Don't confuse "vane angle" with AOA
In Post #20 S&R makes reference to zero vane angle being a long way from zero AOA. This is common for a couple of reasons.
1. AOA vanes measure the angle of the flow where they are located (normally on the side of the forebody). Flow anywhere near the airplane is distorted by the airplane. A change of AOA of one degree will most likely result in a change of vane angle that is significantly more than one degree.
2. The AOA range of interest most often extends on both sides of zero. Often it is found to be easier to deal with a sensor whose "position" takes on only positive values. As such, the vane instrumentation may be set up to have a zero reading correspond to an airplane AOA that is quite negative so as not to have to deal with negative values.
Most often a conversion is provide within the airplane's instrumentation to convert raw "vane angle" to "AOA" that is then used for displays and potentially as part of the control system augmentation. AOA is most typically an indication of the longitudinal angle of the free stream airflow with respect to the fuselage.
A related point of interest is the AOA at which zero lift occurs. For a high speed, clean wing configuration zero lift usually occurs at close to zero AOA. At low speed where trailing edge flaps are often deployed and trailing edge control surfaces such as ailerons may be drooped, zero lift occurs at an AOA that is considerably below zero.
I concur with the previous comments regarding complexity of wing shape and twist such that defining the true zero incidence angle for the wing itself is complex and varies with wing configuration, fuel loading, airplane weight, speed, Mach number, and load factor. It is much simpler to reference AOA to a more stable, less variable geometry reference such as fuselage angle.
Last edited by FCeng84; 5th February 2018 at 21:51.
Reason: Fix typo and add airplane weight as a factor