Part 12 (2/2)
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.
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