BODY-FIXED REFERENCE FRAME FB (BODY AXES, Oxyz)
Any set of axes fixed in a rigid body is a body-fixed reference frame. If the body is not rigid, i. e. if it has articulated parts such as control surfaces, or elastic motions, then the body axes are chosen to be those for which the resultant linear and angular momenta of the relative motions of articulation and elastic distortion vanish. This choice is always possible (see Sec. 5.1). The origin of the body axes is usually the mass center C. A particular set
of body axes with special properties are principal axes of inertia, denoted FP.
Flight vehicles almost invariably have a plane of symmetry (to a good approximation); this plane is chosen to be Cxz, with z directed “downward.”
Body axes play an especially important role in flight dynamics, and there is a tradition of notation associated with them. This is given in Fig. 4.8. Note that the subscript В is dropped when there is no possibility of confusion. The angular velocity of FB relative to Fx is to (p, q, r), and the components of VB are (и, v, w).