Identification and verification of frequency-domain models for XV-15tilt-rotor aircraft dynamics in cruising flight

Frequency-domain methods are used to extract the open-loop dynamics of the XV-15 tilt-rotor aircraft from flight test data for the cruise condition (F=170 knots). The frequency responses are numerically fitted with transfer-function forms to identify equivalent modal characteristics. The associated handling quality parameters meet or exceed level II, category A requirements for fixed-wing military aircraft. Step response matching is used to verify the time-domain fidelity of the derived transfer-function models. The transient responses of the model and aircraft are in close agreement in all cases, except for the normal acceleration response to elevator deflection in cruise. This discrepancy suggests that center-of-rotation corrections in the normal acceleration model are highly sensitive to small changes in flight condition and input form, and should be omitted if a robust model is desired. The utility of the frequency-domain approach for dynamics identification and analysis is clearly demonstrated.