In the preceding sections of this and the previous chapter we discussed aerodynamic characteristics of symmetrical configurations flying with the velocity vector in the plane of symmetry. As a result the only nonzero motion variables were V, a, and q, and the only nonzero forces and moments were T, D, L, and M. We now turn to the cases in which the velocity vector is not in the plane of symmetry, and in which yawing and rolling displacements (/3, ф) are present. The associated force and moment coefficients are Cv, C„ and C„.
One of the simplifying aspects of the longitudinal motion is that the rotation is about one axis only (the у axis), and hence the rotational stiffness about that axis is a very important criterion for the dynamic behavior. This simplicity is lost when we go to the lateral motions, for then the rotation takes place about two axes (x and z). The moments associated with these rotations are cross-coupled, that is, roll rotation p produces a yawing moment Cn as well as rolling moment Ch and yaw displacement j8 and rate r both produce rolling and yawing moments. Furthermore, the roll and yaw controls are also often cross-coupled—deflection of the ailerons can produce significant yawing moments, and deflection of the rudder can produce significant rolling moments.
Another important difference between the two cases is that in “normal” flight— that is, steady rectilinear symmetric motion, all the lateral motion and force variables are zero. Hence there is no fundamental trimming problem—the ailerons and rudder would be nominally undeflected. In actuality of course, these controls do have a secondary trimming function whenever the vehicle has either geometric or inertial asymmetries—for example, one engine off, or multiple propellers all rotating the same
way. Because the gravity vector in normal flight also lies in the plane of symmetry, the CG position is not a dominant parameter for the lateral characteristics as it is for the longitudinal. Thus the CG limits, (see Sec. 3.7) are governed by considerations deriving from the longitudinal characteristics.