Tractor Monoplane
Some of the competitions arranged by the Kite and Model Aeroplane
Association have been for duration models of a minimum weight of 1
lb., capable of rising from the ground under their own power, carrying
a dead weight of a quarter of their own total weight. Such a model
is that here illustrated and described. It has flown repeatedly
for thirty-eight seconds after rising from the ground, while its
hand-launched duration is about half a minute, by no means a small
accomplishment for a 16-oz. model. It will be seen that it has somewhat
larger dimensions than the ordinary rubber-driven model.
The top main spar is of spruce, 4 ft. 6 in. long and ½ in. by ¼ in.
in cross-section, the bottom one being 2 in. longer and ¼ in. by ³/₁₆
in. in section. The bottom member is to be bent by steam approximately
to the shape shown in the side view of the machine (Fig. 113), and
then fitted to make a clean butt joint to the top spar =B= (Fig. 114).
Bearings for the two ⅝-in. gears, details of which are shown in Fig.
115, are bent to shape from No. 20 gauge sheet-brass, a lug being left
projecting to engage with the bottom member of the fuselage. The holes
for the shafts should be drilled so that the gears make a fairly tight
mesh. The spindles are to be of No. 16 gauge piano wire, on to which
the gears are to be soldered. The propeller shaft is continued forward
of the bearing for 1½ in., and is bent back at an angle (as shown in
detail at =A= in Fig. 116) to grip tightly in the propeller boss. The
gears are kept central between the two bearings by means of pieces of
brass tubing, which are slipped on the spindles at each side of the
bearing and soldered in position.
[Illustration: Fig. 113.—Side Elevation]
[Illustration: Fig. 114.—Spar Joint]
[Illustration: Fig. 115.—Gearing]
[Illustration: Fig. 116.—Shaft Attachment]
[Illustration: Fig. 118.—Chassis]
[Illustration: Fig. 119.—Main Plane Attachment]
Elastic hooks, formed from No. 16 gauge wire, are fixed at the rear end
of the fuselage, and serve the double purpose of providing an anchorage
for the bottom spar, which is whipped and glued with the hooks to the
top spar. Fig. 114 shows clearly what is meant. Bamboo, of ³/₁₆-in. by
₧-in. cross-section, is used for the tail skid, which is secured to the
fuselage by thread binding. Piano wire is used for the landing chassis,
and of the same gauge as hitherto used on the spindles.
The triangular side struts of the chassis should first be framed up
from one continuous length of wire, lugs being bent at the point
where they meet the fuselage, to be bound with fine tinned iron wire
and soldered to the spar. Figs. 113, 117 and 118 clearly show its
construction. Make the axle of such a length that a 12-in. wheel-base
may be left after the wheels are placed on. The writer found that
2-in. rubber-tyred disc wheels left nothing to be desired for rising
off short grass. Care should be taken that the measurement from the
periphery of the wheel to the top spar, measured, of course, in a
vertical direction, shall be 9 in. (see Fig. 113).
[Illustration: Fig. 117.—Plan]
[Illustration: Fig. 121.—Kingpost]
[Illustration: Fig. 122.—Wing Bracing Attachment]
As twin gears of equal size are used, the torques of the oppositely
revolving skeins of rubber will be balanced. Hence no bracing will be
found necessary on the motor spar. The gearing, by the way, is bound to
the top and bottom members of the fuselage with tinned iron wire, and
soldered as shown in Fig. 115.
The main plane is of rather a large span, and it is essential that
birch be used, ½ in. by ³/₃₂ in. in section. The wing spars are bent
at their centres, to impart a tapering wing plan analogous to the
Martinsyde monoplane. Seven ribs connect the spars, and these are
cambered to ¾ in. The wing tapers from 10 in. at the centre to 7 in.
at the tips, the centre rib projecting for ½ in. fore and after of the
wing. The tin clips shown in detail in Fig. 119 slip over these, and so
provide a means of adjustment to the centre of pressure of the complete
machine.
Choice of covering must be left to the builder; but yellow Japanese
silk, proofed with varnish, will be found quite suitable, and of a
rather pleasing amber hue. A birch kingpost passes through the fabric,
and to this the wings are braced by No. 35 gauge music wire. Sufficient
tension should be placed on the top wires to give the wings a 3-in.
dihedral angle, as in Fig. 120. The detail illustration of the kingpost
(Fig. 121) is self-explanatory. The bracing wires are anchored to wire
hooks forced through the wing in the manner shown in Fig. 122.
The correct position of the main plane should be found by trial. The
kingpost can then be permanently fixed to the main spar by pinning and
gluing.
No. 18 gauge wire of the music or piano variety should be used for the
tail and rudder. Draw the plan form of the wing full-size on a board;
pins may then be driven partly home on each side of the line at spaces
of about 3 in. The wire may now be pushed between the pins, cut off,
and lapped for ½ in. The two cross ribs can be soldered to the tail
before the tacks are withdrawn. It will be found on releasing the tacks
that the wire will remain true to the shape of the template. This may
seem a rather laborious process; but it is far quicker and easier than
attempting to guess the correct curvature. The rudder may be made to
any convenient shape, preferably that shown. The two ends should be
sprung outwards after the form of the letter =A= to form a clearance
for the rubber hooks, and then soldered to the tail. A very slight
adjustment of this will be found necessary to obviate propeller torque.
[Illustration: Fig. 120.—Front Elevation]
No provision has been made for the adjustment of the lift on the tail.
Indeed, none was found necessary, it being quite an easy matter to
bend the tail flaps up or down to increase or decrease the elevation.
They should always, however, have a slight negative angle to maintain
longitudinal stability. The tail should be bound to the fuselage with
copper wire.
The propeller, of the usual integral type, is carved from the solid
block, which measures 15 in. by ⁵/₈ in. by 2¼ in. It should be made
of right-handed pitch, and must be placed on the right-hand gear, so
that thrust balances torque. On each side, nine strands of ¼-in. strip
rubber, lubricated with diluted soft soap, supply the motive power.
This will stand approximately 600 turns. Vaseline will suffice to
minimise friction on the gears.
It is advisable to test the model down the wind with about 50 per
cent. of the maximum turns. The main plane should be moved forwards to
increase the elevation, and backwards to decrease it.
[Illustration: Fig. 123.—Complete Tractor Monoplane]
Fig. 123 shows the model in perspective.