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 shining 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.

6. _Wire Loops._--Wire is often bent over at its end in the form of a loop, in order to connect with a turnbuckle or fitting. These loops, even when made as perfectly as possible, have a tendency to elongate, thus spoiling the adjustment of the wires. Great care should be taken to minimize this as much as possible. The rules to be observed are as follows:

[Ill.u.s.tration: Wrong shape. Result of wrong shape. Right Shape.]

(_a_) The size of the loop should be as small as possible within reason.

By that I mean it should not be so small as to create the possibility of the wire breaking.

(_b_) The shape of the loop should be symmetrical.

(_c_) It should have well-defined shoulders in order to prevent the ferrule from slipping up. At the same time, a shoulder should not have an angular place.

(_d_) When the loop is finished it should be undamaged, and it should not be, as is often the case, badly scored.

7. _Stranded Wire Cable._--No splice should be served with twine until it has been inspected by whoever is in charge of the workshop. The serving may cover bad work.

Should a strand become broken, then the cable should be replaced at once by another one.

Control cables have a way of wearing out and fraying wherever they pa.s.s round pulleys. Every time an aeroplane comes down from flight the rigger should carefully examine the cables, especially where they pa.s.s round pulleys. If he finds a strand broken, he should replace the cable.

The ailerons' balance cable on the top of the top plane is often forgotten, since it is necessary to fetch a high pair of steps in order to examine it. Don't slack this, or some gusty day the pilot may unexpectedly find himself minus the aileron control.

CONTROLLING SURFACES.--The greatest care should be exercised in rigging the aileron, rudder, and elevator properly, for the pilot entirely depends upon them in managing the aeroplane.

[Ill.u.s.tration: Position in which controlling surface must be rigged. It will be its position during flight.]

The ailerons and elevator should be rigged so that, when the aeroplane is in flight, they are in a fair true line with the surface in front and to which they are hinged.

[Ill.u.s.tration: Position during flight. Position in which controlling surface must be rigged.]

If the surface to which they are hinged is not a lifting surface, then they should be rigged to be in a fair true line with it as ill.u.s.trated above.

If the controlling surface is, as ill.u.s.trated, hinged to the back of a lifting surface, then it should be rigged a little below the position it would occupy if in a fair true line with the surface in front. This is because, in such a case, it is set at an angle of incidence. This angle will, during flight, cause it to lift a little above the position in which it has been rigged. It is able to lift owing to a certain amount of slack in the control wire holding it--and one cannot adjust the control wire to have no slack, because that would cause it to bind against the pulleys and make the operation of it too hard for the pilot.

It is therefore necessary to rig it a little below the position it would occupy if it was rigged in a fair true line with the surface in front.

Remember that this only applies when it is hinged to a lifting surface.

The greater the angle of incidence (and therefore the lift) of the surface in front, then the more the controlling surface will have to be rigged down.

As a general rule it is safe to rig it down so that its trailing edge is 1/2 to 3/4 inch below the position it would occupy if in a fair line with the surface in front; or about 1/2 inch down for every 18 inches of chord of the controlling surface.

When making these adjustments the pilot's control levers should be in their neutral positions. It is not sufficient to lash them. They should be rigidly blocked into position with wood packing.

The surfaces must not be distorted in any way. If they are held true by bracing wires, then such wires must be carefully adjusted. If they are distorted and there are no bracing wires with which to true them, then some of the internal framework will probably have to be replaced.

The controlling surfaces should never be adjusted with a view to altering the stability of the aeroplane. Nothing can be accomplished in that way. The only result will be to spoil the control of the aeroplane.

FABRIC-COVERED SURFACES.--First of all make sure that there is no distortion of spars or ribs, and that they are perfectly sound. Then adjust the internal bracing wires so that the ribs are parallel to the direction of flight. The ribs usually cause the fabric to make a ridge where they occur, and, if such ridge is not parallel to the direction of flight, it will produce excessive drift. As a rule the ribs are at right angles to both main and rear spars.

The tension of the internal bracing wires should be just sufficient to give rigidity to the framework. They should not be tensioned above that unless the wires are, at their ends, bent to form loops. In that case a little extra tension may be given to offset the probable elongation of the loops.

The turnbuckles must now be generously greased, and served round with adhesive tape. The wires must be rendered perfectly dry and clean, and then painted with an anti-rust mixture. The woodwork must be well varnished.