I thought this question sounded familiar - Try this [link]http://www.rcuniverse.com/forum/m_157948/mpage_1/key_osprey/anchor/tm.htm#157948[/link]
and you will find a very detailed thread regarding this subject
Very very difficult to do and as a FYI the ring vortex problem on the full scale has
been solved and I believe testing is due to resume shortly on this amazing aircraft
as the Marines really REALLY want this ship!

Hope this helps