29.Features of flow about the wing of finite span.

Figure 11.27: A sketch of the vortex system generated by a wing of finite span s and chord c, developing a lift force per unit length . The wing tip vortices trailing downstream of the wing are also shown.

The three-dimensional flow over airplane wings can be described quite well by irrotational flow theory, which is the basis for aircraft aerodynamic design. Some of the three-dimensional effects of the flow surrounding an airplane wing of finite span are illustrated in figure 11.27, a perspective view showing a high aspect ratio wing ( ) developing a lift per unit span, , that is uniform along the wing. The circulation around the wing airfoil section can be considered to be that of a bound vortex of strength attached to the wing. Since this vortex cannot end at the wing tips, it trails off downstream from either wing tip as two trailing vortices, each of strength but of opposite direction.

The flow induced by the trailing vortices produces a drag force on the wing, called the induced drag. This is not a viscous effect but one due to the wake region of the wing embodied in the trailing vortex system. The wake region has both vertical and horizontal momentum that balances the wing lift and induced drag.

31.Ways of decreasing of a wave drag and attenuation of wave crisis.

In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.

Wave drag is caused by the formation of shock waves around the body. Shock waves radiate a considerable amount of energy, resulting in drag on the body.

The wave drag at the zero lift condition is reduced primarily by decreasing the thickness-chord ratios for the wings and control surfaces and by increasing the length-diameter ratios for the fuselage and bodies. Also, the leading edge of the wing and the nose of the fuselage are made relatively sharp. With such changes, the severity of the diversions of the flow by these elements is reduced, with a resulting reduction of the strength of the associated shock waves. Also, the supersonic drag wave can be reduced by shaping the fuselage and arranging the components on the basis of the area rule.

The wave drag can also be reduced by sweeping the wing panels. Some wings intended for supersonic flight have large amounts of leading-edge sweep and little or no trailing-edge sweep. The shape changes required are now determined using very complex fluid-dynamic relationships and supercomputers.

30.Comparison of the aerodynamic characteristics of rectangular and swept wings.

Swept wings are preferred in supersonic aircraft to facilitate vertical take off and reduce runway length during take off.

When comparing a rectangular wing and a swept back wing of the same wing area and wing loading (assume all other factors of importance remain constant), the swept back wing has the advantage of:

A: increased longitudinal stability.

B: greater strength.

C: higher critical Mach number.

D: lower stall speed.

Comparison of the aerodynamic characteristics of rectangular and swept wings is shown on the figure: