Well the reason I asked is that its in a robbie and the static tube is nice hidden in the cowling by the MR gearbox in which case i couldn't really see the reason for the dip unless there was a pressure differential that occured as you went into a hover. Do you still say the static tube is being immersed in the downwash even though it is nicely hidden?
Kind of a side question here but if ground effect has nothing to do with a pressure build up under the airfoil what is the other explanation? Ive heard that the tip vortices are reduced or slowed down but then this explanation wouldn't apply to fixed wing aircraft.