In the opinion of the writer, a spruce longeron should be of larger section than one of hard wood, for one or two reasons. Spruce is a soft wood, and the outside fibres are far more apt to get damaged by a fitting which has been bolted home with too much pressure, also the corners may get rubbed or knocked off, which all means a reduction in strength. The use of a spruce longeron precludes any sharp bends in the contour of the fuselage, as this wood does not lend itself to bending, although it may be sprung to an easy curve. By disposing joints in the longerons, it is possible to arrange the lengths so that the bend is contained in one portion. This portion can then be of laminated construction, i.e. it can be built up of a number of layers glued together, and clamped to a block of the required shape until the glue has set. In some cases the longerons from the engine mounting to the rear cockpit, where additional strength is necessary, are of ash, while aft of that, to the stern post, spruce is the material.
Fig. 67.—Longeron sections.
It is usual, in this country at least, to spindle the rails to one of the sections illustrated by [Fig. 67], this spindling or channelling running through from nose to stern post, or the front portion, extending as far as the rear cockpit, is left solid, the tail part only being spindled. This channelling is always stopped at the intersection of the cross-struts with the rails, to provide the abutment for the struts, and the extra material to compensate for any holes necessary for the attachment of the fitting. In the shaping of the rails longitudinally, two methods are available: they may be tapered or gradually diminished from the front to the stern post, or the overall section may be parallel to a point somewhere in the neighbourhood of the pilot’s seat, and from that point diminished to the stern post. The first method is obsolete, as all the fittings vary in size, which makes for undue complication as well as increasing the number of jigs and dies necessary to produce the stampings. The second method only partly obviates this, and the only system which permits of the same size fitting being used right through is that in which the rails are of the same overall section throughout, but this is very rarely used.
Another arrangement consists of keeping the rail of equal thickness for approximately 10 ft. from the engine bearers, and then diminishing in a series of steps to the stern post. By this method only three or four sizes of fittings are necessary. Some fittings are not affected by the taper of the rails, and are made the same size throughout, but in nearly every case the attachment to the rails is accomplished either by bolts or screws. The piercing of the longeron, particularly when this is of spruce, is hardly commendable practice, and certainly in view of the many forms of clip fittings in use appears to be unnecessary. A point which apparently escapes the notice of some designers, is the necessity of some allowance being made for unfair stresses induced by landing shocks and rough handling. There is a tendency to make the tail portion separate from the front, the joint occurring just aft of the rear cockpit, so that in the event of damage due to strains transmitted by the tail skid, this portion can be detached and a new portion substituted, which seems infinitely better than dismantling the whole machine and returning the whole body to the works or depôt. In the design of the body under consideration due regard should be given to the necessity of occasional replacement of a damaged rail. Some fittings afford the utmost facility for this, while others render this procedure a lengthy and difficult operation.
Jointing of Longerons.
A popular method of jointing longerons consists usually of a plain butt joint, clipped with some form of steel tube socket, or by fish-plates flanged to clip the edges of the longerons and bolted through. A spliced joint is sometimes used when timber is not procurable in any great length, this consisting of an ordinary splice from 12 to 18 ins. long, glued and riveted, and afterwards, when the joint is thoroughly set, bound with tape soaked in glue and subsequently doped and varnished. As this is a somewhat lengthy operation the socket method predominates. In modern aeroplanes the size of a longeron rarely exceeds 1½ ins. square, and it will therefore be realized that this construction is all that is possible, as, owing to the slightness of material, no advantage would accrue from the employment of a joint of the halved or scarfed variety.
Diagonal Wood Bracing.
A great deal of the foregoing applies to the second type, so far as the longerons and tail portion are concerned. The diagonal wood bracing is usually of spruce, and is, of course, heavier than wiring. The aluminium or duralumin sheeting has latterly given place to three-ply for the outside covering, which may be ascribed to the saving in weight effected by its use, as a square foot of 20 B.W.G. aluminium, which is the general thickness for this purpose, weighs 8 ozs., while a square foot of 3/32 in. birch three-ply weighs approximately 5 ozs. This gives a saving of 3 ozs. for every square foot of surface covered, and moreover three-ply, properly glued and screwed or copper, nailed to the framework, constitutes by far the better stiffening medium. The disadvantages of this method of construction are: (1) the difficulty of re-truing the front portion should distortion occur; (2) erection is somewhat involved; and (3) it is heavier than the first type, although it affords a more solid mounting for the engine, with a consequent reduction of vibration.