I had the problem, that the propeller wasnt turning at a constant speed when animating the plane. So I found this solution.
If you have the problem, that the propeller is slowing down in the middle of the animation (or is slowly ramping up/down) then hover your mouse over the dope sheet or the animation graph editor and press “T”. A context menue should pop up. There choose “Linear”. After that the propeller should turn at a constant speed from the beginning.