The presence of these vortices produces a downwash flow over the wing, affecting the local angles of attack throughout the wing and, therefore, its lift and overall aerodynamic characteristics. It should be appreciated that the flow over any wing of finite span will be inherently three-dimensional and further complicated by the effects of the vortices that trail behind the wing, as shown in the figure below. In the meantime, it is possible to proceed under the assumption that either measurement or calculation can be used to obtain these integrated results, a particular approach being detailed at the end of this chapter. In practice, the net forces and moments can be measured with a balance (e.g., a scale) or obtained by suitable integration of the effects of the pressures and stresses that act around the surfaces. Likewise, the differences in the pressure distribution between the upper and lower surfaces will contribute primarily to the lift force and pitching moment on the airfoil, the shear stresses having a minor net contribution in that direction. It can be deduced from the figure above that the shear stresses, in the aggregate, will act in a direction primarily parallel to the chord line and so will contribute significantly to the drag force on the airfoil section. The origin of aerodynamic forces on a wing comes from the integrated effects of the distributions of pressure and boundary layer shear stress over its surfaces. In addition, boundary layer-induced shear stresses can be positive (flow moving downstream) or negative (e.g., reversed flow). These distributions are not uniform and can be positive or negative, e.g., a higher pressure pushing inward toward the surface (as shown in red) or a lower pressure pulling outward away from the surface (shown in green). The origin of the net aerodynamic forces on an airfoil or wing, such as lift and drag, comes from the integrated effects of the pressure and the boundary layer shear stress distributions acting over its surface, shown in the figure below. Know the differences between subsonic, transonic, and supersonic airfoil sections.Understand how to calculate lift and other integrated quantities from the pressure and shear stress distributions about a body.Appreciate the effects of flaps and other high-lift devices.Know some of the aerodynamic characteristics of airfoil sections, both in attached flow and with flow separation, and how these characteristics change at different Reynolds and Mach numbers.Be conversant with the various definitions of aerodynamic forces and moments, as well as lift coefficient, drag coefficient, lift-curve slope, maximum lift coefficient, aerodynamic center, and center of pressure.Furthermore, this approach makes it possible to isolate the other more complicated and interrelated effects associated with the finite span of a wing, including the impact of the wing tip vortices and other aerodynamic effects caused by sweepback, twist, planform (chord) variations, and potentially other things. While the concept of a “two-dimensional” wing section may initially sound somewhat artificial, it is possible to mimic a wing of infinite aspect ratio, both experimentally and theoretically, and obtain aerodynamic results that pertain only to the shape of the airfoil section itself. Such two-dimensional airfoils are equivalent to wings of infinite span and aspect ratio. However, before examining the characteristics of finite wings, i.e., three-dimensional wings with finite span and perhaps with twist and planform taper, it is prudent to investigate the aerodynamic characteristics of two-dimensional airfoil sections. Moments will also be produced on the body.Ī body in a flow will potentially produce three forces and three moments. The magnitude of the lift and drag forces depends on many factors, including the size and shape of the body and its orientation to the flow, as well as the Reynolds number (based on a characteristic length) and free-stream Mach number. By definition, the component of this force that acts on the body in a direction perpendicular to the relative free-stream velocity or “relative wind direction” is called the lift, as shown in the figure below. The force component on the body in a direction parallel to the relative wind direction is called the drag. Any body shape that is moved through a fluid will create some form of fluid-dynamic force upon it. ![]() ![]() Understanding the aerodynamic behavior of airfoils and wings is a significant part of the practice of aerospace engineering, and this understanding is critical to the successful design of all aircraft. 26 Aerodynamics of Airfoil Sections Introduction
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