Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 24

If a mechanic tightens a nut by applying a force to a wrench, a twisting action, called a moment, is created about the center of the bolt. This particular type of moment is called torque (pronounced “tork”). Moments, M, are measured by multiplying the amount of the applied force, F, by the moment arm, L:

(1.2)

The moment arm is the perpendicular distance from the line of action of the applied force to the center of rotation. Moments are measured as foot–pounds (ft‐lb) or as inch–pounds (in.‐lb). If a mechanic uses a 10 in.‐long wrench and applies 25 lb of force, the torque on the nut is 250 in.‐lb.

The aircraft moments that are of particular interest to pilots include pitching moments, yawing moments, and rolling moments. If you have ever completed a weight and balance computation for an aircraft, you have calculated a moment, where weight was the force and the arm was the inches from datum. Pitching moments, for example, occur when an aircraft’s elevator is moved. Air loads on the elevator, multiplied by the distance to the aircraft’s center of gravity (CG), create pitching moments, which cause the nose to pitch up or down. As you can see from Eq. 1.2, if a force remains the same but the arm is increased, the moment increases.

Several forces may act on an aircraft at the same time, and each will produce its own moment about the aircraft’s CG. Some of these moments may oppose others in direction. It is therefore necessary to classify each moment, not only by its magnitude, but also by its direction of rotation. One such classification could be by clockwise or counterclockwise rotation. In the case of pitching moments, a nose‐up or nose‐down classification seems appropriate.

Mathematically, it is desirable that moments be classified as positive (+) or negative (−). For example, if a clockwise moment is considered to be a + moment, then a counterclockwise moment must be considered to be a − moment. By definition, aircraft nose‐up pitching moments are considered to be + moments.

Figure 1.8 Balance Lever.