Semi-active flutter suppression using aerodynamic measures

www.luftfahrtforschungsprogramm.de

Objectives of the joint project:

SaFuMa is a joint research project with the overall objective of investigating various passive methods with regard to their potential for flutter suppression on high-aspect-ratio wings and fan rotor blades, and to better understand the underlying physical phenomena.

Passenger aircraft operate in the transonic regime. Although the cruise speed is well below the speed of sound, the acceleration of the airflow over the wing causes local areas of supersonic flow. To maximize aerodynamic efficiency, the wings of modern commercial aircraft have increasingly longer spans and aspect ratios. This reduces drag and thus fuel consumption, but the larger span of the wing also reduces its bending stiffness.

The transonic flow, together with the increasingly flexible wings, is subject to the impact of disturbances (increase in flight velocity, increase in angle of attack, gust impact) to trigger a critical condition known as flutter. The interaction of unsteady aerodynamic forces due to the vibrations of the aircraft wing in its elastic eigenmodes (motion-induced aerodynamic forces) with elastic forces and inertia forces can result in a self-excited oscillating system. Reinforcing the wing structure counteracts this effect but also leads to a higher weight. The same effect can occur with the rotor blades of modern turbofans with high bypass ratios, whose diameter and circumferential speeds are increasingly growing. The rotational symmetry of the blade arrangements is particularly interesting in this context, both for aerodynamics and for the structure.

For high-aspect-ratio wings and future generations of engines, it is therefore necessary to control the aeroelastic phenomena that occur in the critical region. The University of Stuttgart (project lead), the Technical University of Berlin, the Technical University of Munich, RWTH Aachen University, the University of the Bundeswehr Munich, and the German Aerospace Center are jointly investigating various measures, both numerically and experimentally, to make aviation more environmentally friendly and to expand the flight envelope of future commercial aircraft.

Research in this Joint Project:

Flutter is a dynamic aeroelastic phenomenon describing an interaction between unsteady aerodynamic forces, elastic forces, and inertia forces. A deep understanding of the aerodynamic and aeroelastic effects is required for both wing and fan rotor design. In experimental investigations in the wind tunnel, not only the flow variables but also the time-dependent deformations and deflections of the model must be determined. In addition, it must be ensured that the model under investigation is not influenced by scaling or wind tunnel effects. For time-resolved numerical investigations, a fluid solver (CFD) must be coupled with a structural model. Since this coupling greatly increases the complexity and computational effort, a linearized system analysis based on the aerodynamic response to a geometric deformation can be performed as an approximation. Each method is subject to its own assumptions and simplifications. The results of the different approaches are used for comparison and validation.

The University of Stuttgart, RWTH Aachen University, and TU Berlin are investigating the potential for shifting the flutter boundary by applying bumps to the surface of the wing or fan rotor. These local contour bumps on the upper surface of the wing enable to control the supersonic domain and thus also the pressure distribution and the pitching moment of the wing or fan rotor blade. This altered pressure distribution has a direct effect on the coupled fluid-structure system. Based on preliminary work carried out by the project partners on the effects of bumps, it can be assumed that this will shift the limits of the operating range to higher speeds and angles of attack. For the wing, the OAT15A airfoil (known for studies on the related phenomenon of buffet) and the DLR-F25 configuration are being considered in SaFuMa. NACA sections and the NASA Rotor 67 configuration are used for the fan rotor. Both RANS calculations and scale-resolving simulations are applied in the numerical investigations. Wind tunnel experiments are carried out in parallel to validate the results.

Together with the University of the Bundeswehr Munich, the Technical University of Munich is pursuing an approach to improve the flutter characteristics of the wing using control surfaces. Using deflections of spoiler elements and wing trailing edge flaps, it is also possible to influence the shock characteristics on the upper side of the wing. Similar to the effect of bumps, this should result in a wing geometry with a higher flutter limit than that of the reference wing. Both numerical simulations and wind tunnel experiments are being carried out.

The Institute of Aeroelasticity at the German Aerospace Center plays a central role in the field of flutter analysis. Together with the results from the other project partners, linearized analyses and modal structural models are used to determine the flutter boundary.

Several industrial partners support SaFuMa with their experience and their connection to industrial practice and are available for consultation.

Within the joint research project SaFuMa, the University of the Bundeswehr Munich (UniBwM) focuses on the experimental analysis of the effects of spoilers and control surfaces on flow-physical phenomena and the shift of the flutter boundary. Initially, the aerodynamic measures defined within the consortium are investigated on a rigid 2D model to determine optimal positions, sizes, and deflection angles that minimize the amplitude of shock oscillations under transonic flow conditions. The identification of efficient aerodynamic concepts is carried out in close collaboration with the project partners. The most promising configurations will subsequently be experimentally analyzed in the project’s main phase using a freely oscillating 2D wing model and advanced optical flow measurement techniques in the Trisonic Wind Tunnel Munich (TWM). By varying the Mach number and angle of attack, the influence of the aerodynamic measures on the flutter boundary will be quantified. Through controlled variation of the structural eigenfrequency, the coupling between fluid and structure can be adjusted, enabling the evaluation and comparison of the different aerodynamic concepts. To assess the transferability to realistic wings, selected spoiler and/or control surface configurations will also be investigated on a swept 3D wing to determine their potential for suppressing flutter tendencies.