Decambering

Abstract: A decambering approach was developed in 2006 at NCSU for prediction of post-stall aerodynamics of wings and configurations using post-stall data for airfoils as input. This approach can be incorporated in vortex lattice methods (VLM), Weissinger and liftingline formulations. Recently, in 2008, this method was made significantly faster by the use of pre-computed basic and additional loadings, enabling the use of rapid superposition of these loadings for post-stall aerodynamic calculations. The new approach only requires the VLM (or other analysis method) for pre-computing the loadings and does not use the VLM for the iterative post-stall computations. This paper describes the incorporation of this fast decambering approach in flight dynamics simulations at post-stall conditions. The fast decambering method is written as a function that can be called by an ordinary differential equation (ODE) solver for solving the standard fixed-wing equations of motion (EOM). At each time step in the solution of the EOM, the full aerodynamic lift distributions (pre- or post-stall) on the lifting surfaces of the configuration are computed using superposition of stored basic and additional lift distributions. An important modification to the aerodynamic prediction method was in developing a way to simulate the effect of aircraft motion, especially angular velocity, on the lift distributions using superposition of stored lift distributions. The examples in the paper illustrate the use of this approach for simulation of aircraft motion in post-stall conditions. The computations are fast enough for use in real-time simulations.

Abstract: RINKU MUKHERJEE. Post-Stall Prediction of Multiple-Lifting-Surface Configurations Using a Decambering Approach. (Under the direction of Dr. Ashok Gopalarathnam.) A novel scheme is presented for an iterative decambering approach to predict the post-stall characteristics of wings using known section data as inputs. The new scheme differs from earlier ones in the details of how the residual in the Newton iteration is computed. With earlier schemes, multiple solutions are obtained for wings at high angles of attack as the final converged solution depends on the initial conditions used for the iteration. With this scheme, multiple solutions at high angles of attack are brought to light right during the computation of the residuals for the Newton iteration. In general, the new scheme is found to be more robust at achieving convergence. Experimental validation is provided using experimental airfoil lift curves from Naik and Ostowari for three different aspect ratios of rectangular wings. Results are presented from a study of the stall characteristics of wings of different planform shapes and two configurations of a wing-tail and a wing-canard configuration. Results are also presented from a study to investigate possible asymmetric lift distributions when the iterations were started with an initial asymmetric distribution of the decambering. Post-Stall Prediction of Multiple-Lifting-Surface Configurations Using a Decambering Approach

Abstract: Birds morph their wing shape to adjust to changing environments through muscle-activated morphing of the skeletal structure and passive morphing of the flexible skin and feathers. The role of feather morphing has not been well studied and its impact on aerodynamics is largely unknown. Here we investigate the aero-structural response of a flexible airfoil, designed with biologically accurate structural and material data from feathers, and compared the results to an equivalent rigid airfoil. Two coupled aero-structural models are developed and validated to simulate the response of a bioinspired flexible airfoil across a range of aerodynamic flight conditions. We found that the bioinspired flexible airfoil maintained lift at Reynolds numbers below 1.5x105, within the avian flight regime, performing similarly to its rigid counterpart. At greater Reynolds numbers, the flexible airfoil alleviated the lift force and experienced trailing edge tip displacement. Principal component analysis identified that the Reynolds number dominated this passive shape change which induced a decambering effect, although the angle of attack was found to effect the location of maximum camber. These results imply that birds or aircraft that have tailored chordwise flexible wings will respond like rigid wings while operating at low speeds, but will passively unload large lift forces while operating at high speeds.

Abstract: The surface of a rectangular wing is morphed at high angles of attack such that it continues to operate at the reduced coefficient of lift ( C l ) at which the baseline wing operates, but unlike the baseline wing, where the flow is separated, the flow remains attached on the morphed wing. A morphed surface is also generated to operate at a local design 2D (two-dimensional) C l , which is obtained by incrementing the baseline C l by a percentage at pre and post-stall angles of attack. The morphed surface is generated numerically using a novel ‘decambering’ technique, which accounts for the deviation of the coefficients of lift and pitching moment from that predicted by potential flow, analytically, using CFD and implemented experimentally by attaching an external Aluminium skin to the leading edge of the wing. Two different wing sections, N A C A 0012 and N A C A 4415 , are tested on a rectangular planform. The effect of morphing on the aerodynamic performance is discussed, and aerodynamic characteristics are reported. Results indicate that significant improvement in aerodynamic performance is achieved at high angles of attack, especially at post-stall through this active morphed flow surface.

Abstract: A conventional Fowler flap is designed to improve the take-off and landing performances of an aircraft. Because the flight states of general aviation aircraft vary significantly. A Fowler flap with a double-sliding track has been designed, which is capable of changing airfoil camber while cruising and climbing as well as meeting low-speed performance requirements. The aerodynamic characteristics of the variable camber Fowler flap were studied by computational simulation, and cambering was found to be beneficial for improving the lift-to-drag ratio when the lift coefficient was larger than the critical value, below which decambering was more effective; this critical value differed somewhat under different conditions. Taking the mechanism into account, the take-off and landing configurations were optimized on the basis of the GA (W)-1 airfoil with a 30% chord Fowler flap. Compared with reference configuration, the maximum lift coefficient of optimized take-off configuration was increased by 6.6% as well as the stalling angle and the lift-to-drag ratio were increased by 1.3° and 7.58%, respectively. Moreover, the maximum lift coefficient of the optimized landing configuration was increased by 6.3%, and the stalling angle was increased by 1.1°; however, the nose-down pitching moment of both configurations increased. Similar results were attained on a general aviation aircraft wing/body combination. A 3D model of the variable-camber Fowler flap driving mechanism was established in a computer-aided design system, and the results showed that all design configurations could be achieved by the double-sliding track.