Airfoil surface refers to all the surfaces of an aircraft that provide lift. For an aeroplane, it is the wings; for a helicopter, it is the blades. In the latter case, it is called a rotating airfoil surface.
The wings are the main lift surfaces of a plane. The wing moving through the air is in relative motion; the current or flow of air around the moving wing is called relative movement (vector v).
The aerodynamic forces applied to the wing during its movement through the air are:
- the lift (Rz), a force perpendicular to the relative wind, which, being the resultant effect of the pressures exerted on a wing moving in the air, represents the 'useful' component;
- the drag (Rx), a force which, by definition, opposes the forward movement of any body immersed in a fluid, and is the 'detrimental' component.
It is their resultant, called the aerodynamic resultant (R), which finally determines the lift of the plane.