Historical Development of Aircraft Flutter

Introduction A EROELASTICITY, and in particular flutter, has influenced the evolution of aircraft since the earliest days of flight. This paper presents a glimpse of problems arising in these areas and how they were attacked by aviation's pioneers and their successors up to about the mid-1950s. The emphasis is on tracing some conceptual developments relating to the understanding and prevention of flutter including some lessons learned along the way. Because it must be light, an airplane necessarily deforms appreciably under load. Such deformations change the distribution of the aerodynamic load, which in turn changes the deformations; the interacting feedback process may lead to flutter, a self-excited oscillation, often destructive, wherein energy is absorbed from the airstream. Flutter is a complex phenomenon that must in general be completely eliminated by design or prevented from occurring within the flight envelope. The initiation of flutter depends directly on the stiffness, and only indirectly on the strength of an airplane, analogous to depending on the slope of the lift curve rather than on the maximum lift. This implies that the airplane must be treated not as a rigid body but as an elastic structure. Despite the fact that the subject is an old one, this requires for a modern airplane a large effort in many areas, including ground vibration testing, use of dynamically scaled wind-tunnel models, theoretical analysis, and flight flutter testing. The aim of this paper is to give a short history of aircraft flutter, with emphasis on the conceptual developments, from the early days of flight to about the mid-1950s. Work in flutter has been (and is being) pursued in many countries. As in nearly all fields, new ideas and developments in flutter have occurred similarly and almost simultaneously in diverse places in the world, so that exact assignment of priorities is often in doubt. Moreover, a definitive historical account would require several volumes; yet we hope to survey some of the main developments in a proper historical light, and in a way that the lessons learned may be currently useful. It is recognized that detailed documentation of flutter troubles has nearly always been hampered by proprietary conditions and by a reluctance of manufacturers to expose such problems. From our present perspective, flutter is included in the broader term aeroelasticity, the study of the static and dynamic response of an elastic airplane. Since flutter involves the problems of interaction of aerodynamics and structural deformation, including inertial effects, at subcritical as well as at critical speeds, it really involves all aspects of aeroelasticity. In a broad sense, aeroelasticity is at work in natural phenomena such as in the motion of insects, fish, and birds (biofluid-dynamics). In man's handiwork, aeroelastic problems of windmills were solved empirically four centuries ago in Holland with the moving of the front spars of the blades from about the midchord to the quarter-chord position (see the article by Jan Drees in list of Survey Papers). We now recognize that some 19th century bridges were torsionally weak and collapsed from aeroelastic effects, as did the Tacoma Narrows Bridge in spectacular fashion in 1940. Other aeroelastic wind-structure interaction pervades civil