Application of experimental modes for solving helicopter vibration problems

Abstract

Helicopter vibration reduction frequently requires detuning airframe natural frequencies from well-defined frequencies of excitation. If airframe detuning proves impractical, vibration reduction is generally achieved by use of vibration control devices. The most common vibration control device is the fixed or variable-tuned vibration absorber. For either effective detuning or efficient absorber design, accurate information is required with respect to airframe natural frequencies and mode shapes. Due to complexity of the airframe structure, classical finite element models often are inadequate for defining design changes necessary for either detuning or determining an effective location for vibration absorbers. Also, these finite element models are both time-consuming and costly to develop and use. In work at Hughes Helicopters considerable success has resulted from application of classical theory to carefully obtained experimental airframe modes and natural frequencies for establishing effective vibration reduction methods. The manner by which this is accomplished is summarized in the paper. Airframe detuning is achieved through application of stiffness changes in selected aircraft locations or by changes in mass distribution at response solution using modal superposition. In both cases, theory is applied to experimental airframe eigenvectors and eigenvalues.