Low-Frequency Vibroisolation Mounting of Power Plants for New Generation Airplanes with Engines of Extra-High by-Pass 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 the basic source of noise, which is a fan noise. In the forward hemisphere, beside the discrete components at fan blade frequency, long row of discrete components has been observed around the principal blade frequencies as 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 dynamical impact spectrum of power plant, transmitted through mounting to the airframe construction. An airframe typically possesses dozens of oscillation modes in the low-frequency spectrum part. Interaction of some of them with the influence of power plant may generation of discrete low-frequency high-level noise components in cabin. We have designed a new low frequency attachment containing built-in elastic elements with non-linear characteristics and with quasi-zero stiffness zone at proof load by cruise mode.