# What is the reference chord for AoA measurement or calculation in a twisted wing?

Let us assume we have a rectangular, unswept wing with no dihedral and a given wing twist and we want to measure in flight or calculate its $c_L$ vs $\alpha$ (angle of attack) diagram. Of course each airfoil along the span will have a different local angle of attack at every flight attitude. Which y-coordinate along the span must (or should?) be used as a reference?

• Angle of attack is always a local quality. The overall parameter is angle of incidence, and that is measured relative to the airplane's X coordinate. – Peter Kämpf Mar 6 '18 at 20:35
• As already answered, it is a matter of convention. For the whole aircraft, its longitudinal axis is usually used as a reference. The root chord, 25%-span chord, MAC and even the angle of zero lift can all be used. But if the aircraft has an AoA sensor, everything is usually calibrated in terms of this sensor indication (i.e. its local AoA), which may have very indirect relationship to the wing AoA. – Zeus Mar 7 '18 at 1:04