PA28 encountered rotor wash from a helicopter that had preceded them on the landing approach. Momentary loss of control necessitated a go-around.

Narrative

My son (a new pilot) flew me in a PA28-180; so that I could pick my up airplane that had been serviced by the local FBO. I am a 1;200 hour pilot and have been servicing my airplanes at the FBO for the last 8 years. As we approached the turn from downwind to base for Runway XX; the State Police helicopter entered a right base for Runway XX. We acknowledged the State Police call; on the CTAF; and extended our downwind for traffic separation. As we turned from base to final; I could see the State Police helicopter crossing over the threshold for Runway XX; then side-stepped to Taxiway A; past the intersection for Taxiway D. As we crossed over the threshold for Runway XX about 50 FT in altitude; with the State Police helicopter still over Taxiway A; we flew into the helicopter's rotor wash. Our aircraft initially experienced a drop in altitude that required power to be added in order to maintain a proper attitude; then the aircraft rolled right (approximately 30 degree bank attitude); then left; then violently to the right (with a bank greater than 60 degrees). I recovered the aircraft and executed a go-around; at which point I noticed the helicopter was either hovering over Taxiway A; or departing from the taxiway (the helicopter pilot did not make a call on CTAF of his intentions; so I am not sure what part of flight was being conducted at this time). Someone then made a cautionary announcement; on CTAF; to the traffic behind us that a PA28 had just flipped 90 degrees but recovered. When we came back around; we noticed the helicopter had moved from this location.