Line of zero lift

Now an aerofoil may provide lift even when it is inclined at a slightly negative angle to the airflow. And one may well ask, how can an aerofoil inclined at a negative angle produce lift? The idea seems absurd, but the explanation of the riddle is simply that the aerofoil is not really inclined at a negative angle. Our curious chord may be at a negative angle, but the curved surfaces of the aero­foil are inclined at various angles, positive and negative, the net effect being that of a slightly positive angle, which produces lift.

If we tilt the nose of the aerofoil downwards until it produces no lift, it will be in an exactly similar position to that of a flat plate placed edgewise to the airflow and producing no lift, and if we now draw a straight line through the aerofoil parallel to the airflow (Fig. 3.6, overleaf) it will be the inclination of this line which settles whether the aerofoil provides lift or not.

Such a line is called the line of zero lift or neutral lift line, and would in some senses be a better definition of the chord line, but it can only be found by wind tunnel experiments for each aerofoil, and, even when it has been found, it is awkward from the point of view of practical measurements.

Nor is it of much significance in practical flight, except perhaps in a dive when the angle of attack may approach the no lift condition.

Note that for an aerofoil of symmetrical shape zero lift corresponds to zero angle of attack.

Direction

of

airflow

Fig 3.6 Line of zero lift