# Chapter 2

# Aerodynamic Forces: Basic Definitions

## 2.1. Main aerodynamic coefficients

Before we consider the equations (or mathematical models) and the calculation methods that are used to predict the forces to which a vehicle is subjected when it moves within a fluid, it is useful to first introduce certain concepts that are repeatedly reoccurring in aerodynamics.

To illustrate the idea, let us consider a *two-dimensional* airfoil. This means that the wing is supposed to have an infinite span, so that the flow does not have a component of velocity along the dimension perpendicular to the airfoil (wing span). The velocity at *upstream infinity,* meaning a distance upstream large enough that the wing does not have a significant influence on the flow, which is denoted by .

As shown in Figure 2.1, the airfoil is set at an angle of incidence *α*, which means that the chord of the wing (straight line joining the leading edge and the trailing edge) makes an angle *α* relative to the upstream velocity vector.

From now on, we will often use the airfoil as an example. In addition, the definition of wing airfoils (or profiles) forms one of the major objectives in aerodynamics. It is obvious that the following concepts can be applied to any object, whether flying or not, that moves within a fluid. Depending on a physical process, which we will analyze further, the flow around the airfoil exerts a set ...