Taught many years ago at a very respected School: conditions for entry into INCIPIENT VR - KIAS < 20, ROD > 200ft/min and a SIGNIFICANT increase in power. Watch VSI - I assure you you will see a rapid increase in ROD - hold these conditions and you WILL get into VR. What's happening? At the tip decreasing angle of attack (loss of lift) due re-ingestion of tip vortices, at the root increasing angle of attack to beyond stalling angle (more loss of lift), in the middle - an area of blade that is producing a diminishing amount of lift. Modern helos are less susceptable because of better blade design.
GAGS
E86