Retreating Blade Stall No 2
Thread Starter
Retreating Blade Stall No 2
This thread is the second attempt at exploring Retreating Blade Stall (RBS). Lu Zuckerman begins by sying:
>>If you believe that the retreating blade has stalled and further believe that individual blades stall and drop out of orbit and fall due to the stall then try to visualize this. Once the blade has stalled and dropped down over the tail (90-degrees later) it still is attached to a spinning rotorhead and must then immediately get back to the commanded tip path. That means that that blade when it is down over the tail it must fly up until it is now the advancing blade and, will end up being down over the nose, as this is the commanded tip path. If you were to look at the disc from the side of the helicopter the disc would scribe an inverted V or be just the opposite of the cone angle. Can you imagine the vibratory forces that would ensue if the blades had to change their position so radically at anywhere from 250 to 500 times per minute?
Now this may be difficult to comprehend but try to visualize the disc as a single entity. The basic premise in helicopter design is to have an equal distribution of lift across the disc. When the pilot moves his cyclic he alters the lift distribution across the disc and the disc tilts in the direction of cyclic movement. You can visualize this as aerodynamic precession and I think of it as gyroscopic precession. In either case there is a differential of lift across the disc.
In the case of retreating blade stall, the retreating side is generating less lift than the advancing side. This causes either a perturbation of the disc if you accept gyroscopic precession or it is a direct aerodynamic lift that responds 90-degrees later and the result is the disc blows back / flaps back.
I put the comment on the web to find out what other people thought on the subject.<<
Respond if you are game.......
>>If you believe that the retreating blade has stalled and further believe that individual blades stall and drop out of orbit and fall due to the stall then try to visualize this. Once the blade has stalled and dropped down over the tail (90-degrees later) it still is attached to a spinning rotorhead and must then immediately get back to the commanded tip path. That means that that blade when it is down over the tail it must fly up until it is now the advancing blade and, will end up being down over the nose, as this is the commanded tip path. If you were to look at the disc from the side of the helicopter the disc would scribe an inverted V or be just the opposite of the cone angle. Can you imagine the vibratory forces that would ensue if the blades had to change their position so radically at anywhere from 250 to 500 times per minute?
Now this may be difficult to comprehend but try to visualize the disc as a single entity. The basic premise in helicopter design is to have an equal distribution of lift across the disc. When the pilot moves his cyclic he alters the lift distribution across the disc and the disc tilts in the direction of cyclic movement. You can visualize this as aerodynamic precession and I think of it as gyroscopic precession. In either case there is a differential of lift across the disc.
In the case of retreating blade stall, the retreating side is generating less lift than the advancing side. This causes either a perturbation of the disc if you accept gyroscopic precession or it is a direct aerodynamic lift that responds 90-degrees later and the result is the disc blows back / flaps back.
I put the comment on the web to find out what other people thought on the subject.<<
Respond if you are game.......
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I would think that the retreating blade only stalls on the very edges and then quickly recovers from the stall as it moves towards the 6 oclock position. So that would mean that it would dip down momentarily at the 9 oclock position. this would then move the rear of the disk downward and result in the front climbing. The roll would just be caused by the increased drag on the left side of the system right?
[ 05 December 2001: Message edited by: baranfin ]
[ 05 December 2001: Message edited by: baranfin ]
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To: baranfin
"I would think that the retreating blade only stalls on the very edges and then quickly recovers from the stall as it moves towards the 6 oclock position. So that would mean that it would dip down momentarily at the 9 oclock position. this would then move the rear of the disk downward and result in the front climbing. The roll would just be caused by the increased drag on the left side of the system right"?
Response:
The action you describe may be correct but not sufficient to cause a “full stall” When the tips are in the stall region they may do what you stated but not to the extent you stated and this may account for the vibration experienced at the onset of retreating blade stall however in a full stall as much or possibly more than 30% of the blade span is involved and at this time there is sufficient lift differential to cause the flapback / blowback.
[ 05 December 2001: Message edited by: Lu Zuckerman ]
"I would think that the retreating blade only stalls on the very edges and then quickly recovers from the stall as it moves towards the 6 oclock position. So that would mean that it would dip down momentarily at the 9 oclock position. this would then move the rear of the disk downward and result in the front climbing. The roll would just be caused by the increased drag on the left side of the system right"?
Response:
The action you describe may be correct but not sufficient to cause a “full stall” When the tips are in the stall region they may do what you stated but not to the extent you stated and this may account for the vibration experienced at the onset of retreating blade stall however in a full stall as much or possibly more than 30% of the blade span is involved and at this time there is sufficient lift differential to cause the flapback / blowback.
[ 05 December 2001: Message edited by: Lu Zuckerman ]
Thread Starter
Lu, I have taken the liberty of recsuing some more of your theories on RBS from "Ditching" and have posted then here for you:
Lu says: First of all, let’s disregard compressibility as retreating blade stall can occur under several different sets of conditions. I agree that there will be a lift differential. At this point our opinions start to differ. The roll will occur prior to the disc rising over the nose. At the onset of retreating blade stall there is a pronounced vibration followed by a roll to the left (American design). If at this time the pilot has not taken corrective action the disc will not just rise over the nose it will flap (blow) back quite violently and in some cases the blades will contact the tail boom (cone).
The major difference between us is how we view the reasons for roll and blow (flap) back. You say that is a pure aerodynamic response and I agree that aerodynamics is involved. As I have stated previously this is an alternate to gyroscopic theory and I can accept that. However, I believe that the differential of lift or, differential of forces acting on the disc which has gyroscopic characteristics causing it to precess. The roll is aerodynamic but it is the precursor to the flap back, which I believe is the result of gyroscopic precession.
The same condition exists but to a lesser degree when transitioning and the helicopter rolls and the disc blows back. Under these conditions the pilot can take corrective action by applying forward cyclic.
In retreating blade stall the retreating blade(s) are generating less lift than the advancing blade(s).
This lift differential is a perturbing force on the right side of the disc and is acting upward causing the disc to respond as a gyro rotor and the resultant of this upward force is the tilting of the disc up over the nose and down over the tail. The differential of lift will cause the helicopter to roll to the left, which is a precursor to the flapping back or blowing back. In this area there is a pronounced vibration just prior to the blow / flap back.
In both cases there is a differential of lift causing the disc to tilt up over the nose and down over the tail. In one case, the helicopter rolls right and in the other the helicopter rolls left. The roll is the result of aerodynamic forces while the flapback / blowback is the function of lift differential across the disc and the upward perturbing force results in a gyroscopic response.
In the case of the actions resulting from transverse flow effect the movement of the helicopter and the rotor is benign and the pilot can counter these actions with cyclic input. In the case of retreating blade stall the roll is pronounced and there is a lot of vibration. If by that time the pilot has not taken corrective action the disc will flapback violently and most likely chop off the tail boom.
Hopefully you can see that the actions taking place are similar if not identical. The corrective actions by the pilot are different and there is a major difference in what happens to the rotor relative to its’ response. Each condition should be taught in the flight training program and it should be obvious to any pilot that he / she will not experience retreating blade stall at translational lift speeds and therefore, understand the corrective actions that must be undertaken for both situations.
--------------------
The Cat
To Baranfin:
The blade does not actually stall at the tips, it spreads out from the root first due to relative airflow (root area is moving much slower than tip).
Lu says: First of all, let’s disregard compressibility as retreating blade stall can occur under several different sets of conditions. I agree that there will be a lift differential. At this point our opinions start to differ. The roll will occur prior to the disc rising over the nose. At the onset of retreating blade stall there is a pronounced vibration followed by a roll to the left (American design). If at this time the pilot has not taken corrective action the disc will not just rise over the nose it will flap (blow) back quite violently and in some cases the blades will contact the tail boom (cone).
The major difference between us is how we view the reasons for roll and blow (flap) back. You say that is a pure aerodynamic response and I agree that aerodynamics is involved. As I have stated previously this is an alternate to gyroscopic theory and I can accept that. However, I believe that the differential of lift or, differential of forces acting on the disc which has gyroscopic characteristics causing it to precess. The roll is aerodynamic but it is the precursor to the flap back, which I believe is the result of gyroscopic precession.
The same condition exists but to a lesser degree when transitioning and the helicopter rolls and the disc blows back. Under these conditions the pilot can take corrective action by applying forward cyclic.
In retreating blade stall the retreating blade(s) are generating less lift than the advancing blade(s).
This lift differential is a perturbing force on the right side of the disc and is acting upward causing the disc to respond as a gyro rotor and the resultant of this upward force is the tilting of the disc up over the nose and down over the tail. The differential of lift will cause the helicopter to roll to the left, which is a precursor to the flapping back or blowing back. In this area there is a pronounced vibration just prior to the blow / flap back.
In both cases there is a differential of lift causing the disc to tilt up over the nose and down over the tail. In one case, the helicopter rolls right and in the other the helicopter rolls left. The roll is the result of aerodynamic forces while the flapback / blowback is the function of lift differential across the disc and the upward perturbing force results in a gyroscopic response.
In the case of the actions resulting from transverse flow effect the movement of the helicopter and the rotor is benign and the pilot can counter these actions with cyclic input. In the case of retreating blade stall the roll is pronounced and there is a lot of vibration. If by that time the pilot has not taken corrective action the disc will flapback violently and most likely chop off the tail boom.
Hopefully you can see that the actions taking place are similar if not identical. The corrective actions by the pilot are different and there is a major difference in what happens to the rotor relative to its’ response. Each condition should be taught in the flight training program and it should be obvious to any pilot that he / she will not experience retreating blade stall at translational lift speeds and therefore, understand the corrective actions that must be undertaken for both situations.
--------------------
The Cat
To Baranfin:
The blade does not actually stall at the tips, it spreads out from the root first due to relative airflow (root area is moving much slower than tip).
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Retreating blade stall q
Have a quick q for you knowledgeable folks...
Is it harder to get into retreating blade stall with a semi-rigid rotor system? If so, a brief explanation would be appreciated.
Thanks!
Is it harder to get into retreating blade stall with a semi-rigid rotor system? If so, a brief explanation would be appreciated.
Thanks!
Feel free to disagree, but thats my 2cents worth.
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It is my belief that teetering rotors stall earlier than articulated or bearingless rotors, that is they have less maneuver capability if all other factors are equal.
Why? Because the blades are not free flap as they wish, they must flap in pairs, and this flapping can affect angle of attack, probably to the detriment of the maximum lift from that blade.
Why? Because the blades are not free flap as they wish, they must flap in pairs, and this flapping can affect angle of attack, probably to the detriment of the maximum lift from that blade.
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The answer to your question requires an answer to another question:
What do you mean by "harder to get into retreating blade stall"?
If you mean, "Is it harder to put the retreating blade into stall", then the answer IMHO depends more on blade design than the system that allows the blade to flap. To partly address Mr. Lappos' comments, in a semirigid system, the designers' aim is a design that will balance out the dissymetry of lift in the disk through the up-flap of the advancing blade and the down-flap of the retreating blade. In a fully-articulated system each blade finds its own "balance". Regardless of the flapping system, the retreating blade will eventually stall given sufficient forward airspeed. It's the design of the blade itself with features such as twist that increase the airspeeds at which a retreating blade will stall.
If you mean, "Is it harder to get the aircraft to depart controlled flight due to retreating blade stall", then the answer is most probably no. A semirigid system will depart controlled flight sooner than a fully-articulated one. But this is, again, not principally due to the flapping system.
Typically, fully-articulated rotor systems have more than two rotor blades, but only one rotor blade is in a stall condition at any given time. In the airframe I presently fly, there are four blades so when one is in stall, three are not. Retreating blade stall manifests itself simply as a 1/4 vibration that increases in intensity as you increase the stall. So you usually really have to push it to get the aircraft to depart controlled flight.
Semirigid systems, OTOH, have only two blades. When one is in stall, only one other blade is still producing lift, and it's all on one side. So departure from controlled flight happens fairly rapidly and sometimes violently. I can attest to that from personal experience...
What do you mean by "harder to get into retreating blade stall"?
If you mean, "Is it harder to put the retreating blade into stall", then the answer IMHO depends more on blade design than the system that allows the blade to flap. To partly address Mr. Lappos' comments, in a semirigid system, the designers' aim is a design that will balance out the dissymetry of lift in the disk through the up-flap of the advancing blade and the down-flap of the retreating blade. In a fully-articulated system each blade finds its own "balance". Regardless of the flapping system, the retreating blade will eventually stall given sufficient forward airspeed. It's the design of the blade itself with features such as twist that increase the airspeeds at which a retreating blade will stall.
If you mean, "Is it harder to get the aircraft to depart controlled flight due to retreating blade stall", then the answer is most probably no. A semirigid system will depart controlled flight sooner than a fully-articulated one. But this is, again, not principally due to the flapping system.
Typically, fully-articulated rotor systems have more than two rotor blades, but only one rotor blade is in a stall condition at any given time. In the airframe I presently fly, there are four blades so when one is in stall, three are not. Retreating blade stall manifests itself simply as a 1/4 vibration that increases in intensity as you increase the stall. So you usually really have to push it to get the aircraft to depart controlled flight.
Semirigid systems, OTOH, have only two blades. When one is in stall, only one other blade is still producing lift, and it's all on one side. So departure from controlled flight happens fairly rapidly and sometimes violently. I can attest to that from personal experience...
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retreating blade stall
hi folks from a jet jockey but madly in love with helicopters
i am reading a couple of manuals about helicopters, one being learning to fly helicopters by padfield, and i just learnt a couple of things that puzzled me:
why in a retreating blade stall the nose is pitching up? i guess it has to do with gyroscopic precession of the main rotor system but i can not quite get a full grasp on it!
on more advanced helicopters the throttle grisp is gone from the collective and that it is replaced by knurled switches for fine tuning and engine speed levers, how do they work? i am familiar with turbine engines, although i have never flown a turboprop, but i guess it is something similar to a fadec, just wondering what the available options are - SHUTOFF, GROUND IDLE, FLIGHT IDLE? - how do you use them and how do you fine tune
last but not least, at some point it mentions a very good exercise to acquire muscle memory practicing with a broom stick mimicking a collective while seated on a chair and lifting the stick up while adding power - knurles towards your body - spelling out power on, and doinf the opposite calling power off: an old assignment from his early days i guess!
any other advise on which book to read it would be very appreciated!!
many thanks for your help
baobab72
i am reading a couple of manuals about helicopters, one being learning to fly helicopters by padfield, and i just learnt a couple of things that puzzled me:
why in a retreating blade stall the nose is pitching up? i guess it has to do with gyroscopic precession of the main rotor system but i can not quite get a full grasp on it!
on more advanced helicopters the throttle grisp is gone from the collective and that it is replaced by knurled switches for fine tuning and engine speed levers, how do they work? i am familiar with turbine engines, although i have never flown a turboprop, but i guess it is something similar to a fadec, just wondering what the available options are - SHUTOFF, GROUND IDLE, FLIGHT IDLE? - how do you use them and how do you fine tune
last but not least, at some point it mentions a very good exercise to acquire muscle memory practicing with a broom stick mimicking a collective while seated on a chair and lifting the stick up while adding power - knurles towards your body - spelling out power on, and doinf the opposite calling power off: an old assignment from his early days i guess!
any other advise on which book to read it would be very appreciated!!
many thanks for your help
baobab72
The Machine Will Talk To You
Retreating blade stall has been a point of misconception with the advent of the modern higher disk loading rotor systems. In the H-53 series we were provided with an Incipient Blade Stall Chart that provided the pilots with a maximum KCAS for every weight altitude combination. The essence of this chart was to provide the pilot with a set of limiting conditions that would reduce wear and tear on the dynamic components. The machine would actually talk to the crew in the form of increased vibrations, if you exceeded these limits, The same was evident if you maneuvered the machine excessively at any given weight altitude combination. The mark one butt could feel it if you got too aggressive.
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At the risk of upsetting the powers that be, I'd strongly recommend 'Cyclic and Collective' (I know the author so well, I let him sleep with both my ex-wives...)
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The advancing blade has more speed thus generate more lift for a given collective take, while the retreating blade is slower and loses lift capabilities.
Forces are applied like you said 90º offset, so you have lift in the nose and drag in the tail. The helicopter will pitch up and then roll to the side where lift is lost.
I hope I am not making any wrong statement. There are probably more experienced pilots around here that might have real RBS experiences and can help more than me.
Take care.
Forces are applied like you said 90º offset, so you have lift in the nose and drag in the tail. The helicopter will pitch up and then roll to the side where lift is lost.
I hope I am not making any wrong statement. There are probably more experienced pilots around here that might have real RBS experiences and can help more than me.
Take care.
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Wayback when I was just a practising lunatic I did have a go at some freaking high airspeeds in my ol'G5. As predicted it started to roll left and pitch up. But the vibes good boy, jees-suz, and admittedly the old machine was a bit -ahem- tired with all the rod ends, power cylinder bushes etc sort of flying in loose formation, so maybe that helped me recover a bit quicker by amplifying said vibes where the collective was jumping up and down like you wouldn't believe at which point self preservation took over with the lever in auto go-down mode and cyclic slowly retreating. This all followed by some medication from the nearby re-fridge-er-rator.
Believe it or not but apart from the listed reference data above and the book that I devoured to pass my tests which was John Fay's "The helicopter and how it flies", even Mr. Google is an authority on the subject these days. To the point where just reading him I am thinking of a good shot of caffiene whilst I ponder the age old question as to whether i am now fully certified or still - practicsing.
Believe it or not but apart from the listed reference data above and the book that I devoured to pass my tests which was John Fay's "The helicopter and how it flies", even Mr. Google is an authority on the subject these days. To the point where just reading him I am thinking of a good shot of caffiene whilst I ponder the age old question as to whether i am now fully certified or still - practicsing.
In his second book....Shawn is a much better pilot back then as compared to the first version. Another Golden Rule of Helicopter flying proven.
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Having experienced the phenomena several times at altitude (2 bladed system, counterclockwise) what is going to happen after you have reach the limit of the retreating blade is, the aircraft will get a high frequency vibration (seemed like) and right after that (if acceleration is maintained) the aircraft will pitch up and start rolling to the right.
It is not a violent maneuver when it happens but it will pitch up to about a 45 deg nose up and also roll to about 45 deg. The recovery from that is very simple, do nothing. The pitch up will kill some of the forward speed, the aircraft will roll back level and the nose will come down, simultaneously. It will go right back to where it was before the stall if you don't interfere with it.
JD
It is not a violent maneuver when it happens but it will pitch up to about a 45 deg nose up and also roll to about 45 deg. The recovery from that is very simple, do nothing. The pitch up will kill some of the forward speed, the aircraft will roll back level and the nose will come down, simultaneously. It will go right back to where it was before the stall if you don't interfere with it.
JD
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Retreating blade stall
Fijdor,
Fully agree vibration is an early indicator
The stall in fact triggers a stable reaction of the rotor, the blow back tries to slow it down, resisting an "overspeed condition". Blow back will of course provoke pitch up.
Lowering angles of attack( pitch and or speed) helps to releave it
As previously discusssed low rpm autorotation (case of 530 hard landing) can trigger it and not enough cyclic may be available to compensate it during flare
d3
Fully agree vibration is an early indicator
The stall in fact triggers a stable reaction of the rotor, the blow back tries to slow it down, resisting an "overspeed condition". Blow back will of course provoke pitch up.
Lowering angles of attack( pitch and or speed) helps to releave it
As previously discusssed low rpm autorotation (case of 530 hard landing) can trigger it and not enough cyclic may be available to compensate it during flare
d3
It pitches nose up and rolls LEFT on a counterclockwise rotor because the stall occurs past the 9 o'clock position.
Whilst blowback (flapback) resists any increase in speed and has to be overcome with more forward cyclic, it is not the restoring factor in RBS - as the nose pitches up, the relative airflow changes, reducing AoA. Additionally, the reduction in speed caused by the nose up will allow flapforward (assuming nothing is done with the cyclic) to bring the nose down again.
Whilst blowback (flapback) resists any increase in speed and has to be overcome with more forward cyclic, it is not the restoring factor in RBS - as the nose pitches up, the relative airflow changes, reducing AoA. Additionally, the reduction in speed caused by the nose up will allow flapforward (assuming nothing is done with the cyclic) to bring the nose down again.