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Session: Movables
Session starts: Wednesday 19 June, 16:00
Presentation starts: 16:00
Room: Room 1.1

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Carlos Sebastia Saez (Technical University of Munich)
Mirko Hornung (Technical University of Munich)

Abstract:
The potential based Doublet Lattice Method (DLM) is a state-of-the-art tool to model unsteady aerodynamic loads in the flutter analysis. However, it does not account for viscous or non-linear effects. In the subsonic regime, thickness and boundary layer effects may only play a minor role in aeroelastic analysis. This is no longer the case when oscillating control surfaces are involved. In these cases, DLM overpredicts the efficiency of the control surface, as it neglects the effects of an increasing boundary layer thickness and overpredicts the pressure distribution over the control surface. The unsteady flap hinge moment can deviate as much as 20% between theoretical and experimental results. Correcting DLM with higher fidelity methods or wind tunnel experiments can increase the accuracy of the flutter prediction if control surfaces are involved. Most of the related studies are performed on a NACA0012 airfoil. The uncertainties regarding the effect of an oscillating natural transition location or a laminar separation bubble on the aerodynamic efficiency of a sailplane Natural Laminar Flow (NLF) airfoil due to oscillating control surfaces and the resulting hinge moments are still considerable. The goal of this investigation is to reduce uncertainties in the aerodynamic behavior of a modern NLF airfoil due to flap oscillations and correct the pressure distributions predicted by lower-fidelity methods. Wind tunnel experiments at low turbulence intensities and CFD simulations with the Gamma transition model have been conducted to investigate the impact of an unsteady transition on the pressure distribution. The results are compared to DLM predictions and are used to correct the DLM pressure distribution. The chosen correction method is based on a post-multiplication of the aerodynamics influence coefficient matrix by a correction matrix, formulated so that the pressure loads predicted by DLM are equal to the ones of the higher fidelity method. The analysis of the pressure distributions shows good agreement between the experimental and CFD results. The pressure magnitude predicted by DLM shows the most significant deviations near the leading edge and the hinge line, demonstrating the need to correct the DLM results. The impact on the flutter results is analyzed with Nastran.