Model aeroplanes / The building of model monoplanes, biplanes, etc., together with a chapter on building a model airship

by F. J. Camm

Fig. 146.—Side Elevation of Biplane Driven by Compressed Air

In bracing the outriggers, or longerons, some care will be required to ensure their being quite true. It will be easier to finish each section off first, so that they are quite parallel at the joints.

The part plan view of the model ([Fig. 148]) will make the relative position of the various component parts quite plain. The two top tail outriggers pass through the fabric at the point where the spar is located, their front ends being pinned and cross bound to the wing spar, which is made of greater cross-section in the centre, so that its strength is not materially impaired through the piercing of it. Birch is to be used for the wing spars and ribs of the sections indicated.

The planes are ribbed at periods of 6 in. and given a camber of ¾ in., the greatest depth of which is 2¼ in. from the leading edge. It is far easier to impart the camber after the wing framework is made than to camber each rib separately. Each rib should be cut 1 in. longer than necessary, and pinned and glued to the spars, with ½ in. overlapping each of these latter. When the glue is quite set, the pins may be clinched over by supporting the wing on an iron weight and tapping them back flush to the spars.

The full-size section of the camber should be drawn upon a board, with which to check the accuracy of the first rib to be cambered (the end rib).

The ribs are cambered in a jet of steam, the convex or top sides being placed nearest to it. Having cambered the end rib carefully to agree with the drawing, the others may be matched to it. It will thus be easy to ensure that every rib is of the correct curvature, as any mistake in the steaming of the rib will distort the wing spar at the point of its attachment.

If, however, it is thought advisable to camber the ribs first, a wooden bending jig should be made, to enable several ribs to be bent at one operation. The ribs should be tied down to the jig with string, and thus held under the steam jet, being well dried in front of the fire before they are detached from the jig. All three spars pass underneath the ribs.

A very light fabric should be chosen, such as can be obtained from the model-aero accessory warehouses, or an unproofed Japanese silk can be used and varnished when on the wing. If this latter is used, it will be found advantageous to use a yellow hue, as this colour is least affected by the action of the varnish. But the covering of the wings must be left for the time being, for the reason that the sockets to which the inter-struts are made fast must first be attached. Further, the top plane must be covered after the tail outriggers have been assembled, as it is so much easier to make the joint between the wing spars and these latter before the fabric is attached.

To render it unnecessary to refer to the point further, it may be noted that the fabric is brought over the leading spar of each wing to pocket it out. It is much neater to sew the fabric along on the leading edge, as when glue is used an unsightly black smear shows through. The fabric should be stretched from end to end first, the fabric overlaps being glued on the bottom face of each end rib. Drawing-pins should be partially pressed into the ribs to secure the fabric until the glue is set.

At B in [Fig. 148] is shown the method of securing the bottom plane to the inter-struts. Convenient notches are cut in the struts into which the plane is sprung. It will have been noticed from the side elevation ([Fig. 146]) that the width of the inter-struts increases towards the bottom or lower ends, and also that they incline slightly; this is to provide for the entry of the lower plane, since the top plane is attached outside the struts, while the bottom is placed inside them. At C is shown the method of attaching the inter-struts.