Another form of spar construction is that given by [Fig. 34]. Q shows a spar fretted out, the sides being covered with a thin veneer glued and cramped into place, and R the method of making a slotted spar.
[Fig. 35] is the simplest possible form of model aeroplane fuselage, if such it can be called.
Choice of materials and the method of utilising them to the best advantage, so that the machine is strong without being unduly heavy, is a phase of model aeroplaning that calls for some care and judgment. There is a very erroneous impression prevalent among novices that packing-case wood or similar material is suited to the requirements peculiar to model aeroplanes. Nothing could be farther from the truth; and the fact that 50 per cent. of the total marks awarded in competition are for design and construction should show that this matter is of primary importance. The true test of any model is the way it “stands up” to a nose dive, for then the care and forethought of the builder in providing for anticipated eventualities will manifest itself. It is to be feared that those who had lavished much care and infinite pains in the scientific construction of models were woefully handicapped in competition, the flimsy freak that could flutter aloft for a minute or so, with three strands of rubber wound to nearly breaking point, gaining priority over the properly built machine.
There are three salient points to be borne in mind on which the durability of the machine largely depends: (1) its capacity for resisting the torque of the rubber motor; (2) of absorbing the shocks of rough landing; and (3) the provision that has been made for the rigid attachment of the various parts. If the machine is at fault with regard to point one, fuselage distortion is likely to occur, and resulting from this there will be lack of alignment of the surfaces and attendant troubles. Point two calls for suitable bracing of the spar or spars, and careful choice of timber. It is inadvisable to use wood of square cross-section, an oblong section with the greatest measurement placed vertically being preferable. If no arrangement has been made to fix rigidly the wings, chassis, etc. (point 3), these parts are likely to rock or sway when the machine is in the air, and so occasion bad stability, apart from which a couple of landings would shake the machine out of truth.
Fig. 35.—Simple Fuselage
To obviate these difficulties a knowledge of the strength of the various timbers will be found useful, and there is appended a table of the weights of various timbers. The writer prefers birch for fuselage members over 3 ft. 6 in. in length. Although on the heavy side compared with spruce, it will stand a great amount of rough usage. Spruce is also suitable for fuselages up to this length, while maple is more suited for main planes. Bamboo can be used more efficaciously for cross members, struts, etc. Some model-makers use bamboo for planes, the joint of the rib to the spar being by means of glue and cross-binding. Although planes so built are exceedingly strong, it is not possible to make quite so neat a job of them as with spruce or maple.
Another method of building main planes is to use spruce or birch spars with piano-wire ribs, these latter being bound to the former.
For single-spar models the main spar should be tapered fore and aft from a point one-third of the length from the front of the machine. Where it is necessary to pierce the spar of a model aeroplane for the reception of a kingpost or other member, silk tape binding should be used, the joint being soaked with clean, weak glue.
To resist fuselage distortion the spar must be suitably braced in a lateral direction, the outrigger carrying the bracing wires being situated just forward of the centre of the spar. No. 35 s.w.g. is quite strong enough for fuselage bracing. Silk fishing-line or Japanese silk gut is admirably suited for wing bracing, and is not so liable to stretch as the tinned-iron or brass wire sometimes used. Piano wire is generally used for elevators, tail planes, chassis, and propeller shafts, of a gauge ranging from No. 17 s.w.g. to No. 22 s.w.g. A clock-spring or piano-wire protector fitted to the nose of a model aeroplane will also prevent a broken spar should it strike any object during flight.