It's a particular problem with a movable stabiliser, as these trim systems are
very powerful compared to elevator authority.
With a variable incidence tail, however, as the C.G. is moved over comparatively large distances, the incidence of the tail is altered to provide a balancing force and the elevator remains in the streamlined position. Because the tail area is much larger than the elevator area the tail can be moved through a smaller angle to produce the required balancing force; and the elevator, always being 'neutral' to the tailplane, remains available over its full range at all times. This large increase in balancing forces available from a variable incidence tailplane, together with good pitch control from and unrestricted elevator, makes a large C.G. range a practicable proposition.
...
Summary
In dealing with the consequences of having a variable incidence tailplane one basic fact must be kept in mind - it is very powerful. Because the elevator, when in trim, is always slipstreaming the rail it remains available over its full range and can be smaller than the elevator on a fixed tail aircraft. This is simply because the stabiliser can be set to handle the bulk of the demand and the elevator remains to look after the rest of the demand. On a variable incidence tailplane aeroplane, therefore, the elevator is smaller, and consequently less effective in isolation than it is of a fixed tailplane aeroplane.
D.P. Davies
HANDLING THE BIG JETS,
Third Edition
pages 37 & 39
First published in 1967, so this information isn't new!
