16-11-2012, 02:12 PM
Aircraft Engine Attachment and Vibration Control
Aircraft Engine Attachment.pdf (Size: 477.94 KB / Downloads: 58)
ABSTRACT
Controlling the vibration and internal cabin noise levels of fixed wing aircraft has long been a challenge and never-ending trade off of system performance variables. A presentation of the fundamental aspects of vibration and how it relates to fixed-wing aircraft engine attachment is made. Available technologies related to engine vibration treatments are presented with a preferred design approach.
INTRODUCTION AND STATEMENT OF THE PROBLEM
The intent of this paper is to provide the reader with a fundamental background to the engine vibration/noise problem in modern aircraft and present the available solutions that can be used to treat the engine vibration problem. Additionally, a design approach that provides technology options to the aircraft OEM throughout the design and flight test phases of the program is outlined.
All mounting systems need to accomplish two basic functions: 1) constrain motion, and 2) provide vibration isolation and noise reduction. “Constraining Motion” refers to limiting the relative motion between two structures created by thrust, ‘g’ loads, weight, and torque. “Providing isolation” and “reducing noise” involves minimizing the transmission of vibration from one structure to another so as to reduce the transmitted noise into the cabin area.
To provide the first basic function, the mounting system must be stiff to minimize relative motions. In order to minimize transmitted vibration (or noise), the mounting system must be dynamically soft (Reference 1). This inherent problem sets up competing objectives that require compromise and flexibility in the engine attachment design. This basic issue, along with the need for longer service lives and reduced costs, is the reason for new technology development.
ENGINE VIBRATION SOURCE
In an aircraft engine installation, an imbalance in the rotating machinery creates oscillating forces applied to the structure, thus generating structural vibration, as depicted in Figure 1. The consequence of the rotating imbalances of the engines manifests itself through the structural vibration of the fuselage, which induces noise in the cabin as shown.
NOISE
The noise at frequencies related to engine vibrations is usually produced at levels much higher than noise produced by sources such as external airflow, air conditioning, or accessories. These sources generally create the broadband noise levels, whereas the engine rotating imbalance creates specific tones of their fundamental frequencies and harmonics. Figure 3 below, shows a typical noise spectrum of the measured data of the internal cabin noise spectrum. Clearly shown are the tonal penetrations of the engine vibrations. This higher noise, produced by the engine vibration through the structure, presents the most likely need for isolators in an attachment system.
TRANSMISSIBILITY AND EFFECTIVENESS
Transmissibility is a common term used when discussing vibrating systems, but is more correctly used with rigid masses and foundations that do not exist on aircraft.
Effectiveness is a comparison of an attachment system’s performance with an ‘isolator’ to that of a hard mount, and is a more appropriate term to describe the vibration isolation (or noise reduction) realized.
Figure 4 is a representative curve of effectiveness. The figure plots the ratio of mount stiffness (Km) and structure stiffness (Ks) to the amount of noise or vibration reduction realized. This shows that as the mount stiffness decreases relative to the structure, greater isolation performance is realized. As the figure shows, passive systems generally provide up to 10 dB reductions and active systems are effective enough to produce 25 dB reductions.