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.......

:D :D

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 ]

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 ]

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).

:) :)

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!

At the risk of getting my a$$ handed to me, I'm going to out on a limb and say I don't think the type of rotor head should make any difference. Regardless of head type, at some forward speed the retreating blade will stall (will be unable to produce equal lift to that of the advancing blade). So, to answer your SPECIFIC question, I'd say its just as easy to get into retreating blade stall if you go too fast, regardless of the head type (I suppose thats why we have performance graphs which tell us stuff like Vrbs for a particular set of environmental factors).

Feel free to disagree, but thats my 2cents worth.

HP

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.

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...:ouch:

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

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.:ok:

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...)

Shawn.....never dreamed you had AIDS!



















Aviation Induced Divorce Syndrome!

Mmmm ....

HEY .... isn't AIDS the primary qualification for holding an ATPLH ?? :confused:

:{

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.

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.

Shawn,

I got your first book, along those from Ray Prouty's; is your's latest book different from the first (apart from talking about helicopters)??

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.

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

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

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.

Pitch up and rolls RIGHT on a counterclockwise, 2 bladed system, here that would be a B205 this is what it did to me and I know the difference in between left and right.

JD

maeroda:
'Cyclic and Collective' recycles 'The Art and Science of Flying Helicopters', plus added about 30% more content (a chapter on piston engines in helicopters, for example), plus, if my rapidly failing memory serves me right, about 50% more bad jokes.
And yes, SASless is right - I became a much better pilot by the time the second book was written. Not sure how, or why. Now trying to make sure I capture all those memories of my supposed expertise.

It is a bit more complicated in the Chinook.....but with great effort and persistence...you can discover the joys of RBS in the old girls (thinking "A" Models here).

I seem to recall it was the Aft Head that would "stall" before the Forward head did.....which made for much less pitching and rolling....and usually just a lot of vibration was felt.

One experience that stuck with me was getting into RBS at about 46 Knots....no typo....46 knots......when we lofted off with an underslung load of 16,800 pounds. The Wokka would lift it....but did not like to fly with it very fast.

Our normal underslung load weight was 8,000 pounds in those days with the "A" Model.

The coning angle must have been about like an American Football Referee signaling a Touchdown!

fijdor - why do you think it is call retreating blade stall? That is on the left in a counterclockwise rotor and that is the way it rolls. If the blade on the left stalls, please explain how the aircraft rolls to the right.

If your 205 did something different then either every book I have read about helicopter aerodynamics (including what is taught to both the UK and US military) is wrong or you suffered something other than RBS.

You may have to reread the paragraphs that you missed in all those books.

Here is the part you missed, reading this might help you understand why the aircraft rolls to the right in a counterclockwise rotor sys.

"The spinning main rotor of a helicopter acts like a gyroscope. It has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resulting action occurring 90 degrees from the applied force."

The sequence goes like this 1) vibration 2) pitch-up 3) roll to the right in that order

Something happens on the left (RBS is the applied force in this case), somewhere in between 180 deg to 360 deg, nothing says it happens at exactly 270 deg, reaction will occurs somewhere in between 180 deg to 000 deg. Hence the pitch up and roll right.

Now I am NOT an Aeronautical Engineer, I am a helicopter pilot that has been flying helicopter for a living. When I tried to calculate the exact amount of times I had RBS, I ran out of fingers and toes. Once I got tired of playing with this RBS and I knew the when, the how, speed, what to do etc I simply resumed the work I was doing and stayed away from it.

JD

Sorry fidor but that is complete horse**** and I can only assume you are trying to wind me up.

I am not an aeronautical engineer either and, like you, I have been flying helicopters for many years (30 in fact) and I have experienced RBS myself in both clockwise and counterclockwise rotors.

It always, always, always rolls towards the retreating side because, if you look at where the highest AoA and the lowest speed is, it is just past the 270 position - hence this is where the blades stall.

According to your 'precession' theory (the rotor does not act as a gyroscope because it does not have enough mass and loads of hinges) the nose should pitch down if the RBS happens near to the 180 position.

Crab I am not trying to wind you up, this is not the intention here.(take note, English is my second language) I have been flying commercially for 35 years myself and to say that the aircraft was rolling to the left well I would be lying to you. Below is an explanation on how I understand it and also how other people understand it as well, it works for me. Now if the reason below is not the right explanation/reason for it then too bad because next week, time off is over and I will hook up the longline and get back to work moving drills to pay for the goodies and retirement

The point is this, you have a blade that will stop flying somewhere around the 9 o'clock position and will fully recover by about 7 to 6 o'clock position, using the Gyroscopic precession principle, the result of this will be FELT 90 deg later (counterclockwise) in our situation here it means the 6 o'clock position to the 3 o'clock position. (roughly)

It means the rear end and the rear right of the rotor disc is NOT flying anymore at that stage, it has LOST lift, that is the result of the stall 90 deg later but the front end and the left front of the rotor disc IS STILL creating lift, still flying The back end and the right rear of the aircraft will fall because of the lost of lift, the front and front left of the aircraft will go up because of the lift still created hence the pitch-up and roll right.

Jacques

Fidor - try this on for size:

From the 12 o'clock position towards the tail, the blade experiences a reduction in TAS and begins to flap down, this increases the angle of attack such that the stall is already beginning by the time 9 o'clock is reached and continues until the blades reaches it's low point in the 7 to 8 o'clock position - therefore giving pitch up and roll left.

In terms of your precession argument, my explanation is that the blade begins to stall in the 10 o'clock and the effect is felt 90 degrees later in the 7 o'clock.

Your explanation assumes the blade stalls at the 9 o'clock or later and then starts to flap down - the reality is that it was flapping down before it got to the 9 o'clock.

The US ARMY's Fundamentals of Flight manual show this with illustrations of a 2 bladed rotor system that looks exactly like a 205 and quite explicitly states that it will pitch up and roll left.

The rotor behaves like a gyro in a vacuum but in air the aerodynamic forces determine its behaviour - there will always be a difference of opinion in this because it is taught differently both sides of the Atlantic - we say phase lag, you say precession.

Crab quoting Fort Rucker....and not CFS???

Diagrams are much easier in two colors....Black and White!

Miracles do happen on that dusty road.

Ok Crab i will see when I have some good long "time off" if this new theory (to me) will fit on, so far even with the wrong one (in theory) things have been going pretty good. I like my job :)

Thanks for the infos.

JD

Cheers Fidor, fly safe:ok:


Sasless - Ah dun see the light massah, ah dun see the light:)

Ran across this video a while ago....

fijdor
just something for you to contemplate - I have had the bell 47 often roll uncommanded to the right but not in RBS, this was in a deliberate VRS. You have a couple of the ingredients high DA, rolling to the right. But nose coming up is a mystery. How are you loaded, with slighlty aft C of G and are you fairly heavy say with a hook load?

What I did and mainly to see how long I could stay in VRS was after experimentation, I set up with wind slightly from port side with fore and aft and lateral loading at neutral (C of L). I then entered VRS without applying any recovery, in fact deliberately using collective to keep it in.

After a short while gyroscopic precession takes over and the aircraft rolls to the right, and hey presto, escapes the recirculating air and recovers as the disc bites into fresh air.

The hook load would help to keep you straight and level and thus in it for longer, due to pendular effect. With a slightly aft C of G your tail would sink faster than the nose giving the illusion of nose lifting, the aircraft would then recover rearwards.

Or it may try to and if you then went forward on the cyclic it would have the effect of keeping it in VRS or at least the downdraft, so to speak, and it would feel quite a while to get airspeed up and a lessening of descent rate.

With a turbine it may pay to be concerned about not getting a heap of wind up the choof.

Vibrations at VRS entry are always there but absolutely nothing like the vibrations at incipient RBS.


Anyway just a thought. what sort of aircraft was it?

cheers tet

Interesting idea Tet, since some people seem interested in this RBS, the theory , recovery from it and things connected to it, I may as well state the the conditions, location, aircraft and the kind of work I was doing when I encounter this condition.

Job itself was in the Chilean Andes, the top of them to be precise, I spent months surveying the top from West to East back and forth, thousands of kilometers flying the bird (magnetic survey) attached to a 100 ft line below me and following the terrain at 200 ft agl. Aircraft was a Single Hyd Bell 205 with 205 blades and -17. working gross weight was about 8000 lbs including 800 lbs of fuel which would last almost 2hrs up there . Altitude range we worked at was from 16,000 ft to 21,500 ft PA . In Northern Chile (desert) with a temp of roughly +5c to – 2c would equal to 20,000 to 25,000 ft Density Altitude, in there somewhere.

Starting at about 15,000 ft PA, the RBS would start to happen at the VNE limitations calculated as per the flight manual, 8000 gw 115 kt VNE reduce VNE 3kts per thousand ft, ex. 21,000 ft vne, in between 50 and 55kt, going past the vibration a that time the nose would pitch up and the aircraft would roll to the right. At first I tried to recover using the controls but later on realized that the aircraft would go back to where it was before the stall happened without doing anything to it excepted to get the bird back on the line and not hitting anything solid with it

Like I said earlier in my post, it is not a violent maneuver since being at that altitude everything happens slowly, Here is where I believe the "gyroscopic precession" theory fits, things happens so slowly that the reaction to it (RBS) happens way later, you can see the whole thing happening in slow motion, the pitch up and roll, no matter what you do to the controls, it will finish what it has started before your inputs can act on it. Now by the time you have reached the 25,000 ft DA there is not much limit left in the forward speed and in actual fact, no limit left at both end of the airspeed indicator, because slowing down below 20 kt ,you would loose tail-rotor authority (LTE) and have to deal with the little spinning problem. This, you need patience and good timing to come out of it but out of subject here.
Since the available speed range from min to max was roughly 30/35 kts, I did see a lot of RBS and LTE early on the job until I figured out all the limitations.

Aircraft was well within the limits weight wise, RBS, 99% of the time would happen on a almost straight and level section of the job since I could get some quick kilometers in (relatively speaking) by maintaining the airspeed at the vibration level of the RBS.

Now for the VRS theory I don't think it is a factor here, at least not according to my experience dealing with it. I can say this though, VRS, WAS a great tool at getting this job done but that is another story.

Never experience this condition again later on in my career.

Now that's my story and I am sticking to it :8

JD

Fidor - I hadn't appreciated what high DAs you were operating at when you encountered the RBS - it may well be that the reduced level of aerodynamic damping because of the thin air did alter the handling characteristics.

IIRC we are talking about Lock Number here which is the relationship between blade inertia and aerodynamic damping and is the reason that phase lag changes with increasing DA.

It seems conceivable that a blade flapping down due to the stall condition might flap further down and therefore further round the circle of the disc because the thin air takes longer to affect its path, possibly even past the 6 o'clock position which would indeed account for your roll to the right.

Crab, you are a good man.:ok:

JD

Crab - withdraw?
 

Crab to Fijdor: I suppose you withdraw that remark now?

Compressibility issues
 

Just wondering with those high density altitudes Fjidor is talking about, could 'there be compressibilty issues with the advancing blade coming into supersonic speeds?

High DA would not alter the aerodynamics or forces to the extent the aircraft would roll in the opposite Direction. I would have to see a very detailed Test Report proving it to be the case before I would believe it.

The High DA would generate a lower IAS for the onset of RBS...but the stalling region would still be focused on the same region of the rotor system. Lateral CG would have to be way off to the side and thus very noticeable in cyclic stick position for normal flight if one was to suggest lateral CG as the cause for the deviation from the rule.

If the Bell 205 goes low "G" the tail rotor thrust will roll the airframe right.