rotor stall in dynamic turn

[chr]

Hi

most of the helicopters may have rotor stall during vertical descent with more than 500-700 ft/min and fwd speed less than 30 kts but this only matter when your blades are with positive angle of attack , so if you are in the same conditions but has your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .

You Tube

[JohnDixson]

The n/rev roughness/vibration below 30 kts ( and in my experience* ) in a descent at more like 800-1300 fpm, the phenomena you refer to isn’t stall, but the advancing blade going thru the tip vortex created by the preceding blade.
The vibration/roughness can be attention getting, if one is successful in doing this in a steady state manner ( need non-gusty wind conditions ).
*Typical flight load survey test plans at Sikorsky would include a so-called “ rough approach” for each condition ( weight/center of gravity ). The vibratory structural loads in some areas, in some machines can be high enough to influence some structural lives. In my experience the CH-54 was particularly “interesting” when put into this condition.
BTW-nothing sacrosanct re the rate of descent I quoted for the Sikorsky machines-it will depend on the specific rotor and weight. We would typically do the flight loads survey work at design and max or alternate gross weight.
Chr: title of your post included stall in a dynamic turn, but your post did not include a turn but a straight descent, and my response was based upon that. Did you intend to discuss stall in turns?

[[email protected]]

CHR - are you referring to VRS when you talk about rotor stall in a vertical descent with low speed and high RoD?

Only the roots are stalled in Full VRS.

The Blackhawk in the video has forward speed (airflow over the rotor) even in his spiral descent so quite unlikely to encounter VRS.

Rotor stall in a high AoB level turn is something else entirely.

[gipsymagpie]

Originally Posted by [email protected]

CHR - are you referring to VRS when you talk about rotor stall in a vertical descent with low speed and high RoD?

Only the roots are stalled in Full VRS.

The Blackhawk in the video has forward speed (airflow over the rotor) even in his spiral descent so quite unlikely to encounter VRS.

Rotor stall in a high AoB level turn is something else entirely.

Well said crab. There might be some retreating blade stall if you loaded up the rotor in a steep bank perhaps at high DA.

I am a bit more worried about when he bunts the machine roughly half way through and the rear disc gets awfully close to the top deck. A Bell 429 recently chopped its own tail off in the USA as a result of harsh manoeuvring at low G. Even in a fully articulated (UH60) or semi rigid (Bell 429) you can end up with some weird disc versus fuselage attitudes at low G. Not as bad as an R22 obviously!

[fdr]

Originally Posted by chr

Hi

most of the helicopters may have rotor stall during vertical descent with more than 500-700 ft/min and fwd speed less than 30 kts but this only matter when your blades are with positive angle of attack , so if you are in the same conditions but has your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .
https://www.youtube.com/watch?v=USmOqlj9Hag

Thanks for the fun video... As far as the commentary in the OP, the post can be edited, and you may want to open up any book on helo aerodynamics, and remove the confusion of your post. JD and crabby have been very polite..

Search for retreating blade stall, settling with power, vortex ring state, and open up a RHC AFM and read the notes on low rpm rotor stall.

Rotor stall is a very specific situation, and to be avoided. That a rotor stall event has occured is usually identifiable by the wreckage that the dead bodies are found in, and the unique bent blade shape.

Fly safe

[SASless]

Considering the complexities of helicopter rotor aerodynamics I would suggest everyone be polite when discussing the many aspects of the matter.

We see these threads started fairly often and we see posts with varying degrees of sophistication and technical knowledge.

I would prefer reading discussions complete with astute knowledgeable responses that serve to fairly advance the discussion and some times downright debate.

I have no doubt there are some among us that do know what they are talking about, there are some who think they know what they are talking about.....and there are some like me who know when to sit quietly and occasionally nod their head as if we understood what was being talked about.

My knowledge comes from Prouty, the Sikorsky Blue Book, and various other sources....and as an Instructor I taught the subject but even then I knew my own limitations.

I also passed on to my Students the advice I was given very early in my learning....."There is no such thing as a stupid question other than the question not asked.".

The rest of the advice was to carefully consider the answers as some of them might be absolute rubbish.

One has to consider the sources and compare what they have to say and how it was said and sort out the wheat from the chaff.

[Rotorbee]

I don't get it, what ch really wants to find out.

Could it be, that he is looking for something similar like an accelerated stall in a FW?

Well, in general, helicopters just don't do this. You would have to pull back on the cyclic in a turn and that would bleed of the speed faster then you can say gobbledygook, since there is nothing that keeps pushing forward like a propeller or jet engine.

In a descending turn? I don't think so. Help me here. I can't find a combination of cyclic an collective, where one can increase the g without giving up something else. Mostly speed. Even diving and then pulling g's will not stall the rotor.

Then he talks about the numbers in VRS. Like the old crustacea said, in VRS parts of the rotordisk are stalled, but never all. Same thing goes for autorotations.
Actually, even under normal circumstances, some parts of the rotor disk may be stalled.
Then there is retreating blade stall. Also something where part of the rotordisk stalls. Part of the retreating side, that is. Something to be avoided, because it shakes the dentures lose. And parts of the ship, too.

A full stall of the rotor must be avoided at all cost. First sign will be a decay of the rotor RPM and if no action is taken, followed by an automatic folding of the blades upwards. This leads, depending on altitude, to a prolonged near death experience which will end in most cases in a full death experience. The reason of a full stall is a too high angle of attack for the whole disk, but there isn't enough power available to continue turning the rotor at normal RPM, because of the added drag. Therefore the rotor RPM will decay and the centripetal force isn't strong enough to keep the blades in their normal path. They will bend upwards and finally break.

Even the stalling a rotor sitting on the ground isn't a very clever idea. This can lead to quite a bit of flapping of the blades and lost parts, like the whole tail.
There are rumours that pilots have managed to regain control of a completely stalled helicopter, but frankly I have no reason to believe it. Wasn't there, didn't do it, didn't get the t-shirt.

[Baldeep Inminj]

Originally Posted by chr

Hi

most of the helicopters may have rotor stall during vertical descent with more than 500-700 ft/min and fwd speed less than 30 kts but this only matter when your blades are with positive angle of attack , so if you are in the same conditions but has your collective fully or almost down (0 or very little pitch ) this shouldn't happen ?
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed .
https://www.youtube.com/watch?v=USmOqlj9Hag

There are few misunderstandings with your statement, but that is not surprising as this is a chewy subject for many people and you are not alone! I lecture internationally on helicopter operations and principles of flight so I know a little about this - nobody knows everything, believe me...and a lot of PoF is what we 'think' is happening. There is also a lot of poetic licence used to make concepts easier to understand, for example, if you are familiar with the 'vector diagram' often used to show forces acting on a blade, it is actually drawn wildly out of proportion to the true vectors (a vector having both magnitude and direction), but it allows us to explain the concept in a more simple and easy to understand way.

I shall add a few comments to your statement, which I believe is referencing Vortex Rings state fronm the parameters you mention. If you are implying Retreating Blade Stall (I don't think so?) then I can explain that also.

most All of the helicopters may have partial or complete rotor stall during vertical descent with more than 500-700 ft/min and fwd airspeed less than 30 kts, but many aircraft have far higher requirements and need RoD in excess of 1500ft/min with airspeed below 10 kts, depending on muliple factors but predominantly aerofoil cross section, sweep charcteristics and disc loading. but this only matter when your blades are with positive angle of attack - wrong, this only occurs when the 2 conditions you mention are met, and there is also power applied. Positive AoA is not a required factor for this condition. A helicopter in autorotation with the collective fully down and zero pitch applied has a positive AoA due the relationship between the chordline and the relative airflow. Remember the RAF is modified due to the RoD flow from under the aircraft and this creates a positive AoA, thereby creating lift, and also drag that acts forward of the blade (at 90 degrees to lift or parallel to the RAF, whichever you prefer) to accelerate it - in short, the position of the collective dictates pitch angle,but it does not dictate Angle of Attack. - , so if you are in the same conditions but have your collective fully or almost down (0 or very little pitch ) this shouldn't happen ? Kind of - if the collective is fully down you cannot get into VRS, but is has nothing to do with pitch or AoA, it is because there is no power applied.
My consideration are in regards to approach to land technique with very dynamic 180 degree turns in forward flight like for ex. on below video (40 sec.) - this rotor didn't stall because the airflow was from below (desending) and it has some fwd speed . Some mention that in VRS it is the root that is stalled - this is true, but a little misleading. The root of a blade is always stalled (too little airpseed and no workable aerofoil cross-section, but in VRS, whilst the root stall is larger, the tip's may also stall due to the recirculation adding to the induced flow driving the AoA to beyond the critical AoA, but it is not a certaintly.

Cheers

[SASless]

Meanwhile.....Sasless mouthing spit bubbles sagely nods in agreement.

That is due to a couple of lines of that post by Baldeep Inminj.....

Kind of - if the collective is fully down you cannot get into VRS, but is has nothing to do with pitch or AoA, it is because there is no power applied.

​​​​​​​The root of a blade is always stalled (too little airpseed and no workable aerofoil cross-section....

​​​​​​​If you are familiar with the 'vector diagram' often used to show forces acting on a blade, it is actually drawn wildly out of proportion to the true vectors (a vector having both magnitude and direction).

One will note that no colored pencils were harmed in the creation of that explanation.

​​​​​​​Well done BI!

[fdr]

Originally Posted by SASless

Meanwhile.....Sasless mouthing spit bubbles sagely nods in agreement.
That is due to a couple of lines of that post by Baldeep Inminj.....
One will note that no colored pencils were harmed in the creation of that explanation.
​​​​​Well done BI!

SASless and JohnDixson were both involved in UH60 test, and I would imagine that the only two items that would be of concern in that video is the rapid longitudinal cyclic application at low speed which leads to reduced TPP/tail clearance, but was still within limits... and the possible pitch link loads during what is nearly the UTTAS RTT manoeuvre, which IIRC is still not within the design limits of the UH60 when performed to the original specs. The stall of the blade in high speed flight is almost always a retreating blade event, with predictable consequences. Doing that from an underspeeding NR could end badly, as in doing the 1.1 max autorotation at min Nr in autorotation, in a left turn and recovering with a pitch up before adding torque. (that test point gives very high root bending loads and pitch link loads as well, and the vibration in my experience is pretty close to the edge of fun.

Wayne Johnson wrote the NASA TM 81182/ USAAVRADCOM TR-80-A5 Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics, which is more or less the basis of his later theory manuals, but it has the equations in there handwritten, which is quite a piece of art, whether we follow his maths or not, it is good. It is also free... around p97 the conversation, and it is that, gets to aerodynamic analysis, and that runs for 30 pages. The late sections add the inertial and aerodynamic effects are good reading, as is every bit on stability and control. I need coffee when I read his stuff, and then aspirin afterwards, but there is a lot of hidden gems in those pages.

All up, keeping Nr in the normal region for the operation is desirable. Like every part on a helicopter, limitations are there for good reasons.

Johnson -, W. (1980). A Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics Part 1: Analysis Development.

[SASless]

fdr

Thank you for the compliment, but sadly Brother Dixson and I have not shared a common cockpit and it were he that did the UTTAS Project Flying.

I did not work for Sikorsky but did work for a Contractor to Sikorsky at the West Palm Beach Completion Center out in the swamp near West Palm Beach.

He is among the experts all things Sikorsky as he is very knowledgeable and always willing take time to translate complicated issues into language I can mostly understand.

[fdr]

I'm not sure, but since Shawn Coyle went west, Ray's lecture series is short of presenters, would think that SASless and "Brother Dixson" would be ideal candidates for that.

In the meantime, I am chasing down how to explain a fanjet performance that goes sideways with adiabatic characteristic fan curves, unless the underlying efficiency was pretty shabby, which is not what is proposed.

[FH1100 Pilot]

Originally Posted by Rotorbee

I don't get it, what ch really wants to find out. Could it be, that he is looking for something similar like an accelerated stall in a FW? Well, in general, helicopters just don't do this. You would have to pull back on the cyclic in a turn and that would bleed of the speed faster then you can say gobbledygook, since there is nothing that keeps pushing forward like a propeller or jet engine. In a descending turn? I don't think so. Help me here. I can't find a combination of cyclic an collective, where one can increase the g without giving up something else. Mostly speed. Even diving and then pulling g's will not stall the rotor.

Ray Prouty wrote about this very thing. While it might be theoretically possible, you can't really stall a rotor disk because, as you said, the aircraft just slows down. But that doesn't mean that the individual blades can't stall.

A full stall of the rotor must be avoided at all cost. First sign will be a decay of the rotor RPM and if no action is taken, followed by an automatic folding of the blades upwards. This leads, depending on altitude, to a prolonged near death experience which will end in most cases in a full death experience.

True that! I don't believe that I have ever read anything on this forum that has made me (inappropriately) laugh out loud - until now. Well done, Rotorbee!

[[email protected]]

Rotorbee - we used to teach steep turns (60 deg AoB) in both the Gazelle and the Lynx and you can reach a steady state holding 2g with the speed stable (about 60 kts in the Gazelle and 80 Kts in the Lynx).

You do sacrifice speed initially during the entry but eventually it is power that is holding you up rather than flare effect.

In both aircraft this steady state was achieved at almost max continuous power - in more powerful aircraft you could probably manage steady state at either higher AoB or higher speed.

No coloured pencils required, no international qualifications, just an ex-mil QHI who still flys both types.

[JohnDixson]

As to the impact of the flying in that video, there is nothing in that video that should concern the Pprune audience, the unit maintenance chief, or anyone else I can think of*, Obviously an empty aircraft, the pullup wasn't particularly aggressive etc etc. There wasn't any stall ( by a huge margin ) so control loads were within reason.
*Now, if that was a US Army ship, there might have been some administrative issues as their definition of prohibited aerobatic flight is very restrictive, and this maneuver went beyond it, by a good margin.
Think I've mentioned this in some PMs, or maybe even here, but the subject of doing some public aerobatic flight came up as the ship was getting close to delivery in 1978, and the review of flight manual limits was under review with the Army. It was the clear position taken by the Army leadership not to do anything like that, and the flight manual limits reflected very restricted maneuver limits. Their reasoning was that they have a large number of young aviators who have graduated from a training syllabus that included zero acrobatic training in any aircraft, much less a helicopter. They saw the certitude of some imitation by this group if the aerobatic capability of the Blackhawk were to be made public, either by Sikorsky or by the Army, and thus the limits in their publications.
Certainly, during the required development and qualification of the helicopter to the rigorous standards contained in AMCP 706-203 ( The Army Engineering Design Handbook, Part 3, 1972 ), the maneuver envelope was very fully developed. The main rotor flapping hinge offset is 25% more than the original Blackhawk, the S-67 ( basically an S-61 head geometry-wise ) so the available hub moment is up accordingly. Those interested in more detail can send a PM.

[Rotorbee]

Well crab, no argument there. Certainly an empty Blackhawk is capable of holding a 2g turn. Probably my example came out wrong, now that I thought about it for a while (note to self, think first, write second).

FH1100. Thank you for the flowers, one aimes to please. Since you mentioned Prouty, I found the article and realised, that my brain hides things from me.
But since chr does not give us any more clues, what he actually wants to know, we can rest the case. It does not lead anywhere.

[JohnDixson]

Crab, did you/are you flying the Westland SeaKing? Did that 61 naval version have the Cruise Guide system/guage installed?

[[email protected]]

Hi John,

Yes, still flying the Sea King and yes they are fitted with the cruise guide system and gauge - it was useful when heavy on a SAR shout since so many variables could affect your ability to reach the calculated VMax but the CGI didn't lie - but then neither did the rattling of the fillings in your teeth

[[email protected]]

The root of a blade is always stalled (too little airpseed and no workable aerofoil cross-section....

BI - I assume you are taking about the inboard section where there is no actual aerofoil and not the inner part of the blade itself where, at least on the advancing side, there is both speed and an aerofoil section.

I have only ever heard reference to the root of the blade in P of F terms as only including the parts where the aerofoil section exists but then I am just an ex-mil QHI who doesn't lecture internationally........

[fdr]

JD; can you clear some of your mail?