I had an encounter while landing in a Cessna 337 a year or two ago, with the rotorwash from a little Robinson R22. It was night, and the Robinson was about fifteen feet in the air, nearly invisible, and taxiing clear of the runway. I passed behind it, landing, and nearly thought I was sideways. Correction required power and nearly full control input until clear.
I've ended up flying close, under helicopters at night, while in a fixed wing light airplane, when the helicopter had no lights on. The results were dramatic, roughly akin to flying through a large dust devil, crossed with a microburst.