Fig. 15. Details of Ribs and Struts, Curtiss Biplane
Struts. Before going into the detail of the construction of the remainder of the main cell and its attached framing, a brief description of its parts and their relation to one another will make matters clearer. The upright struts, Fig. 15, which hold the two planes apart, fit at each end into sockets, which are simply metal cups with bolts projecting through their ends. Fig. 16. Those at the bottom of the front row of struts pass through the eyes of the turnbuckles and connections for the wire trussing, then through the flattened ferrules of the main ribs, and finally through the beam, all being clamped together with a nut. Those at the top go through the turnbuckles first, then through the beam, and finally the rib ferrule. The bolts at the back row of struts must go through the full thickness of the main ribs, and so must be longer. The drawings. Figs. 15 and 16, show the method of attachment of both the main and the small ribs and illustrate a neat method of attaching the turnbuckles—instead of being strung on the socket bolt one after another, they are riveted to the corners of a steel plate which alone is clamped under the socket.
Fig. 16. Details of Metal Parts of Curtiss Biplane
Beams. The beams are jointed at each strut connection, the ends being cut square and united by a sheet-steel sleeve, a pattern of which is shown in Fig. 16, clamped on by two small bolts. The hole for the socket bolt is drilled half in each of the two abutting beams. As it is very difficult to obtain long pieces of wood sufficiently straight grained and free from knots for the purpose, this jointed system considerably cheapens the construction. Both beams and struts are of spruce, but to give additional strength, the beams of the middle section may be ash. Special aero cloth, rubberized fabrics, or light, closely-woven duck (racing yacht sail cloth of fine quality, this being employed at first by the Wright Brothers in their machines) forms the surfaces of the wings. The front edge of each section of the surface is tacked to the beam and the rear edge is laced over the rear wire already referred to, this wire being stretched taut through the holes in the rear tips of the ribs, both main and small. After the cloth is stretched tight, it is tacked to the small ribs, a strip of tape being laid under the tack heads to prevent the cloth from pulling away from under them. If the aeroplane is intended to be taken apart very often, the standard design as shown by the large drawings, Figs. 11 and 12, may be modified so as to make it unnecessary to unlace the cloth each time. This is arranged by regarding the two outer sections at each end of the plane as one, and never separating them. Additional main ribs are then provided at the inner ends of these sections, and are attached directly to the beams, instead of being clamped under the strut sockets. In taking the machine apart, the struts are pulled from the sockets, leaving the latter in place. It will then be an advantage to shorten the main planes somewhat, say 3 inches on each section, so that the outer double sections will come under the "12-foot rule" of the Express Companies.
Running Gear. Three wheels are provided—one in front under the outrigger and two under the main cell for starting and landing. Two beams extend from the front wheel to the engine bed and serve to carry the pilot's seat, as will be seen from the elevator, Fig. 12. A third beam runs back horizontally from the front wheel and on rough ground acts as a skid. The rest of the running gear is made of steel tubing, the pieces being joined simply by flattening the ends, drilling and clamping with bolts; no sockets or special connections of any kind are necessary here. If desired, the wheels may be carried in bicycle forks and may be fitted with shock absorbers, some idea of the various expedients adopted by different builders for this purpose being obtainable from the sketches. Fig. 40 in "Types of Aeroplanes." Two separate tubes, one on each side of the wheel make a simple construction and will probably serve just as well. The details of the running gear will be given later.
Outrigging and Rudders. For the outriggers and the frames carrying the front horizontal or elevating rudder and the rear vertical rudder and tail, or horizontal keel, either spruce or bamboo may be employed. Bamboo will be found on machines turned out by the Curtiss factory, and while it is the lighter of the two, it is not generally favored, as spruce is easier to obtain in good quality and is far easier to work. At their ends, these outriggers are fitted with ferrules of steel tubing, flattened and drilled through. The outriggers are attached to the main framework of the machine by slipping the ferrules over the socket bolts of the middle section struts, above and below the beams. It is preferable, however, to attach the rear outriggers to extra bolts running through the beams, so that when the machine is to be housed the tail and rudder can be unshipped and the triangular frames swung around against the main frame, considerably reducing the space required.
The tail, horizontal and vertical rudders, and the ailerons are light frames of wood, covered on both sides with the same kind of cloth as the main planes or wings. These frames are braced with piano wire in such a manner that no twisting strains can be put on them. The front horizontal rudder, which is of biplane construction like the main cell, is built up with struts in the same way. Instead of being fitted with sockets, however, the struts are held by long screws run through the planes and into their ends, passing through the eyes of the turnbuckles.
DETAILS OF CONSTRUCTION
Main Planes and Struts. It is preferable to begin with the construction of the main planes and their struts and truss wires, the ribs already described being the first step.