Vortex Ring and other Heli War Stories..

[InTgreen]

Hi all,

Long time reader, very infrequent poster....

As a low hour twin turbine operator, working in a range of theatres.. be it mountains, coastal and sea, I greatly enjoyed a thread on Vortex Ring a few weeks back.. What I propose is that a thread be started that goes into some of these helicopter specific phenomina, in a range of theatres. For example, I had heard about the fact that hovering in up-drafting air is the same as a descent at high power and at low airspeed, but the previous thread made me think on a few point that I had not considered since ground school many moons ago. I think that a thead like this would be a good refresher for everyone - especially myself, and if some of the more experienced guys out there could illustate these 'war stories' with real experiences, it would bring the dangers of opereating helicopters back to the front of our minds, where it should be!

It would make a lot more interesting reading than all my old notes!!!!

Regards

TGreen.

[Shawn Coyle]

old notes???
You mean someone actually took notes during basic training??
They might be worthwhile fleshing out and reviewing here.

[MD900 Explorer]

InTGreen

There was this on time back in Nam..........Cheltenam i recall.......blah blah blah. :E

I was doing an S-61 type rating, and out on a sortie, the boss said the magic words "You have control, and do take us down", (Brain still in non-flying pilot mode) so off i went and took control and everything was going great, doing a ready state of decent and speed was good, and i was thinking we were ok, when all of a sudden the flipping intrument panel went mental and tried to shake it'self out of the cockpit, and i turned around and said, "You know, I'm not happy with this, we are in VRS (or was it settling with power :E), so i am going around.

At which point all I heard was, from the boss was "I have control" and "What the hell are you doing?? This is normal attitude and vibration on landing".

Needless to say, I am not flying S-61's now :{

MD :ok:

[InTgreen]

MD

During training on the S-61(ish), in the hover, pulling 35% matched tq in the updrafting air off Pen-y-Fan in South Wales (sorry for any spelling errors). The main job I had as the Co-Pilot was pushing all the blo@dy engine instruments back into the panel as they vibrated themselves out!! It was this little experience that got me thinking about vortex ring in the hoover....:uhoh:

[212man]

Some how I don't think that will be vortex ring; sympathetic airframe vibration perhaps? Even the smooth ones have it in some regions (not sure about Wales though, is that a region or a country?)

[NickLappos]

That vibration in and around translational lift is each rotor blade bumping into the shed vortex from its preceeding blade, called Blade Vortex Interaction (BVI). It is not at al near VRS, and is actually a good indicator that you are far away from VRS.

The panel in a 61 is isolated to help quell this beast, as is the panel in a BO-105. The vibration is n/rev (about 20 Hz in most helos) which feels like a low speed dentist's drill, and dithers the seats and gages at high frequency. It is worse in some aircraft depending on blade spacing and the natural modes of the airframe (212man calls it sympathetic airframe vibration, which is quite correct).

The vibration in vrs is more of a buffeting at about 1 to 3 times per second, which tosses the aircraft and occupants like turbulence. Look at those rings in the last set of flow depictions at this link. The set is the last triple set of vortex rings, at 1050 feet per second descent rate (for a Robbie). See how the flow jumps around at about once per second.

http://www.s-92heliport.com/vrs.htm

[InTgreen]

Thanks Nick....

On a slight tangent, I was under the impression that some of the problems experienced by the V-22 during trials were Vortex Ring related... At the sort of ROD stated in those examples(3400 fpm), it make that threoy unlikley does it not????

Tgreen

[CyclicRick]

Vortex ring sounds a doddle....try collective bounce in a 205 with 1,5 tonnes on the hook..now thats scary! :uhoh:

[NickLappos]

InTgreen,
The V-22 accident where VRS was cited had rates of descent at the 5000 fpm level, well within the VRS boundary. The other aircraft in the flight, as lead, experienced a hard landing, and was reported to need extensive work to make it flyable. Since it is a tough modern airframe, we can believe it hit pretty hard.

[212man]

I thought with the V-22 the additional factor was it was being rolled too, thereby putting the down going nacelle at a higher rate of descent still.

I gather the subsequent trials were quite exciting, with departures past 90 degrees of bank!