Anyone know what exactly causes this? I have heard the air in front of the prop starts vortexing, so you lose your pull. Heres my dilema, I have a Katana GS 40 profile with a Saito 56 swinging an apc 13x4/sullivan spinner nut. On calm days when I get in a hover the prop will get loud and wash out. I can power up to overcome this but it makes me nervous when low to the ground.

I really like the plane/engine combo but I also have an OS 70 as well.
My guess is the only fix is to go to a larger motor and prop. Any other suggestions? Thanks for an help.