1. DRAG REDUCTION OF AIRPLANES - INDUCED DRAG

2. Four Mani Drag are encountered in aerodynamics
Skin friction drag
Form drag
Wave drag
Induced drag

3. REDUCTION OF SKIN FRICTION DRAG
Ski friction drag can be reduced in one of the two ways
Either laminar flow is maintained by postponing transition, this is so called laminar flow technology
Either to reduce the surface shear stress generated by the turbulent boundary layer.

5. REDUCTION OF FORM DRAG
Form drag is kept to minimum by preventing boundary layer separation
Streamlining is vitally important for reducing form drag
Form drag is the main contribution to the overall drag for bluff bodies like cylinder.
The case of the streamlined body skin friction drag, form drag being less than 10% of the overall drag.
For bluff bodies even minimum streamlining can be very effective.

6. REDUCTION OF INDUCED DRAG
Induced drag falls as the aspect ratio of the wing is increased.
It was also show that for a given aspect ratio elliptic shaped wing have the lowest induced drag.
The winglet has been developed as a device for reducing induced drag without increasing aspect ratio.

8. On all subsonic wings there is a tendency for a secondary flow to develop from the high pressure region below the wing round the wing tip to the relatively low pressure region on the upper surface. This is the part of the process of forming the trailing vortices.

9. REDUCTION OF WAVE DRAG
For supercritical aerofoil it was found that keeping the pressure uniform over the upper wing surface minimized the shock strength there by reducing the wave drag.
Waisted fuselages of this type are now a common feature of aircraft designed for transonic operation.