If the C.G is moved aft it results in a larger nose down pitching moment
which has to be compensated for by placing a download on the tail plane. This
can be achieved by deflecting the elevator up, although this
results in high trim drag at higher airspeeds. On large aircraft small
movements of the variable incidence stabilizer allow the trimming to be carried
out, but with reduced trim drag.
An upward deflection of the elevator is required to trim the aircraft,
so think of the stabilizer as a large elevator with its trailing edge moving in
the same direction as the elevator. To achieve this you must therefore lower
the leading edge of the stabilizer to produce the necessary download.
Note: Once in a trimmed condition on fully powered flying controls the
elevator will then align itself with the stabilizer to reduce the tail load and
also the loads acting on the hinges and servo actuator. On power assisted
controls there is however no guarantee that this occurs due to downwash and
configuration effects that can alter the flow over the surface. It is also fair
to say that trim drag also increases fuel consumption.