The stabilator trim tab is mechanically linked to the fuselage, as shown schematically in Figure 8.18. The tail is trimmed by moving the attachment point of the link to the fuselage. With the stick free and the hinge moment trimmed to zero, the entire tail is floating around the pivot line. If the fuselage angle of attack is disturbed, the tail will tend to maintain a constant angle of attack. However, the tab, being linked to the fuselage, will deflect, thereby changing the aerodynamic moment about the pivot line. This, in turn, will cause the entire tail to float to a different position at which the moment is again zero. Thus, to determine the free elevator factor for a stabilator, the linkage to the fuselage must be taken into account. The tail cannot be treated as an entity, as was just done for the horizontal stabilizer-elevator configuration. We begin by setting Equation 8.32 equal to zero and substituting
For the stick-fixed case, where the incidence angle is fixed, the tail lift curve slope in the downwash of the wing is simply
a,( 1 – e«)
By comparison to Equation 8.45, it follows that for the stabilator configuration, the free elevator factor is given by
p = , (1 – тке)Ь і
bx – b2ke
Using the numbers previously estimated for the Cherokee 180, the value for Fe is estimated to be
Fe = 1.30
•Thus, the effectiveness of the stabilator in providing longitudinal static stability is not degraded by freeing the stick; indeed, it is actually improved!
Stick-Free Static Margin
The effect of freeing the elevator on the neutral point can be determined simply by multiplying a, by the free elevator factor, Fe, and using this effective tail lift curve slope in the expressions previously derived for the stick-fixed case.
If Fe is less than 1.0 the neutral point shifts forward, thereby decreasing the static margin for a given center-of-gravity position.