Vortex Ring Autorotation
 

A question that has always troubled me.

If you are in fully established vortex ring state, why would entering auto not instantly exit the state?

Once the lever is fully lowered and the green arrow is going in the right direction, surely the vortex must disappear?


I am genuinely interested.
I am also very interested to know whether anyone has any wind tunnel smoke videos of a rotor in normal flight and in auto and vortex ring so I could see the actual effect visually?

I am having trouble visualising how the state could survive auto.

thanks.

Tourist, I believe it does exit VRS once the collective is fully lowered and the disc enters autorotative state.

However, as the IAS is very low and ROD very High, raising the lever again would put the rotor straight back into VRS.

The recovery therefore is to lower the lever fully and apply fwd cyclic to restablish IAS before raising the collective.

That's my understanding. Now where's a Test Pilot when you need one..........

You need low IAS, a high rate of descent and power applied to create VRS. Therefore to enter AR will eliminate VRS. But as stated above, if you apply power later on and you have still a low IAS and a high rate of descent you will find you immediately again in VRS...

Tourist - if you do search on pprune you should find posts by Nick Lappos about this - it is also referred to as windmill brake state.

So yes - fully lowering the lever will get you out of VRS but you need lots of height and the Nr will increase as the RoD flow increases through the rotor.

Dear Tourist
May I present you with the LINK to a post from years ago, written by Nick Lappos.
BTW, the whole thread is worth to read. Learned a lot from it and the banters between Lu Zuckermann (RIP) and Nick (not) are almost classics.

That's all agrees with my knowledge, however as we all know from actually flying in fully developed VRS, it actually takes quite a while to get full control back to the cyclic after you enter auto. You can be holding a lot of sick forward for quite a while before the nose drops.

The other thing is that fully developed VRS has a higher rate of descent than auto so fully lowering should instantly reduce your descent?

Thanks Rotor, that is indeed an interesting thread. I've started reading it but have not found the VRS section yet.

But then how do zero speed autos work?

Zero speed auto has no engine. To get back into VRS you need power.


Guessing - as I am no test pilot


I suspect that if you could hold the lever up long enough in a zero speed auto then you could get back into VRS, but you bleed energy so quickly that without an engine you can't hold the air flow long enough.


To have truly the same situation, you are talking about Auto from high hover. If you are high enough, you put the nose down and get some speed, so you avoid going straight down. If you are low enough, then straight down and cushion with the lever. In the latter case there is no VRS as the air does not get to recirculate (ground is close and it disperses the downdraught). Between the two heights, - you bend / break the machine and damage / kill those on board (hence the HV curve).

The link should take you directly to post 177.
He explains nicely, why you will not get into VRS while flaring or in a quick stop.

Not true though is it. When you raise the lever you are taking power from the stored energy in the head. The air does not know or care where the power comes from....

For some reason your link takes me somewhere else, but I tracked it down, thanks.
I don't think that is relevant to my question though.

If fully lowering the lever instantly gets you out of VRS, then you should have instant full control over pitch.

You could then rapidly lower the nose 30 degrees or so exactly like a recovery from a zero speed auto and climb away once past lets say 30kts.

Plus of course VRS is not instant to establish and you are descending away from your previous vortex so should be able to pull hard for at least a couple of seconds exactly as you might in the very bottom section of a zero speed auto to the ground.

I'm not saying all this is actually what happens, I am just trying to establish what is actually going on in my own mind.

Thanks crab, lots of interesting links to look through there, but unfortunately most of the interesting stuff Nick Lappos links too are so old they don't work.

Tourist


Exactly my point - if you could take power long enough you would get VRS. in light helicopters anyway, not enough stored energy for you to reverse the flow long enough to get into meaningful recirculation.

Zero speed autos are just that - no IAS. This could mean going backwards over the ground, at a set recovery height (I think 1200' for the Gaz piece) you select significant nose fwd to regain a positive trajectory over the ground if you wish or leave it to land with no fwd speed at all. Switching the engine off during the descent makes it an EOL and no longer an auto.

I get what Tourist is asking: If the RoD in VRS is *higher* than an autorotative descent, wouldn't lowering the pitch completely and entering a zero-speed auto reduce your RoD and return cyclic control?

Answer: P-r-o-b-a-b-l-y...but there are a lot of variables. If you've allowed FDVRS to develop, then transition back to zero-airspeed auto is going to take some time. And when you're plummeting to earth at 2000 fpm or greater you don't really have a lot of time to play with. If it's "just" IVRS, then lowering the collective fully and pushing the nose down is going to give you a serious rate of descent.

These discussions are largely theoretical. Prouty admits that the air and airflow through a rotor is so chaotic that wind-tunnel testing of VRS models is not really super-accurate. The airframe of the little model helicopter does not move in the environment like a full-scale helicopter in the real world. The wind tunnel air is pretty constant, unlike that produced by Mother Nature.

Secondly, nobody gets into FDVRS way up high with lots of altitude and time in which to recover. If you ever did, then it really doesn't matter which technique you choose.

Mind your head
 

Whilst entering auto from a VR state may theoretically remove one of the contributing factors, I'd suggest that you consider the design of the particular rotor head. Entering auto from level flight, the rate of descent builds up slowly. Imagine the shock loading on the head if you already have a rate of descent of 2,000 ft per minute and suddenly flatten the blade pitch to present its full surface area to the RoD airflow. It was the great Bill Barrell who advised me (many years ago) that the technique was inappropriate for teetering heads particularly. I think his exact words were "Don't be a f#*king idiot!"
:=

I would think it would wind the Nr up very quickly if you introduced that much airflow suddenly into the disc from below with zero collective pitch!

Tourist - you don't carry out the EOL from a zero speed condition, you adjust to normal auto speed before that and only use the zero speed to help you reach your aiming point. A constant attitude EOL is still done from 30-40 kts IAS so lots of forward speed.

However, raising the lever in a zero speed auto will simply decay the Nr, way before you get to any chance of opposing the upward flow through the disc and revisiting VRS - you would have slowed and stalled the whole rotor and properly fallen out of the sky. It would reduce your RoD a bit, but only temporarily.

There is nothing in the rules that says you cannot finesse entry into autorotation by lowering the correct amount to stop the Nr going crazy.

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

Doing the helo conversion one of the demonstrations was VRS. A climb to 14,000 in the Huey (with parachute) and damned if the instructor (Jerry Hardy) could get the aircraft to comply, pulling collective to max permitted merely reduced the ROD.

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!

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.;)

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.

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.

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.

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.

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.

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

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)

http://www.pprune.org/[IMG]http://ww...y59e_thumb.pnghttp://www.dumpt.com/img/links.php?f...ofrepfy59e.pnghttp://www.dumpt.com/img/files/9m8lg...y59e_thumb.png
http://www.dumpt.com/img/links.php?f...ofrepfy59e.png

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.





http://www.dumpt.com/img/viewer.php?...m1zhvqbkqr.png

No such thing as a VortexRingAuto :ugh: they're not synonymous :8

Ancient British Graph here:

http://www.dumpt.com/img/files/kr189...0hrl_thumb.png

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

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,

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


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.

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,

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.

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?

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).

Vortex Recovery
 

Mornin' [email protected]
My point exactly; Auto's & Vortex states, don't mix :O but are mentioned as a form or part of a recovery method from Vortex :ooh: 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 :8

HappyHappyLandings :cool:

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.

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.