An active-control surface modal (ACSM) device has been developed as an improved means of alleviating buffet of an aircraft. The ACSM device is a "smart" actuator system that includes an array of antagonistic piezoelectric actuators installed within a rudder or other aircraft control surface (see Figure 1). These actuators are used to deform the control surface in what amounts to controlled vibration modes (see Figure 2), the frequencies and mode shapes being chosen to affect unsteady aerodynamic damping to suppress the dynamic effects of buffet.

Figure 1. Piezoelectric Actuators are built into a control surface in a configuration to enable controlled deformation of the surface.
Buffet is a turbulent airflow phenomenon that originates from the separated flow shed behind aerodynamic lifting surfaces of an aircraft. An instability in this flow gives rise to random fluctuations of pressure that excite vibrations of the tail surfaces of the aircraft. These vibrations contribute to fatigue damage and thus to life-cycle cost and maintenance time.

Prior to the development of the present ACSM, both passive and active methods of alleviating buffet were considered. The passive methods included the use of fences and other modifications to minimize the strengths of shed vortices. Active methods included, variously, the use of hydraulic or piezoelectric actuators to superimpose, upon control motions, oscillations of the affected control surfaces about their hinges to affect aerodynamic damping. The two major shortcomings of this prior active method are the following: (1) For a given aircraft, the flight-control system and the buffet-load-alleviation control system must use the same degree(s) of freedom of the same control surface(s), so that the availability of the control surface(s) for each role (flight control or alleviation of buffet) is reduced. (2) Because it is difficult to make a massive control surface oscillate about its hinge at a high frequency, operation is limited to relatively low frequencies.

The use of piezoelectric actuators in an ACSM device to deform a control surface (in contradistinction to trying to make the control surface oscillate about its hinge) makes it possible to use modal frequencies that are high enough to encompass the frequency spectrum of buffet, gusts, and flutter. Although the ACSM is an integral part of the rudder or other control surface, it can be actuated according to control laws independent of those of flight-control system; hence, the control surface can respond to flight-control commands as well as the dynamic buffet-load-alleviation signals.

Figure 2. These Examples of Mode Shapes are typical of rudder deformations generated by an ACSM to reduce buffet loads.
The vertical tail of the F-18 airplane was selected for benchmark case study for a demonstration of the ACSM approach to alleviation of buffet loads. Buffet-load data generated by NASA Langley Research Center in experiments on a 16-percent-scale model of the F/A-18 airplane were used in the study. Frequency-response analyses were performed for open- and closed-loop-control test cases. In the closed-loop case, two acceleration signals measured at leading and trailing edges of the tail tip section were used as the feedback signals to command the actuators. These analyses showed that the use of the ACSM device would reduce peak tail-root stresses by 60 to 80 percent; this would translate to an eight-fold enhancement of the fatigue life of the vertical tail. The maximum total actuator power needed to effect this reduction was found to be 182 W. It was also concluded, from the results of these analyses, that gust loads could be alleviated and flutter suppressed by use of the ACSM device.

This work was done by K. Appa and J. Ausman of Appa Technology Initiatives, K. S. Khot of the Air Force Research Laboratory, and Martin Brenner of Dryden Flight Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.

DRC-01-25


This Brief includes a Technical Support Package (TSP).
"Smart" Actuators for Alleviating Buffet of Aircraft

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This article first appeared in the December, 2001 issue of NASA Tech Briefs Magazine.

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