Abstract
The mounting of the auxiliary power unit (APU) and gear box in Aircraft/Helicopter is discussed in the paper. The APU is usually connected to a Gear box to drive other components. The APU is exposed to shock and vibration environments. The shock and vibration concerns dealing with the design of a stiff support system to achieve target frequency. A frequency margin criterion is selected to ensure fundamental system frequencies are at least 15% away from dominant vibration inputs such as harmonics of helicopter blade passing frequency. A lumped parameter dynamic model is formulated to perform the modal analysis and 1g load static analysis. Modal analysis will help to identify the mounting frequencies, while static analysis will help to identify distribution of the load at each mount, driving the design of supports and identifying design equivalent loads for shock and vibration input. A detailed finite element analysis was performed to validate the lumped mass finite element model. The lumped mass model approach helps in quick turnaround in the preliminary phase of the design, to optimize the mount locations and identify required stiffnesses. The connection stiffness between the APU and Gear box was found to be the controlling stiffness for the mounted frequencies. Hence a short-term (compromise) solution and long-term (lower risk) approach were provided to achieve the target frequency.
