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Control surfaces, such as trailing-edge flaps, provide a means to dynamically alter the aerodynamic characteristics of aircraft for primary flight control, secondary vibration control, and even higher frequency noise control. While the development of several novel technologies has been explored, many practical implementation barriers still exist for a single actuation system to serve all three of these objectives. This is particularly true for rotorcraft, where the demands of the harsh rotary and vibratory environment are severe in terms of actuator force and displacement, bandwidth limitations, life cycle concerns, and physical volume available. Accordingly, it has been assumed that on-blade active controls of a rotorcraft would be subject to the most stringent requirements in the subsonic flight regime, and if a control surface actuation technology could survive here, it could be reasonably applied to a fixed wing aircraft. A brief account of the current state-of-the-art for rotorcraft blade controls follows. Helicopter rotors typically operate in a highly unsteady aerodynamic environment. In forward flight, the rotor blade sections experience large variations in angle-of-attack over one revolution. This is the primary source of performance degradation, such as high vibration and retreating blade stall. Actively changing the angle-of-attack of the blade sections as a function of blade azimuth has been shown to significantly alleviate vibration levels, as well as improve aerodynamic performance of the rotor (Straub et al., 2000). The change in angle-of-attack can be accomplished in a variety of ways. Implementation of high bandwidth hydraulic actuators in the rotating frame has demonstrated the ability to actively change the root pitch of the rotor blades and has since been demonstrated in both scale models (Lorber et al., 2001) and full scale tests (Arnold & Strecker, 2002). Another approach is to vary the aerodynamic forces on the blades by dynamically changing the geometry of the airfoil sections. This can be accomplished through actively controlling blade twist, airfoil camber, or through the use of trailing-edge flaps (Hall & Wereley, 1993). Recent advances in adaptive materials have led to a variety of schemes for on-blade actuation in these areas (Chopra, 2000). Some of these include piezoelectric innovations such as adaptive twist of the rotor blade (Chen & Chopra, 1997; Chen et al., 2001; Shin et al., 2005), trailing-edge flaps (Straub et al., 2001; Fulton, 2000; Fulton, 2005), and active camber control (Konstanzer et al., 2001; Nissly et al., 2005). In comparing these approaches to active rotor systems, there are potential drawbacks, however. For instance, implementation of hydraulic systems in the rotating frame of

Wind Tunnel Testing of Pneumatic Artificial Muscles for Control Surface Actuation