Compliant Airfoils Research Articles

Load alleviation of feather-inspired compliant airfoils for instantaneous flow control

Birds morph their wing shape to adjust to changing environments through muscle-activated morphing of the skeletal structure and passive morphing of the flexible skin and feathers. The role of feather morphing has not been well studied and its impact on aerodynamics is largely unknown. Here we investigate the aero-structural response of a flexible airfoil, designed with biologically accurate structural and material data from feathers, and compared the results to an equivalent rigid airfoil. Two coupled aero-structural models are developed and validated to simulate the response of a bioinspired flexible airfoil across a range of aerodynamic flight conditions. We found that the bioinspired flexible airfoil maintained lift at Reynolds numbers below 1.5 × 105, within the avian flight regime, performing similarly to its rigid counterpart. At greater Reynolds numbers, the flexible airfoil alleviated the lift force and experienced trailing edge tip displacement. Principal component analysis identified that the Reynolds number dominated this passive shape change which induced a decambering effect, although the angle of attack was found to effect the location of maximum camber. These results imply that birds or aircraft that have tailored chordwise flexible wings will respond like rigid wings while operating at low speeds, but will passively unload large lift forces while operating at high speeds.

Aerodynamic-driven topology optimization of compliant airfoils

A strategy for density-based topology optimization of fluid-structure interaction problems is proposed that deals with some shortcomings associated to non stiffness-based design. The goal is to improve the passive aerodynamic shape adaptation of highly compliant airfoils at multiple operating points. A two-step solution process is proposed that decouples global aeroelastic performance goals from the search of a solid-void topology on the structure. In the first step, a reference fully coupled fluid-structure problem is solved without explicitly penalizing non-discreteness in the resulting topology. A regularization step is then performed that solves an inverse design problem, akin to those in compliant mechanism design, which produces a discrete-topology structure with the same response to the fluid loads. Simulations are carried out with the multi-physics suite SU2, which includes Reynolds-averaged Navier-Stokes modeling of the fluid and hyper-elastic material behavior of the geometrically nonlinear structure. Gradient-based optimization is used with the exterior penalty method and a large-scale quasi-Newton unconstrained optimizer. Coupled aerostructural sensitivities are obtained via an algorithmic differentiation based coupled discrete adjoint solver. Numerical examples on a compliant aerofoil with performance objectives at two Mach numbers are presented.

Experimental and Numerical Investigations on Nonlinear Aeroelasticity of Forward-Swept, Compliant Wings

At the beginning of aviation history, aeroelastic static instabilities represented a problem in operating monoplane aircraft. After being discovered, they were systematically avoided, since they would have led to large deformations and structural failure. A new idea (active aeroelasticity) reverts this approach and utilizes static instabilities to realize wing morphing instead of avoiding them. Another innovative idea (compliant systems) deals with structures designed to achieve large deformations within the elastic range of the material. Joining those two ideas leads to a novel class of airfoil structures (active aeroelastic, compliant airfoils) which enable operation at and beyond aeroelastic instabilities. Such structures need a new modeling approach, which includes nonlinearities of structural and aerodynamic kinds. In this paper, a non linear analysis of aeroelastic bending divergence (a phenomenon which concerns forward-swept wings) is presented, initially based on so-called low-fidelity models. Such models are, to some extent, inaccurate but allow a good insight into the physical behavior of the phenomenon and are very useful in preliminary design. The results of wind-tunnel tests follow, which were performed to investigate the aeroelastic response of a compliant airfoil model near divergence. Finally, high fidelity simulation results based on state-of-the-art methods (finite element method and fluid-structure-interaction) are shown and discussed. Those tools allow the prediction of the system response more accurately and are therefore well suited to the detailed design of active aeroelastic, compliant airfoils.

Camber effects in the dynamic aeroelasticity of compliant airfoils

This paper numerically investigates the effect of chordwise flexibility on the dynamic stability of compliant airfoils. A classical two-dimensional aeroelastic model is expanded with an additional degree of freedom to capture time-varying camber deformations, defined by a parabolic bending profile of the mean aerodynamic chord. Aerodynamic forces are obtained from unsteady thin airfoil theory and the corresponding compliant-airfoil inertia and stiffness from finite-element analysis. V– g and state-space stability methods have been implemented in order to compute flutter speeds. The study looks at physical realizations with an increasing number of degrees of freedom, starting with a camber-alone system. It is shown that single camber leads to flutter, which occurs at a constant reduced frequency and is due to the lock in between the shed wake and the camber motion. The different combinations of camber deformations with pitch and plunge motions are also studied, including parametric analyses of their aeroelastic stability characteristics. A number of situations are identified in which the flutter boundary of the compliant airfoil exhibits a significant dip with respect to the rigid airfoil models. These results can be used as a first estimation of the aeroelastic stability boundaries of membrane-wing micro air vehicles.

Nonlinear aeroelastic behavior of compliant airfoils

Since the beginning of aviation and up to the present time, airfoils have always been builtas rigid structures. They are designed to fly under their divergence speed in order to avoidstatic aeroelastic instabilities and the resulting large deformations, which are notcompatible with the typically low compliance of such airfoils. In recent years, research onairfoil morphing has generated interest in innovative ideas like the use of compliantsystems, i.e. systems built to allow for large deformations without failure, in airfoilconstruction. Such systems can operate in the neighborhood of divergence and takeadvantage of large aeroelastic servo-effects. This, in turn, allows compact, advancedactuators to control the airfoil’s deformation and loads, and hence complement oreven replace conventional flaps. In order to analyze and design such compliant,active aeroelastic structures a nonlinear approach to static aeroelasticity is needed,which takes into account the effect of large deformations on aerodynamics andstructure. Such an analytical approach is presented in this paper and applied to acompliant passive airfoil as the preliminary step in the realization of a piezoelectricallydriven, active aeroelastic airfoil. Wind tunnel test results are also presented andcompared with the analytic prediction. The good agreement and the observedbehavior in the wind tunnel give confidence in the potential of this innovative idea.

Modal Synthesis of Flexible Mechanisms for Airfoil Shape Control

The use of `compliant' or flexible mechanisms constitutes a very promising option for the successful realization of shape-adaptable airfoil structures. Achieving large deformations by exploiting structural flexibility instead of employing conventional mechanisms with moveable parts offers several advantages: absence of backlash and wear, no need for lubrication, reduced noise, smooth geometry changes, a lighter design, and reduced manufacturing costs. As a counterpart, compliant systems are more complex to analyze and design due to their inherent coupled behavior. Established design procedures, developed in the framework of kinematics design, are of limited use for application to shape-adaptable structures due to differences in requirement priorities. This study presents a novel synthesis procedure for compliant shape-adaptable airfoils. Initially devised for the so-called belt-rib airfoils, the procedure can be extended to the general case of compliant airfoils with an external flexible skin (optionally reinforced by frames) and a system of internal stiffeners. After the description of the core synthesis algorithm, which is based on a modal approach, its integration in a global design procedure is discussed. Application to the case of a belt-rib airfoil is then considered. The compliant structure resulting from the procedure combines the characteristic qualities of a compliant system (smooth deformation pattern, absence of moveable parts, low weight) with the low load-dependence of kinematics which is typical of conventional mechanisms.