Hi welliewanger,
I've seen the same Bernoulli effect is in a science museum, where a disc of plywood is sucked onto the ceiling over the spot where a stream of air is flowing from the ceiling.
The air flow down wash from the blades has to be replaced with an air flow onto the top. Near the ceiling, this replacement airflow is squeezed between the "disc" of your helicopter blades and the ceiling, so you get a drop in pressure over the top of the blades.