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?

  • $\begingroup$ 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. $\endgroup$ Mar 6 '18 at 20:35
  • $\begingroup$ 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. $\endgroup$
    – Zeus
    Mar 7 '18 at 1:04

In short: it’s up to the designer to pick any definition as long as the design and analysis always obeys that definition.

Usually, it’s a general practice to define a root chord and then calculate the rest of the local aoa‘s in relation to the root chord. This makes wing-strip calculations simpler. It’s also used in helicopter rotor blades and propellers.

On the other hand, there are numerous simplified calculations (that calculate wing or aircraft level properties) that may consider the mean aerodynamic chord. In this case, of course, the mac location would be used.

  • $\begingroup$ Thanks, that's what I suspected. I was curious to know if it was a matter of convention. $\endgroup$ Mar 7 '18 at 10:02

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