a tendency to beat it to the ground. Both
propellers must be released at exactly the same time. It should be
carefully watched while it is flying, and if it persists in turning,
say to the right, the fault will probably be that the left propeller
is more effective, or the planes on the left side of the machine are
elevated more. For straight flights it is most important that all the
planes should be in perfect alignment; but to succeed in making a
twin-propelled model fly perfectly straight is largely a matter of
perfect construction.
The model can be steered by means of the elevator (looking at the
machine from the propeller end); if the elevator is lowered on the left
side it will fly round to the left, and vice versa. The adjustment
necessary to effect this can be accomplished by means of the slot in
one of the elevator uprights.