A helicopter produces downwash equal to its weight. If you fly through it, it can have disasterous effects on a light aircraft, so treat it with respect. If the helicopter is behind you, it is unlikely to have any effect, but if its ahead or has crossed your path you need to allow sufficient time for the wash to sink. A colleague of mine was killed after a large helicopter flew over the runway he was landing on when his aircraft rolled in the downwash.