DRAG

In aerodynamics, drag refers to forces that oppose the relative motion of an object through the air. Drag is generated by every part of the airplane (even the engines!). How is drag generated?...

Drag is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field or an electromagnetic field, where one object can affect another object without being in physical contact. For drag to be generated, the solid body must be in contact with the fluid. If there is no fluid, there is no drag. Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid. If there is no motion, there is no drag. It makes no difference whether the object moves through a static fluid or whether the fluid moves past a static solid object.

Drag is a force and is therefore a vector quantity having both a magnitude and a direction. Drag acts in a direction that is opposite to the motion of the aircraft. Lift acts perpendicular to the motion. There are many factors that affect the magnitude of the drag. Many of the factors also affect lift but there are some factors that are unique to aircraft drag.

Types of Drag

There are different types of drag. The most commonly encountered are:

1. Parasitic Drag
2. Induced Drag

Parasitic Drag comprises of

• Form Drag, which is the result of the aerodynamic resistance to motion due to the shape of the aircraft,
• Skin Friction Drag, which is due to the smoothness or roughness of the surfaces of the aircraft, and
• Interference Drag, which may occur where surfaces with different characteristics meet (e.g. wing and fuselage)

Induced Drag is a secondary effect of the production of lift. It is also called "drag due to lift" because it only occurs on finite, lifting wings. Induced drag occurs because the distribution of lift is not uniform on a wing, but varies from root to tip. For a lifting wing, there is a pressure difference between the upper and lower surfaces of the wing. Vortices are formed at the wing tips, which produce a swirling flow that is very strong near the wing tips and decreases toward the wing root. The local angle of attack of the wing is increased by the induced flow of the tip vortex, giving an additional, downstream-facing, component to the aerodynamic force acting on the wing. The force is called induced drag because it has been "induced" by the action of the tip vortices. The magnitude of induced drag depends on the amount of lift being generated by the wing and on the distribution of lift across the span. Long, thin (chordwise) wings have low induced drag; short wings with a large chord have high induced drag. Wings with an elliptical distribution of lift have the minimum induced drag. Modern airliners use winglets to reduce the induced drag of the wing.

Click the video below to learn more about Aerodynamic Drag:

Credit: NASA EDU