Controller Requirements
The requirements for the stability and performance of fight control systems are specified in the certi-fication requirements for U.S. civilian aircraft, Federal Aviation Regulations (F.A.R.), Parts 23 and 25, and in the US military specifications MIL-F-8785C based on aircraft flying qualities (F.Q.s). For US Military aircraft the flying qualities are categorised by flying quality levels as described in Table 7.1. Requirements for damping ratios for the long-period and short-period modes are shown in Tables 7.2 and 7.3 respectively. Because the long-period is always a slow acting mode which gives the pilot plenty of time to control there is no specification on the natural frequencies required in this mode.
The short period natural frequency however is a critically important parameter. The short period frequency requirements for accurate flight control are determined by the n : α ratio. Fig. 7.3) shows the regions of acceptable ωnSP vs the acceleration sensitivity nZα = n/α from MIL-F-8785C for a flight phase of Category B. To determine this ratio we consider the state-space equation of the short period approximation
x = Ax+Bu