I'm assuming we are discussing a helicopter flying through a relatively large fixed wing wake turbulence? If that is the case, I flew perpendicular across the lead in lights at Honolulu international airport, during the busy part of the afternoon. after holding north of the centerline for traffic I was cleared to cross the extended centerline and proceed "feet wet" south of the field. I pulled in the torque and crossed the centerline, when suddenly the helicopter was "grabbed" and thrown pretty violently downward as if i hard hit a pocket of strong turbulence. aircraft went to ~0.0G and maybe even slightly Negative G. this was at 200 Feet above the water. As quickly as it started, it let us go. and the aircraft recovered to straight and level. It was at this point i realized what I had just done and put the puzzle pieces together in my head... oops. aircraft was an OH-58D with the Bell "soft-in-plane" rotor system, im not sure of the size of the fixed wing traffic because it was already on the runway and basically out of sight. but my guess would be 747 or larger. probably the hardest spank i have felt inflight with this helicopter. luckily we went perpendicular to the FW flight path and minimized the duration. probably lost 100ft total but started at 200AGL, which made things interesting.