**Aerodynamic Center:** A point, near the quarter-chord location, about which the pitch coefficient is constant.

**Angle of Attack:** The angle of an airfoil, measured from the chord line, to the air flow.

**Angle of Zero Lift:** The angle of attack, measured from the airfoil's chord line, at which there is no lift.

**Boundary Layer:** Air flowing along an airfoil does not have a uniform velocity. The molecules of air that are in direct contact with the airfoil surface are not moving. At some small distance from the airfoil surface, the local air velocity is uniform. The thickness of the disturbed airflow is known as the boundary layer.

There are three basic types of boundary layers; laminar, turbulent, and separated. Laminar boundary layers are thin and create the lowest drag. At some point, a laminar boundary layer will become turbulent. The thickness and drag increases more rapidly. Separation occurs when the boundary layer detaches from the airfoil surface and eddies are created. Boundary layer thickness and drag increase very rapidly.

**Boundary Layer Transition:** The point at which the boundary layer is said to transition from laminar to turbulent or separated and from turbulent to separated. In AeroFoil, the transition points are given in dimensionless units of length, x / chord. A value of 0.0 represents the leading edge. 1.0 represents the trailing edge.

**Camber:** Also known as the camber line or mean line. The mid-point of an airfoil thickness profile. A line mid-way between the upper and lower surfaces of an airfoil.

**Chord:** The distance from the leading edge to the trailing edge of the airfoil. The chord line is a straight line between the leading and trailing edge and is used as a standard reference in all airfoils.

**Drag Coefficient:** A measure of an airfoil's resistance to air flow. In standard reference nomenclature it is generally listed as Cd.

Total wing drag = dynamic pressure x Wing Area x Drag Coefficient

**Dynamic Pressure:** The amount of energy in the air flow. In standard reference nomenclature it is generally listed as either Q or q.

Dynamic pressure = 1/2 x air density x velocity^{2}

**Free Stream Velocity:** The velocity of air flow, undisturbed by the airfoil.

**Inviscid:** Having no viscosity. An ideal fluid.

**Lift Coefficient:** A measure of the amount of lifting force that an airfoil can generate. In standard reference nomenclature it is generally listed as Cl.

Total wing lift = dynamic pressure x Wing area x Lift Coefficient

**Pitch Coefficient:** A measure of the twisting force that an airfoil will generate in flight. A negative value implies that the leading edge of the airfoil will be twisting downward. Unless otherwise specified, the pitch coefficient is referenced to a point located on the airfoil chord, 25 percent back from the leading edge. This point is known as the quarter-chord. In standard reference nomenclature it is generally listed as Cm or Cm25. The pitch coefficient about the Aerodynamic Center is generally listed as Cm0.

Total Wing Pitch = dynamic pressure x Wing area x Chord length x Pitch Coefficient

**Potential Flow:** Also known as ideal flow. The airflow that would exist if air had no viscosity.

**Reynolds Number:** A dimensionless number that is the ratio of a fluid's inertial forces to its viscous forces.

Re = chord length x velocity / kinematic viscosity

**Stagnation Point:** A point at the leading edge of an airfoil where the airflow divides. Any molecule of air that is above the stagnation point will pass over the top surface and any that is below the stagnation point will pass under the bottom surface.

**Stall:** The point of maximum coefficient of lift. The lift coefficient will increase as the angle of attack is increased up to its maximum value. After the stall, the coefficient of lift will decrease with increasing angle of attack.

**Thickness Profile:** The distance between the upper and lower surfaces of the airfoil. The thickness profile is independent of the chamber line. Combining a thickness profile with a chamber line is one method of deriving an airfoil.

**Viscosity:** The measure of a fluid's resistance to shear. Oil has a higher viscosity than water, which has a much higher viscosity than air.