During the lifetime of any given aircraft program, the addition of numerous external pods and stores (items stored on the underside of the wing) may be required to meet the needs of unanticipated mission profiles. Aerodynamic turbulence induced by these stores can result in large dynamic loads on aircraft structures, for example downstream airfoils. Structural vibration is typically a direct result of forcing inputs, or of broadband forcing inputs driving structural resonances. In some cases, such vibration may produce stresses beyond the design threshold and result in premature, fatigue induced failure.
One means of mitigating flow-induced loads is structural vibration control (SVC). While it has been shown that SVC is a viable solution, it is expensive and heavy, requiring large numbers of transducers and tremendous amount of electrical power. Another technique is active flow control (AFC), the feasibility of which has been demonstrated in previous studies. This research aimed at determining the complementary effect of AFC and SVC systems at reducing structural fatigue. To that end, we fabricated two scaled SVC systems. These systems were combined with AFC modules in subsonic and transonic wind tunnels tests. Data revealed that the systems can work effectively together and the potential for full scale implementation exists.
In order to maximize fatigue inducing stress, the SVC hardware was designed to resonate with disturbances produced by the flow control module. A variety of tip masses provided the means to fine tune the structural resonant frequency. The test structure was a simple vertical airfoil (fin) with piezoelectric bimorph actuators and strain sensors mounted at the base. Placement of the sensor/actuator pairs ensured their greatest influence on the first and second bending modes only. Expected wind tunnel pressure loads were not known exactly, however, an analysis was developed to predict fin response. These models demonstrated that the increased aerodynamic loading at transonic airspeeds would require a second fin with lower surface area and higher stiffness. Several LQR and LQG controllers were designed to effect 5-10% damping in the fundamental structural modes of both fins.
Wind tunnel experiments consisted of an SVC airfoil mounted downstream of the AFC module. The module contained a pod, specifically designed to produce shedding vortices. Pod to airfoil spacing was adjusted to ensure the maximum aero-elastic coupling between the airflow and the fin. The module also used a piezoelectric diaphragm to expel air pulses from the anterior of the pod to perturb the development of the vortices.
Experimental data was recorded for a number of different test parameters including: airspeeds, SVC control authorities, tip masses, and AFC module positions and pulse frequencies. In all cases, the SVC system performed extremely well and actuator saturation was not encountered even for the highest controller gains and highest tested Mach number (0.95). Likewise, flow control modulations produced a reduction in excitation of the airfoil. At certain parameters, the combined SVC and AFC system resulted in a maximum 85% reduction of peak bending moment at the resonant frequency. More importantly however, these experimental data showed that a combined SVC/AFC system provides vibration suppression with much lower weight and power requirements than an equivalently effective, independent SVC system. Possible future experiments could examine the potential of collaborative control methods including the use of adaptive feed forward architectures.
