AOA fixed-pitch propeller
 

Could an expert please help me understand whether we reference TAS or IAS when considering the angle of attack of a fixed-pitch propeller blade?
I have both Bob Tait and ATC text book, Bot Tait refers to TAS and ATC refers to IAS.

I would think TAS is correct because we are only looking at the speed of the relative airflow, note the pressure, but I am very confused and can't really come to a conclusion I can understand.

Thank you

Don't use IAS, as it is subject to system indication errors on the aeroplane but those don't exist on the blades.

You can use any of CAS, EAS or TAS - so long as you use the same for both forward velocity of the aircraft and rotational speed of the blade, you'll get the same answer.

I think that if I was working it out as an aerodynamicist - which I have had reason to do in the past, I'd use TAS. The reason for that is that TAS is easy to work out from geometry and engine speed of the blade, and similarly the path to reduce IAS to TAS for the aircraft is straightforward and well understood and allows you to remove system indication errors and compressibility in one place.

Thank you Genghis, that's very helpful.