R22 rotor decay below 50 kts

[Weads]

It seems to me that there are a lot of knowledgeable people here that can help with a question that I have been struggling with. Working towards my CFI and I have a question about why NR decay of the R22 decreases below 50kts. It makes no sense to me since at lower airspeeds you would have more air moving up through the blades since your descent would be greater. What's the reason for this? Thanks Scientific equation? To clarify for some people that are complete jerks I'll rephrase the question. They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some). Why does nr decay occur in zero airspeed autos. When I say zero airspeed I'm referring to a high decent rate and no forward airspeed. Thanks again

[Twist & Shout]

It seems to me that there are a lot of knowledgeable people here that can help with a question that I have been struggling with. Working towards my CFI and I have a question about why NR decay of the R22 decreases below 50kts. It makes no sense to me since at lower airspeeds you would have more air moving up through the blades since your descent would be greater. What's the reason for this? Thanks Scientific equation?
If English is your first language and you believe that questions makes any sense, I'd consider a path other than instruction.

[Twist & Shout]

Just to be clear, on the information provided:
You are flying along and descending. Your NR is decaying so unless this decay is arrested, soon you'll be dead. You slow to 50kts (inferred) and the rate of decay decreases. So you will live slightly longer.

Brief over, lets go flying.

[Twist & Shout]

Sorry. I feel guilty for being harsh.

If you rephrase the question so that it accurately depicts the situation you need clarification on, I'll personally do my best to help you understand the phenomenon you are experiencing.

[rjtjrt]

Good on you Twist and Shout, for admitting that and correcting it.
I was about to post a comment on how people are being severely discouragd for asking a question.
And Weads, if you don't know something never be afraid to ask, as you have. Well done and keep your enquiring mind going.

[Gordy]

I suspect you are talking about being in the flare of an auto possibly?

Below 50kts you are on the back side of the power curve. Is this what you are maybe hinting at?

Help us out a little and maybe explain your question a little better....

[LRP]

I think you're asking a question about rotor rpm in autorotation. If you review your books about autorotation you'll see that the rpm is governed by the relationship between the driving and the driven regions of the rotor disk. I know nothing about R-22's but generally in a properly rigged aircraft the maximum rpm will be obtained right around the max r/c airspeed (correspond to the minimum rate of descent airspeed for autorotation), as you slow below that airspeed rpm will decrease, as you go faster rpm will decrease. The rpm should stay in the green unless you exceed the max airspeed for autorotation.

Specific aircraft performance and limitations can vary significantly but generally most helicopters fall in these parameters.

If that wasn't your question, disregard.

[Weads]

LRP that's fairly close to what I was hinting at. When you slow down the stall angle of the blade will move out is that right? Appreciate the insight.

[jeepys]

If your nr decays then it means your aoa is too much therefore the drag created will slow the rotor continuously until either you lower the lever or you crash.
If the aoa is too high then you run the risk of stalling the blades/aerofoil. Once you stall it just goes from bad to worse and recovering the nr will become impossible beyond a certain point. I guess this is the point you are referring to.
Forward speed is irrelevant, once you stall the rotor it is quite often game over.

[Pofman]

Weads, you should know from your basic POF that the RPM are a function of IAS and weight. Higher speed or weight needs more disc loading. So, in an ungoverned helicopter in auto you demonstrate that the RRPM for a given weight at high speed then, at steady speeds, the slower you go the RRPM decrease. Show this to your student early in the flight then repeat at the end and note the difference with weight reduction. The important point from this is that the minimum weight allowed for flight is so that you will always have the minimum RRPM allowable for autorotation should you have the engine failure( provided you lower the lever promptly!)

[[email protected]]

Weads - do you mean that following an engine failure (or throttle chop) the Nr decays slower at 50 kts than at other speeds?

If so then it will be because 50 kts is about min power speed for a Robbie and you will therefore have least collective pitch applied to maintain straight and level at that speed - therefore Nr decay will be slower (but still bloody fast).

[Twist & Shout]

Weads

Are you starting to see the importance of being concise in your explanations, especially in instructional situations.
Correct use of technical jargon.
And avoidance of "double use". You say the NR decay decreases. Do you mean the NR is still reducing, but decreasing at a lower rate? (I suspect you didn't mean this.)

Try phrasing the complete question again.

You can see there are pleanty keen to help.

[LRP]

LRP that's fairly close to what I was hinting at. When you slow down the stall angle of the blade will move out is that right? Appreciate the insight.
Don't get too wrapped up in the "why". If you want to do the work, you can research why the limitations are there for a particular airframe, but when you're instructing you don't want to teach your personal theories or anecdotal evidence.
Rotor rpm in steady state autorotation is the result of the balance of driving and driven regions of the disk. The actual value will vary due to density altitude and gross weight. For any given set of conditions you (again generally) will see maximum rpm at your best R/C airspeed (also best L/D airspeed), the steady state rpm will decrease as you go slower, and will also decrease you go faster up to your max airspeed for autorotation. The rotor rpm should stay in the "green" throughout the approved envelope. This is valid for steady state rpm, you may see the rpm go beyond limits while in transit between airspeeds.
If you want to go farther than that you're going to have to do a lot more "learnin".

[Arm out the window]

Also for the R22, the practical situation is different to a theoretical steady state rotor RPM in auto because you will almost always be 'carrying' some collective, that is, having it raised a bit to stop the rotor RPM overspeeding.

Looking at your edited first post now, I'm still a bit confused about exactly what you're asking, and if you're open to a bit of advice, be aware there are some very smart, experienced and helpful helicopter experts around this forum and you won't get much help being rude (if that's not your intention, as it well may be by the looks):

To clarify for some people that are complete jerks I'll rephrase the question.

They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some).

[Twist & Shout]

They make a book that would be great for reading it's called the fundamentals of instruction that describe things never to do to a student (it might be worth reading for some).

Touché

It also gives advice on being clear and unambiguous.

I've apologized for my original "attitude" (complete jerkiness?)
We are trying to help.
You seem to have gone from 50kts to 0 IAS.
"Rotor decay" implies a decreasing (changing) RPM

I think most will agree, in a constant IAS autorotation (autorotation wasn't mentioned in the original question BTW) the RRPM will remain constant.

When changing speeds the RRPM will vary first, and possibly dramatically depending on the rate of IAS change due to the loading/unloading of the disc which causes "flare effect" due to the law of conservation of angular momentum.

Secondarily, once a steady (different) IAS has been achieved and stabilized, the RRPM will stabilize. Potentially at a slightly different RRPM.

Thirdly gross weight and air density will affect what stabilized RRPM will result from a certain collective pitch condition/IAS combination.

My (distant past) experience of R22 autorotations. Suggests the flare effect to be dramatic, and therefore problematic if not taken into account, or beneficial if anticipated and used correctly.
While the difference in RRPMin steady state autorotation at normal speed range. (0-80kts) is undetectable. And masked by the small amount of collective required to maintain the RRPM in the safe range.

In case you haven't been exposed to it yet. The "auto revs" are setup during mtce by varying the "flat pitch" (collective fully down) pitch angle by adjusting the pitch link lengths.

This flat pitch RRPM is designed to be safe at the configuration that produces the lowest steady state RRPM - in this case a very light all up weight. This would result in a RRPM overspeed if a flat pitch autorotation was maintained in an AC at maximum gross weight. Hence the need to have some collective applied to reduce the RRPM to an acceptable level.

Have a great day.

[[email protected]]

Weads, I think I now understand what you are asking.

In a vertical autorotative descent, the air is slowed by the rotor disc (extracting energy from it) and the Nr will be say 100% with the lever probably on the bottom stop. The effective rate of descent (as experienced at the rotor disc) is less than the actual rate of descent as the column of air is slowed and can only pass through the disc at a certain rate - an equivalent parachute of the same size was a phrase used many years ago in Brit Mil teaching.

Now, when you increase speed, you are moving the aircraft into a faster moving mass of air (not slowed by the rotor) so the effective rate of descent goes up, increasing the Nr and usually allowing you to add a little collective to keep it in the green. However the actual rate of descent reduces.

Your text books may refer to factors A, B and C where A is the tilting of the disc that reduces its area to capture the moving air and so increases RoD, B is the change in inflow angle from increasing speed which also increases RoD but C is the effect of moving into faster air which increases rotor thrust and decreases RoD. So Factor C outweighs factors A and B.

This keeps happening until you get to your best auto speed (60kts I think for R22), beyond which the disc tilt and inflow angle outweigh the increase in airflow - lots of drag now comes into the mix requiring more nose down to overcome.

Some aircraft have a VNE in autorotation because the combined effect of factors A and B (and their effect on the inflow angle) means that the inflow angle gives too much rotor drag and not enough driving force to keep the Nr high enough.

[cattletruck]

Great saving post crab as I think we're all a bit lost with this thread :ok:.

[Twist & Shout]

Yes, I've learned my lesson.
Back in the cave for me.

[Paul Cantrell]

Weads asks: Why does nr decay occur in zero airspeed autos. When I say zero airspeed I'm referring to a high decent rate and no forward airspeed. Thanks again

Here is the drag curve for the Robinson R22: http://copters.com/aero/pictures/Fig_2-23.gif

(from Helicopter Aviation (http://copters.com/aero/drag.html)).

As you know, in the R22 the bucket airspeed (minimum drag) is 53 kts at 104% Nr. In both powered and unpowered flight, drag goes up as you increase or decrease airspeed.

The other thing you need to know is that rotor drag decreases as you decrease RPM. This is in fact why minimum sink in the R22 occurs at 90% RPM. That's the slowest you can safely turn the rotor system in flight according to RHC.

When you autorotate at speeds below or above 53 kts, the total drag increases. The result is that the excess drag will cause the Nr to decrease. As the Nr decreases, rotor drag also decreases as we mentioned above. At some point the decrease in rotor drag balances the increase in total drag, and the rotor will maintain that RPM in steady state autorotation.

I think there is also an effect due to the shift of relative wind to be more from below the aircraft causing an increase in angle of attack as you descend more vertically, and this has the same effect as raising the collective would. However, I'm not prepared to defend that statement in open forum!

[[email protected]]

Paul, I think your graph is a power required curve, usually used to demonstrate the different sources of drag that must be overcome with power.

I'm not sure it is valid in that form for autorotation since your rate of descent at zero speed would have to be significantly higher than at bucket speed.

That graph should be drawn with a discontinuity on the left hand axis since the total drag curve is additive (ie the sum of the other 3 curves at any point on the horizontal axis).

The drag according to your graph for a zero speed auto would be probably twice that at bucket speed because the curve would be more pronounced.

That would mean you would need twice as high a RoD at zero speed compared to bucket speed. I haven't flown a helicopter yet where that is true.

[AnFI]

Crab; "....your rate of descent at zero speed would have to be significantly higher than at bucket speed."
but it IS
you are muddled again

[[email protected]]

It is higher, by between 3 - 500 f/min, but it would need to be almost double to overcome the amount of drag suggested by the graph.

That is what I meant by significantly.

[AnFI]

Can anyone here produce the power curve for an autorotating helicopter?
It would be interesting to see it.
Just how different would it be to the power curve for straight and level that PC brings?
(a Rate of Descent against Airspeed would amount to the same thing if anyone has that, anyone? Nick?)

[LRP]

total drag, power required, rate of descent, should look about the same.

[[email protected]]

They possibly are, but the graph needs to be drawn correctly.

The power required curve, as I understand it, is often taken from the fuel flow measurements in forward flight - easily measured and quantified. Every one I have seen has a much steeper curve than the one PC has shown - hence the need for the discontinuity in the vertical axis.

The Rate of descent equivalent is probably calculable with some physics and maths but may not be so easily compared.

[AnFI]

Crab I'm not sure what YOU mean by a discontinuity , and yes it would normally be steeper approaching the y axis , the gentle arrival at the y axis is typical for a power curve that includes ground effect (which probably should be considered as a 'special case').

LRP yup I have always thought that was a reasonable estimation. (Although probably unimportantly slightly different, curious as to how different, if anyone actually has a real graph of RoD against airspeed in autorotation). The reason I asked was that I was disconcerted that Crab would think that the rate of descent at zero airspeed would only be slightly greater than at 'bucket speed' (Vy) whereas the power required at zero would be significantly greater at zero airspeed (surely?). Since the power required at zero airspeed is so high compared to at Vy one would surely expect the Rate of Descent to be accordingly greater, since Rate of Descent is equivalent to power input for autorotation.

The rate of descent is of course directly equivalent to Power (supplied).
For each 330lbs that an aircraft weighs the VSI reading gives Horse Power directly.
ie "10" on a VSI (with units of 100fpm) IS 10HP per 330lbs.
so a RoD of 2500fpm is 25Hp per 330lbs, so for an aircraft weighing 3300lbs that would be 250Hp

[[email protected]]

Crab I'm not sure what YOU mean by a discontinuity

discontinuity (dɪsˌkɒntɪˈnjuːɪtɪ)
n, pl -ties
1. lack of rational connection or cohesion
2. a break or interruption
3. (Mathematics) maths
a. the property of being discontinuous
b. the point or the value of the variable at which a curve or function becomes discontinuous

See #2

When you did your FI course you probably drew the power required curve and explained the different elements - the 3 elements are additive and that makes the graph very tall if you draw it correctly - therefore in order to get it on a normal page/board/slide, a discontinuity is inserted in the Y axis to compress the graph.

BTW - there isn't a 'gentle' arrival at the Y axis - if you include ground effect, the power required curve dips down again as it approaches the Y axis.

[Arm out the window]

The 'standard' power curve being discussed here isn't really relevant, is it? Induced drag has to do with tip vortices and recirculating flow, surely, whereas in auto there is none of that, just some reduction in rate of descent airflow a la the 'parachute' effect Crab mentioned, unless I'm sadly mistaken (which is entirely possible of course).
In a 60 kt auto you might come down at 1500 - 1700 fpm, say, but there's no way you're doing 3400 fpm at zero speed, is there? More like the extra 300 fpm or so sounds about right.

[[email protected]]

AOTW - yes that was my only point regarding PC's use of the power require/drag curve.:ok:

[AnFI]

doesn't any one have a 'power curve' for autorotation ? It would be useful
i do not believe that it is only a matter of 300fmp in addition to the Vy RoD

Crab, no I don't draw it with a break, it's not too tall for my piece of paper, and anyway that would only be a break of pen to paper and not one in the underlying physics surely? (AND if it were THAT tall then it would represent a bigger RoD)
my idea of a discontinuity was as in Def3 (Mathematics), and there isn't one of those, there is no intstantaneous step change.

I rather thought your explanation was more in line with Def 1 "lack of rational connection or cohesion"

[Arm out the window]

i do not believe that it is only a matter of 300fmp in addition to the Vy RoD


Just have a look at the VSI next time you do one, I'm sure you'll be convinced then ... no offence AnFI, but that's what happens.

[AnFI]

Aotw
No Offence recieved, I am not absolutely certain about it and you sound quite sure, I had always intended to do my own graph of this but have just never got around to it. You could well be right but I am just very doubtful, I did try and take a reading the other day but it was 'off the clock' and didn't have time to do it properly with a stopwatch and altimeter etc

[Arm out the window]

I'm most often doing them in Robbies these days and therefore carrying some collective which may have an effect.

However, both with the speed either at 20 kt or less, or at 60 (and using the same rotor rpm), I'm sure there isn't that much of a difference in ROD. Once the VSI is stable you can read it off easily enough and I'm sure it's not hugely greater at airspeeds close to zero than at 50 - 60 KIAS. However I will check next time I'm doing autos.

[[email protected]]

AnFI - so you are not absolutely certain, yet feel free to criticise those who are - and then regard yourself as a 'legend'...........................

[AnFI]

Crab your inputs are really painful. Aotw is being modest and conciliatory , as was I in offering the small degree of doubt required to accomodate the respect of aotw's opinion, I'm not really all that doubtful, aotw will maybe try it and come back with a revised figure I presume, then we'll (you'll) know...
perhaps someone here has a graph of Power vs Airspeed in Auto?
NL has a library of excellent graphs ... someone?

as for "regard yourself as a 'legend' " I don't think that could be further from the truth, perhaps you could pm me the evidence for that or retract it

[MOSTAFA]

Leg-end more like.

[Paul Cantrell]

I apologize for the long reply: was busy getting ready for Christmas (and a broken N1 tach generator) so while I had time to read a couple people's postings, I didn't have time to put together a researched reply:

Paul, I think your graph is a power required curve, usually used to demonstrate the different sources of drag that must be overcome with power.

Yes, as I mentioned, this is the power required curve for the R22 as published by Robinson Helicopter Company.

I'm not sure it is valid in that form for autorotation since your rate of descent at zero speed would have to be significantly higher than at bucket speed.

I think it should be valid in autorotation, except maybe at exactly zero airspeed. I went back and checked Prouty's red book (Performance, Control, Stability?) to see if I had missed anything major, and the only thing I saw that jumped out at me is that he mentions that you are in Ring Vortex State when doing a zero speed autorotation. I hadn't thought about that, and I'm not sure exactly what the effect of that at zero airspeed might be, but by 10-15 knots I would expect rotor power required to be the same in autorotation as in powered flight.

That graph should be drawn with a discontinuity on the left hand axis since the total drag curve is additive (ie the sum of the other 3 curves at any point on the horizontal axis).

As I said, this is Robinson's chart and a quick check seems to me to show that it is in fact showing total drag as the sum of the other three drag curves (I checked like... 4 points)...

The drag according to your graph for a zero speed auto would be probably twice that at bucket speed because the curve would be more pronounced.

I roughly measured the total drag curve, and I get a ratio of 1.4:1 between zero airspeed and bucket (53 kts) airspeed. That would mean from 1,500 ft/min at bucket airspeed to 2,100 ft/min at zero airspeed. That does seem slightly higher than my experience, but not as much as you're saying (it's not 2:1 on the drag curve). It's been a few years since I've been flying R22s but 1,800-1,900 ft/min is about right for a vertical autorotation in an R22 if I remember correctly. It's certainly close... One point Prouty makes is that in a vertical descent, parasite drag is in fact acting in your favor, i.e. it's having the effect of slowing your fall. So that might be one difference. At zero airspeed the power on curve parasite drag is zero, but the effect of the "vertical parasite drag" is that less thrust is required from the rotor and therefore less power. I suspect at the vertical speeds we are talking about that this is not a huge effect and could probably be ignored.

AnFI asks: Can anyone here produce the power curve for an autorotating helicopter?

I was looking in the Prouty book for exactly that, and didn't find one. However he does have a rate-of-descent curve for autorotation and it looks very similar to the total drag curve. The rates were very high, but it was for a 20,000 pound helicopter which an R22 is definitely not! :-)

LRP says: total drag, power required, rate of descent, should look about the same.

I agree with this. I'm sure there are some differences in autorotation because of upflow relative to the disk, but from a blade element standpoint I would expect the drag equations to be pretty much the same in powered and unpowered flight. I mean, the angle of attack is similar (according to Prouty), the airspeed is exactly the same if you run the RPM at the power on RPM... Therefore the relative wind the blade is seeing (at least the spanwise average) must be the same... Not really sure where the difference would be except maybe some differences in the wake; but even then I'm not really sure it would be different...

Crab says: They possibly are, but the graph needs to be drawn correctly.

Like I said, this is from the Robinson Helicopter company and purports to be the drag curves of the R22 and I believe it to be accurate.

Crab continues: The power required curve, as I understand it, is often taken from the fuel flow measurements in forward flight - easily measured and quantified. Every one I have seen has a much steeper curve than the one PC has shown - hence the need for the discontinuity in the vertical axis.

Just a guess, but as you increase disk loading and solidity, you would expect the steeper curve you mention. The R22 almost certainly is near best case from an efficiency standpoint - it has very low disk loading, only two blades, and even the parasite drag is significantly lower than many helicopters of it's era. So maybe that's the difference compared to other drag curves you are familiar with. I tried to get the R44 drag curve from the company a while back and they said they would but then didn't - the engineer probably told the rep I talked to to go take a hike :rolleyes:

Crab continues: BTW - there isn't a 'gentle' arrival at the Y axis - if you include ground effect, the power required curve dips down again as it approaches the Y axis.

I have no way of knowing whether Robinson considered ground effect when they produced this curve. I could go back and look at the original one to see if they mention that, but I'm too lazy! :hmm:

Arm out the window: The 'standard' power curve being discussed here isn't really relevant, is it? Induced drag has to do with tip vortices and recirculating flow, surely, whereas in auto there is none of that, just some reduction in rate of descent airflow a la the 'parachute' effect Crab mentioned, unless I'm sadly mistaken (which is entirely possible of course).

I think it probably is relevant. Average angle of attack across the disk is similar in powered flight and in autorotation according to Prouty. If the angle of attack is the same, and the airspeed is the same (RPM hasn't changed) then I guess I would expect tip vortices to be similar. As I mentioned, Prouty claims that at zero airspeed in autorotation you are actually in Ring Vortex State which says recirculation is taking place... I'll bow to any aerodynamicists who care to enter the fray, but from a first approximation I'm still of the opinion that the power required curve is basically the same power on or off (except for the VRS thing - that caught me by surprise).

In a 60 kt auto you might come down at 1500 - 1700 fpm, say, but there's no way you're doing 3400 fpm at zero speed, is there? More like the extra 300 fpm or so sounds about right.

Unless I did a bad job of measuring the graph, I actually think the 1.4:1 ratio of drag between 0 and 53 is probably pretty close. I'm not sure where the 3400 number came from - like I mentioned at the top, 1.4:1 means 1500 vs 2000 which is in the right ballpark...

[chopjock]

P.C.
I went back and checked Prouty's red book (Performance, Control, Stability?) to see if I had missed anything major, and the only thing I saw that jumped out at me is that he mentions that you are in Ring Vortex State when doing a zero speed autorotation.

Uh?...

[AnFI]

PC "However he does have a rate-of-descent curve for autorotation " that would do since it is what we are talking about and is the equivalent of Rate of Descent

I agree with PC that a Power Curve should give a reasonable approximation to the Power (and therefore rate of descent) required in autorotation, it might not be exactly the same, but probably good enough (as many recognised design writers also state)

Crab, i think using fuel flow would give a slightly unhelpful error in that the engine whilst producing no useful power might still have a fuel flow (in the region) of 60% of it's maximum. That would not translate into Rate of Descent in terms of the proportionality you mention.

I'm not buying the vortex ring state bit in autorotation, (really?), even if it is Prouty (or you are sure that is what he's saying?). Vortex perhaps, but VRing, I doubt.

MOSTAFA quite correct

also I found a published curve here FWIW

[MOSTAFA]

AnFi. - I don't recall you ever proving anything right, the only thing you appear to have a degree in, is obfuscating the issue - I certainly wasn't referring to Crab. Being another post graduate of, I still respect the Central Flying School for the knowledge and enthusiasm it imparts.

[Birdy2]

and the only thing I saw that jumped out at me is that he mentions that you are in Ring Vortex State when doing a zero speed autorotation
You are safe from VRS in a virtical decent while in autorotative condition.
Just be sure to be flairing ( arresting your decent rate with collective) at ground level, or you get past translation speed before you pull up collective.
Dureing a vertical decent, your decent rate is too high for a ring to develop.

[AnFI]

MOSTAFA ??? I was agreeing with your reference to me, what are you talking about??? "I certainly wasn't referring to Crab." did I say I thought you were? You alumni of CFS seem to share some common characteristics. You ARE enthusiastic, and self confident, perhaps slightly challenged in comprehension?
I think you'll find Crab is talking nonsense again.

WRT your confusion about whether I have "ever proved anything right"
Refer to the graph I have posted above and observe how it DISproves crab's assertion, and thereby YOU, wrong.
Rates of descent in Zero Speed Auto are not a matter of only 3-500fpm greater than at Vy.

MOSTAFA You actually got something to say on the topic or are you just talking part in the Mod supported insult campaign? If you don't think VR is something you can get trapped in for 4000ft then you are not in the self congratulatory self re-enforcing delusion club. Nothing to say on the topic? just here to throw insults at me to support your school friend? I'll probably get in trouble again for your uncalled for, off topic, irrelevant pathetic misunderstanding. Mod please feel free to Mod out MOSTAFAs uncalled for insults, then you can delete this paragraph too ?

[Evil Twin]

Erm...

To be in VRS you must have three things, bit like a fire triangle. ROD <3-400 fpm/AS >12-15 its(below translation)/Power in use.

In autorotation, you have no power in use, therefore cannot be in VRS. Simples...Surely?

[Senior Pilot]

or are you just talking part in the Mod supported insult campaign? If you don't think VR is something you can get trapped in for 4000ft then you are not in the self congratulatory self re-enforcing delusion club. Nothing to say on the topic? just here to throw insults at me to support your school friend? I'll probably get in trouble again for your uncalled for, off topic, irrelevant pathetic misunderstanding. Mod please feel free to Mod out MOSTAFAs uncalled for insults, then you can delete this paragraph too ?

As has been pointed out to you on a number of occasions, AnFI, there is no moderator support for you nor against you. Moderation here is unbiased.

And as also has been pointed out to you, it is time to recognise that you get responses based on what you post. When you choose to be rude or disparaging to others it is not unexpected that you be treated in a like manner.

Any further comments along these lines criticising the moderating of PPRuNe will be removed, as will off topic poor-me rants (apart from this which is left for others to see your attitude and judge accordingly).

[GS-Alpha]

How can any trained helicopter pilot believe they can be in vortex ring state at any speed during autorotation? Putting aside the basic physics of a rotory wing, try thinking about it practically? Surely pilots would be trained to avoid zero airspeed autos at all cost if vortex ring was even a remote possibility? Why would you want to be hurtling towards the ground at an uncontrolled rate of descent when you've suffered an engine failure? I've flown the R22 in autorotation and autogyros which are always autorotating. They most definitely do not enter vortex ring state during autorotation at any speed, and the ROD at zero airspeed is nothing like double that at 53kt.

[[email protected]]

Paul Cantrell As I mentioned, Prouty claims that at zero airspeed in autorotation you are actually in Ring Vortex State which says recirculation is taking place... My copy of Prouty doesn't say that, he highlights the difference between VRS (power applied with moderate RoD), and windmill brake state where all the flow is from beneath the disc and the rotor is extracting the energy to keep turning from that flow. Will there be recirculation at the tips - yes but it doesn't mean you are inducing a flow top to bottom with power.

You may well be correct that the R22 has a different drag curve to other helos with higher disc loading.

There is no doubt that the RoD in a zero speed auto is higher than at bucket speed - the only element for discussion is how much. In every helo I have flown (with widely differing disc loadings and AUMs) the RoD at bucket speed is between 1800 and 2500 f/min and the zero speed RoD doesn't vary by more than 500 f/min from that figure when in steady state.

I mentioned fuel flow graphs only because of the identical shape on the ones I have seen to the power required curve - since the power required curve is exactly what it says, then in powered flight the engines(s) are always providing driving force (if only to overcome rotor profile drag).

[AnFI]

Thank you to the moderator for pointing out that I get a little bad tempered in responding. Quite right - very sorry.

In the humblest way possible I would like to state my view that the RoD in Auto at Zero Speed is not a matter of only (300 to) 500fpm more than at Vy.
I have included a graph which shows that clearly. I could of course be wrong, it only is my view, and not my graph.
I'll report back with a substantiated figure. Unless AOTW is going to get there first? (not easy because it maybe 'off the clock' and would need a stop watch)
(Then perhaps people can stop sending me threats?)

[Arm out the window]

Unless AOTW is going to get there first?

On a couple of weeks off except for Friday, may get to do some then.

[[email protected]]

Having had play at zero speed autos today in a 365 I might have to up my upper end of RoD towards 1000 ft/min greater than bucket speed - 2300 f/min at Vy and 3100 ft/min at zero speed was today's comparison.

Not surprising I suppose that a higher disc loading helo should need a higher RoD to keep the blades turning and perhaps that smaller 'parachute' is more sensitive to forward speed (or lack of it) in auto - hence the greater variation.