Aerodynamics is a branch of fluid dynamics concerned with studying the motion of air, particularly when it interacts with a solid object, such as an airplane wing.

# Airfoils

An airfoil is any part of an aircraft (primarily the wing and tail surfaces) that deflects the air through which it moves to produce a desired reaction. The term "reaction" as used here refers to Newton's third law of motion: "whenever one body exerts a force on a second body, the second body reacts on the first with a force opposite in direction but equal in magnitude." In the case of an airfoil, the airfoil pushes air downward while the air pushes the airfoil upward.

An airfoil-shaped body moving through a fluid produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag.

# Drag

Drag comes in two varieties: parasitic drag and induced drag.

## Parasitic Drag

Parasitic drag is the resistance offered by the air to anything moving through it.

Parasitic drag can be further classified into form drag, skin friction, and interference drag. Glider form drag is reduced by making the frontal area of all parts as small as practical. Skin friction is reduced by reducing the surface area. Interference drag occurs where two parts join, such as the wing and fuselage, and is reduced with fairings to streamline the intersection.

Parasitic drag increases with the square of speed. (i.e. double the speed and parasite drag increases four times.)

## Induced Drag

The other kind of drag is induced drag and is a by-product of the generation of lift. Even if parasitic drag were reduced to zero, the aerodynamic force on a wing deflecting the relative wind would not be perpendicular to the direction of flight, and the greater the wind deflection the greater the deviation of that force from perpendicular. Sometimes it is helpful to describe the total aerodynamic force (excluding parasite drag) as "true lift" perpendicular to the "average" relative wind. This explanation recognizes that the relative wind is deflected by the airfoil and its direction after deflection, called "downwash", is no longer parallel to the flight path. Thus its "average" direction lies somewhere between the flight path and the downwash angle.

Induced drag increases as the amount of deflection of the relative wind increases.

# Lift

Aerodynamic lift is the force on an airfoil perpendicular to its flight path.