Quantitative feedback theory with a scheduled gain for full envelope longitudinal control

Two approaches to control law design for the unstable, nonlinear, short-period dynamics of the F-16 aircraft using nonlinear quantitative feedback theory (QFT) are presented, The first is a conventional QFT design, whereas the second uses a variable gain before applying QFT. Large uncertainty templates result from using a set of linear time invariant equivalent models to accurately represent the nonlinear aircraft dynamics over a wide operating envelope. It is shown that including the variable gain, which Is inversely proportional to speed, has the beneficial effect of reducing the size of these templates. Once the modified templates have been obtained, the traditional QFT method is applied to yield a controller, which meets the same performance criteria, while using lower control effort. The gain-scheduled QFT controller is endowed with the usual, good robustness qualities of QFT designs while requiring lower gain at high speeds than the fixed-gain controller. The reduced gain provides improved phase margin and eliminates a tendancy for excessive elevator and pitch rate oscillations at high speed.