Originally Posted by
albatross
Used to do a thing called "laser survey"
Hovering at 3-6 thousand feet maintaining position +_ 12 inches over a 10 amp gyro stabilized laser pointed veritically and impacting on a 24 inch screen on the bottom of the helicopter. The laser dot could be seen on a 4 inch tv screen in the cockpit...just keep the dot in the middle of the screen...that part took a lot of practice! LOL Some guys never could do it but with 4-6 hours of training most could ...some could never stop climbing, some could, some could also descend while keeping the dot centered and one eye/hand genius I knew would happily do hover turns as he climbed, hovered, descended and smoked a cigarette.
Sometimes ( a lot actualy ) you would be sitting there in a hover and the vsi would flicker downwards.
Add a little power ..vsi would flicker to 1-200 fpm and then WHAM you would be in fully developed Vortex Ring State...lots of fun but we could never figure out how it developed so quickly. Had lots of time to explore it as we plummeted downwards with the VSI pegged and the cyclic like a wet noodle..( once you lost the laser you had to go back to groundlevel hover over the laser to get the point back in the screen then climb vertically back up. The surveyors used to laugh when the helicopter rapidly disappeared verticaly downwards out of the field of view of their transits...the told me that they detected no downward movement of the helicopter before the abrupt downward departure and they had the helicopter in the crosshairs of a 50x theodolite.
Basically we were using the helicopter as a very tall stadia rod.
So anyone have an idea as to why the helicopter would suddenly enter Vortex Ring State from a stable hover? We were very light..usually only the pilot aboard and perhaps 3/4 fuel - AS350D in my case but we also did it using 500D, Gazelle and Alouette.
One thing I did not mention was that the above scenario used to happen after you had been in a stable hover at 2-6000 agl in light or no wind conditions for 3-4 minutes ..so you had been hovering +_ 12 inches creating a downflow of rotor down wash unaffected by wind thereby creating a column of down flowing air. That might be a clue as to what caused the sudden downwards departure into VRS. I understand that this case was not something you normally do in ops but it was a very interesting scenario.
Perhaps I was creating my own local downdraft?