In this lecture Prof H. Bijl talks about theory on airfoils, starting with the airfoil parameters and how changing them affects their aerodynamic properties. Next an in depth look of the airflow around an airfoil is examined, and an expression is derived for the normal force and lift on the wing as a function of the pressures. With the knowledge on how the air pressure behaves around an airfoil the critical pressure coefficient is explained, and finally derived.