GEARED TABS

The coefficient 62 dominates the hinge moment of a control, and hence the control force. It gives the rate at which the hinge moment increases with
control angle. The need for reduction of b2 by aerodynamic means was referred to in Sec. 6.5. One such means, which is very effective, is the geared or servo tab. The geometry of such a tab is illustrated in Fig. 6.26. The angle of the tab relative to the control surface is determined by the rigid link AB. When arranged as shown, downward movement of the control is accompanied by an automatic upward movement of the tab. The hinge moment caused by the tab is then of the sense which assists the control movement. If В were moved to the upper surface of the tab, so that AB crossed HH, then the opposite effect would be obtained. This arrangement, known as an antiservo, or antibalance tab can be used when a control is otherwise overbalanced, or too closely balanced. It provides a means of achieving a zero or positive b1 without any detrimental effect on b2, as follows. The balance, cb (Fig, 6.21), is chosen large enough so that bx becomes zero or positive. The control will then have b2 either too small or even positive. This is then corrected by introducing an antiservo geared tab.

Suppose that, when the elevator moves through an angle 8e, the tab displacement is — yde. y, called the “tab gearing,” is positive for a servo tab and negative for an antiservo tab. The hinge-moment coefficient will then be

Che — b0 + bjo+ b2de + b38t

(6.7,6)

The servo tab thus in effect reduces the value of b2 by the factor