C. P. van Dam

The aerodynamic design of multi-element high-lift systems for transport airplanes

Abstract High-lift systems have a major influence on the sizing, economics, and safety of most transport airplane configurations. The combination of complexity in flow physics, geometry, and system support and actuation has historically led to a lengthy and experiment intensive development process. However, during the recent past engineering design has changed significantly as a result of rapid developments in computational hardware and software. In aerodynamic design, computational methods are slowly superseding empirical methods and design engineers are spending more and more time applying computational tools instead of conducting physical experiments to design and analyze aircraft including their high-lift systems. The purpose of this paper is to review recent developments in aerodynamic design and analysis methods for multi-element high-lift systems on transport airplanes. Attention is also paid to the associated mechanical and cost problems since a multi-element high-lift system must be as simple and economical as possible while meeting the required aerodynamic performance levels.

Recent experience with different methods of drag prediction

Abstract During the recent past the process of engineering design has changed entirely because of rapid developments in computational simulations. In aerodynamic design, computational fluid dynamics (CFD) methods are slowly superseding empirical methods and design engineers are spending more and more time applying CFD tools to analyze and predict the aerodynamic characteristics of vehicles. The purpose of this paper is to review recent experiences in CFD-based drag prediction with an emphasis on flow solutions governed by the Euler and Reynolds-averaged Navier–Stokes equations. For these types of flow solutions, various drag analysis methodologies are outlined and applied to determine the drag of components of as well as whole-body airplanes, helicopters, and ground-based vehicles at subsonic and transonic flow conditions. The review demonstrates that although significant progress has been made, CFD-based drag prediction still faces a number of hurdles that must be dealt with before it will become more widely accepted.

Aerodynamic Analysis of Blunt Trailing Edge Airfoils

The adoption of blunt trailing edge airfoils for the inboard region of large wind turbine blades has been proposed. Blunt trailing edge airfoils would not only provide a number of structural benefits, such as increased structural volume and ease of fabrication and handling, but they have also been found to improve the lift characteristics of airfoils. Therefore, the incorporation of blunt trailing edge airfoils would allow blade designers to more freely address the structural demands without having to sacrifice aerodynamic performance. Limited experimental data make it difficult for wind turbine designers to consider and conduct tradeoff studies using these section shapes and has provided the impetus for the present analysis of blunt trailing edge airfoils using computational fluid dynamics. Several computational techniques are applied, including a viscous/inviscid interaction method and three Reynolds-averaged Navier-Stokes methods.

Active Load Control for Airfoils using Microtabs

Micro-electro-mechanical (MEM) translational tabs are introduced for active load control on aerodynamic surfaces such as wind turbine rotor blades. Microtabs are mounted near the trailing edge of rotor blades, deploy approximately normal to the surface, and have a maximum deployment height on the order of the boundary-layer thickness. Deployment of the tab effectively changes the sectional chamber of the rotor blade, thereby changing its aerodynamic characteristics. A tab with tab height to blade section chord ratio, h/c, of 0.01 causes an increase in the section lift coefficient, C 1 , of approximately 0.3, with minimal drag penalty. This paper presents a proof of concept microtab design and the multi-disciplinary techniques used to fabricate and test the tabs. Computational and experimental wind tunnel results for a representative airfoil using fixed as well as remotely actuated tabs are compared. Although the specifics of load control limitations, including actuation and response times will require further research, the results presented demonstrate the significant potential for using microtabs for active load control.

Experimental Analysis of Thick Blunt Trailing-Edge Wind Turbine Airfoils

An experimental investigation of blunt trailing-edge orflatback airfoils was conducted in the University of California, Davis aeronautical wind tunnel. The blunt trailing-edge airfoil is created by symmetrically adding thickness to both sides of the camber line of the FB-3500 airfoil, while maintaining the maximum thickness-to-chord ratio of 35%. Three airfoils of various trailing-edge thicknesses (0.5%, 8.75%, and 17.5% chord) are discussed in this paper. In the present study, each airfoil was tested under free and fixed boundary layer transition flow conditions at Reynolds numbers of 333,000 and 666,000. The fixed transition conditions were used to simulate surface soiling effects by placing artificial tripping devices at 2% chord on the suction surface and 5% chord on the pressure surface of each airfoil. The results of this investigation show that lift increases and the well-documented thick airfoil sensitivity to leading-edge transition reduces with increasing trailing-edge thickness. The flatback airfoils yield increased drag coefficients over the sharp trailing-edge airfoil due to an increase in base drag. The experimental results are compared against numerical predictions obtained with two different computational aerodynamics methods. Computations at bounded and unbounded conditions are used to quantify the wind tunnel wall corrections for the wind tunnel tests.

Computational Investigations of Small Deploying Tabs and Flaps for Aerodynamic Load Control

The cost of wind-generated electricity can be reduced by mitigating fatigue loads acting on the blades of wind turbine rotors. One way to accomplish this is with active aerodynamic load control devices that supplement the load control obtainable with current full-span pitch control. Techniques to actively mitigate blade loads that are being considered include individual blade pitch control, trailing-edge flaps, and other much smaller trailing-edge devices such as microtabs and microflaps. The focus of this paper is on the latter aerodynamic devices, their time-dependent effect on sectional lift, drag, and pitching moment, and their effectiveness in mitigating high frequency loads on the wind turbine. Although these small devices show promise for this application, significant challenges must be overcome before they can be demonstrated to be a viable, cost-effective technology.

Unsteady computational investigations of deploying load control microtabs

Flow around an airfoil with a deploying microtab device has been numerically simulated by solving the unsteady turbulent compressible Navier-Stokes equations with the OVERFLOW-2 solver. Using a Chimera/overset grid topology, microtabs were placed at 95% ofchord of a symmetric NACA 0012 airfoil. Microtab heights on the order of 1% of chord, deployed on the order of one characteristic time unit were utilized. The unsteady effects of tab deployment time, deployment height, and freestream angle of attack on aerodynamic responses were also investigated. Validation studies with experimental results for static deployed microtabs and a dynamically deployed spoiler were also performed to ensure accurate temporal and spatial resolution of the numerical simulations.

Design-oriented high-lift methodology for general aviation and civil transport aircraft

A high-lift system design methodology that can be incorporated during the early stages of aircraft development is presented and thus has the potential to provide a superior and more cost-effective vehicle than one developed utilizing traditional linear design methods. The present methodology offers two different levels of e delity: one applicable to the conceptual design stage and the other to the preliminary design stage. The underlying e ow solver couples a three-dimensional nonlinear Weissinger method with two-dimensional viscous data to provide fast and accurate aerodynamic predictions for high-lift cone gurations. Several test cases that illustrate the capabilities of this hybrid e ow solver are presented.

Drag prediction at subsonic and transonic speeds using Euler methods

A technique for the evaluation of aerodynamic drag from flow field solutions based on Euler equations is discussed. The technique is based on the application of the momentum theorem to a control surface enclosing the configuration and it allows the decomposition of the total drag into induced drag and wave drag. Consequently, it provides more physical insight into the drag sources than the conventional surface-pressure integration technique. The induced drag is obtained from the integration of the kinetic energy of the trailing vortex system on a wake plane and the wave drag is obtained from the integration of the entropy jump over the shocks. The drag-evaluation technique is applied to three-dimensi onal steady flow field solutions for the ONERA M6 wing as well as an AR-7 wing with an elliptic spanwise chord distribution and a NACA 0012 section shape. Comparisons between the drag obtained with the present technique and the drag based on the integration of surface pressures are presented for several Euler codes.

Airfoil Drag Prediction and Decomposition

The accuracy and the consistency of numerical techniques for the prediction of the aerodynamic drag of airfoils in viscous transonic and subsonic flows are explored. Attention is paid to the calculation of the total drag as well as to the decomposition of the drag into its physical components: viscous drag and wave drag. Two different Reynolds-averaged Navier-Stokes solvers are used to generate the flowfield solutions for the NLF(1)-0416 and the RAE 2822 airfoils. The results show that wake integration can produce results comparable with those the often-used surface integral technique, thus demonstrating that wake integration has great potential in simplifying drag calculations for more complex problems such as multielement airfoils or complex three-dimensional configurations