Low-frequency vibroisolation mounting of power plants for new-generation airplanes with engines of extra-high bypass ratio

Abstract

Compliance of aircraft with new noise standards defines the tendency to switch to extra-high bypass-ratio engines. Substantial noise redistribution has occurred in the aircraft of the new generation. While jet noise has been reduced dramatically, the engine still remains to be the basic source of noise, which is a fan noise. In the forward hemisphere, besides the discrete components at fan blade frequency, long row of discrete components has been observed around the principal blade frequencies as a result of shockwave influence. This phenomenon is called “buzz-saw noise”. Fan shaft frequency reduction is one of the necessary measures for shockwaves control. Due to the decrease of frequency, the vibration spectrum shifts towards the low-frequency range. Such components will determine the dynamical impact spectrum of power plant, transmitted through mounting to airframe construction. An airframe typically possesses dozens of oscillation the modes in the low-frequency spectrum part. Interaction of some of them with the influence of the power plant may generate discrete low-frequency high-level noise components in the cabin. We have designed new low-frequency attachments containing built-in elastic elements with nonlinear characteristics and with a quasi-zero stiffness zone at proof load by the cruise mode.