I've been reading about it regarding trim changes and such, but I don't exactly know what a VIT is? Can someone explain that to me?
Incidence is the angle at which an aerodynamic surface is mounted to the remaining structure of an aircraft. Variable means that this angle can be changed in flight. Normally, a control surface is split between a forward part, called the stabilizer, and the rear part, called the elevator. On aircraft with less powerful wing flaps, the stabilizer incidence is fixed and only the elevator can move, while more complex aircraft can also move the stabilizer.
A third way is to do away with a separate elevator and to move the whole tail surface. This is a variable incidence tailplane.
Benefits of a full-flying tail surface
- By avoiding a contour break due to a flap deflection, all-moving control surfaces can avoid the shocks which would otherwise occur at high subsonic speed. This is especially beneficial for supersonic aircraft.
- At supersonic speed, the uncambered surface will produce less drag, so for supersonic speed the all-moving control surface is more efficient.
- By moving all of the tail surface at high speed, an all-moving control surface will produce the highest rate of moment change over time possible. To produce also the highest sustained moment change, however, it needs to be larger than a comparable stabilizer-elevator combination because it forfeits the benefits of variable camber.
- The all-moving control surface has no gaps which could add radar reflections, allowing for a more stealthy design and slightly reducing drag.
- Lower mechanical complexity. This is somewhat offset by the need for much beefier hydraulics.
Some gliders have full-flying horizontal tail surfaces for reduced complexity. They have a slight back sweep to shift their aerodynamic center and their center of gravity to the same location. This makes the tail light and simple and avoids the added drag of the elevator hinge gap. However, such tail configurations have caused a number of crashes in the early phase of a winch launch when the pilot did not keep the stick neutral. At low speed a fully deflected tail surface has separated flow on the suction side, rendering the tail less effective. Only when the flow suddenly attaches as speed picks up the tail effectiveness is regained. In combination with the pitch-up moment of the tow line this lead to very steep angles of attack right after take-off which the pilot could not correct with the still ineffective tail, and some gliders stalled, rolled and crashed as a consequence. Therefore, modern gliders avoid this feature.
Schempp-Hirth Cirrus with full-flying tail (picture source)
It is basically what its name implies: an horizontal stabilizer with a variable angle of incidence (not to be confused with angle of attack). It might be used for trimming purposes only (like in most modern airliners and a few GA aircraft, control with the trimming system) or for pitch control (in most if not all supersonic, non-delta-winged fighters, controlled by pilot's input).
You can read more here: https://en.wikipedia.org/wiki/Stabilator (especially the Airliners section).