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Low-order equivalent system approaches to analysis of aircraft pitch dynamics are reviewed. Equivalent systems show promise for reducing the transfer functions of complex, highly augmented aircraft into familiar and interpretable parameters. The merits of such approaches are discussed in light of the flying qualities military specification and the desire to retain the classical criteria for defining acceptable pitch characteristics. Several potential problems with interpreting the equivalent systems parameters are reviewed. Some high-order transfer functions are shown to produce low-order system parameters which may not be equivalent to their classical counterparts and which may be difficult to physically interpret. CAP ) =F s = g h HOS = K LOES = M M, = n Nomenclature control anticipation parameter longitudinal stick force acceleration due to gravity altitude high-order system transfer function gain low-order equivalent system aerodynamic pitching moment divided by pitch moment of inertia bM/di where /=d e , q, w, or a normal acceleration pitch rate Laplace operator, a±ju first-order lag time constant time constant of lag between flight path and attitude responses steady-state velocity vertical velocity aerodynamic force along vertical flight path axis dZ/di where i=d e or w angle of attack vertical flight path angle elevator deflection damping ratio of equivalent short-period mode damping ratio of basic vehicle short-period mode pitch attitude time delay time delay of equivalent system frequency of equivalent short-period mode frequency of basic vehicle short-period mode frequency of second-order lag a 7 5, f. »Sp e T w *p (a)

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