Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / The helicopter is a prime example of a nonlinear multi-body dynamic system that is subjected to
numerous forces and motions to which the system must react. When a helicopter, with a
conventionally articulated rotor head, is resting on the ground with its rotor spinning, a condition
called ground resonance can develop. Ground resonance is a specific self-excited oscillation of
the helicopter and is caused by the interaction between the main rotor blades and the fuselage
structure. Inertia forces of the blades perform an out-of-phase lagging motion, which reacts with
the elastic landing gear of the helicopter. For certain values of the main rotor angular velocity, the
frequency of these inertia forces coincides with a natural vibration frequency of the fuselage
structure. If this occurs, the inertia forces of the lagging blades produce oscillations of the
fuselage, which then further excite the lagging motion of the blades. This interaction is
responsible for an instability of conventionally articulated main rotor helicopters, which is called
ground resonance.
The ground resonance phenomenon is investigated by means of a classical analytical approach
in which the ground resonance equations are derived from Euler-Bernoulli beam theory and
verified with results in literature. These equations are required to discuss ground resonance
stability in further detail and determine the specific regions in which the phenomenon occurs.
These results are incorporated in a simplified numerical model using an elastic multiple-body
dynamics analysis program called DYMORE to simulate the South African Rooivalk Combat
Support Helicopter. DYMORE is a program that offers nonlinear multi-body dynamic analysis
code, using the finite element method, which was specifically developed for helicopter modelling.
The complexity of helicopter modelling generally requires large amounts of computing power to
ensure reasonable processing time. In order to prevent excessive computational time, the
numerical model will be simplified in terms of aerodynamic and structural aspects. The scope of
the numerical investigation is, therefore, limited to the ground resonance phenomenon without the
effect of aerodynamic forces and representing the fuselage as multi-body beam structures of
specified stiffness.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2522 |
| Date | 03 1900 |
| Creators | Eckert, Bernd |
| Contributors | Van Niekerk, J. L., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 4137428 bytes, application/pdf |
| Rights | University of Stellenbosch |
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