# What is the relation between the Lift Coefficient and the Angle of Attack?

I'm trying to understand the relation between the Lift Coefficient and the Angle of Attack using PEP software for the A320 IAE. However, I notice that often for a given Angle of Attack, there could be more than one Lift Coefficient ("slightly different" but different).

Is it an error or a real issue? If yes, what are the standard conditions assumed for the Cl vs AOA graph?

• The airfoil of most aircraft is not identical along the wing, so naturally there would be a difference in Lift coefficient at different points along the wing. – Taher Elhouderi Nov 3 '15 at 8:01
• What do you mean by 'slightly different'? Like a 0.001 difference, or a difference of 0.1? – ROIMaison Nov 3 '15 at 8:21
• 0.1/0.2 of difference – Afe Nov 3 '15 at 17:45

Normally the relation is linear within the practical range of angles of attack used in flight. However, the Mach number and the Reynolds number will cause shifts. For the Mach effect, use the Prandtl-Glauert rule in subsonic flow up to Mach 0.7: $$(\alpha - \alpha_0)_{Mach} = (\alpha - \alpha_0)_{incompressible}\cdot\sqrt{1-Mach^2}$$