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Tourist
19th Feb 2016, 05:59
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.

DOUBLE BOGEY
19th Feb 2016, 06:15
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..........

FICH
19th Feb 2016, 06:21
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...

19th Feb 2016, 06:40
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.

Rotorbee
19th Feb 2016, 06:42
Dear Tourist
May I present you with the LINK (http://www.pprune.org/rotorheads/158490-helicopter-urban-myths-6.html#post1717027) 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.

Tourist
19th Feb 2016, 07:24
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..........

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?

Tourist
19th Feb 2016, 07:26
Dear Tourist
May I present you with the LINK (http://www.pprune.org/rotorheads/158490-helicopter-urban-myths-6.html#post1717027) 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.

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

Tourist
19th Feb 2016, 07:27
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...

But then how do zero speed autos work?

John R81
19th Feb 2016, 07:36
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).

Rotorbee
19th Feb 2016, 08:21
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.

Tourist
19th Feb 2016, 10:00
Zero speed auto has no engine. To get back into VRS you need power.


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....

Tourist
19th Feb 2016, 10:09
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.

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.

Tourist
19th Feb 2016, 10:30
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.

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.

John R81
19th Feb 2016, 10:53
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.

Thomas coupling
19th Feb 2016, 12:37
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.

FH1100 Pilot
19th Feb 2016, 15:04
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.

Cylinder Head
19th Feb 2016, 16:19
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!"
:=

19th Feb 2016, 17:03
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.

Tourist
19th Feb 2016, 22:35
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!


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.


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.


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

megan
19th Feb 2016, 23:57
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.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.

rotorfossil
20th Feb 2016, 06:34
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
20th Feb 2016, 08:16
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.;)

20th Feb 2016, 09:10
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
20th Feb 2016, 10:43
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
20th Feb 2016, 12:05
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.

20th Feb 2016, 12:59
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
21st Feb 2016, 10:23
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.

21st Feb 2016, 10:50
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/reports/arc/rm/3117.pdf

AnFI
21st Feb 2016, 14:05
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://www.dumpt.com/img/files/9m8lguhgcjofrepfy59e_thumb.pnghttp://www.dumpt.com/img/links.php?file=9m8lguhgcjofrepfy59e.pnghttp://www.dumpt.com/img/files/9m8lguhgcjofrepfy59e_thumb.png (http://www.dumpt.com/img/viewer.php?file=9m8lguhgcjofrepfy59e.png)
http://www.dumpt.com/img/links.php?file=9m8lguhgcjofrepfy59e.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?file=c810f7u3zbm1zhvqbkqr.png

Vertical Freedom
21st Feb 2016, 15:00
No such thing as a VortexRingAuto :ugh: they're not synonymous :8

AnFI
21st Feb 2016, 15:42
Ancient British Graph here:

http://www.dumpt.com/img/files/kr1898v326yqba8u0hrl_thumb.png (http://www.dumpt.com/img/viewer.php?file=kr1898v326yqba8u0hrl.png)

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

Sokol
21st Feb 2016, 16:07
[...]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,

22nd Feb 2016, 05:10
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.

Sokol
22nd Feb 2016, 22:43
[...]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
22nd Feb 2016, 22:56
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
22nd Feb 2016, 23:16
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
23rd Feb 2016, 00:32
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
23rd Feb 2016, 02:20
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:

Tourist
23rd Feb 2016, 02:50
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
23rd Feb 2016, 06:04
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.

23rd Feb 2016, 06:55
Sokol - while blade shapes have changed significantly, the disc loading and therefore the downwash has changed enormously, mostly due to the gas turbine engine with its far superior power to weight ratio.

You can either take a large area of air (big rotor disc) and accelerate it a small amount (uses less power) or have a smaller area of air (smaller rotor) and accelerate it a greater amount (uses more power) to achieve the same rotor thrust to balance the mass of the aircraft. Modern rotor discs can be smaller because the engines are more powerful - hence faster downwash speeds.

Light pistons such as the R22 still have relatively low disc loading, hence they are more vulnerable to VRS because their downwash speeds are slower and it is easier to catch the rotor wake up in a descent.

The biggest change in helicopter blades design is not the lack of taper but the change in aerofoil sections, moving away from symmetrical to cambered shapes, especially with composite blades where that section can be altered along the length of the blade. Additionally, the lift is varied along the blade using twist rather than taper so the root is at a higher pitch angle than the tip.

Sokol
23rd Feb 2016, 09:47
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.

Lower stiffness of the Blades, higher tendency to twist.

The biggest change in helicopter blades design is not the lack of taper but the change in aerofoil sections, moving away from symmetrical to cambered shapes, especially with composite blades where that section can be altered along the length of the blade. Additionally, the lift is varied along the blade using twist rather than taper so the root is at a higher pitch angle than the tip.

There is a difference between high mass flow and high velocity, read about it.
Plus: Composite(plastics) blades have a tendency to higher stiffness, so the AOA in the center of the disc is not the same as with an Steel/Wooden Blade.
Disc load: Same helicopter, MTOW and empty(pilot and fuel are abord...) which gets earlier into VR?

Greetings,

Wirbelsturm
23rd Feb 2016, 10:51
https://www.youtube.com/watch?v=pAwxHGBA2_I

Unfortunately height is not always available!

The guy who gets thrown out of the forward personnel door is a good friend of mine. Spent a fair bit of time in plaster! The crewie, who jumps from the cargo door to pull the guys out, has a broken spine!

The cause was attributed to 'settling with power' over a hot concrete surface with light winds and strong thermals.

Not a nice place to be.

Doesn't help the debate but shows the unintended consequences! :sad:

ShyTorque
23rd Feb 2016, 10:57
Lower stiffness of the Blades, higher tendency to twist.

I think all helicopter pilots of my background are aware of that - we all got to see the film clip of the Whirlwind blade taken from a camera on the rotor head many years ago.

But what point are you trying to make by the above statement? You said that older designs of MR blades changed in profile along their length. I pointed out that that wasn't always the case.

Sokol
23rd Feb 2016, 11:06
[...]But what point are you trying to make by the above statement?[...]

That sometimes materials (or more precisely: thier usage) are the reason too. So you are right.

Note: The question was originally why the helicopters in the document get into VR this early. I answered this question as questioned.

Greetings,

SimonK
23rd Feb 2016, 17:46
If anyone's interested (probably not ;)), this is what happened to me:

Puma HC1, 6000' at night for an incipient VRS demo. The only way my friendly QHI could get her in was by going backwards and obviously down, I watched the VSI like a hawk and it was as we passed >800' rod and 0kts on the asi that we properly entered. I think it caught him out judging by the swearing coming from the RHS and the immediate and instant reaction was the aircraft went from approx 10 degrees nose up to what felt like 90 degrees nose down, but must have been at least >45 degrees. Simultaneously the aircraft rolled 180 degrees and this was definite: the town that was on the rhs was now on my side. I honestly thought I was about to die and the recovery took a surprisingly long time. We actually came out of sorts at 1500', with a full balls width of slip and 30kts airspeed. I guess the facts are: we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees.

I guess some of the exact detail may be lost to time but I wrote it all down afterwards and remember it clearly to this day. I've been told by several Puma pilots of old and bold that you can't get a Puma into full VRS, but I promise that is what happened. After my 'incident' all incipient VRS was done in the wonderful MSHATF.

Certainly don't want to ever get into that again :D

23rd Feb 2016, 18:52
Sokol Disc load: Same helicopter, MTOW and empty(pilot and fuel are abord...) which gets earlier into VR? The empty one - rotor thrust must equal mass so the heavier helo has to accelerate the air down quicker using more AoA and giving a faster downwash speed - you point is?

ShyTorque
23rd Feb 2016, 19:45
SimonK, I'm not sure why your Puma QHI was trying to demo that, it certainly wasn't in the OCU syllabus!

Having said that, from personal experience of a similar nature, I can confirm that it certainly IS possible to get a Puma into FDVRS, even when you're not trying. I wasn't flying when the aircraft entered; but although I was in the LHS (but not acting as QHI), I did eventually "assist with diagnosis and recovery action" before we finally hit the ground with the VSI pegged hard on the bottom stop, in hostile territory too. We went from around 10,000 feet at max continuous power (job required a very high OGE hover) to ground rush very quickly indeed. It was twenty five years ago but I still recall it quite easily. Something I wouldn't wish to repeat in a hurry.

Sokol
23rd Feb 2016, 20:19
you[r] point is?

Weīll get onto that later, another question:

Which helo (full or empty) decelerates faster in descent?

Greetings,

23rd Feb 2016, 20:31
Sokol - greetings
Which helo (full or empty) decelerates faster in descent? that wholly depends on what forces are acting upon it - another pointless question???

If you mean 'will a heavier aircraft take more power to arrest a descent than a light one?' then the answer is yes but that is't pertinent to the discussion about VRS - more pertinent to SWP.

SimonK
23rd Feb 2016, 21:12
Hi ShyTorque,

No it wasn't in the OCU syllabus but it was for the same thing you were doing! Think it was a Fooked up demo basically but there you go, did a few corkers myself when I was a QHI aswell....but not quite that bad ;)

Cheers!

ShyTorque
23rd Feb 2016, 21:33
The "old and bold ones" who tried to tell you it wasn't possible possibly never actually got a low enough IAS, just less than 40 kts when the ASI went to sleep.

Backwards and down trying to hover up there was always bad for the grey hair count; that's also what happened to us.

I used to aim to achieve the high hover from a gently climbing straight quickstop, far more predictable and quieter on the ground, too.

AnFI
23rd Feb 2016, 23:10
Quite right TOURIST

You wouldn't even need to do as much as 30degrees and 30kts, before you re-applied power and climbed away.

and you are right you can make vortex ring from autorotation, but it's a fairly extreme scenario. If you auto vertically at zero airspeed (ie zero wind day and vertical) and you just pull hard at the bottom there is a strong chance of just 'falling through' as you make a vortex rather than significantly arrest your descent. If you are just off zero speed at say 10kts airspeed its still a pretty hairy arrival anyway, but worth staying composed, unlikely to kill you if you stay flat. Depends on the helicopter. (rotor energy / power required gives useful 'hover time')

AnFI
23rd Feb 2016, 23:31
There is something odd going on here:

ShyTorque: "from around 10,000 feet " "before we finally hit the ground with the VSI pegged hard on the bottom stop"

and

SimonK: "6000' at night for an incipient VRS demo." "we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees."

and

Democritus (I think): '5000ft lost unable to get out of the condition' with a test pilot

It seems something is going on because these stories don't square with the many people who do fully developed VR demonstration regularly, without any big deal.

Is it possible that there are helicopters with these terrible characteristics?
If so we need to find out.
Can anyone shed some light on it?

Sokol
24th Feb 2016, 09:15
that wholly depends on what forces are acting upon it - another pointless question???[...]

You are a very patient man.

[...]If you mean 'will a heavier aircraft take more power to arrest a descent than a light one?' then the answer is yes[.][...]

Speaking of power: The power you need depends on the forces on the Helo as you said. Mechanical force is mass multiplied by acceleration. Following this law and your statement:

The empty one - rotor thrust must equal mass so the heavier helo has to accelerate the air down quicker using more AoA and giving a faster downwash speed[.]

The lighter helo has to decend faster to get in the same state as the heavier helo, right?

Greetings,

24th Feb 2016, 14:21
Sokol - greetings The lighter helo has to decend faster to get in the same state as the heavier helo, right?
No.

If the heavy helo has to displace more air (same size disc) quicker to balance the mass (in the hover) then the speed of the downwash must be higher. That is where F = MA comes in.

If you require a rate of descent of at least half your downwash speed to get into VRS, then the heavy helicopter must have to descend at a faster rate than the lighter one.

If your Vi is higher (as it is in the heavy helo) then your Vd has to be higher to reach the required ratio for VRS.:ok:

Sokol
25th Feb 2016, 17:30
If the heavy helo has to displace more air (same size disc) quicker to balance the mass (in the hover) then the speed of the downwash must be higher.

So you are saying that the lighter helo wouldīnt have a downwash speed as high as the heavy helo?
(You can refer this question right to the which helo decelerates faster question)

We are up to the point, explaining why there are so raw windtunnel tests.

Greetings,

26th Feb 2016, 05:28
So you are saying that the lighter helo wouldīnt have a downwash speed as high as the heavy helo? yes:)

wouldn't:ok:

Sokol
26th Feb 2016, 09:56
Sorry, I haveīnt specified my question enough.

I meant the initial downwash(when you start pitching up), both Helicopters are in the same pitch, refered to the helicopter and not to the environment.

(the environment then is the key factor to get to the point)

Greetings,

26th Feb 2016, 14:14
Sokol - sorry, I don't understand what you mean.

Sokol
29th Feb 2016, 20:45
If you descend with the Heavy and Light helo at the same speed, pitching up both the same manner you get exactly the same initial downwash speed on both helos (refered to the helicopters)

The only difference is the deceleration rate, at early stages the heavy helo simply falls through the VR, with the higher integral downwash compared to the light to the environment.
Also the deceleration rate plays a role. Wich is for shame not linear, now try to get an nonlinear speedchange in an windtunnel.

Disc load itself is only a mathematical simplification for power calculations since the blades on varying modern day helicopters are nearly equal.
A smaller disc is actually providing a VR, since the outer vortex has to be smaller, the proper reasons for higher disc loads are:

Smaller diameter provides landing nearer to obstacles
Materials, for shure(since we have seen that tapered blades also provide VR)
Blade systems (with more blades the diameter decreases)

Greetings,

1st Mar 2016, 06:57
Sokol - I'm still not sure what your point is regarding VRS.

As for your assertions about rotors, might I suggest you read Prouty to better understand how helicopters have developed over the years.

Can I ask, are you in fact a helicopter pilot?:ok:

Rotorbee
1st Mar 2016, 14:49
Oh, please Crab. You certainly read his profile. Ray Prouty wasn't a pilot either (AFIAK). We all know there are many stick jockeys out there that still believe in gyroscopic precession in a rotor. Being a helicopter pilot is not really a proof of understanding helicopter aerodynamics.

Sokol, I agree with you, that the disk loading is just a mathematical simplification, but the downwash speed on a helicopter with a lower disk loading is really lower than one with a higher disk loading.
An approximation for this in a stable flight condition is:
square root(M(helicopter)/(2x(air density)x(disk area))
And that has nothing to do if you are going up or down or around the church spire.

The higher the mass of the helicopter or the lower the air density or the disk area, the higher the downwash speed.

Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds.

1st Mar 2016, 15:36
Rotorbee - no, I don't make a habit of reading people's profiles but now I have, I understand Sokol's viewpoint - ie theoretical rather than practical.

You are right that not all pilots understand P of F but plenty of engineers think they know more about cockpits and flying than most pilots:ok:

Rotorbee
1st Mar 2016, 16:18
Give that man a cigar.
So true, so true, and not only in aviation. :D
Try IT. Everyone is expert in that world.

Sokol
2nd Mar 2016, 09:07
Oh, please Crab. You certainly read his profile.Thatīs true. I am no pilot, doesīnt that allow me to ask pilots about thier expierience? I am shure that is one of the main problems in the aviationworld.

(e.g. understanding that and why an lighter helo gets into VR faster came until this discussion)

but the downwash speed on a helicopter with a lower disk loading is really lower than one with a higher disk loading.From a theoretical point of view: I can design you a blade with higher length that has the same characteristics as the shorter one, my problem is only the (not superstiff) materials. And that was a point I always mentioned.

Blade load would be a more appropriate term. But I do agree that the formula gets really complex though.

The higher the mass of the helicopter or the lower the air density or the disk area, the higher the downwash speed.True. In stable flight.

Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sledsEmbarassing that I didīnt thought of it. :ugh:

You are right that not all pilots understand P of F but plenty of engineers think they know more about cockpits and flying than most pilotsI do know a bit more about aerodynamics, thank you for sharing your flying expierience with me :ok:

Try IT. Everyone is expert in that world.This is mainly an communication problem of people who areīnt able to change thier opinion(for example about engineers, short introduction into it: https://youtu.be/BKorP55Aqvg)

Greetings,

2nd Mar 2016, 13:09
Thatīs true. I am no pilot, doesīnt that allow me to ask pilots about thier expierience? I am shure that is one of the main problems in the aviationworld, Sokol - This is a Professional Pilots Forum but you are more than welcome to ask any questions you like:ok:

However cleverly you design the blades - a heavier helicopter always has to displace more air than a lighter one in a stable, free-air hover - hence the higher downwash speed:)

Rotorbee
2nd Mar 2016, 19:00
Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds
Embarassing that I didīnt thought of it.

See Sokol, that's why you should read Prouty ... and some NACA papers ... or Castles and Gray who did wind tunnel research on VRS. I don't really see a problem with wind tunnels. It is better than real live, because you can keep the helicopter in that area with the highest turbulence for any length of time, while flying in there on purpose and staying there is according to researchers rather difficult. Prouty thought, that the pilots got only a highly transit state during the test flights.

At the same downwash speed, the heavier ship will fall faster into or through VRS. Is it that what you mean? In that case, that would be obvious. Otherwise I still don't get your thing with the constant downwash speed either.

By the way, the formula would not be that much different. There is a similar formula that uses the rotor diameter. Blade load or disc load is more or less the same thing. It only gets ugly, when you want to find out the regions where the blade stalls at what Vd and it gets even worse if you have to take blade twist into account.

Who knows, if this (https://books.google.ch/books?id=lTkiuG4RaFIC&lpg=PA403&ots=0teQ5DFGnC&dq=vd%2Fvi%20vortex%20ring%20state&hl=de&pg=PA402#v=onepage&q=vd/vi%20vortex%20ring%20state&f=falsehttp://) brings us any further.

Known that video for a long time. Been in IT for a long time. Had to deal with smart-arses in IT for a long time. It's not a communication problem, believe me.

SimFlightTest
2nd Mar 2016, 19:04
Crab,


While a heavier aircraft does displace more air in a hover than a lighter one, this does not imply a higher downwash speed. The rotor diameter can be increased, hence resulting in more air being displaced for the same downwash.


Imagine two rotorcraft of the same weight, but one has a rotor diameter of 1 mile, while the other has a diameter of 1 inch. The downwash for the 1 mile rotor would barely be perceptible, while the 1 inch rotor will essentially be a rocket.

2nd Mar 2016, 20:36
SimFlightTest - yes I know that and if you read back a few posts you will see that the discussion was qualified by the 2 helicopters having the same size of disc.

Sokol
3rd Mar 2016, 19:42
See Sokol, that's why you should read Prouty ... and some NACA papers ...Well, thanks fo that advice. As my mayor is in turbofan engines I couldīnt think of reading about it.

However: Thanks for literature recommendation.

I don't really see a problem with wind tunnels.The problem occurs simply in scale....

At the same downwash speed, the heavier ship will fall faster into or through VRS. Is it that what you mean?The thing is only the point and time you are referring to. The downwash referred to the environment is higher on the heavier helo after an infinitessimal short time period after inducing the same controls on both helos.

In that case, that would be obvious.Not for a man that only refers to the environment 3 times during layout. And thought (honestly) first about an magnetofluid like behavior.

Blade load or disc load is more or less the same thing.Interesting. If I do build an helo with 2 Blades and an helo with 5 Blades of the same shape as the 2 bladed one, that changes nothing.

Who knows, if this (https://books.google.ch/books?id=lTkiuG4RaFIC&lpg=PA403&ots=0teQ5DFGnC&dq=vd%2Fvi%20vortex%20ring%20state&hl=de&pg=PA402#v=onepage&q=vd/vi%20vortex%20ring%20state&f=falsehttp://) brings us any further.Thanks for that document. I have to read firstly through it, looks a bit like a bernoulli-based calculation, well I think Navier stokes would be more appropriate since the lift isīnt equal over the diameter.

Greetings,

Rotorbee
4th Mar 2016, 05:41
Interesting. If I do build an helo with 2 Blades and an helo with 5 Blades of the same shape as the 2 bladed one, that changes nothing.

OK, my bad. I was fixated on the 2 bladed head. There is BTW, an interesting piece in the Prouty, how to design a helicopter and how to decide on the number of blades.

Not for a man that only refers to the environment 3 times during layout. And thought (honestly) first about an magneto-fluid like behaviour.
Would you care to explain? I don't get it. Both.


The problem occurs simply in scale....
Why? VRS occurs in every size of rotor. The drone web-community is full of tips and tricks for avoiding VRS and so are the RC sites.

The thing is only the point and time you are referring to. The downwash referred to the environment is higher on the heavier helo after an infinitessimal short time period after inducing the same controls on both helos.

Ok, real world behavior. In the infinitessimal short period of time after a control input absolutely nothing happens. Rotors suffer from a phase lag, because the blade is not stiff. OK, that was unfair. The point I want to make here is, that helicopter design involves to the day still trial and error. Why computed fluid dynamics get better and better, airplane aerodynamics are easy compared to a helicopter. Have you ever watched that video of a rotor blade in flight (https://youtu.be/Ug6W7_tafnc)? Worth a look. At every instant the blade flexes and therefore AoA and lift change at every point along the blade. And it's going round. And its going forward. And the fuselage does have some influence. And behind the rotorhead everything changes again. Everything you calculate will still be further from the real world than on an plane.

The VRS is, while quite good understood (but not completely), not a very important subject in helicopter aerodynamics. I believe there wasn't any fundamental research on this for decades. Despite some self-proclaimed pilot gods opinion, it does not kill that many people. CFIT is way more important, but that has nothing to with aerodynamics.

If you are interested:
Shawn Coyle: Little Book of Autorotations
Shawn Coyle: Cyclic & Collective
These are as close to the bible for pilots as it gets.

4th Mar 2016, 06:30
Interesting. If I do build an helo with 2 Blades and an helo with 5 Blades of the same shape as the 2 bladed one, that changes nothing.
let's hear it for rotor solidity ratio........ how many helicopters with a small, two-bladed system do you see?

Sokol - yes scale is an issue with wind tunnels but quite a well understood one - have you studied Reynolds numbers?

Sokol
4th Mar 2016, 07:18
Sokol - yes scale is an issue with wind tunnels but quite a well understood one - have you studied Reynolds numbers?Well, Reynolds numbers are for laminar incoming flow. Which you canīt expect fully on an rotorhead. (Prandtl)

There is BTW, an interesting piece in the Prouty, how to design a helicopter and how to decide on the number of blades.for shure it is :ok:

Would you care to explain? I don't get it. Both.Engines: Environment only happens on Intake and exhaust. Intake is one variable. Exhaust is driven by two: core and bypass. That are the only three references to the environment, the rest is (mostly) thermodynamics.

Magnetofluid: a long story. Oil in your pan is for example a magnetofluid. If your oil gets hot, little bosses occur. This are convection rolls. These can be vectored by an induced magnetic field.
Private message for more information.

Why? VRS occurs in every size of rotor. The drone web-community is full of tips and tricks for avoiding VRS and so are the RC sitesIt is not the size itself, it is more the reynolds numbers as mentioned above, So you MAY get an exact VRS behavior, but the triggering part is a little different.

OK, that was unfair.Honestly: That was it planned to be, thanks for your patience.

The point I want to make here is, that helicopter design involves to the day still trial and error.So it is an interesting topic to discuss about. Thanks for the informations you gave me.

let's hear it for rotor solidity ratio........ how many helicopters with a small, two-bladed system do you see?

Well, i have seen some. But mostly photos and well, they wereīnt a single group of parts anymore.

Greetings,

Rotorbee
4th Mar 2016, 08:08
I am waiting for that PM. I know magneto fluids, I did not know that hot oil is one. How that has anything to do with a rotor will be interesting.

Helicopter aerodynamics is very interesting. We can beat each other over the heads for hours and then find out that nobody was right ... or all ... or some. May take a few years sometimes.

you MAY get an exact VRS behavior, but the triggering part is a little different.

... go on ...

4th Mar 2016, 14:03
Well, Reynolds numbers are for laminar incoming flow. Which you canīt expect fully on an rotorhead turbulent flow as well - high Reynolds number.

You made a point about scale in wind tunnel tests which is why I mentioned Reynolds numbers - I didn't say it was applicable to VRS and flow around a real rotor system.

Sokol
7th Mar 2016, 08:13
... go on ...

Well, triggerspeed should have a high variation. (Mostly be higher as predicted) As I said: that is more a problem in reynolds numbers.

In the VRS itself the behavior should be nearly the same as I would call it an "nearly enclosed system". But I have read a bit more prouty to enforce this thesis though.

Helicopter aerodynamics is very interesting. We can beat each other over the heads for hours and then find out that nobody was right ... or all ... or some. May take a few years sometimes.

Well said. The main problem should be the prove of Navier-Stokes, the only thing an engineer can ashame an physician for:E

Greetings,

7th Mar 2016, 10:55
the only thing an engineer can ashame an physician for
why are you picking on the Doctors now?:E