On Monday, September 23, 2019 at 5:24:13 PM UTC-7, wrote:
We should get an aero engineer to comment but in simple terms the AOA of the prop can become negative with a windmilling prop(depending on pitch design and rotational speed). If that happens you get negative thrust.

Here is a study of the subject:
http://www.peter2000.co.uk/aviation/misc/prop.pdf
Conclusion: it depends.

In this paper the same conclusion is reached (note the date):
http://naca.central.cranfield.ac.uk/...report-464.pdf

Tom