If it is necessary to bore holes in the spars for the purpose of receiving, for instance, socket bolts, then their places should be marked before being bored and their positions confirmed by whoever is in charge of the workshop. All is now ready for the sail-maker to cover the surface with fabric.

ADJUSTMENT OF CONTROL CABLES.--The adjustment of the control cables is quite an art, and upon it will depend to a large degree the quick and easy control of the aeroplane by the pilot.

The method is as follows:

After having rigged the controlling surfaces, and as far as possible secured the correct adjustment of the control cables, then remove the packing which has kept the control levers rigid. Then, sitting in the pilot's seat, move the control levers _smartly_. Tension the control cables so that when the levers are smartly moved there is no perceptible s.n.a.t.c.h or lag. Be careful not to tension the cables more than necessary to take out the s.n.a.t.c.h. If tensioned too much they will (1) bind round the pulleys and result in hard work for the pilot; (2) throw dangerous stresses upon the controlling surfaces, which are of rather flimsy construction; and (3) cause the cables to fray round the pulleys quicker than would otherwise be the case.

Now, after having tensioned the cables sufficiently to take out the s.n.a.t.c.h, place the levers in their neutral positions, and move them to and fro about 1/8 inch either side of such positions. If the adjustment is correct, it should be possible to see the controlling surfaces move.

If they do not move, then the control cables are too slack.

FLYING POSITION.--Before rigging an aeroplane or making any adjustments it is necessary to place it in what is known as its "flying position."

I may add that it would be better termed its "rigging position."

In the case of an aeroplane fitted with a stationary engine this is secured by packing up the machine so that the engine foundations are perfectly horizontal both longitudinally and laterally. This position is found by placing a straight-edge and a spirit-level across the engine foundations (both longitudinally and laterally), and great care should be taken to see that the bubble is exactly in the centre of the level.

The slightest error will a.s.sume magnitude towards the extremities of the aeroplane. Great care should be taken to block up the aeroplane rigidly. In case it gets accidentally disturbed while the work is going on, it is well to constantly verify the flying position by running the straight-edge and spirit-level over the engine foundations. The straight-edge should be carefully tested before being used, as, being generally made of wood, it will not remain true long. Place it lightly in a vice, and in such a position that a spirit-level on top shows the bubble exactly in the centre. Now slowly move the level along the straight-edge, and the bubble should remain exactly in the centre.

If it does not do so, then the straight-edge is not true and must be corrected. _This should never be omitted._

In the case of aeroplanes fitted with engines of the rotary type, the "flying position" is some special att.i.tude laid down in the aeroplane's specifications, and great care should be taken to secure accuracy.

ANGLE OF INCIDENCE.--One method of finding the angle of incidence is as follows:

[Ill.u.s.tration]

First place the aeroplane in its flying position. The corner of the straight-edge must be placed underneath and against the _centre_ of the rear spar, and held in a horizontal position parallel to the ribs. This is secured by using a spirit-level. The set measurement will then be from the top of the straight-edge to the centre of the bottom surface of the main spar, or it may be from the top of the straight-edge to the lowest part of the leading edge. Care should be taken to measure from the centre of the spar and to see that the bubble is exactly in the centre of the level. Remember that all this will be useless if the aeroplane has not been placed accurately in its flying position.

This method of finding the angle of incidence must be used under every part of the lower surface where struts occur. It should not be used between the struts, because, in such places, the spars may have taken a slight permanent set up or down; not, perhaps, sufficiently bad to make any material difference to the flying of the machine, but quite bad enough to throw out the angle of incidence, which cannot be corrected at such a place.

If the angle is wrong, it should then be corrected as follows:

If it is too great, then the rear spar must be warped up until it is right, and this is done by slackening _all_ the wires going to the top of the strut, and then tightening _all_ the wires going to the bottom of the strut.

If the angle is too small, then slacken _all_ the wires going to the bottom of the strut, and tighten _all_ the wires going to the top of the strut, until the correct adjustment is secured.

Never attempt to adjust the angle by warping the main spar.

The set measurement, which is of course stated in the aeroplane's specifications, should be accurate to 1/16 inch.

LATERAL DIHEDRAL ANGLE.--One method of securing this is as follows, and this method will, at the same time, secure the correct angle of incidence:

[Ill.u.s.tration: FRONT ELEVATION and PLAN.]

The strings, drawn very tight, must be taken over both the main and rear spars of the top surface. They must run between points on the spars just inside the outer struts. The set measurement (which should be accurate to 1/16 inch or less) is then from the strings down to four points on the main and rear spars of the centre-section surface. These points should be just inside the four centre-section struts; that is to say, as far as possible away from the centre of the centre-section.

Do not attempt to take the set measurement near the centre of the centre-section.

The strings should be as tight as possible, and, if it can be arranged, the best way to accomplish that is as shown in the above ill.u.s.tration, _i.e._, by weighting the strings down to the spars by means of weights and tying their ends to struts. This will give a tight and motionless string.

However carefully the above adjustment is made, there is sure to be some slight error. This is of no great importance, provided it is divided equally between the left- and right-hand wings. In order to make sure of this, certain check measurements should be taken as follows:

Each bay must be diagonally measured, and such measurements must be the same to within 1/16 inch on each side of the aeroplane. As a rule such diagonal measurements are taken from the bottom socket of one strut to the top socket of another strut, but this is bad practice, because of possible inaccuracies due to faulty manufacture.

The points between which the diagonal measurements are taken should be at fixed distances from the b.u.t.ts of the spars, such distances being the same on each side of the aeroplane, thus:

[Ill.u.s.tration: Points A, B, and C, must be the same fixed distances from the b.u.t.t as are Points D, E, and F. Distances 1 and 2 must equal distances 3 and 4.]

The above applies to both front and rear bays.

It would be better to use the centre line of the aeroplane rather than the b.u.t.ts of the spars. It is not practicable to do so, however, as the centre line probably runs through the petrol tanks, etc.

THE DIHEDRAL BOARD.--Another method of securing the dihedral angle, and also the angle of incidence, is by means of the dihedral board. It is a light handy thing to use, but leads to many errors, and should not be used unless necessary. The reasons are as follows:

The dihedral board is probably not true. If it must be used, then it should be very carefully tested for truth beforehand. Another reason against its use is that it has to be placed on the spars in a position between the struts, and that is just where the spars may have a little permanent set up or down, or some inaccuracy of surface which will, of course, throw out the accuracy of the adjustment. The method of using it is as follows:

[Ill.u.s.tration]

The board is cut to the same angle as that specified for the upward inclination of the surface towards its wing-tips. It is placed on the spar as indicated above, and it is provided with two short legs to raise it above the f.l.a.n.g.es of the ribs (which cross over the spars), as they may vary in depth. A spirit-level is then placed on the board, and the wires must be adjusted to give the surface such an inclination as to result in the bubble being in the centre of the level. This operation must be performed in respect of each bay both front and rear. The bays must then be diagonally measured as already explained.

YET ANOTHER METHOD of finding the dihedral angle, and at the same time the angle of incidence, is as follows:

A horizontal line is taken from underneath the b.u.t.t of each spar, and the set measurement is either the angle it makes with the spar, or a fixed measurement from the line to the spar taken at a specified distance from the b.u.t.t. This operation must be performed in respect of both main and rear spars, and all the bays must be measured diagonally afterwards.

[Ill.u.s.tration]

Whichever method is used, be sure that after the job is done the spars are perfectly straight.

STAGGER.--The stagger is the distance the top surface is in advance of the bottom surface when the aeroplane is in flying position. The set measurement is obtained as follows:

[Ill.u.s.tration]

Plumb-lines must be dropped over the leading edge of the top surface wherever struts occur, and also near the fuselage. The set measurement is taken from the front of the lower leading edge to the plumb-lines. It makes a difference whether the measurement is taken along a horizontal line (which can be found by using a straight-edge and a spirit-level) or along a projection of the chord. The line along which the measurement should be taken is laid down in the aeroplane's specifications.

If a mistake is made and the measurement taken along the wrong line, it may result in a difference of perhaps 1/4 inch or more to the stagger, with the certain result that the aeroplane will, in flight, be nose-heavy or tail-heavy.

After the adjustments of the angles of incidence, dihedral, and stagger have been secured, it is as well to confirm all of them, as, in making the last adjustment, the first one may have been spoiled.

OVER-ALL ADJUSTMENTS.--The following over-all check measurements should now be taken.

[Ill.u.s.tration: The dotted lines on the surface represent the spars within it.]

The straight lines AC and BC should be equal to within 1/8 inch. The point C is the centre of the propeller, or, in the case of a "pusher"

aeroplane, the centre of the nacelle. The points A and B are marked on the main spar, and must in each case be the same distance from the b.u.t.t of the spar. The rigger should not attempt to make A and B merely the sockets of the outer struts, as they may not have been placed quite accurately by the manufacturer. The lines AC and BC must be taken from both top and bottom spars--two measurements on each side of the aeroplane.

The two measurements FD and FE should be equal to within 1/8 inch. F is the centre of the fuselage or rudder-post. D and E are points marked on both top and bottom rear spars, and each must be the same fixed distance from the b.u.t.t of the spar. Two measurements on each side of the aeroplane.

If these over-all measurements are not correct, then it is probably due to some of the drift or anti-drift wires being too tight or too slack.

It may possibly be due to the fuselage being out of truth, but of course the rigger should have made quite sure that the fuselage was true before rigging the rest of the machine. Again, it may be due to the internal bracing wires within the lifting surfaces not being accurately adjusted, but of course this should have been seen to before covering the surfaces with fabric.

FUSELAGE.--The method of truing the fuselage is laid down in the aeroplane's specifications. After it has been adjusted according to the specified directions, it should then be arranged on trestles in such a way as to make about three-quarters of it towards the tail stick out unsupported. In this way it will a.s.sume a condition as near as possible to flying conditions, and when it is in this position the set measurements should be confirmed. If this is not done it may be out of truth, but perhaps appear all right when supported by trestles at both ends, as, in such case, its weight may keep it true as long as it is resting upon the trestles.

THE TAIL-PLANE (EMPENNAGE).--The exact angle of incidence of the tail-plane is laid down in the aeroplane's specifications. It is necessary to make sure that the spars are horizontal when the aeroplane is in flying position and the tail unsupported as explained above under the heading of Fuselage. If the spars are tapered, then make sure that their centre lines are horizontal.