Vortex Ring In Autorotation

[bugdevheli]

Dr Wayne Johnson states in his book "helicopter Theory" That it is recommended that sufficient foreward speed should be applied to avoid the vortex ring state when making a vertical auto from low altitude following an engine failure! Does this suggest that certain helicopters have sufficient inertia to provide enough power to put them in a settling with power condition?
Bug

[wg13_dummy]

If he is on about a real engine failure, it is impossible to get into vortex ring.

I think what he may be refering to is when carrying out a PFL. It is strongly advisable to get forward speed on prior to raising the collective for the over shoot.

When carrying out a constant attitude EOL (which is pretty much vertical or at least with low G/S), I would suggest it is impossible to get into vortex ring!
Remember the old vortex ring state 'triangle of fire'. Low airspeed, high RoD, power on. Take one of those factors away and you remove the risk of the state.


And I'm sure that Nick will add that from low level, getting into full Vortex ring is very hard! It takes quite a bit of height for it to develop into full blown vrs.

[slowrotor]

Vortex ring state is impossible in unpowered autorotation.
Johnson must be talking about the transition from autorotation to the use of rotor inertia just before touchdown (powered from inertia).
The power from inertia is no different than if the power came from an engine, therefor VRS might be possible in theory.

I wondered about that myself for sometime.

[NickLappos]

The maneuver in question is not an autorotation, therefore VRS is entirely possible. For a drop from a hover of less than perhaps 500 feet, there would only be only a powered descent (albiet a very under-powered powered descent) as the rotor rpm serves as the "engine" power.

This is of course a recipe for disaster, as the ROD will be near infinite at "landing".

I have done intentional zero knot descents that passed slowly through 1000 fpm then through VRS and then to 6000 fpm autorotation.

[Hidden Agenda]

Oh dear me….it seems another urban myth has been shattered.

[oldbeefer]

Nick - I think you are going to confuse an awful lot of people here. We do 'zero speed autos' all the time and, although the ROD is higher than a 65 knotter, it is not excessive (perhaps 2500 fpm for your favourite French heli!) with a near normal Nr. OK, so we convert to a normal auto at 1000ft, but perhaps you could expand your explanation to help us lesser mortals understand what goes on.

[Overt Auk]

Now,

I dont feel fit to hold a candle to some of the contributers to this forum when it comes to technical matters but as I understand it:

Vortex ring occurs when the air gets confused as to whether it is passing down through the disk, as in normal flight or up through it, as in auto-rotation.

Descending, power off, whether at 100 kts or zero kts, does not risk vortex ring: the air goes up. Converting a zero speed auto into a normal auto does not risk vortex ring, just a very high rate of descent until normal forward speed is gained.

Arresting a vertical auto by pulling up the lever is, however a problem. The air does not know whether the engines are working or not. If there is sufficient inertia in the blades then air will be forced back down through the disk and "settling with power" will ensue even though the power is coming from the stored kenetic energy in the blades.

Expecting incoming.

OA

[NickLappos]

Both Overt and oldbeefer have it right.

In an engine failure from 100 or 200 feet, you might call that an "autorotation", but it is not. Until the ROD goes very big so that the flow pushes up through the rotor [when the rate of descent is perhaps 1 1/2 times the downwash velocity (1000 fpm for a robie)] the rotor is in powered flight, powered by the rotor energy due to the inertia.

The point I am trying to make is that the rotor does not care if the engine is running, that is not the way to tell if an autorotation is taking place. In a powered, engine off descent, it is possible to get into VRS at the point in the descent where the downflow is just turning into upflow, as Overt says.

[wg13_dummy]

Ah ha. Light bulb moment!

Thanks Nick.

Sounds like a great question for a 6 monthly. ;)

[topendtorque]

I wonder if the original quote / question refers to the sometimes sinking feeling one encopunters right at the bottom.

You know ~ Everything seems set up right, then - during the flare stage - whoosh downhill and pick it up with a quick collective flick- then one wonders s*^& what happened.
In my mind mostly I put it down to a sudden undetected wind shift, and thus losing effective lift, not incipient VRS.

During auto training I always demonstrate that it takes about three hundred feet of airspace to translate from full powered on flight - to stabilised autorotational flight. Therefore we don't hover below between three hundred feet and twenty feet, OK?

Of course with advanced training, as covered elsewhere, one can conduct a successful EOL from in between those heights given that a fair bit might be going for you, but you weren't stabilised in auto when doing it.

[wg13_dummy]

Ref your bit about the sink. Surely thats down to the loss of flare effect?

Ref your bit about 300' - 20'. Avoid curve springs to mind and the explanation of factors A, B and C equates to where you can and cannot successfully carry out an EOL.

[topendtorque]

"Ref your bit about the sink. Surely thats down to the loss of flare effect?"

Yes, but lost (i think) because I suddenly encountered a bit of a tailwind and the set up flare was suddenly irrelavant.

Other times when in nil to light wind (especially in a 47) and a flare was initiated a bit early , then laid off a bit and the resultant downwash was then flown down into, when the collective was checked and it dropped like a bomb, I have often wondered whether it was VRS. Certainly felt like it

"Ref your bit about 300' - 20'. Avoid curve springs to mind and the explanation of factors A, B and C equates to where you can and cannot successfully carry out an EOL."

Yes, training, good conditions, spot available and luck, otherwise don't fly in the avoid curve.

I'm not a fan of vertical EOL's and we do tend to merge our terminology in this forum.
For instance I always address an EOL as an Auto. of course the EOL will 1. always have the mandatory, touchdown,
2. not always the usual termination area (say a flare commenced from one foot of altitude for every knot of airspeed) and,
3. only sometimes the developed autorotational flight - at any airspeed.