In investigating the handling qualities related to longitudinal dynamics, many workers in the field separate the problem into two parts, associated

Fig. 12.11 Effect of phugoid damping, on pilot rating (from ref. 12.15).

with the short-period response and phugoid response. Attempts are then made to correlate pilot opinion with the various parameters or with the characteristics of these two modes.

First consider the phugoid response. This mode was discussed at length in Chapter 9, and approximations to the period and damping were given in Sec. 9.2. For conventional fixed-wing airplanes the period is very long and not a significant factor in pilot rating. The damping is important however, and some experimental results (ref. 12.15) are shown on Fig. 12.11. These were obtained in flight under instrument conditions. As the damping of the phugoid mode decreases more attention must be devoted to controlling the associated low-frequency motion, which can be excited by movement of the aircraft controls or by gusts. It is seen that, generally speaking, a divergent phugoid mode (a negative £p) must be avoided. The same study that produced these results found that under visual flight conditions, a reduction in the damping from.32 to —.12 had little influence on pilot ratings.

Studies of the effect of the short-period response on pilot ratings have been made using variable stability aircraft (ref. 12.15). Although a range of results have been noted for various tasks and aircraft, the general pattern is as illustrated on Fig. 12.12. It shows a typical plot of pilot “iso-opinion” curves from such an experiment. The solid lines represent curves of constant pilot rating as the values of o)n and £ are altered. The regions of satisfactory, acceptable, poor, and unacceptable handling qualities are indicated along with the pilot comments for the various areas in the unacceptable region.