Influence of aileron oscillation and gap size on aerodynamic control derivatives

Kruijk, T.R. (2015) Influence of aileron oscillation and gap size on aerodynamic control derivatives.

Abstract:In the broad scope, this project is about the control of unmanned aircraft when applying a harmonic oscillation to the ailerons. It concerned a phenomenon that arises when oscillations are applied to flaps in airfoils. More specifically, it focused on defining a procedure to predict real-life values of two aerodynamical parameters concerning ailerons (@cl=@� and @cm=@�) as a function of the aileron oscillation frequency (reduced frequency k) and the gap size between aileron and airfoil (�) using the theoretical background as the starting point. For simplicity, a 2D situation has been considered throughout the project. The theoretical background used here is the potential theory as defined by Theodorsen [?]. For simplicity, a at plate is assumed as this is well described by the potential theory. The circulatory and non-circulatory parts of the flow have been considered separately in order to closely investigate the influence of each contribution. The lift force and moment are functions of three main variables: the heave, the rotation and the aileron rotation and the first and second derivative of each. As the case of an oscillating aileron is considered, the heave and rotation of the airfoil itself and their derivatives are assumed to be zero and are left out of the equation. The aileron motion is prescribed by a harmonic oscillation, which means that all relevant motions have been described. For the model results, determined using MATLAB, a reference case has been defined which defines all other unknowns like atmospheric conditions, geometry and frequency range. This case refers to an aircraft flying at sea level with a Mach number of 0.2 and an aileron size of 20% of the airfoil chord. The investigated range of the reduced frequency is between 0 and 2. It turns out that for low frequencies, the circulatory part of the flow is the dominant contributor to both control derivatives and that for higher frequencies, the non-circulatory part of the ow becomes dominant. A sensitivity analysis has been performed to investigate the influence of several parameters. An empirical relation by Wright & Cooper [?] has been used to check the validity of the quasi-steady model when the main variable is the aileron location using zero oscillations. Due to a lack of time only the theory considering flap oscillations has been applied, gap flow has not been considered here. Next, high-fidelity analyses using CFD have been performed for the same reference case. Here, both variables (k and �) have been considered. Considering the computation grid, there were two main problems. First of all, for each gap size a different mesh is needed as the geometry changes. This has been solved by using four different meshes, using 0, 2, 4 and 6% of the flap chord as the gap size. Finally, as an oscillation is applied to the aileron, the mesh is not constant over time. The mesh has been set up such that a harmonic oscillation can be applied to the aileron, specifying the maximum deflection, equilibrium position, oscillation frequency and hinge location in the configuration file. In each analysis, four oscillation periods have been calculated to get rid of any possible entrance effects, each oscillation period has been considered using at least 25 time steps. Analyses have been performed for each gap size using a reduced frequency of k = 0:2 and k = 1 and for 11 di�erent frequencies using a gap size � = 2% of the aileron chord. Varying frequency is easier in this case, as no new mesh is needed for each increment in frequency. After performing the analyses, it turned out that when varying gap size the results were quite odd. One would have expected some kind of regression in the maximum value of cl with increasing gap size as a larger gap implies a larger leakage flow, but this is not what followed from the results. Zooming in around the gap showed surprisingly that no gap flow was present at all, which highly questions the CFD results. The analyses in which only the frequency was varied produced more logical results, a similar progress for the control derivatives as a function of k was observed as in the theoretical model. Finally, despite the sometimes questionable results, correction functions have been set up which correct the theoretical values of the control derivatives to the \real-life" values as simulated by CFD as a function of reduced frequency k and gap size �, respectively. Closely examining the flow field showed that the incompressibility assumption was valid and that that flow separation indeed occurred at the trailing edge, but the limitations of the potential theory and the quality of the CFD results limit and question the use of these specific results. However, when including theory about gap flow in the MATLAB model and using better practical results (from wind tunnel tests or from more accurate CFD calculations) the procedure in general might give a pretty good prediction of the desired parameters in future.
Item Type:Internship Report (Master)
Instituto Tecnológico de Aeronáutica, Brazil
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Aerodynamics, flap motion, control derivatives, aileron oscillation, mesh movement
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