A more accurate method is to drop a plumb line from the leading edge of the center section and adjust until the line is at the correct distance ahead of the point on the fuselage where the leading edge of the lower wing meets it. This point may be determined by measuring the distance from the inside of the front hinge to the leading edge of the lower wing and then laying off this distance on the body from the front of the hinge on the lower longeron. Better still, if the hinges are at the same distance from the leading edge on both top and bottom wings, the plumb line may be dropped from the front side of the hinge on the center section and the stagger measured back to the hinge on the lower longeron (Fig. [38]-b). This has the advantage of setting the plumb line out far enough to clear the fuselage. Also the measurements are easily made.
Next, adjust the two front wires until one cross distance is exactly the same as the other cross distance (Fig. [38]-c).
3. Alignment of Leading Edge.—(a) Upper Plane.—The leading edges of the upper and lower planes of one wing should next be made perfectly straight. By standing on a step ladder, placed 15 to 20 ft. to one side, and sighting along the leading edge of the upper plane, any bow or warp can be easily seen. This should be straightened out by loosening or tightening the front landing wires. The edge should be brought in exact line with the leading edge of the center section.
(b) Lower Plane.—After the leading edge of the upper plane has been made straight, sight along the leading edge of the lower plane. If there is no warp in the plane, this edge should also be straight.
(c) Align the opposite wing in the same manner.
4. Getting Both Wings the Same Height.—Place a small tack exactly in the middle of the leading edge of the center panel.
Measure from this tack to similar points at the lower ends of the intermediate and outer struts (Fig. [39]). Make these distances the same on each side by raising or lowering one wing or the other, or by raising one wing and lowering the other wing, all the while keeping the leading edges of both wings perfectly straight.
5. Dihedral.—The method of setting the wings of a machine at a dihedral angle is as follows:
Place two tacks in the leading edge of the upper plane, one tack near the tip of each wing and exactly the same distance out from the tack in the center section. Stretch a string tightly between the two outer tacks, until there is no sag in the string.
A dihedral angle of 178° means that each wing has been raised 1°. To set the wings of a machine at a dihedral angle of 178° for example: