The term flutter represents one of the central phenomena in the field of aeroelasticity and describes a self-excited oscillation, which, in the worst case, can lead to a destruction of aircraft components and ultimately to a destruction of the aircraft itself. Especially in the transonic flow regime, the risk of flutter is high. Hence, the design process of an aircraft has to emphasize on preventing such critical conditions. Measures to control the flutter behavior are an important field of research in aeronautics. One possible approach is the use of spoilers.
While operating in cruise flight, most of today’s airliners encounter transonic flow conditions with velocities close to the speed of sound. When exceeding the critical Mach number, supersonic regions (Mach>1) terminating with a shock wave appear on the surface of the aircraft structures, for example on the wing. A sudden increase in pressure and a reduction in velocity indicates the shock wave. Dependent on the shock intensity, flow separation can also occur behind the shock. These aerodynamic effects are fundamental for the aeroelastic behavior of aircrafts: A sudden decrease in the flutter stability boundary and thus a reduction in the critical value of the flutter coefficient, above which flutter can occur, can be observed. This phenomenon is referred to as the transonic dip.
Spoiler deflection on an aircraft wing (Source: Wikipedia CC BY-SA 4.0)
Ensuring flutter stability is of great priority in the design process of an aircraft. Considering the transonic dip region, the question arises whether appropriate measures which influence the shock formation can increase the flutter stability boundary. Examples for such measures are technologies like suction and blowing in the boundary layer as well as morphing devices creating surface bumps. However, one disadvantage of these technologies is their constructional complexity. Another possible method is the use of spoilers. As an elementary wing component, they are used to increase the aerodynamic drag and to reduce lift. But, is it possible to exploit them as a flutter suppression device as well?
Computational grid around an airfoil with spoiler used for CFD simulations
The department of Aeroelastic Simulation investigates the potential of spoilers used as a technology for semi-active flutter suppression. This means that the spoiler is statically deflected at critical conditions, but then no further dynamic actuation is needed. To that aim, fundamental studies are conducted by means of airfoil models. Due to the complex aerodynamic effects in the transonic flow regime, high-fidelity CFD (computational fluid dynamics) methods have to be applied in order to resolve the unsteady aerodynamic loads acting in the transonic dip region. For this purpose, the flow field needs to be discretized by a fine grid such that in each of the grid cells the Reynolds-averaged Navier-Stokes equations (RANS equations) physically describing the aerodynamics are solved.
The investigations with the computational models show that even small spoiler deflection angles (angle ≤ 10°) have a great influence on the properties of the transonic flow field. Due to the spoiler deflection, the flow is decelerated on the surface of the airfoil and thus a shock formation may be prevented. Regarding the aeroelastic behavior, the flutter stability boundary is increased in the region of the transonic dip. However, the changes in the flutter stability boundary are very sensitive towards the spoiler deflection angle and an increase in the flutter coefficient is not achieved in every case. In order to ensure a robust behavior, the spoiler deflection needs to be scheduled depending on the Mach number. Based on the previous results, computational models for a three-dimensional wing will be considered in the next studies in order to further assess the potential of spoilers used as a semi-active flutter suppression device.
Author: Martin Schmalz, Department Aeroelastic Simulation, DLR-Institute of Aeroelasticity