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.

32.High-lift devices. Influence of high-lift devices to the aerodynamic characteristics of a wing.

A wing designed for efficient high-speed flight is often quite different from one designed solely for take-off and landing. Take-off and landing distances are strongly influenced by aircraft stalling speed, with lower stall speeds requiring lower acceleration or deceleration and correspondingly shorter field lengths. It is always possible to reduce stall speed by increasing wing area, but it is not desirable to cruise with hundreds of square feet of extra wing area (and the associated weight and drag), area that is only needed for a few minutes.

It is also possible to reduce stalling speed by reducing weight, increasing air density, or increasing wing CLmax . The latter parameter is the most interesting. One can design a wing airfoil that compromises cruise efficiency to obtain a good CLmax , but it is usually more efficient to include movable leading and/or trailing edges so that one may obtain good high speed performance while achieving a high CLmax at take-off and landing. The primary goal of a high lift system is a high CLmax; however, it may also be desirable to maintain low drag at take-off, or high drag on approach. It is also necessary to do this with a system that has low weight and high reliability.

This is generally achieved by incorporating some form of trailing edge flap and perhaps a leading edge device such as a slat.

Flaps change the airfoil pressure distribution, increasing the camber of the airfoil and allowing more of the lift to be carried over the rear portion of the section.

Leading edge devices such as nose flaps, Kruger flaps, and slats reduce the pressure peak near the nose by changing the nose camber. Slots and slats permit a new boundary layer to start on the main wing portion, eliminating the detrimental effect of the initial adverse gradient.