Later Developments

An interesting inertial coupling development that came after the great rush of interest in the 1950s was the finding that moderate amounts of sideslip could add to the problem (Stengel, 1975). Perturbation motions about combined sideslip and rolling equi­librium solutions are less stable than perturbations about pure rolling motions.

Also important to the inertial coupling problem are some developments in related fields of airplane dynamics. Chapter 9, on “Spinning and Recovery,” notes the advent of the advanced bifurcation analysis method for study of stall-spin divergence, steady spinning, and wing rock. Bifurcation analysis is also able to predict jump phenomena in rolls or two equilibrium states for the same control surface angles (Schy and Hannah, 1977).

The 1977 study by Schy and Hannah was extended a year later to include nonlinear variations of the stability derivatives with angle of attack (Young, Schy, and Johnson, 1978). The authors correctly observed that the main utility of the bifurcation analysis method as applied to inertial coupling in rolls is to predict the flight conditions and control surface angles for which jumps may occur. These combinations should be examined in detail in complete time history solutions.