Shape memory wires contract when they are converted to the austenitic phase upon heating and they expand when they become martensitic upon cooling. Thus an electric current will make the wire contract, because of Joule heating, and cooling by the surrounding air will make it expand, when the current is switched off. This phenomenon be used for adapting the shape of an air foil to the extant flight conditions. A model is shown in the figure. The model consist of a "slice" of an airfoil which bends so as to minimize the drag of the airfoil under the constraint of a fixed lift force.

straight wing	bent wing

The optimal shape of the air foil for given angles of incidence and different speeds of the incoming air is stored in a data bank calculated from aerodynamics. The necessary current for the realization of the shape is calculated from the Müller-Achenbach theory of shape memory which was recently adapted by Seelecke for quantitative predictions.

Basically that theory is a statistical thermodynamic theory of activated processes. The theory combines rate laws for the phase fractions with the energy balance that governs the temperature of the wires.

The suitability of the theory for quantitative simulation of shape memory behaviour is shown by a feedback control problem which exhibits good agreement between experiment and the prediction of the theory.