Vortex Ring Autorotation

[rotorfossil]

I think the idea of entering auto to get out of VRS is a bit academic. Vortex ring accidents almost always occur near the ground so there isn't time for that sort of recovery. I seem to remember way back that a Sycamore at CFS/H was lost in that sort of scenario and it was reckoned that the rrpm was setting off around the dial for the second time due to the high rate of descent. Maybe memory getting hazy now after 50 odd years!

[ShyTorque]

Not when I went through Wallop. I still remember with some horror the zero speed EOL. It was near enough vertical in the Gazelle.

Having instructed in the military Gazelle, I think you're confusing zero speed with constant attitude, or were doing it wrong. A zero speed auto to the ground / EOL would result in a negative ground speed in anything other than zero headwind and that was definitely not taught.

Don't forget that helicopter ASIs don't register properly at very low IAS and it may appear that the speed is zero when it's not.

[[email protected]]

And, having instructed at Wallop on the Gazelle, I know you are getting confused as Shy says.

A zero speed auto is used when you are close to your only available (or least worst) landing area - bringing the speed back allows you to keep the point in view as it moves up the windscreen. When you judge the aiming point to be in the right place, you adjust the speed back to 60 kts (in the Gaz) ready to carry out your variable flare EOL.

A constant attitude EOL was flown at 35 - 40 kts and could be quite steep if the wind was strong (but never backwards) - this was used when you couldn't get enough speed for a VF EOL or when one was an unsuitable choice (night reversionary for example). They could look scary but were quite straightforward and lots of fun.

As for finessing entry to auto - you are talking about recovery from VRS which, due to the rapid height loss, doesn't really allow for finesse - if your normal auto RoD is 2000'/min and your VRS RoD is 4000'/min getting into a proper auto/windmill brake state is going to be tricky.

The advice on lowering the lever in VRS is to try and unstall the root and lessen the tip votices to give you some semblance of control - it doesn't have to be all the way to min pitch.

[Tourist]

Ok, my fault, but we are slightly getting away from my original question here. (Shy yes you are probably correct about the speed. It was a while ago!)

I am well aware of standard recovery techniques.

My question is merely this.

Why is vortex ring not instantly solved as soon as auto is entered?
The vortex should be disappearing above and behind you.

Yes you would still be slow with a high rate of descent, but with full control recovered, adding 30-40kts should be very quick plus it does take a little while for a new VRS to form as we all know from trying to enter it.

[ShyTorque]

Tourist, Prouty explains that an aircraft can achieve "windmill brake" conditions; i.e. the rotor is being driven by the upwards airflow and falls at a rate roughly equivalent to one where a parachute of similar diameter was fitted, rather than rotors (don't ask me to do the maths on that....). Therefore a state of VRS still exists in autorotation (if recovery actions aren't taken by then) and may be the final "symptom" of fully developed VRS.

He also states that there is evidence that a true vertical descent in fully developed VRS may result in a lower ROD than one at a 70 degree descent angle, i.e. one with some forward motion. It appears that as the aircraft is recovering, a transitory stage of a higher ROD may have to be passed through.

[[email protected]]

If you accept that VRS is caused by a strong vortex at the tips as the upflow opposes the normal downward flow that you (in normal flight) are trying to induce with collective - and that secondly the root ends of the blades are stalled because of the very high AoA - - -then lowering the lever fully will remove both conditions.

There was always talk of a bubble of air trapped beneath the rotor in FDVRS which would move randomly, burst and reform - it was this that caused the random pitch roll and yaw in theory.

Whether that has been proven, I don't know but it would still be interesting to see what effect going from a small parachute (rotor disc enveloped in FDVRS) with only a small part producing lift, back to your normal size parachute (lowering the lever to establish normal autorotation) would have, especially transiently.

It is this part that would answer your question Tourist - does it happen like a switch or are there a few interesting stages before normal auto is resumed.

[Tourist]

Crab

Yes, that is what I have been wondering and why it would be really nice to see some wind-tunnel/smoke videos. I am quite sure they must exist, but I can't find them anywhere.

Shy

I need to do some reading about windmill brake to make sure I understand it. Do you have any valid links? I can't find any that still work from the old thread referenced earlier.

[[email protected]]

Tourist - googling it will lead you to lots of maths It is about how the blades extract energy from the air passing through them to keep them turning - exactly as in autorotation.

However try this

http://naca.central.cranfield.ac.uk/...rc/rm/3117.pdf

[AnFI]

Wow Crab

That's a really quaint peice of history, Britain at it's best etc

Interesting to note:

1 that there was such a violent nose down tendancy in those earlier types and since no horizontal tail surfaces then one presumes (i think you can see it) that the Center of Pressure from the Arm and Area of the tail boom is way aft of the rotorhead. (they describe running out of aft cyclic to stay in VR)

2 they still describe roughness, even though a really good vortex ring state can be hyper smooth, maybe the slipstream of around the boom? Or not fully formed.

3 the center of the phenomenon is centered symetrically around the zero airspeed axis in the Brit version. UNLIKE the center of the phenomenon shown in Rotorbees graph (Origin US Military?) which seems to show a maximum at about 65degrees angle of incidence (VERY SURPRISING THAT, maybe not true, see Brit version below)




On reflection the US MIL version that Rotorbee shows would be the right zone for VIBRATION but is not showing where VR is. It is effectively showing where the vibration zone is, on the edge of VR, rather than the VR itself. Whereas the Brit version is showing where you would find VR.

By using 'Vi' in the graph you can see that it is possible to sustain VR to higher ROD by adding power (more Vi) and increasing the ROD than you can stay in VR to. Hard to stay in FDVR, never seen any evidence that it is hard to get out of, only hearsay.

[Vertical Freedom]

No such thing as a VortexRingAuto they're not synonymous

[AnFI]

Ancient British Graph here:



though it looks like the "Vortex Ring State" label
should be in the place they've put "Region of Roughness"

[Sokol]

[...]Why is vortex ring not instantly solved as soon as auto is entered?[...]

Aeroelastics.

You get into VRA because the main Rotor blades pump too much air for the environment.(Same thing happens when you heat up oil and those little bosses occur - another story)
This happens mainly because the Rotor blade is elastic itself and steered by the inner part. So the inner part has an higher AOA for a short time which supports the outer vortex with the energy it needs. (Would be interesting if any K-Max pilot has gone into an VR)
Then you press the stick forward because the airstream around the tail boom is more directional. (As seen in the paper, backward movement increases the vortex in front which is seen by increasing angle in 4.2)
IAS will help you now by pumping the Vortex away.

When you simply pitch to auto you will gain no IAS. And the Airflow has to cope with the new AOA firstly, as i said, from the inner (part of the)blade to the outer (part of the)blade. This does brake your rotor. After you got out of the Vortex you still have to cope with an high rate of descent, which you cant do just by pitch. It yust shortens your time to evade obstacles.

Greetings,

[[email protected]]

Vertical Freedom - try reading the thread instead of just the title. No-one is saying that VRS and autorotation are the same


AnFI - I'm not holding that paper up as definitive but highlighting that this is not a new issues and was investigated many years ago. However it is interesting to note the rates of descent are much lower in the old Brit document, due to the much lower disc loading (and power available) of those early helicopters. Modern aircraft have higher disc loadings and stronger downwash so the RoD required to catch up with that downwash is higher.

[Sokol]

[...]note the rates of descent are much lower in the old Brit document, due to the much lower disc loading (and power available) of those early helicopters.[...]

This is not the Reason.

The reason is to be found in the shape of the early blades, of whom are designed to distribute the same lift(by massflow) on each diameter.
As you can see in the document the blades thicken up to the inside as seen from above.
Modern blades wont do that, so the inner part of the blade has an lower lift resulting from the lower speed, this does not support the outer vortex. Ergo: higher rate of descend possible without getting into VR.

Greetings,

[ShyTorque]

The reason is to be found in the shape of the early blades, of whom are designed to distribute the same lift on each diameter.
As you can see in the document the blades thicken up to the inside as seen from above

Not the case for the blades of many older helicopters. They were very simply made, with a constant profile along their length.

The Whirlwind, which I flew very early in my career, was reputedly able to enter VRS at a rate of descent not much over 300ft/min. The blades were made from a simple, hollow metal spar, with metal pockets bonded on to form the constant blade profile along its length.

My present aircraft's RFM tells me to avoid rates of descent of over 900ft/min at low speed.

[n5296s]

The blades were made from a simple, hollow metal spar, with metal pockets bonded on to form the constant blade profile along its length.

Sounds like a good description of the R44 blades!

I wonder what the REAL figure is for the R44? I'd really like to know how much margin I have when I'm flying a steep approach at 300 ft/min - will I plummet earthwards at 310 or is it (which I'm pretty sure I've seen somewhere) above 500 before bad things happen?

[7478ti]

Vortex Ring State and Windmill Brake State nicely explained

A good description of all this, and some very useful diagrams, are found in Wayne Johnson's book "Helicopter Theory", page 99 section 3-1.2.2 for Vortex Ring State, and on page 101 section 3-1.2.4 for Windmill Brake State. As I recall, at least some of this clear explanation originally derives from Prof. Norm Ham's excellent work (a colleague of Prouty).... (when both Wayne and Mike Scully were a graduate students of Norm's, back at MIT Course 16, in the late 60s and early 70s).

[Vertical Freedom]

Vortex Recovery

Mornin' [email protected]
My point exactly; Auto's & Vortex states, don't mix but are mentioned as a form or part of a recovery method from Vortex here?

If in a Vortex state of some description; leave the Power On, apply more Power if You can (red) whilst smartly shoving the stick forward..maintain same heading....get back Your translational plus airspeed ASAP

Thou machine, needs You to stay at, or above ETL always (unless confined approach,slinging,film,fire) unless your coming into ground effect for a landing (terrain?)

Avoiding Vortex is the Best option, ETL is a big Key, a Saviour...You'll never get Vortex at or above ETL

HappyHappyLandings

[Tourist]

VF

You are still not reading.

Nobody is putting forward a vertex ring recovery here. I am merely asking why something doesn't work.

I think everybody here knows how we do recover and I'm not interested in the usual advice about avoiding it in the first place. This is an aerodynamic question, not a practical one.

[rotorfossil]

N5296s. The real figure is higher, but Robinsons have to set a conservative figure for liability reasons. The problem is that that the ASI readings are unreliable at low speeds and the VSI suffers from large lags. From a practical point of view, all you can say is that you won't get VRS at 30kt/300fpm, but under certain conditions of sudden changes of flight path it is POSSBLE to get it at lower air speeds and higher rates of descent.