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Leading
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Trailing
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Fiig. 3.52    Stick-free permissible center of gravity travel.
3.3.16  Stick-Free Margin
    The stick-free margin is defined as
                                              H; = NL - Xc8                                     (3.137)
                               = -~ ),-                    (3.138)
For a stick-free stable airplane, the stick-free margin (H;) is positive.
   With stick-free, the permissible center of gravity travel (Ax') is modified as
shown schematically in Fig. 3.52.
3.3.77  Trim Tab
    A tab is a small flap hinged at the trailing edge of the elevator (see Fig. 3.53).
A deflection of the tab alters the pressure distribution over the entire horizontal
tail surface and hence gives a control over the hinge-moment coefficient. Thus, a
deflection of the tab also affects the floating characteristics of the elevator. As we
will discuss soon, the stick force directly depends on the floating characteristics of
the elevator, hence on the hinge-moment coefficient. Thus, a tab deflection affects
the stick force. The condition Cti - O corresponds to zero stjck force, and a tab
used to achieve this condition is called trim tab*
    Assuming that the hinge moment depends linearly on tail angle of attack, ele-
　
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