CONTRIBUTIONS OF A TAIL

There is an approximate method for evaluating the contributions of a tail surface, which is satisfactory in many cases. This is based on the concept of the lag of the downwash. It neglects entirely the nonstationary character of the lift response of the tail to changes in tail angle of attack, and attributes the result entirely to the fact that the downwash at the tail does not respond instantaneously to changes in wing angle of attack. The downwash is assumed to be dependent primarily on the strength of the wing’s trailing vortices in the neighborhood of the tail. Since the vorticity is convected with the stream, then a change in the circulation at the wing will not be felt as a change in downwash at the tail until a time At — IJV has elapsed, where lt is the tail length (Fig. 6.10). It is therefore assumed that the instantaneous downwash at the tail, e(t), corresponds to the wing a at time (t — At). The corrections to the quasistatic downwash and tail angle of attack are therefore

Де =——– a At

do.

CL& OF A TAIL

The correction to the tail lift coefficient for the downwash lag is

h де

V да

The correction to the airplane lift is therefore

* ~ Л deSt

AC r = a A ——— *

VdaS

Cma OF A TAIL

The correction to the pitching moment is obtained from ДCLt as Mm = – VHACLt

dCm lt де

—- = —aA’jf———

da Vda

OF A TAIL

The correction to at produces a change in the elevator hinge moment

де. lt