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).

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.

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.

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.

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.