Closed Loop Criterion

Closed loop criterion uses an additional cascade TF with the pilot’s model [15]: .

This term signifies integration at low frequency. The two forms used are given as

No HQ boundaries are given. The additional term is necessitated due to the use of 3DOF equations for computation. It might be necessary for the pilot to perform this integration to avoid the droop at frequencies lower than the BW frequency. The closed droop should not be more than —3 dB for Levels 1 and 2. The closed loop resonance should not be greater than 3 dB for Level 1, and 9 dB for Level 2 (over the range 0-10 rad/s).

10.4.1.3 Pitch Rate Response

The aircraft-control system’s state responses to a unit step can be obtained [16]. In addition, its response to turbulence can be obtained and RMS values can be computed. All the four longitudinal states are plotted. From the pitch rate response, several time-domain specifications can be studied. These specifications are standards from the conventional control system requirements: rise time, transient peak ratio, etc. The pitch rate peak ratio is computed as

(max value — steady-state value)/(steady-state value — min value)

The peak ratio signifies the damping of the system. A tangent line (extending from the steady-state horizontal line to the time axis) is drawn at the point of maximum slope. The effective time delay is computed as

time of the intersection of the tangent at maximum slope with the steady-state line — time of the intersection of the tangent with the time axis

The limits are given in Tables 10.3 and 10.4. The criteria are applicable to conventional and many pitch augmentation systems.