The original version of this paper was presented and distributed as part of the 37th Israel Annual Conference on Aerospace Sciences Proceedings. Without attempting a thorough review of the burgeoning literature, the results of a representative sampling of recent papers dealing with smart materials and structures as actuators in aeronautical systems are summarized here. Their potential for improving performance, handling qualities in a stall, and increasing fatigue life is discussed briefly as requiring relatively slow-acting shape and shape-distribution changes. A similar review is made of applications for improving aeroelastic divergence, flutter instabilities, and tail buffeting on fixed-wing aircraft; and reducing vibrations, improving external acoustics, and providing flight controls for rotating-wing aircraft - all of which require a high-frequency response. The status of some of the most promising developments is noted and the remaining problems are touched on. Two approaches, which have not been given substantial attention elsewhere, are reviewed; these are: developing concentrated, namely nondistributed, piezoelectric actuators in helicoidal configurations, on the one hand, as a way to improve force - deflection output; and using control surfaces purposefully designed to be marginally unstable and stabilized by smart structures, on the other hand, as a means of reducing the force - deflection combinations required of smart-structure actuators.

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Preface I consider it a great honor t o have been asked t o deliverthe Annual Nikolsky Lecture. The distinguished speakers who preceded me in this series make me all the more aware that this is a special opportunity. Unlike m y predecessors I was not privileged t o know Alexander Nikolsky. Among my friends, however, are a number who were Professor Nikolsky's students and colleagues. I, therefore, asked a few of them if they could provide me with some anecdote appropriate for this occasion. Considering that biographies reviewing the exceptional career of Alexander Nikolsky were given by previous speakers, I asked for an anecdote in a lighter vein.David Hazen and Courtland Perkins of the National Academy of Engineering came t o my rescue, with the following story recalling how Professor Nikolsky became Chief of Structures for Sikorsky Aircraft. Professor Nikolsky was a structures engineer for the company, in the days when Sikorsky was building fixed-wing aircraft. Among them was a flying boat, big for its day, with four engines on a highmounted wing. Professor Nikolsky was in charge of the structural design for the wing flap. Wing flaps were a particular challenge for flying boats, and when Professor Nikolsky's flap design was first flight tested, he a s the designer, along with several other high ranking engineers including Igor Sikorsky himself, rode in the cabin when the first flap deployment was undertaken a t altitude. I n those days instrumentation consisted largely of eyes pealed to the portholes. All the men in the cabin watch carefully as, on Signal, the pilot put the flap down 5 " , then lo", then 15'.

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