Aerodynamic and aeroelastic amplification in adaptive belt-rib airfoils

Abstract

The belt-rib concept for variable-camber airfoils was developed at DLR (German Aerospace Centre) in the framework of the Adaptive Wing project (ADIF). Based on the idea of exploiting distributed structural flexibility – instead of using articulated mechanisms – in order to produce the required large geometry changes, the belt-rib concept implements a new design philosophy for light shape-adaptable structures in which the basic requirements of large allowable geometrical changes, high load-carrying capability and low weight are explicitly taken into account. Among several options for the activation of a belt-rib airfoil, the use of multifunctional materials like piezoceramics or shape memory alloys is a very attractive one, in particular as a consistent solution with the solid-state construction principle (virtual absence of moveable parts) of the structure. However, the high performance requirements (high actuator work, particularly in presence of high aerodynamic loading) are not likely to be met by multifunctional materials which are available to date. A very promising solution consists in exploiting aerodynamic and aeroelastic amplification effects in order to reduce the energy requirements of the actuator system. A study of such effects is presented in this paper, based on a modal theory of the airfoil mechanics. Due to its modal formulation, the theory can be applied to a conventional airfoil equipped with an additional control surface for camber adaptation as well as to a belt-rib airfoil, despite of the essential differences in the design philosophy. Aerodynamic amplification effects are described by means of an aerodynamic amplification factor, which is a function of the airfoil kinematics and of the aerodynamic load distribution. Quantitative results are presented for the case of the ADIF benchmark structure, the AIRBUS A340 landing flap. Further, aeroelastic amplification effects are analysed and an aeroelastic amplification factor is introduced as a function of properly chosen structural and aerodynamic operators. Some final remarks about the role of this contribution in relationship to the state of the research as well as on perspectives opened by the exploitation of the described amplification effects close the paper.