Now glue on each side two plates of plywood 1.5 mm., 130 mm. broad and 500 mm. in length. With the aid of a cord or a wire 2 mm. diameter and the king-post made of a tube 1.5 metres long, pull up the points of the flanges in such a way that the thread, stretched between the points, passes at a height of 230 mm. above the central boxed portion.
A temporary block, 40 mm. in height, will separate the flanges at 300 mm. from the ends. Take care to see that the arch of the flaiiges, as checked by the thread, is approximately equal to the right and to the left hand. Next, provide both faces of the spar from end to end with strips of 1.5-mm. plywood (nailing it with fine nails 8 mm. long in a zigzag at every 15 mm.). Place the plywood strips side by side without overlap.
Altogether this 6-metre-long spar requires 1 square metre of plywood. It weighs 15 lbs. Construct in the same way the spar of the rear wing, but on a span of 4 metres, and with a. curve of 180 mm. under tlle thread. This spar weighs 11 lbs.
To facilitate the folding of the wings for transportation along the road, place some blocks at the same distance apart as on the front wing.
Let everything dry for 12 hours before taking out the screw rods and the blocks. You will be surprised at the stiffness of these beams. They give the impression and a perfectly correct impression, of really solid bits of stuff, to which one could trust one's life. You can make these two spars in one day.
Cover a board measuring 30Omm. x 1,500 mm. with white paper, and mark out on it the profile of the rib as follows :
Draw a straight line (Fig. 23) at 50 mm. from the lower edge. On this line draw 15 perpendiculars spaced 100 mm. apart, and mark them in accordance with the drawing given. For example, 2-0 marks the point of the leading edge, .7 and 8.1 are the respective distances to the line of the lower side of the upper side of the wing, and so on right up to the tail of the rib, of which the trailing edge is 3 cm. above the line. This line is the chord of the wing.
Join all the points together, and this will give you the form of the profile of the wing.
Join all the points together, and this will give you the form of the profile of the wing. Two laths in 6-mm. x 12-mm. material are held between nails 2 mm. long, of which you will have cut off the heads, and which mark out the lines required.
At 320 mm. from the leading edge mark off the line for the axis of the bolts of the spar. Place the 6-mm. x 12-mm. laths on either side of this line, leaving a free space of 70 mm. This is where you will thread the spar on to the ribs.
Join the two flanges by a web of plywood 1.5 mm thickness. Keep the grain in the position shown by the arrows. Nail it every 25 mm with nails of 8mm. length. Dismount it and the rib is now retained in form.
Now nail the leading edge and the four gussets. In this way construct 22 ribs. With a cutting compass lighten the ribs. This should prove a simple matter, and should remove about 20 grammes from each web, it is very little, but it will lighten the set of ribs by half a kilo, and that is certainly proper aviation practice.
The rib weighs 160 grammes. It requires ten minutes to nail it up. The ribs which are not of the main series , -the ribs 8 and 9- each one repeated four times over, will be, designed and constructed in the same manner in accordance with the' drawing (Fig. 24).
One can prepare the webs and laths in five hours. All 31 ribs can be nailed up, in one afternoon, although you should allow several hours for rubbing up with sandpaper. The batch of 18 ribs weighs 3 kilos. One square metre of plywood of 1.5 mm makes webs for eight ribs.
You will find it'very easy to nail two nails in three seconds, but if you utilise a box of nails put on a slant. This slope will have the effect of making the nails roll until their heads are all pointing downhill. With a pair of pinchers you can then easily pick up each nail, and bring it under the hammer with the point about 1 mm. from the wood; at your first blow the nail is stuck into the wood. Take away your pincers,@ and with one more blow the nail is driven home.
You will get along quite fast, and will avoid damaging your thumb and fmgers. Take the pot of glue well away from the box of nails.