Iterative controller design for planar Poiseuille flow by model unfalsification and controller redesign is the topic of the paper. The main contribution is to show that model-unfalsification-based iterative design can be useful in flow control problems. The a priori knowledge on the dynamics of the sampled system is obtained from the analytic approximation of the Navier-Stokes equations by a Galerkin method. Pole-positions, expected model orders and feasible dynamic variations are valuable prior knowledge which can be taken into account in the uncertainty-model unfalsification-based iterative design scheme presented.