Part 12 (1/2)
WASHERS.--Under the bolt-head, and also under the nut, a washer must be placed--a very large washer compared with the size which would be used in all-metal construction. This is to disperse the stress over a large area; otherwise the washer may be pulled into the wood and weaken it, besides possibly throwing out of adjustment the wires attached to the bolt or the fitting it is holding to the spar.
LOCKING.--Now as regards locking the bolts. If split pins are used, be sure to see that they are used in such a way that the nut cannot possibly unscrew at all. The split pin should be pa.s.sed through the bolt as near as possible to the nut. It should not be pa.s.sed through both nut and bolt.
If it is locked by burring over the edge of the bolt, do not use a heavy hammer and try to spread the whole head of the bolt. That might damage the woodwork inside the fabric-covered surface. Use a small, light hammer, and gently tap round the edge of the bolt until it is burred over.
TURNBUCKLES.--A turnbuckle is composed of a central barrel into each end of which is screwed an eye-bolt. Wires are taken from the eyes of the eye-bolt, and so, by turning the barrel, they can be adjusted to their proper tension. Eye-bolts must be a good fit in the barrel; that is to say, not slack and not very tight. Theoretically it is not necessary to screw the eye-bolt into the barrel for a distance greater than the diameter of the bolt, but, in practice, it is better to screw it in for a considerably greater distance than that if a reasonable degree of safety is to be secured.
Now about turning the barrel to secure the right adjustment. The barrel looks solid, but, as a matter of fact, it is hollow and much more frail than it appears. For that reason it should not be turned by seizing it with pliers, as that may distort it and spoil the bore within it.
The best method is to pa.s.s a piece of wire through the hole in its centre, and to use that as a lever. When the correct adjustment has been secured, the turnbuckle must be locked to prevent it from uns.c.r.e.w.i.n.g.
It is quite possible to lock it in such a way as to allow it to unscrew a quarter or a half turn, and that would throw the wires out of the very fine adjustment necessary. The proper way is to use the locking wire so that its direction is such as to oppose the tendency of the barrel to unscrew, thus:
[Ill.u.s.tration]
WIRES.--The following points should be carefully observed where wire is concerned:
1. _Quality._--It must not be too hard or too soft. An easy practical way of learning to know the approximate quality of wire is as follows:
Take three pieces, all of the same gauge, and each about a foot in length. One piece should be too soft, another too hard, and the third piece of the right quality. Fix them in a vice, about an inch apart and in a vertical position, and with the light from a window s.h.i.+ning upon them. Burnish them if necessary, and you will see a band of light reflected from each wire.
Now bend the wires over as far as possible and away from the light.
Where the soft wire is concerned, it will squash out at the bend, and this will be indicated by the band of light, which will broaden at that point. In the case of the wire which is too hard, the band of light will broaden very little at the turn, but, if you look carefully, you will see some little roughness of surface. In the case of the wire of the right quality, the band of light may broaden a very little at the turn, but there will be no roughness of surface.
By making this experiment two or three times one can soon learn to know really bad wire from good, and also learn to know the strength of hand necessary to bend the right quality.
2. _It must not be damaged._ That is to say, it must be unkinked, rustless, and unscored.
3. Now as regards keeping wire in good condition. Where outside wires are concerned, they should be kept _well greased or oiled_, especially where bent over at the ends. Internal bracing wires cannot be reached for the purpose of regreasing them, as they are inside fabric-covered surfaces. They should be prevented from rusting by being painted with an anti-rust mixture. Great care should be taken to see that the wire is perfectly clean and dry before being painted. A greasy finger-mark is sufficient to stop the paint from sticking to the wire. In such a case there will be a little s.p.a.ce between the paint and the wire.
Air may enter there and cause the wire to rust.
4. _Tension of Wires._--The tension to which the wires are adjusted is of the greatest importance. All the wires should be of the same tension when the aeroplane is supported in such a way as to throw no stress upon them. If some wires are in greater tension than others, the aeroplane will quickly become distorted and lose its efficiency.
In order to secure the same tension of all wires, the aeroplane, when being rigged, should be supported by packing underneath the lower surfaces as well as by packing underneath the fuselage or nacelle. In this way the anti-lift wires are relieved of the weight, and there is no stress upon any of the wires.
As a general rule the wires of an aeroplane are tensioned too much.
The tension should be sufficient to keep the framework rigid. Anything more than that lowers the factor of safety, throws various parts of the framework into undue compression, pulls the fittings into the wood, and will, in the end, distort the whole framework of the aeroplane.
Only experience will teach the rigger what tension to employ. Much may be done by learning the construction of the various types of aeroplanes, the work the various parts do, and in cultivating a touch for tensioning wires by constantly handling them.
5. _Wires with no Opposition Wires._--In some few cases wires will be found which have no opposition wires pulling in the opposite direction.
For instance, an auxiliary lift wire may run from the bottom of a strut to a spar in the top plane at a point between struts. In such a case great care should be taken not to tighten the wire beyond barely taking up the slack.
[Ill.u.s.tration: Distortion of upper wing caused by auxiliary lift wire being too tight.]
Such a wire must be a little slack, or, as ill.u.s.trated above, it will distort the framework. That, in the example given, will spoil the camber (curvature) of the surface, and result in changing both the lift and the drift at that part of the surface. Such a condition will cause the aeroplane to lose its directional stability and also to fly one wing down.
I cannot impress this matter of tension upon the reader too strongly.
It is of the utmost importance. When this, and also accuracy in securing the various adjustments, has been learned, one is on the way to becoming a good rigger.