Vibration of an Inflatable Fabric Wing Model

The vibrational characteristics of an inflatable, fabric wing model were found both experimentally and through the use of measured influence coefficients. The model tested had a semispan delta wing planform with 67.6° sweepback of the leading edge and a nearly uniform thickness. The first eight modes of vibration were experimentally determined for a range of internal wing pressures. The exciting force consisted of a loudspeaker located near the lower surface of the wing, driven by the amplified output of a variable-frequency, function generator. Flexibility influence coefficients were measured with the wing model subdivided into 10 and 18 sections and the natural frequencies and associated mode shapes for the first four modes of vibration determined by a matrix iteration procedure. The results obtained through experimental excitation and through the influence coefficient method were in reasonably good agreement. I. Introduction AT the present time there is a considerable amount of -£^- interest in inflatable space structures and winged reentry vehicles that can be packaged in small containers for the launching operation and expanded when outside the earth's atmosphere. There have been several interesting investigations concerning plate-like fabric structures and their uses for space and re-entry vehicles. Leonard et al.1 studied collapse loads and developed an analytical theory for the stress and deflection in inflatable fabric structures with particular application to platelike structures, such as could be used for re-entry vehicles. McComb2 developed a linear theory for inflatable plates of arbitrary shape, and Stroud3 compared the results obtained through application of this theory with the experimentally determined deflections and natural frequencies of vibration for a square Airmat plate with pinned and fixed boundaries. The highly swept, delta planform design of the wing model used for this investigation was chosen because of its similarity to the planform for a re-entry vehicle. Knowledge of the static and dynamic characteristics of the primary structure of a vehicle is a necessary prelude to detailed design studies of landing loads, gust response, and similar aeroelastic phenomena. The natural frequencies and mode shapes of a system can be found either experimentally or through the exact or approximate solution of equations, which mathematically represent the system. McComb2 developed a linear, small-deflection theory for the elastic behavior of inflatable plates. His theory, however, does not include the added stiffness effects due to the curved edges of the wing or wing model, which, particularly at low internal pressures, have a noticeable effect upon the behavior of the structure. Influence coefficients, either computed or measured, have been used successfully for finding approximate static deflections and vibrational characteristics of many types of elastic structures. This paper is concerned with an experimental investigation of the vibrational characteristics of an inflatable wing model. The resonant frequencies and mode shapes are determined both experimentally and through the use of measured influence coefficients. neoprene cement and paint dacron cover ply dacron inner ply lower surface r upper surface