Stabilization of a flight control system with varying numbers of right half-plane poles and zeros

Data for a linearized model of the STOL/maneuver technology demonstrator (SMTD) with stabiliser used to control pitch-rate are examined. Six flight conditions were considered, ranging from 0.1 to 0.9 Mach, and from zero to 40000 ft. The six cases comprised one with one right half-plane (RHP) zero and one RHP pole, two cases with two RHP poles, one case with one RHP zero and three RHP poles, and two cases with one RHP pole. The problem was to stabilize the system over this range of flight conditions by means of a single, fixed compensator, i.e. without any scheduling or identification. Quantitative feedback theory (QFT) was applied in a straightforward manner to obtain such a design with phase margins ⩾50° and gain margins ⩾6 dB (only one case with 6 dB, the five others ⩾20 dB). The technique is simple and systematic, with little cut-and-try. It can be applied by anyone with classical control background since the Nichols chart is the principal design tool. Time and frequency domain simulations are included