How do you simulate prop wash from a choper ?

It depends where the subject is of course. Water might be the easiest to portray, but the hardest to do in modelling (my opinion). You always see people ducking when they go towards a chopper, holding on their hats, skirts etc... If you have any grass, the impact point of the air is straight down and away from the helo, for a hovering helo that is. This fact will always send light things flying around....

Just a few thoughts, hope it helps!

Wow - and I thought fire and smoke were the hardest to model.
Prop wash - lets see - think think think - over land ie an LZ in VN take your rice or tall grass and make a 'crop circle' use a small board and flatten out the grass in an outward fashion. Let it get rough around the edges. Make sure any figures are holding thier lids and thier cloths are appropriately blown around.
Over water - again try the same circle method of rippled water. I'd read through the ship building forum and or try to pick up a FSM article on making wakes on ships and apply the same method to a circluar pattern in the wash area. Not a ship/water guy so I'm not a lot of help.
The dust and or water spray - go digital. Maybe the dust can be represented with a 'smoke' machine hidden underneth with little nozzles all around the LZ. Use a train locomotive style smoke maker. I know micro-mark has them.
Let us know how it goes.

Thanks. This will help alot.

Happy New Year All