Entry into autorotation on a helicopter with twin rotors

[Dave_Jackson]

Does anyone have any information related to a non-simultaneous rotor passage through the vortex ring state?

Of specific concern is the possibility of roll in a helicopter with side-by-side rotors.

Dave

[somepitch]

wasn't that a problem for the v-22?

[oldbeefer]

See this thread http://www.pprune.org/forums/showthread.php?t=294936

[Graviman]

Dave, VRS cannot happen in autos (except possibly in the flare). You need to be in a powered descent initially about 1/2 auto sink rate, although -20kts sink is not uncommon once developed. I suspect coaxial yaw authority would not be reduced, if that is your question. Machine just needs more power that engine can put out until you find clean air.

[Dave_Jackson]

Thanks for the references to the V-22. Like the V-22, the concern relates to non-simultaneous entry of two rotors into the vortex ring state, but, I should have explained the request better. This inquiry is related to a conventional helicopter, with a side-by-side configuration, and specifically one with a very low disk-loading.

The side-by-side Focke-Wulf Fw 61 (http://avia.russian.ee/helicopters_eng/fw-61.php) was the first to prove that a helicopter could autorotate. However, I have been unable to find information on the VRS for helicopters with very low disk loading. The application under consideration (http://www.unicopter.com/ElectrotorSimplex.html) will have a disk loading of only 1.4 lb/ft-sq. and an autorotative descent rate of about 800 ft/min.

Mart,

I agree with much of what you are saying except that Leishman says "From a piloting perspective, entry into autorotative flight requires transition through the vortex ring state. The vortex ring state is known to exhibit considerable unsteadiness at the rotor, and this can lead to significant blade flapping and a loss of rotor control"

My concern is whether it would be possible, under any condition, to put such a lightly disk-loaded craft into a VRS?

Dave

[CGWRA]

You need power going to the blades to get into vortex ring. From what I read that v22 that ran into it was doing a decent wayyy over the maximum rate of decent limits of the aircraft. The lighter the loading on the disk of course the less likely you are to encounter it. The same is true for how shallow the decent is.

[IFMU]

CGWRA,

I thought it was harder to get into VRS with high disk loading, but then much sportier once you were there.

-- IFMU

[Dave_Jackson]

IFMU,

Quoting Leishman, again, he says "For traditional helicopters with low disk loading, the region of the flight envelope affected by the VRS is generally small, and has not overly constrained practical operations."

My concern is a newbie trying an impractical operation. :uhoh:

Dave

[Boslandew]

At the end of a session in the Chinook sim at Aberdeen, when the Training Captains had run out of other ways to frighten you, they would sometimes force the sim into vortex ring at low-level and leave you to extricate yourself. It was surprisingly difficult to get it to enter VR - it required a massive flare at relatively high speed and then a power application - gross mishandling under normal conditions. On the occasion I remember the fwd rotor stayed clear and the rear rotor entered VR, then cleared due to our recovery action but, BAM, re-entered it again. The fwd-aft pitch changes were violent and my overcontrolling made it worse. Eventually the TC had had enough and he froze the sim motion. An eye-opener just how quickly it built up, how extreme the motion was and how difficult it was to recover without instruction and practice.
I'm surmising but to have that happen in a helicopter with two side by side rotors or the V22 would, I suspect, quickly render it uncontrollable

[Graviman]

Boslandew, was there any way of VRS recovery in the Chook? Maybe a sideslip? What sink rate was needed to VRS that Chinook wind? I gather from SASless post's that take-off at "Max Possible Weight" was a bit hairy, as rotors were getting close to stalling. I can't imagine VRS of rear being much fun either.

[Boslandew]

Hi Graviman
I last flew the Chinook in 1984 so my memory is a bit rusty. I must stress that this only happened in the simulator and that it took an extreme manoeuvre to get into VRS. I don't think the Chinook was any more prone to it than other helicopters. The problem was that invariably, even if both rotors entered VRS which was not always the case, the fwd rotor came out of it first giving an indication that you were clear. Due to the very confused airflow under such conditions, the rear rotor might clear or it might not but then was prone to entering VRS again, hence the BAM as it did so. If you consider that fwd/aft ac pitch changes were made by differential collective inputs on each rotor and yaw changes by tilting each rotor in opposite directions, if one rotor is reacting and the other not, in addition to enormous power output differences, you have considerable control problems even before the rear rotor goes in and out of the condition.
As regards recovery, to my embarrassment, my co-pilot flew a quick recovery and I think he did it by a side-slip, separating the airflow through the two discs and allowing the rear rotor to recover being less affected by the airflow from the front disc.
I have no idea what sink-rate was required. The Chinook was operated to Performance 'A' conditions with attitude changes kept to a minimum and made slowly for passenger comfort either from the airfield or from the platforms to which we flew with slow descent rates and always with forward speed. I would doubt that under those circumstances anyone got anywhere near VRS in line service.
Because all flights were Perf 'A' in accordance with the graphs, even at MTOW, 48,500 lbs, performance, even One Eng Inop for base checks, was adequate and the blades were nowhere near stalling - the aircraft would not have been certificated if they had been.
I once flew an underslung load trial and due to an error in the measuring system of the water tank we were carrying the aircraft was nearly two tons heavier than planned. Every gauge was re-lined, we were on the two and a half minute power limit but there was no adverse reaction from the rotors.
Finally, the rear rotor going in and out of VRS, produced the most extreme movements I experienced, albeit in the simulator, in my 35 years heli flight. It was horrendous, of a scale to cause airframe damage I would. think

[Graviman]

Interesting read Boslandew, the more i learn about the Chook the more impressed i am. That redlined flight must really stick in the memory. It's about time i got a little heli flying in, although i can't see myself ever being anywhere near these sorts of limits!

[oldhornet]

Hi all,

New to the group and found it interesting. Thought I was one of very few to have experienced this in a 47. In the 70's in Germany returning from the UK with lots and lots of tentage on board (gw was probably around 37000 or 38000), I allowed the airspeed to drop to 0 spiralling down through an opening in the cloud layer. I only realized something was wrong by the geometry of my arm on the thrust lever. It felt way too high. When I came back inside and saw the airspeed at 0 and the rate of descent pegged down, I sideslipped. Left stick and right pedal and immediately felt the tremendous increase in g forces as we slid out into clean air. scared me pretty good. I demoed VRS with every student I had at Wolters in the 60's in TH-55s but this was a lot more frightening.

When I returned to base, our standards guy wouldn't believe me. I gave up trying to convince him after a few days.

Thanks to all for your input.