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:
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 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.
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 butt 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 butt 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 assume 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.