Hi and welcome!
Those prop discs are pretty thin in IL2, as I recall. When I used to do this, I would hand paint and blur a new propellor on a new layer above the image. Adjusting the transparency of this would help blend it all together.
I'd suggest starting with an elliptical selection in the shape and size of the prop disc, then use a feathered brush to make a couple of broad, sweeping marks. Blur and adjust opacity to taste.