Can You Treat Composite Beams or Wings Mathematically like Isotropic Wings?

August 28, 2008 by aeroway

The demand for composite materials is growing each day. This demand stems from the mechanical properties of taking two or more materials and combining them to form an enhanced material exhibiting properties all constituents. In the aerospace industry, composites are widely used for control surfaces such as ailerons, flaps, stabilizers, rudders, as well as rotary props, and fixed wings. These fibrous composites can provide a high strength for control surfaces at a fraction of the weight compared to homogeneous metallic materials. The versatility in the design of fibrous composites is notably important. The stiffness of a particular composite beam can be altered simply by adjusting the fiber orientation and stacking sequence.
Aerospace designs are rigorously optimized for weight more so than most disciplines of engineering. With this in mind, the industry has been drawn towards the attractive characteristics of composite materials. Composites provide many advantages over homogenous type materials some of which are found in their mechanical properties; such as weight, strength, stiffness, corrosion resistance, fatigue life, over homogeneous metallic materials.
Although the overhead manufacturing costs of composites is relatively high compared to their metallic counterparts, the cost savings in the future, particularly for the aerospace industry, can be staggering. The savings in fuel cost could possibly offset the increase in manufacturing costs.
Aeroelastic flutter is one of the most critical safety concerns in aerospace designs. The design and optimization of aerodynamic structures requires a strong emphasis on aeroelastic tailoring, so that the predicted instabilities are avoided. Aeroelasticity is the study of the mutual interaction between fluid flow and structure, which enters various disciplines including aerodynamic flow, solid mechanics and vibrations.
There exist many different types of flutter that is, bending torsion flutter, binary flutter, stall flutter, panel flutter and single degree of freedom flutter. For simplicity bending-torsion flutter will be considered and other types of flutter will only be considered if time permits. The effective beam rigidities namely bending, torsion and coupled bending-torsion rigidities are used to model such wings. These effective rigidities calculated in detail in http://www.compositecalculator.com
Generally a wing can undergo bending, torsion and coupled bending-torsion displacements, if there is an offset of the mass axis (CG) from the elastic axis (EA). For composite wings, another coupling between bending and torsion arises. A material coupling is formed from the anisotropy of composite material. A bending-torsion material coupling is created from symmetric laminates, or in the case of thin-walled box-wing structures, Circumferentially Asymmetric Stiffness (CAS) configurations (Armanios et al, 1995). This coupling between bending and torsion, whether it’s from the physical geometry of the wing or a result of material coupling, can experience this flutter phenomenon. Bending-torsion flutter is a pitch-plunge instability generated at a specific applied dynamic pressure.

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