Sung Nam Jung

Aeroelastic response of composite rotor blades considering transverse shear and structural damping

The effects of transverse shear deformations and structural damping on the flutter phenomena of a composite rotor blade in hover have been investigated using the finite element method. First-order shear deformation theory with rotary inertia effects and a damped element model of composite laminates are employed for the structural formulation. A quasisteady aerodynamic theory with a dynamic inflow model is used. Torsion-related out-of-plane warping and noncirculatory aerodynamic components are also incorporated in the formulation. Using these structural and aerodynamic tools, several numerical studies are carried out, first to validate the current approach and second to show the effects of transverse shear deformations and structural damping on the aeroelastic stability of a composite rotor as a function of fiber orientation. The predictions derived by the frequency analysis of this model are found to be more accurate than those given by an alternative approach compared with experimental data. It is shown that the transverse shear flexibility tends to lower the frequency of the rotor, and generally has a destabilizing effect on the lag mode and a stabilizing effect on the flap mode. It is also presented that the magnitude of structural damping can be controlled by changing ply orientation angle. CT cd c, EA

Forward Flight Stability Characteristics for Composite Hingeless Rotors with Transverse Shear Deformation

The shaft-fixed aeroelastic stability behavior of a soft-in-plane, composite hingeless rotor blade in hover and in forward flight has been investigated by using the finite element method in both space and time. The non-classical structural effects, such as anisotropy, transverse shear, and torsion warping, are incorporated in the structural formulation. Timoshenko-type shear correction coefficients, which take into account the bending-shear and extension-shear couplings, are introduced to consider the nonuniform distribution of shear across the section of the blade. The aerodynamic model in the current aeroelastic analysis is formulated to allow either quasi-steady or unsteady two-dimensional aerodynamics. The Leishman-Beddoes model based on an indicial response method is employed to consider the unsteady aerodynamic effects. The effects of compressibility and reversed flow are also incorporated. Finite element equations of motion undergoing moderately large displacements and rotations are derived using the Hamiltons principle. Numerical simulations are carried out to validate the current analysis with other literature. The influence of composite couplings, transverse shear deformation, and unsteady aerodynamics on the aeroelastic behavior of soft-in-plane helicopter blades is investigated. Numerical results illustrating a great potential to improve the aeroelastic stability are presented for a blade with positive tension-pitch coupling. The transverse shear deformation is seen to have a stabilizing effect on the lag mode stability for cases with elastic couplings. Overall, the transverse shear effects become larger at higher forward speeds. It is also seen that the lag mode stability is significantly influenced by the unsteady aerodynamic effects.

Shear Correction Factors for Thin-Walled Composite Boxbeam Considering Nonclassical Behaviors

In this study, shear correction factor for thin-walled composite boxbeam is investigated with considering the anisotropic coupling effects in its laminated walls parallel to the external shear load. We first consider a generally orthotropic laminated beam whose cross section is rectangular. The beam does in-plane shear deformation by the applied shear force, so that there arises the effect of extension-shear coupling which does not arise in the conventionally studied out-of-plane shear deformation. The shear correction factor is defined by the energy considerations with taking into account the coupling effect. Then the procedures are extended to the case of laminated thin-walled boxbeam, where the coupling plays a significant role due to the geometric characteristics of boxbeam. The shear correction factors for the representative boxbeams which are the core structural components of modern composite helicopter rotor blade are presented with the change of their laminate sequences.

Effect of transverse shear on aeroelastic stability of a composite rotor blade

A formulation, which considers the effects of transverse shear flexibility, torsion warping, and two dimensional in-plane behavior, is extended to analyze arbitrary lay-up geometry including antisymmetric configuration. Numerical simulations are performed for a specific antisymmetric configuration to identify the transverse shear behavior on the aeroelastic stability of composite rotor

Development of Smart Missile Fins with Active Spoiler

For conventional missiles, electric or hydraulic actuators are mounted inside the missile fuselage to activate the aerodynamic control surfaces. These internally mounted actuators occupy considerable volume which otherwise can be used for payload or additional fuel. Reducing the size of the internal actuators and hence lowering the total actuator weight may improve the overall performance of missile significantly. The goal of this research is to develop a light-weight, low cost smart missile fin capable of surviving the supersonic operating environment while providing necessary performance comparable to existing missile fins. In this study, in an alternative to facilitate realistic design concepts and to generate enough actuation forces required for missile controls, smart fins with trailing-edge-mounted retractable wedge are considered. The retractable wedge stretches in or out appropriately to decrease the applied pitching moment of the missile fin. For the theoretical calculation, a commercial CFD software package is used to obtain the forces and the moments generated by the fin with retractable wedge. The piezoelectric actuation mechanism that is applicable to the wedge type actuator is also investigated.

Development of Program for Modeling of Cross Section of Composite Rotor Blade

Generally, modeling procedure of cross section of composite rotor blade is complicated and time-consuming, because it is made up of various stiffeners and multiple layers of composite materials. For efficient modeling of cross section of composite rotor blade, a modeling program so called KSec2D, which provides a user friendly GUI, is developed by using a 2D modeling algorithm based on set operation. By the developed program KSec2D, a modeling of complicated cross section of rotor blade is carried out. Through the demonstration, the usefulness of developed program in modeling procedure of cross section of composite rotor blade is verified.

Transverse shear behavior on the aeroelastic stability of composite rotor blades

The aeroelastic stability of a composite hingeless rotor blade, idealized as a laminated thin-walled box-beam, has been investigated using a finite element formulation based on Hamilton’s principle. First-order shear deformation theory and quasi-steady aerodynamic theory have been employed for the analysis. In order to consider the sectional distribution of shear stresses for the box-beam in an effective manner, Timoshenko beam assumption has been made and the formula of shear correction factor of isotropic box section has been used to describe the motion. Three dimensional stress analysis of composite box-beam by using a detailed finite element analysis program has been performed to identify the distribution of shear of the box section and to correlate the results of shear correction factor of isotropic material with that of composite material. Free vibration tests of rotating composite box-beams have showed fairly good agreement between the current results and the experimental data. Transverse shear behavior on the aeroelastic stability has been studied for a specific box-beam configuration. The results displayed in this article have revealed that the transverse shear coupling has a significant role on the flutter boundary of the rotor, especially in case of anti-symmetric configuration.

Aeroelastic Stability Analysis of Hingeless Rotor Blades with Composite Flexures

The flap-lag-torsion coupled aeroelastic behavior of a hingeless rotor blade with composite flexures in hovering flight has been investigated by using the finite element method. The quasi-steady strip theory with dynamic inflow effects is used to obtain the aerodynamic loads acting on the blade. The governing differential equations of motion undergoing moderately large displacements and rotations are derived using the Hamilton’s principle. The flexures used in the present model are composed of two composite plates which are rigidly attached together. The lead-lag flexure is located inboard of the flap flexure. A mixed warping model that combines the St. Venant torsion and the Vlasov torsion is developed to describe the twist behavior of the composite flexure. Numerical simulations are carried out to correlate the present results with experimental test data and also to identify the effects of structural couplings of the composite flexures on the aeroelastic stability of the blade. The prediction results agree well with other experimental data. The effects of elastic coupling such as pitch-flap, pitch-lag, and flap-lag couplings on the stability behavior of the composite blades are also investigated.