Aerodynamic modelling of an active flow control system for flapless flight control in the preliminary design stages
For highly dynamic control manoeuvres blown circulation control aerofoils are able to provide the required control moments without any complex kinematics as conventional flaps would imply. Blowing over a rounded Coandă trailing edge entrains the baseline airflow around the aerofoil to the desired direction and modifies circulation strength. Basically this system acts aerodynamically similar to a conventional flap where the momentum vector is bended upwards or downwards. The scope of the research activities on this flapless flight control concept is primarily the applicability on low aspect ratio flying wing configurations which are, into the bargain, designed laterally unstable. Here, the current concept promises to be suitable also for yaw control when air flow momentum is controlled differentially on the wing half spans. The high mass flow demands during this control mode could be met during cruise flight when the engines are operating at high power settings. For substantive conclusions a global system model is essential including pressurised air sources, ducts and finally the control moment authorities by trailing edge blowing. The latter implies special challenges when computational efficiency and flexibility in terms of parameter variation is crucial for preliminary design stages. Even if vast amounts of fundamental wind tunnel experiments have been performed so far numerical modelling is still necessary to fill the gaps of missing systematically collected data. By means of wind tunnel data the control force and moment reactions can be predicted qualitatively but rarely quantitatively for an arbitrary application with specific geometry. For rapid and automated calculation of double-slotted circulation control aerofoils a tool has been implemented in Matlab. An automatic mesher generates a structured two-dimensional grid processing the baseline aerofoil coordinates, a desired Coandă radius and the given slot heights. The Reynold’s Averaged Navier-Stokes Equations incorporating the Menter SST turbulence model are solved to obtain the control force and moment reactions as a function of blowing rate. The extrapolation to a finite wing by extended lifting line theory results in an aerodynamic data set which enables the evaluation of the aerodynamic performance of a given flapless flight control system. After a short description of the modelling tool it is applied on a test case consisting of a low aspect ratio flying-wing configuration. A final performance assessment allows concluding statements about applicability and design drivers for the given test case.