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The aeroelastic
analysis of laminated composite wings is vital to the prevention of failures
induced by oscillatory motion. The aeroelastic instabilities will change, however,
when a crack has initiated in a wing structure and must be accounted for
by adjustment to the structural and dynamic model.
An aeroelastic normal mode
analysis greatly depends on the free vibration modes of the wing. To achieve
accurate results, a new Dynamic Finite Cracked Element (DFCE) (refer to Chapter
2) is implemented. From the previous chapters, both the DFE and DFCE both show
excellent accuracy for preliminary coarse meshes.
Composite wings consist of two
types of bending-torsion couplings: geometric and material. Geometric coupling
originates from an offset of the centre of gravity (CG) axis from the elastic
axis (EA), where material coupling arises from material anisotropy. Geometric or
material couplings can cause flutter instabilities in a wing. Wings modeled as
beam assemblies can produce various couplings.
Since the incentive of this
chapter is to study the aeroelastic flutter and divergence of a defective wing,
only bending-torsion couplings are considered. To achieve purely bending-torsion
behavior in a composite beam or wing structure, specific laminate stacking
sequence must be considered (i.e. symmetric or unidirectional unbalanced
laminates). The beams used in this chapter, to approximate a wing, are assumed
to be based on classical laminate theory with solid rectangular cross-section
and unidirectional plies.
In Figure 5 (refer to http://www.aeroway.ca/CR1.html), much like the divergence speed plot, the
flutter speeds tend to be sensitive to ply angle and crack ratio. When the crack
ratio is gradually increased from no crack up a/b =0.6 a drop in flutter
speed is observed for most ply angles, except for the unidirectional plies set
in the region of 96 degrees to 146 degrees. In this region, a flip flop in this
trend is seen, where the flutter speed increases with a larger crack.
The influence of a static crack on a laminated composite wing
is significant in both the free vibration modes as shown previously in (http://www.aeroway.ca/CR1.html)
and on the flutter and divergence speeds observed in this blog.
A reduction in the
divergence speeds occurs when the crack size is increased for most
unidirectional ply angles considered. Whereas, the flutter speeds are observed
to increase in specific ranges of ply angles.
The V-g method has proven to be an
excellent method for the extraction of the flutter speeds and readily extended
to more complex formulations particularly ones that include unsteady flow.
AerospaceSteve
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