A cambered aerofoil generates no lift when it is moving parallel to an axis called the zero-lift axis (or the zero-lift line.) When the
angle of attack on an aerofoil is measured relative to the zero-lift axis it is true to say the
lift coefficient is zero when the angle of attack is zero.[1] For this reason, on a cambered aerofoil the zero-lift line is better than the chord line when describing the angle of attack.[2]
When symmetric
aerofoils are moving parallel to the
chord line of the aerofoil, zero
lift is generated. However, when
cambered aerofoils are moving parallel to the chord line, lift is generated. (See diagram at right.) For symmetric aerofoils, the chord line and the zero lift line are the same.[3]
Kermode, A.C. (1972), Mechanics of Flight, Chapter 3, (p. 76, eighth edition), Pitman Publishing
ISBN0-273-31623-0
Notes
^Anderson, John D. Jr, Introduction to Flight, Section7.4 (fifth edition)
^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. Kermode, A.C., Mechanics of Flight, (p.76, eighth edition)