Aeroplane Construction and Operation

by John B. Rathbun

Holes should never be drilled in the longitudinals since these members may be either in tension or compression, depending upon the angle at which the elevator flaps are set. The hole not only destroys the strength at the point at which it is drilled, but this reduction also extends to a considerable distance on either side of the hole, owing to the fibrous nature of the wood. In steel members the effect of the hole is purely local and does not usually extend much beyond the edge of the hole. Considering the wood beam as consisting of a series of parallel fibers, it will be seen that severing any one of the fibers will decrease the strength of the wood through a distance equal to the length of the cut fiber, or at least through a distance equal to the natural shear value of the resins that bind the fibers together.

Fuselage fittings are almost numberless in the variety of design. They must be very light and strong, must be applied without drilling the longerons, and should be simple and cheap to construct. They are usually made of sheet steel of from 0.20 to 0.30 point carbon, and may be either bent or pressed into shape. At the points where the struts are joined to the longitudinals, the fittings connect struts and wires in three planes, the vertical struts and fore and aft wires; the transverse wires and horizontal struts, and the top and bottom wires that lie in a horizontal plane. There are at least 6 connections at every strut, four of the connections being made to the stay wires or cables. A simple connection is therefore very hard to design.

Fig. 29 shows a typical fuselage "panel" and the interconnected members in their usual relation. LU and LL are the top and bottom longitudinals at the right, while LU' and LL’ are the longitudinals at the right hand side. The vertical struts SV and SV’ separate the top and bottom longitudinals, while the horizontal struts SH and SH’ separate the right and left hand sides of the fuselage body. The wires w-w-w-w brace the body fore and aft in a vertical plane. The wires t-t lie in a horizontal plane, produce compression in the horizontal struts SH-SH', and stiffen the frame against side thrust. The transverse rectangle SV-SH-SV’-SH' is held in shape by the transverse stay wires W-W, this rectangle, and the stays resisting torsional stress (twisting), act against the struts composing the sides of the rectangle. In some European machines, the wires WW are eliminated, and are replaced by thin veneer panels, or short wood knee braces as shown by Fig. 30. The section shows the longitudinals L-L-L-L and the struts SV-SV’-SH-SH’ braced by the veneer sheet or diaphragm D. This diaphragm is well perforated by lightening holes and effectually resists any torsional stress that may be due to motor torque, etc. Since the transverse wires W-W in Fig. 29 are rather inaccessible and difficult to adjust, the veneer diaphragm in Fig. 27 has a great advantage. In this regard it may be stated that wire bracing is not a desirable construction, and the substitution of solid veneer is a step in advance.

Wire bracing has always seemed like a makeshift to the author. The compression and tension members being of materials of widely different characteristics are not suitable in positions where a strict alignment must be maintained under different conditions of temperature and moisture. The difference in expansion between wire and the wood compression members produces alternate tightness and slackness at the joints, and as this is not a uniform variation at the different joints, the frame is always weaving in and out of line. Under the influence of moisture the wood either swells or contracts, while the wire and cable maintain their original lengths and adjustments. The result is that a frame of this kind must be given constant attention if correct alignment is desired.

The adjustment of a wire braced wood fuselage should be performed only by a skilled mechanic, as it is easily possible to strain the members beyond the elastic limit by careless or ignorant handling of the wire straining turnbuckles. In the endeavor to bring an old warped fuselage back into line it is certain that the initial tension in the wires can be made greater than the maximum working stress for which the wires were originally intended. Shrinkage of the wood also loosens the bond between the wooden members and the steel fittings unless this is continually being taken up. Some form of unit construction, such as the monocoque body, is far more desirable than the common form of wire trussed wood body.

Fuselage Details of De Havilland V. Single Seat Chaser. A Rotary Le Rhone Motor Is Used in a Circular Cowl. The Diagonal Bracing in the Front Section is Reinforced by Laminated Wood Plates Instead of by Wires. Dimensions in Millimeters.

Fuselage Fittings. In the early days of aviation the fuselage fittings on many machines were made of aluminum alloy. This metal, while light, was uncertain in regard to strength, hence the use of the alloy was gradually abandoned. At present the greater part of the fittings are stamped steel, formed out of the sheet, and are of a uniform strength for similar designs and classes of material.

The steel best adapted for the fittings has a carbon content of from 0.20 to 0.30, with an ultimate strength of 60,000 pounds per square inch, and a 15 per cent elongation. The steel as received from the mill should be annealed before stamping or forming to avoid fracture. After the forming it can be given a strengthening heat treatment. A lower steel lying between 0.10 and 0.15 carbon is softer and can be formed without annealing before the forming process. This material is very weak, however, the tensile strength being about 40,000 pounds per square inch. Fittings made of the 0.15 carbon steel will therefore be heavier than with the 0.30 carbon steel for the same strength. The thickness of the metal will vary from 1/32" to 1/16", depending upon the load coming on the fitting.

A typical fuselage strut fitting is shown by Fig. 31-A in which L-L-L are the longerons, d is the fitting strap passing over the longerons, S and So are the vertical and horizontal struts respectively. The stay wires are fastened to ears (b) bent out of the fitting, the wires being attached through the adjustable turnbuckles (t). The struts are provided with the sheet steel ferrules marked (F). There are no bolts passing through the longitudinals L-L', the fitting being clamped to the wooden member. This is very simple and light fitting. Fig. 31-B is a similar type, so simple that further discussion is unnecessary.

Fig. 32 shows a fuselage strut fitting as used on the Standard Type H-3 Biplane. We are indebted to "Aerial Age" for this illustration. This consists of a sheet metal strap of "U" form which is bent over the longitudinal and is bolted to the vertical strut. At either side of the strut are through bolts to which bent straps attach the turnbuckles. These straps are looped around the bolts and form a clevis for the male ends of the turnbuckles.