Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 96

The air approaching the leading edge of an airfoil is first slowed down and then speeds up again as it passes over or beneath the airfoil. Figure 3.19 compares two local velocities with the flight path velocity V₁ and with each other. As the velocity changes, so does the dynamic pressure “q” and, according to Bernoulli’s principle, so does the static pressure “P.” Air near the stagnation point at the leading edge of the wing has slowed down, so the static pressure in this region is higher than the ambient static pressure. Air that is passing above and below the airfoil, and thus has speeded up to a value higher than the flight path velocity (both V₂ and V₃ are greater than V₁), will produce static pressures that are lower than ambient static pressure. So as “q” increases, “P” static pressure decreases and a greater pressure differential is realized.

Figure 3.18 Effect of pressure disturbances on airflow around an airfoil.

Figure 3.19 Velocity changes around an airfoil.

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.

Figure 3.20 shows a symmetrical airfoil (a) at zero AOA and the resulting pressure distribution (b) at a positive AOA and its pressure distribution. Arrows pointing away from the airfoil indicate static pressures that are below ambient static pressure; arrows pointing toward the airfoil indicate pressures higher than ambient. Note in (b) that the largest magnitude arrow pointing away from the symmetrical airfoil is near the point of the highest velocity (dynamic pressure), so this is the point of greatest pressure differential between the static pressure over the wing and ambient pressure, as static pressure is at its lowest point.