AbstractThis paper investigates the problem of obtaining a state-space model of the disturbance evolution that precedes turbulent flow and the associated increase in skin-friction drag on aircraft surfaces. This problem is highly challenging since the flow system is governed by nonlinear, partial differential-algebraic equations (the Navier-Stokes equations) for which there currently exists no efficient controller/estimator synthesis techniques. In this paper it is shown how a sequence of model approximations can be employed to yield a linear, low-order state-space model, to which the standard tools of control theory can be applied. One of the novelties of this paper is the application of a numerical routine that converts a system of differential-algebraic equations into one of ordinary differential equations. This enables straightforward satisfaction of boundary conditions whilst dispensing with the need for parallel flow approximations and velocity-vorticity transformations. The efficacy of the model is demonstrated by the synthesis of a Kalman filter that clearly reconstructs the characteristic features of the flow, using only wall velocity gradient (shear) measurements obtained from a high-fidelity nonlinear simulation.