Flight Theory and Aerodynamics. Joseph R. Badick

Flight Theory and Aerodynamics - Joseph R. Badick


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target="_blank" rel="nofollow" href="#ulink_1c183b5f-f8a1-557c-abec-73f5a9d43078">Figure 3.20 Static pressure on an airfoil (a) at zero AOA, and (b) at a positive AOA.

      At a point near maximum thickness, maximum velocity and minimum static pressure will occur, note that V2 is greater than V3. This difference in velocity results in pressure differential, and is due to the cambered nature of the airfoil, increased angle of attack, or both. If the airfoil was a symmetrical airfoil at zero angle of attack then the pressure would be equal on both the top and the bottom of the wing. Because air has viscosity, some of its energy will be lost to friction and a “wake” of low‐velocity, turbulent air exists near the trailing edge, resulting in a small, high‐pressure area.

Schematic illustration of the components of aerodynamic force. Schematic illustration of the pressure forces on (a) nonrotating cylinder and (b) rotating cylinder.

      Pressure Distribution on a Rotating Cylinder

      Now consider if the wind tunnel is stopped, and the cylinder begins to rotate in a motionless fluid. We begin to see the factors of viscosity and friction at work. The more viscous the fluid, the more it is resistant to flow, and since air has viscosity properties it will resist flow. Similar to a wing, the surface of the cylinder has some “roughness” to it, so as the cylinder turns some molecules adhere to the surface. The closer to the surface of the cylinder (airfoil), the greater the possibility the molecules are drawn in a clockwise direction by viscosity, so now substituting air we see the velocity increase in the direction of rotation above the cylinder. This circular movement of the air is called circulation.


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