Cruise flaps are trailing-edge flaps operated at small deflection
angles for the purpose of reducing drag at offdesign conditions.
The deflection of a cruise flap results in a shifting of the low-drag
region (bucket) of the drag polar for an airfoil, as shown in Fig. 1.
Flap deflection moves the leading-edge stagnation point, which
affects the pressure distributions along the airfoil upper and lower
surfaces. Figure 2 shows that for natural-laminar-flow (NLF) airfoils,
there is a small region at the leading edge in which it is most desirable
to locate the stagnation point [1,2]. Doing so results in favorable (or
less adverse) pressure gradients over the upper and lower surfaces,
even at offdesign coefficients of lift. Without the cruise flap, either
the upper or lower surface would have experienced loss of laminar
flow at these offdesign conditions. Thus, when scheduled correctly, a
cruise flap can result in a large range of Cl values over which low Cd
is achieved. For this reason, several NLF airfoils have been designed
with cruise flaps [3–5]. Cruise flaps have also been successfully used
on high-performance sailplanes for several decades.
So basicly, automated cruise flap ensures that the wing STAYS inside
the low drag bucket for a larger portion of the flight reducing drag.
I would guess the main advantage would be whenever you are
above or below your optimum altitude or airspeed. In other words, most of the time..
XPM