| Summary: | Identification of modal parameters is crucial especially in aerospace applications
whereby the interactions of airflow with aircraft structures can result in undesirable
structural deformations. This structural deformation can be predicted with knowledge of
the modal parameters. This can be achieved through conventional modal testing that
requires a known excitation force in order to extract these dynamic properties. One
particular challenge using this technique is that it can be experimentally complex because
of the need for artificial excitation and it also does not represent actual operational
condition likewise external disturbance loads, e.g instruments and surrounding noise,
aerodynamic loads and turbulence. Therefore, a good understanding of the dynamic
properties and application of the technique to implement this experimental work needs to
be generated.
Basic approach is to conduct Ground Vibration Testing (GVT) and use the modal data to
determine the flutter onset. The unknown sources or dynamic excitation can be difficult
to be applied using the conventional testing. As the industry has expanded with
implementation of modal testing, there is a need to have different technique applied
which convenient and easy to use. So this is where technique such as Operational Modal
Analysis has its application in the field. Not just that, the opportunities of using hybrid
composite material can be an attractive prospect for aerospace application.
This study utilize Operational Modal Analysis (OMA) on extracting the modal
properties. Operational Modal Analysis acquires information about the dynamic
characteristics of a structure in terms of natural frequencies, damping and mode shapes
without the need for explicit measurement of input vibration inducing loads. This
technique is yet to be applied on composite structures in the subsonic range within a
wind tunnel environment. Therefore in the current work, it was implemented and
demonstrated on a cantilevered hybrid composite plate and wing model exposed to low
speed airflow in a wind tunnel. To do so, a single contactless sensing system via a laser
vibrometer as well as an accelerometer as reference was employed to measure the
vibration response in subsonic speed. Experimental Modal Analysis (EMA) and computational analysis using Finite Element Analysis (FEA) were also conducted as
baseline reference.
Results from the extensive experimental works had successfully shown that OMA can be
implemented in subsonic range on both models and provide modal data with some level
of accuracy. From the experiment testing, the testing technique and data handling as well
as the aerodynamics effects from the airflow play a major role in affecting the modal
parameters. For composite thin plate model, the structure is too light which may affects
the modal data. As for wing model which has a stiffer structure, the mode shape for some
modes were classified as unidentified as these modes displayed combination of bending
and torsional modes, which may indicate coupling of these two modes due to aeroelastic
effects. Due to the same reason, the frequencies of the modes extracted are found either
decrease or increase with air speed especially on merging modes that will lead to flutter.
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