THE AERODYNAMIC ZERO

If a symmetrical aerofoil is at zero angle of attack, it yields no lift, whereas a cambered one will lift when its chord line, i. e. the straight line from extreme leading edge to trailing edge, is parallel to the general airflow. But a cambered profile can be moved to some negative angle at which it, too, will produce no lift This angle is most important and is known as the aerodynamic or absolute zero angle of attack for a particular aerofoil. The more cambered the mean line, the more negative, relative to die chord line, will the absolute zero angle be. On graphs of lift against drag for aerofoils in one family the more cambered profiles’ lift curves are always to the left, i. e. towards the negative side, as shown in Figure 7.4. However, the slope of the lift curve on such graphs, for one aerofoil family, is the same in each case. This remains true so long as the camber is not so great that streamlining breaks down. As shown, the more cambered aerofoil of a family tends to reach a higher value of Cl before stalling than the less cambered, but the geometric angle of attack at which the stall occurs is earlier. Only if the angle of attack is measured from the absolute zero in each case does the more cambered profile stall later. This suggests some important practical points for the design and construction of model wings.