Rotor downwash and light fixed wing
I asked this on Private Flying and was directed to ask here for more, err, accurate information...
I fly from a busy military airfield with big(gish) helicopters - Pumas & Merlins mostly. ATC often give 'caution rotor downwash' along with a clearance - e.g. on Monday I both took off and landed behind a Merlin using the runway, and before it took off the Merlin hovered mid-runway for about 20 seconds in virtually nil wind.
I'm familiar with the AIC on wake vortex but there doesn't seem to be an equivalent document for rotor downwash. A few questions which I hope you can answer...
a. I assume the rotor downwash travels out sideways after hitting the ground... how far, in still wind? Are there persistent vortices which last for seconds/minutes?
b. What difference does it make if the helo is in ground effect or not when hovering?
c. What are the dangers associated with hovering, taking off and landing helos? (and I suppose hover taxiing, too)
So far I've followed broadly the same avoidance technique as with wake vortex i.e. take off before the point where the helo did and stay above its flightpath, and land long again keeping above the helo's flightpath.
Please be gentle with my lack of understanding of helicopter principles of flight...
Tim