Some measurements in the R.A.E. Bedford 3 ft. wind tunnel on a typical high tailplane illustrate a Mach number effect on the tailplane rolling moments due to sideslip which has been apparently overlooked previously. Fig. 1(a) shows that the increase in tailplane rolling moment due to sideslip is much larger than the increase in the fin lift curve slope a1F, Fig. 1(b). This extra increase in rolling moment is associated with the increase in the tailplane lift curve slope a1T, Fig. 1(c). In fact the rolling moment on this high tailplane varies roughly as the product of the fin and tailplane lifts, as Fig. 1(d) shows. A qualitative explanation follows.

Consider the idealised high tailplane shown in Fig. 2. At zero geometric incidence and positive sideslip the fin lift will induce positive and negative incidence distributions on the starboard and port sides of the tailplane.