A very modern type of steel construction is that developed by the Sturtevant Company. The members of the Sturtevant fuselage are in the form of steel angles and channels, similar in many respects to the sections used in steel buildings and bridges. The joints are riveted and pinned as in steel structural work. The longitudinals are angles and the struts are channels. Crystallization of the steel members is prevented by the use of special pin-connected joints provided with shock absorbing washers. Owing to the simplicity of the riveted joints, there is practically no weight due to connections, and since the weight of connections is a large item in the total weight of a fuselage, the Sturtevant is a very light structure. According to G. C. Loening, engineer of the company, the fittings of a large wood fuselage weigh at least 60 pounds. This is almost entirely saved with the riveted connections.
Truss Type Fuselage of Curtiss R-4 Biplane, Showing Motor and Front Radiator Mounted in Place. It Will Be Noted That the Upper and Lower Longerons Are Channeled Out for Lightness and Hence These Members Are of the "I" Beam or Channel Form. Propeller Flange Is Shown Projecting Through the Radiator Opening.
A novel type of wood fuselage has been described by Poulsen in "Flight." Eight small longitudinals are used which are held in place by three-ply wooden formers or diaphragms. Wire bracing is used in a longitudinal direction, but not transversely in the plane of the diaphragms. The cross-section is octagonal, and the completed structure is covered with fabric. For the amateur this offers many advantages since the wiring is reduced to a minimum and all of the members are small and easily bent to shape. It is fully as light as any type of body, for the connections are only thin strips of steel bolted to the diaphragms with small machine screws. No formers are needed for the deck, and the machine can be given a close approximation to the ideal stream-line form with little trouble.
Truss Type Fuselage. We will now take up the construction of the truss type of fuselage in more detail, and investigate the merits of the different types of connections used in fastening the frame together. Like every part of the aeroplane, the fuselage must either be right or wrong, there is no middle course. Fig. 23 shows a side elevation of a typical truss type fuselage built up with wood longitudinals and struts, the tension members being high tensile strength steel wire and cable. L and L’ are the upper and lower longitudinals, S-S-S are the vertical struts, and T-T-T are the horizontal cross struts which run across the frame. The engine bed is the timber marked B at the front of the body. The upper wing is attached to the body through the "cabane" struts C, and the chassis connections are shown at D. The stern post E closes the rear end of the body in a knife edge and acts as a support for the rudder and the rear end of the stabilizer. F is the seat rail which carries the seats and supports the control yokes.
All cross bracing is of high tensile strength steel wire, or of high strength aviation cable, these strands taking the tensile stresses while the wood struts are in compression. In the forward portion, double stranded cables are generally used, with solid wire applied to the after portions. The longitudinals are of ash from the motor to the rear of the pilot's seat, while the rear longitudinals are generally of spruce. In some machines, however, the entire length of the longitudinals is ash. The latter arrangement makes a heavier, but stronger body. The struts are usually of spruce as this material is stiffer than ash and much lighter.
Diagram of Typical Truss Type Fuselage, Showing Principal Members in Place.
Fuselage of Hansa-Brandenburg Fighting Biplane. See Page 268. Figs. 27-28-29-30. Fuselage Details. (Truss Type)
Both the struts and longitudinals are frequently channelled out for lightness, as shown by Fig. 27, the wooden member being left rectangular in section only at the points where the connections are made with the struts and cables. The channelling-out process, if correctly followed, gives very strong stiff members with a minimum of cross-sectional area and weight. Many captured German machines, on the contrary, have solid longitudinals of rectangular section, wrapped with linen fabric. This fabric strengthens the construction and at the same time reduces the chances of splintering the wooden members in a hard landing. The fabric is glued to the wood and the entire wrapping is then given several coats of a moisture repelling varnish. In the older types of fuselage, the longitudinals were often of the "laminated" class, that is, were built up of several layers of wood glued together in a single rectangular mass. This reduced the tendency toward splitting, but was very unreliable because of the uncertainty of the glued joints when exposed to the effects of heat and moisture. Laminated longitudinals are now seldom used, particularly in the region of the motor where water and oil are certain to wreck havoc with the glued up members.
As the stresses rapidly diminish toward the tail, it is the general practice to taper down the section of the longitudinal toward the rear and to reduce the section of the struts. The longitudinals are generally kept constant in section from the motor to the rear of the pilot's seat, the taper starting at the latter point and continuing to the rear end. For example, if the longitudinal section at the motor is 1 1/4" x 1 1/2", the section at the rear will be 1" x 1", the width of the struts corresponding to this taper. While tapering is very desirable from the weight standpoint, it makes the fitting problem very difficult since each fitting must be of a different dimension unless the connections can be designed so that they are adjustable to changes in the section of the longitudinals. In one machine, the width and depth of the longitudinals are kept constant, the variation in weight and section being accomplished by increasing the depth of the channelling as the rear is approached. With this design, the same fittings can be applied from one end to the other.
Figs. 27-28-29-30. Fuselage Framing Members and Details.
Since the loading of the struts is comparatively light, they can be much reduced in section by channelling or by chamfering, as shown by Fig. 28. If the width and thickness is maintained, much of the interior material can be removed without danger of reducing the strength. Sketch (A) in Fig. 28 shows a very common method of strut reduction, the strut being of rectangular section throughout its length, but tapered in such a way that it is thickest at the center (d) and thinnest at the two ends (e). To obtain the correct relation between the center end thickness requires very careful calculation. As shown, the strut is attached to the upper and lower longitudinals by sheet steel fittings or "sockets." Sketch (B) shows a simple method, the rectangular strut being chamfered off at each of the four corners, and left full size at either end where the fittings connect it with the longitudinals. This form is not correct from a technical standpoint, but is generally good enough for lightly loaded struts, and has the advantage of being cheaply and easily constructed. In sketch (C) a channelled strut is shown, the center portion being channelled out in a manner similar to the channelling of the longitudinals. This lightening process is most commonly adopted with the large heavily loaded struts in the front portion of the fuselage, and at the points where the motor bed is suspended or where the wings and chassis are attached to the body. The black dots at the ends of the struts indicate the bolt holes for the fittings, it being permissible to drill holes in the ends of the struts but not in the longitudinal members. If the strut is large enough to resist the bending stresses at the center it will generally allow of holes being drilled near the ends without danger of strength reduction. Again, the struts are always in compression and hence the bolts may be depended upon to partly take the place of the removed material in carrying the compressive stresses.