What is the incidence angle of a propeller in the terminology of the Wright Brothers and how did they obtain it?
According to various scholars, the Wright brothers were referring to &quot;angle of attack&quot; when they used the term &quot;incidence.&quot; They actually published an article by the same title, so the fact is well-established.
References include the following:

In Wilbur's time, the &quot;angle of incidence&quot; was what we now call the &quot;angle of attack&quot; -- the angle at which the wing of an airplane meets the wind (wright-brothers.org).

and

In the Wright's own words: &quot;The lift is thus balanced against the normal pressure on the resistance surfaces ... Therefore, the lift ... at the given angle of incidence (angle of attack) is to the pressure on a square plane of equal area at 90° as the sine of the (...)
(wright.edu)

The angle of attack of a propeller is defined in various ways, a summary of which is here.

The angle at which this air (relative wind) strikes the propeller blade is its angle of attack.