SPRING TABS

The effect of the “speed-squared law” on control forces at high speeds has led to the development of the “spring tab.” The effect of this device is to

mitigate the influence of speed. Figure 6.27 shows the principle. The system functions as follows. When a force is applied through the control rod to the control lever, the latter rotates through some angle 6. The control surface would rotate through the same angle, and the tab not move at all, if the control lever were rigidly connected to the surface. However, this is not so, and the torsion bar twists through some angle ф. The surface displacement is then д = в — ф. The movement of the control lever relative to the surface (angle ф), causes the tab link to move and deflect the tab, just as though it were a geared tab. Now with all other factors equal, an increase in speed will require an increase in the control-rod load to hold the same surface angle. But an increase in this force introduces extra twist into the torsion bar, and hence increases the tab deflection. Thus, as the speed increases, an increasing proportion of the hinge moment is balanced by the tab, and a decreasing proportion by the pilot or control system. In effect, the system behaves like a geared servo tab, the gearing of which increases with speed.