Learning to fly in the U.S. Army

by E. N. Fales

When a machine is already assembled, the first thing to do is to loosen all wires except the landing wires. This is very important, for if one wire is tightened against another wire, an unnecessary and possibly a dangerous strain may be put upon some member. The bracing wires connecting tops of center section struts should be tight enough to hold the shape of the center section when bracing wires are tightened up.

Fig. 38.—Center section and undercarriage alignment.

(a) Machines Having No Stagger.—In machines having no stagger, the struts of the center section should be perpendicular to the propeller axis. As the upper longerons are usually parallel to the propeller axis, they may be used as a base line.

Align one side of the center section first, then the other side, and lastly the front.

From a point at the lower end of one of the front center section struts (the center of a bolt head for example), measure forward on the longeron a certain distance. From the same point (center of bolt head) measure back on the longeron exactly the same distance.

Move the upper end of the strut forward or backward by loosening one of the bracing wires and tightening the other, until the distance from the two points on the longerons to some point on the center line at the top of the strut (center of bolt head) are exactly the same. The strut will then be perpendicular to the propeller axis. Tighten both wires evenly until fairly tight. Measure the cross distances (the diagonal distances between similar points at the upper and lower ends of the front and rear struts), and align the other side of the center section until its cross distances are the same as those on the opposite side.

Align the front of the center section by loosening one cross wire and tightening the other, until one cross distance is exactly the same as the other cross distance.

(b) Machines Having Stagger.—In machines having stagger, the shape and position of the center section strut fittings usually determines the amount of stagger the machine was designed to have (Fig. [38]-a). The JN-4 has 10⅝-in. stagger, i.e., a plumb line dropped from the leading edge of upper panel should be 10⅝-in. from leading edge of lower panel.

Adjust the wires on one side of the center section until the struts and that side are in their correct positions as shown by the shape of the fittings. Tighten the wires, measure the cross distances, and adjust the wires on the other side of the center section until the cross distances are exactly similar to the first set.