(a) Find the natural sine of 1° (0.0175).
(b) Multiply this by the distance in inches between the center tack and one of the outer tacks. The result will give the rise, in inches, of the string over the tack in the center section.
Raise the wings equally, keeping the leading edges perfectly straight, until the proper rise shows over the center section.
Fig. 39.—Alignment diagrams.
6. Alignment of Trailing Edge (Angle of Incidence).—(a) Lower Plane.—The trailing edge should be brought parallel to the leading edge. This can be done by bringing the rear spar in line with the leading edge.
Stand squarely in front of the center of the machine 15 to 20 ft. away. Sight under the leading edge of the lower plane; move forward or backward until the fittings under the rear spar are just visible. Raise or lower the trailing edge by loosening or tightening the rear landing wires, until all of the fittings on the rear spar appear equally under the leading edge.
(b) Upper Plane.—After aligning the trailing edge of the lower plane, place a ladder in front of the center of the machine, and sight under the leading edge of the upper plane. If there is no warp in this plane, the trailing edge should align with the leading edge.
The objection to this method is that since there are no fittings next the body on the rear spar, there is room for considerable error in the angle of incidence.
Reversing the process and finding the angle of incidence at each set of struts secures the alignment of the trailing edge and removes the liability to error. To set wings at correct angle of incidence proceed as follows (Fig. [39]): Place the airplane in rigging position, i.e., level up the top longeron or engine bearers. Set the corner of the straight-edge against the center of the rear spar, level up the straight-edge, and measure from the top of the straight-edge to the center of the front spar or to the lowest point of the leading edge. This must be done next the body and under each set of struts. (It is useless to make such a measurement between the struts because of possible warping of the wings.) Unless the wings have a washout or washin the measurements must agree, thus making the angle of incidence the same all along the wing. Then the trailing edge must necessarily be parallel to the leading edge.