Wing Flutter Analysis Using Computational Fluid-Structure Interaction Dynamics

Wing flutter plays a significant role in the performance and life of lifting surfaces such as aircraft wings. It is an instability that causes the wing to no longer be capable of damping out random vibration, and it occurs at the point called the critical speed. Currently, the determination of this critical speed poses a large challenge for aircraft designers, as there is no method that can quickly calculate the conditions that will cause the wing flutter instability. This paper presents wing flutter analyses using computational fluid-structure interaction dynamics. The computed results reveal the potential speed and accuracy of the computational method, which will allow designers to rapidly determine whether their vehicle will be capable of operating safely within its design envelope.

Higher Harmonic Control: A Historical Perspective

Higher harmonic control (HHC) is an approach for achieving reduced helicopter vibration by controlling the vibratory rotor airloads in such a way that the fuselage excitation is minimized. This paper is a historical look at how a program aimed at helicopter vibration reduction started as an outgrowth of fixed wing flutter suppression at NASA Langley Research Center, proved the HHC concept on aeroelastically scaled wind tunnel models and went on to demonstrate viability in full-scale flight testing on the OH-6A helicopter in 1982. Following the OH-6A flight tests, the helicopter research community was stimulated to prove the effectiveness of HHC on different configurations through analysis, wind tunnel tests, and flight tests. All of these investigations have shown HHC to be effective in reducing vibration to levels not attainable with conventional vibration control methods and without any detrimental side effects. HHC development has progressed to the point that the technology provides one more option to address the ever-present vibration problem in helicopters. The literature demonstrates that helicopter ride quality equivalent to that of fixed wing aircraft is attainable with application of HHC.

Multifidelity Sensitivity Study of Subsonic Wing Flutter for Hybrid Approaches in Aircraft Multidisciplinary Design and Optimisation

A comparative sensitivity study for the flutter instability of aircraft wings in subsonic flow is presented, using analytical models and numerical tools with different multidisciplinary approaches. The analyses build on previous elegant works and encompass parametric variations of aero-structural properties, quantifying their effect on the aeroelastic stability boundary. Differences in the multifidelity results are critically assessed from both theoretical and computational perspectives, in view of possible practical applications within airplane preliminary design and optimisation. A robust hybrid strategy is then recommended, wherein the flutter boundary is obtained using a higher-fidelity approach while the flutter sensitivity is computed adopting a lower-fidelity approach.

A semi-analytical approach for flutter analysis of a high-aspect-ratio wing

We present a hybrid, semi-analytical approach to perform an eigenvalue-based flutter analysis of an Unmanned Aerial Vehicle (UAV) wing. The wing has a modern design that integrates metal and composite structures. The stiffness and natural frequency of the wing are calculated using a Finite Element (FE) model. The modal parameters are extracted by applying a recursive technique to the Lanczos method in the FE model. Subsequently, the modal parameters are used to evaluate the flutter boundaries in an analytical model based on the p-method. Two-degree-of-freedom bending and torsional flutter equations derived using Lagrange’s principle are transformed into an eigenvalue problem. The eigenvalue framework is used to evaluate the stability characteristics of the wing under various flight conditions. An extension of this eigenvalue framework is applied to determine the stability boundaries and corresponding critical flutter parameters at a range of altitudes. The stability characteristics and critical flutter speeds are also evaluated through computational analysis of a reduced-order model of the wing in NX Nastran using the k- and pk-methods. The results of the analytical and computational methods are found to show good agreement with each other. A parametric study is also carried out to analyse the effects of the structural member thickness on the wing flutter speeds. The results suggest that changing the spar thickness contributes most significantly to the flutter speeds, whereas increasing the rib thickness decreases the flutter speed at high thickness values.