Flexible Matrix Composite Skins for One-Dimensional Wing Morphing

Abstract

Morphing aircraft wings require flexible skins that can undergo large strains, have low in-plane stiffness and very high out-of-plane flexu ral bending stiffness. The large strain capability is especially important for gross morphi ng applications such as span change where the skins may be required to undergo axial st rains of the order of 50% or greater. Low in-plane stiffness allows morphing to be accomplished at a reasonable energy cost while high bending stiffness ensures that skin sections b etween supports do not suffer from significant out-of-plane deformation under aerodynamic pressure loads. For some morphing applications (for example, wing span-, chord-, or camber-change), the required deformation is mostly one-dimensional. In such a c ase, a Flexible Matrix Composite (FMC) skin is proposed as a possible solution. A FMC comprises of stiff fibers (for example fiberglass) embedded in a soft high-strain capable matrix material (for example, silicone). The idea is to align the matrix-dominated direction along the morphing direction. This allows the skin to undergo large strain at low ener gy cost. However, the high-stiffness in the fiber-dominated direction, along with applied tensi on along the fiber-dominated direction is critical in providing the membrane skin with a larg e out-of-plane stiffness and consequently, the ability to withstand aerodynamic pressure loads. An analysis for a FMC skin panel under in-plane axial loads and out-of-plane pressur e loads is developed, and this is validated against experiment. The analysis is then used to c onduct design studies. A comparison of the FMC skin to a skin comprising of just the matri x material illustrates the importance of the fiber's stiffness in resisting out-of-plane def ormation under pressure loading. The influence of the matrix and fiber properties and ap plied tension on the out-of-plane deformations and morphing capability is examined in detail.