UAflyer,

I think it is most likely the spiral propwash, as you suggest. One experiment you could use to find this out is to let the tail go up as soon as it can, so that the plane is taxiing with almost no angle of attack. If this does not get rid of the left yaw, then the yaw is not being caused by the 'P-factor'. My understanding is that the spiral propwash is the strongest of the forces that can cause left yaw. Since your plane is a scale model, you may not want to mess with the looks, but there is one modification that can help, if you don't mind the appearance. If you can even out the fin area above and below the thrust line, then the yaw effect will be reduced or eliminated. You could do this by removing some fin area from the top, and/or adding some on the bottom of the fuselage. If you want to make a prop change, you need to go to a lower pitch, rather than a higher pitch. In any case, I am a little surprised that you don't have enough rudder authority to overcome the yaw. Make sure you have at lease the recommended throw. Another thing you can do is to throttle up gradually, so that you don't have the full-strength propwash until you have some airspeed to give stronger yaw stability. I think you will notice that this problem goes away as the airspeed goes up ( I use the term airspeed, even though the plane is still on the ground, because it is the relative speed of the air moving past the fin that gives yaw stability ).

Good luck,

banktoturn