Overview

Frequently, three-DoF models, as described in the preceding chapter, do not model in sufficient detail the vehicle dynamics. Hence we may add two attitude degrees of freedom to the three translational equations and call the composite a five-DoF simulation. For a vehicle that executes skid-to-turn maneuvers (an intercept missile), pitch and yaw attitude dynamics are incorporated. For a bank-to-turn aircraft, the yaw angle of the missile is replaced by the bank angle. Euler's law formulates the differential equations for the two attitude angles. However, the increase in complexity is significant and approaches that of a full six-DoF simulation. To maintain the simple features of a three-DoF simulation and at the same time account for the attitude dynamics, the transfer functions of the closed-loop autopilot replace Euler's equations. This implementation is called a pseudo-five-DoF simulation. The word pseudo conveys the meaning of approximating the attitude dynamics with the linear differential equations of the transfer functions.

Pseudo-five-DoF simulations are popular models for concepts that are only loosely defined. During preliminary design, the vehicle's aerodynamics may be sketchy, the autopilot design rudimentary, and the guidance and navigation implementations uncertain. These are good reasons to match these notional systems with the simple pseudo-five-DoF models. If you want to find out whether a simulation has this pseudo characteristic, look for these telltales: trimmed aerodynamics, angle-of-attack as the output from a transfer function, body rates not obtained by solving the Euler's equations, and the absence of controls and actuator models.

Using the CADAC environment...