A propulsion and lift shafting system in the air cushion vehicle(ACV) is a flexible multi-elements system which consists of two aeroderivative gas turbines with own bevel gears, four stage lift fan reduction gear, two stage propulsion reduction gear, air propeller and lifting fans of large capacity.
In addition, the system includes the multi-branched shafting with multi-gas turbine engines and thin walled shaft with flexible coupling. Such a branched shafting system has very intricate vibrating characteristics. Especially, the thin walled shaft with flexible couplings can lower the torsional natural frequencies of shafting system to the extent that causes resonances in the range of operating revolution.
In the first part of this study, to evaluate vibrational characteristics some analytical methods for the propulsion and lift shafting system are studied and calculating equations are derived. Also torsional natural frequencies and mode shapes of the system are analyzed and the results are compared with potential sources of excitation.
The analysis, including natural frequencies and mode shapes, for five operation cases of the system is conducted using ANSYS code with a equivalent mass-elastic model. The results of torsional vibration analysis are compared with the results of the vibration test conducted during Sea Trial.
In the second part of this study, axial vibration analysis is conducted on a propulsion and lift shafting system for the air cushion vehicle using ANSYS code. The shafting system is totally flexible multi-elements system including wood composite material of air propeller, aluminum alloy of lift fan and thin walled shaft with flexible coupling.
The results of analysis contains the axial natural frequencies and mode shapes of the shafting system taking into account an equivalent mass-elastic model for shafting system as well as the three-dimensional models for propeller blade and fan impeller. Such a flexible shafting system has very intricate vibrating characteristics. Especially, axial natural frequencies of flexible components such as propeller blade and impeller of lift fan can be lowered to the extent that causes a resonance in the range of operating revolution. The results for axial vibration analysis are presented and compared with the ones of axial vibration test for lift fan conducted during Sea Trial.
In the third part of this study, for the study of lateral vibration on a propulsion and lift shafting system for an air cushion vehicle, lateral vibration analysis is conducted using ANSYS code. The analysis includes lateral natural frequencies, mode shapes and harmonic analysis of the shafting system taking into account a three-dimensional models for propulsion and lifting shaft system. In case of ACV the yawing and pitching rate of craft will be quite high. During yawing and pitching of craft, significant gyroscopic moment will be effected to the shafting and will generate high amplitude of lateral vibration. So, such a shafting system has very intricate lateral vibrating characteristics and natural frequencies of shafting must be avoided in the range of operating revolution. The control method of lateral vibration is also included in this study.
This thesis includes evaluation of excitation frequency source and analysis, measurement and control of torsional, axial and lateral vibration on a propulsion and lift shafting system for an air cushion vehicle.
It is considered that the performed study and the derived approach on the vibrational problems for ACV can be used for future development of ACV shafting system.