Figs. 27 to 30.—Various Forms of Fuselage
In bracing such frames as those dealt with, fine No. 35 s.w.g. (Standard Wire Gauge) should be used, fixed to small hooks bound in suitable places on the spar; the hooks should be made from No. 22 s.w.g.
[Fig. 27] shows at G a perspective view of a T-frame propeller bar and support. As there shown, the propeller bar fits into a slot cut in the spar end. If the spar is hollow, the channel should be filled with hard wood, such as birch, before the slot is cut, to strengthen the spar at this point. So much for twin-screw fuselages.
[Fig. 28] is a perspective view of a boat-shaped tractor fuselage, it being understood that a tractor machine is one with the air-screw in front. A model built on such lines is extremely neat in appearance and has a pleasing aspect in the air. The three longerons are attached to a three-way brass bearing at the front end, and are simply bound together at the rear, the hook for the elastic being inserted between the two top members and turned round the end of one of them for security, as shown at H. The bottom member should be cut 1 in. longer than the two top ones, to compensate for the shortening due to the curve, which is effected by compressing the bottom member to the same length as the top ones. The curved cross members are of bamboo, bent to the required shape over a lamp flame; or they could be made from piano wire. Their shape should be drawn full-size to use as a template during the bending operation. As will be seen, a skid is used to protect the tractor screw from damage. This should extend for 2 in. beyond the bearing, and must be attached to the bottom longitudinal directly beneath the first cross member, so that the latter absorbs the shock of landing. At the point of intersection between the skid and the axle, the former should be bound to the latter with fine florist’s wire and neatly soldered.
A two-membered fuselage can be adapted from this design by omitting the bottom member and skid. Such a fuselage would be suitable for a light machine.
It is an essential point with tractor models to fit a chassis; the purpose thus being twofold. First, it protects the propeller, and secondly, it obviates the characteristic tendency of tractor machines to ascend “nose first,” by keeping the weight low (in technical language, providing a low centre of gravity). Hand-launched tractor machines that are unprovided with a landing gear are seldom successful and notoriously troublesome. Furthermore, the centre of thrust (literally the axis centre, or centre of rotation of the bearing) should always be above the centre of resistance. The centre of resistance can usually be taken (although not quite accurate) as being on a level with the planes.
An exceedingly strong two-membered tractor fuselage of the fusiform or cigar-shaped type is that shown by [Fig. 29], the bearing, which is bracketed and cut from brass, being shown in detail at I. In this instance the greatest width of the top spar should be disposed horizontally, the bottom member, with the two cross members, providing rigidity and a girder-like form of construction. The bottom member need be only one-half the weight of the top one, as it will be in tension and so acting as a tie. Silver spruce should be used throughout.
A simple single-spar chassis, consisting of a hollow spar, is shown by [Fig. 30]. A kingpost and bracing is fitted underneath the spar, to counteract the tendency of the twisted skein to bow it. The chassis should be (and this applies to all models) of piano wire of from No. 17 s.w.g. to No. 18 s.w.g.
A twin-screw propeller-behind fuselage of the cantilever type that is exceedingly strong, although more difficult than it appears to construct, is shown by [Fig. 31]. This can be made exceedingly light from a hollow spar (packed solid at the point where the kingposts are let through), and braced with No. 35 s.w.g. piano wire. The bracing is the difficult operation, as the tension on each wire requires to be very delicately adjusted to maintain the truth of the spar.
The box-girder type of fuselage shown by [Fig. 32] is more suited to models which aim at an accurate representation of some prototype. It is intricate in construction yet of neat appearance, the difficulty being in the adjustment of the large number of bracing wires necessary. The illustration gives a view of a Blériot type of fuselage, J being a detail of the cross member and compression-strut joint.