Quote
A curious thing happens though when the model plane starts to fly. The inflow begins to pick up speed as the model picks up speed. The prop gets more efficient, that is, the drag on the prop is reduced as the plane picks up speed. We know this as the unloading of the prop. At some point as the inflow air speed increases, the low pressure in front of the prop turns the inflow to the direction of the prop rotation. The result of this is to decrease the effective angle of attack (pitch) of the prop. If the initial pitch of the prop is too low, the increased air speed can reduce the pitch to zero or to a negative value where the prop becomes a brake. To compensate, speed planes run very high pitches.
some good reading from quote source here.
http://www.clspeed.com/tech.htm