Helicopter downwash certainly has the capability to damage light aircraft and injure or kill the occupants. On the ground you can damage sensitive bits or flip the whole thing over. If you look at the accident history you will see a steady trickle of accidents. There are a lot more cases of damage that do not get recorded as accidents. As suggested, generally, the damaging downwash goes down, but it can be reflected. If the wind is blowing the effects will be taken downwind. If the helicopter moves it leaves most of the disturbance behind it.
I believe that the "official" view in-flight is to regard relatively small helicopters (compared to airliners) as HEAVY for the purposes of separation. The Merlin would be such an example.[You can check that yourself]
In practice though, I suggest the main risk is that on the airfield helicopters will be more likely to cause problems. There are several main reasons:
- Their proximity to light aircraft at or close to the airfield.
- A helicopter requires a lot of power (i.e. downwash) to hover. Hovering or hover-taxying creates much more downwash than when ground taxying (on wheels), for example. Hover power means more downwash (more than it does to fly at a moderate speed for example).
- Downwash is dissipated less quickly in the hover too.
Perhaps the worst situation occurs at or close to take-off or landing for the light aircraft. The helicopter could be close and hovering AND the light aircraft is airborne at low level with little room for error and often not much power margin.
The impact of the helicopter is very significantly affected by the way the helicopter pilot chooses to operate it. Helicopters are very flexible and can change the way they do things more than a fixed wing.