rotor stall in dynamic turn
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rotor stall in dynamic turn
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 .
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 .
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?
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?
Last edited by JohnDixson; 17th Mar 2023 at 14:21. Reason: Misunderstanding?
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.
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.
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.
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.
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!
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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
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
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
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.
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.
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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.
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.
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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
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
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.
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
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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!
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!
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
*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.
Last edited by JohnDixson; 21st Mar 2023 at 20:06. Reason: missing letter
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.
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.
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
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
The root of a blade is always stalled (too little airpseed and no workable aerofoil cross-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........
JD; can you clear some of your mail?