The two ribs on the extreme ends are of a different pattern, because of the tapering of the wing in plan.
The leading edge and the trailing edge (in treble strips glued together), with the small spar, triangulate the framework, which does not need any other stiffener. And so, in the interior of the wing, except for the nailing of the plywood, there is not a single piece of metal. No fittings, no wires, no turn-buckles. The wing is supported on the pylon by its centre at a place where the block is shown, while the system of bracing cables, joined to the wing, keeps it steady but allows it to pivot about the axis.
The pivoting is controlled by the cable which joins the lever of the control column to the small spar, to which this cable is fixed at the spots marked by the four blocks. A spring, attached as shown, pulls down the wing in front.
The Main Spar
The spar is made of two flanges in 15-mm. x 60-mm. material, planed up to points and curved and maintained in correct form by two webs of 1.5-mm. plywood, of which the grain is vertical to the depth of the spar. The depth of the spar is 130 mm. (see Fig. 21).
It is perhaps difficult to find ordinary pine as long as 6 metres free from knots. This great length, on the other hand, is rather inconvenient for delivery. Let us start, then, with lengths of 3 metres 20 mm., which can be glued solidly together in the middle on the bevel.
Plane up at the same time the ends of the two 15-mm. x 60-mm. planks to a length of 400 mm., and they are then placed side by side and fixed together by a bolt. Take care that the surface is quite fiat and regular. After planing it pass a file or rasp over it in order to take off the polish.
Join the bevels together with glue and align carefully the two planks, which will be temporarily fixed by two small nails. Glue all over this and press it carefully between two blocks in two vices or two strong screw presses. Leave it for about 24 hours in which to dry.
Next, prepare for the two wings two blocks which can be of good pine of of beech. All the holes are of 1lmm. diameter. Get ready also five screwed 10-mm. rods 180 mm. long.
Assembling the Spar
After removing the two flanges out of their presses and smoothing their four faces correctly, drill a 11-mm. hole in the middle and two holes 500 mm. apart at 1 metre 300 mm. on both sides of the centre. Stated more exactly, these double holes will be 3 mm. closer to the centre in the upper flange.
Place the two planks on two trestles and glue them together. Make certain that these two planks are quite parallel end to end.