Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 99

Consider the pressure distribution about a symmetrical airfoil at zero angle of attack (AOA) (Figure 3.23a). The large arrows show the sum of the low pressures on the top and bottom of the airfoil. They are at the center of pressure (CP) of their respective surfaces. The CP on the top of the airfoil and the CP on the bottom are located at the same point on the chord line. The large arrows indicate that the entire pressure on the top and bottom surfaces is acting at the CP. Because these two forces are equal and opposite in direction, no net lift is generated. Note also that the lines of action of these forces coincide, so there is no unbalance of moments about any point on the airfoil. Figure 3.23b shows the pressure distribution about a symmetrical airfoil at a positive angle of attack (AOA). There is now an imbalance in the upper surface and lower surface lift vectors, and positive lift is being developed. However, the two lift vectors still have the same line of action, passing through the CP. There can be no moment developed about the CP. We can conclude that symmetrical airfoils do not generate pitching moments at any AOA. It is also true that the CP does not move with a change in AOA for a symmetric airfoil.

Now consider a cambered airfoil operating at an AOA where it is developing no net lift (Figure 3.24a). Upper surface lift and lower surface lift are numerically equal, but their lines of action do not coincide. A nose‐down pitching moment develops from this situation. When the cambered airfoil develops positive lift (Figure 3.24b), the nose‐down pitching moment still exists. By reversing the camber, it is possible to create an airfoil that has a nose‐up pitching moment. Delta‐wing aircraft have a reversed camber trailing edge to control the pitching moments.

Figure 3.23 Pitching moments on a symmetrical airfoil (a) at zero AOA and (b) at positive AOA.

Figure 3.24 Pitching moments on a cambered airfoil: (a) zero lift, (b) developing lift.

Figure 3.25 Flaps extended pitching moments.

Source: U.S. Department of Transportation Federal Aviation Administration (2016a).

Aerodynamic pitching moments also occur when the pilot changes the camber of an airfoil during flight, as retracting or deploying flaps. When the trailing edge flaps are moved, the chord line changes resulting in a new AOA. When the AOA moves a pitching moment may develop, with initial flap settings the airplane may initially “balloon” due to the immediate increase in lift, then a general nose‐down pitching moment will develop. As shown in Figure 3.25, care must be taken during large power changes and significant trim settings (go‐around/missed approach) as increased pitching moments may be experienced. In some aircraft, a significant nose‐up pitching moment may occur with full deployment of trailing edge flaps as downwash over the horizontal stabilizer increases tail‐down force (airplane nose goes up).