Neal-Smith Approach

The connection between excessive lead requirements for control and poor pilot ratings is the basis for the Neal-Smith criterion, dating from 1970. A lead-lag pilot model is assumed, with a fixed time delay of 0.3 second. When this pilot model is combined with the dynamics of the airplane, the model parameters can be adjusted to meet bandwidth and other closed-loop requirements. The resultant pilot model phase lead and closed-loop resonance are compared with pilot opinions to establish acceptable boundaries (Figure 21.5).

The Neal-Smith approach is an important contribution to the rationalization of flying qualities requirements since it makes direct use of the mathematical pilot model. The method has shortcomings in that the required pilot lead is strongly dependent on the required bandwidth, an arbitrary starting point (Moorhouse, 1982).

Figure 21.5 Neal-Smith criterion for pitch control. Acceptable short-period behavior occurs below the boundary established by closed-loop peak resonance ratio, the abscissa, and pilot model lead, the ordinate. The hatched boundaries are more restrictive limits proposed for large transports. (From Mooij, AGARD LS-157, 1988)