CONTRIBUTIONS OF A TAIL

As illustrated in Fig. 5.3, the main effect of q on the tail is to increase its angle of at­tack by (ql,/u0) radians, where u0 is the flight speed. It is this change in a, that ac­counts for the changed forces on the tail. The assumption is implicit in the following derivations that the instantaneous forces on the tail correspond to its instantaneous angle of attack; i. e., no account is taken of the fact that it takes a finite time for the tail lift to build up to its steady-state value following a sudden change in q. (A method of including this refinement has been given by Tobak, 1954.) The derivatives obtained are therefore quasistatic.

Cz of the Tail

Zq

By definition, CZq = (dCzldq)0 = (2u0lc){bCzldq)0, and, from (5.1,1), (dCz/dq)0 = — (ЭCL/dq)0. The change in tail lift coefficient caused by the rotation q is

ql,

AQ, = aJSa, = a, —

u0

and the corresponding change in airplane lift coefficient is

Therefore

and

(5.4,2)

Cmq of the Tail

The increment in pitching moment that corresponds to AC/ ( is [see (2.2,9)]

Hence