Unsteady Aerodynamics of Flaps
With the advent of smart structures it is now becoming increasingly feasible to use smart compliant airfoil surfaces or several trailing edge mounted flaps on rotor blades
Figure 8.40 The effects of a nonsteady free-stream Mach number variations on the unsteady lift for an airfoil with a constant AoA at a mean value of Mo = 0.5. (CFD calculations courtesy of Arun Jose.)
as a means of individually controlling the aerodynamic environment on each blade – see Section 6.12. Actively controlling the blade airloads as a function of blade azimuth position offers tremendous possibilities in improving helicopter rotor performance, as well as reducing blade loads and vibration levels and perhaps even reducing rotor noise. See Lorber (2000) and Friedmann (2004) for a good summary of what may be possible. However, practical concern of active blade adaptation through the application of flaps, airfoil camber or other chordwise shape changes is the availability of suitable low mass, high force actuators. These actuators must also be mounted inside the rotating blade and must be used to drive the aerodynamic surfaces at relatively high physical and reduced frequencies.
Theoretical studies of these types of moving flap problems using advanced helicopter rotor models require the use of a suitably formulated time-domain theory for the blade section aerodynamics. An unsteady aerodynamic theory is required, first because the local camber actuation frequency may be several multiples of the rotor rotational frequency and, second, because high resolution predictions of both the unsteady airloads and acoustics need to be made. In addition, because the local effective reduced frequencies based on active camber motion may become large, incompressible assumptions may not be adequate despite the exact analytical neatness of the theory.