Étude expérimentale et numérique de l'interaction aérodynamique entre deux profils : application au risque aéronautique du décrochage profond

摘要

Deep stall is a specific type of airplane stall, in which the horizontal tail, driving the pitching moment, is inside the detached wake of the main wing. The tail loses its efficiency, leading to a stable pitching equilibrium position with a high angle-of-attack, without any easy recovery procedure. The aim of the study is to characterize the aerodynamic associated to that phenomenon in order to propose an identification and recovery procedure. A bibliographical analysis of deep stall flight dynamics and of airfoil interaction is provided. This defines the approach consisting in a two-dimensional flow characterization based on an aeronautical reference configuration. The experimental set-up, developed especially for that study, consists in two NACA 23 012 airfoils, with adjustable angles-of-attack and spacing. The instrumentation is made of strain gauges balances, for forces measurements, and a particle image velocimetry system which is not time-resolved, for flow measurements. Aerodynamic coefficients, obtained for a wide range of angles-of-attack, show the interaction between the airfoils on the stall of the downstream airfoil. The analysis of velocity fields gives the width and the axial development of the airfoils wakes with angle-of-attack and brings to a parametric study of configurations where a downstream potential effect is felt on the upstream airfoil. Phase-averages of velocity fields lead to the synthesis of flow time-development. It is obtained from the localization of the centers of Von Kármán vortices, shed from the upstream airfoil, for detached flow configurations. The Strouhal number built on the shedding frequency shows a good accordance with the bibliographical data available. With these results, a potential model of flow forcing on the downstream airfoil, by Von Kármán vortices shedding from the upstream airfoil, explains the lift coefficient alteration imposed by the interaction, for a moderate angle-of-attack of 15°. However, that model is invalidated for a larger value of angle-of-attack of 30°. Flow numerical simulations, giving time-resolved fields, provide experimental developments for the angles-of-attack 0° and 15°, but meaningful discrepancies appear for 30°, which could be explained by a three-dimensional flow structure. The whole set of results is used, concurrently with real aircraft data, inside a longitudinal flight model in order to analyze the airplane dynamical behavior. The simulations evidence an important default of the damping ratio of the short period mode during the installation of deep stall stable equilibrium. Criteria for the identification of the dynamic leading to that equilibrium provide a rapid detection of deep stall and the implementation of a recovery strategy.