Retreating Blade Stall No 2
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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.
'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!
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.
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.
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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
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
Last edited by fijdor; 7th May 2012 at 17:53.
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.
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.
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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
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.
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.
Diagrams are much easier in two colors....Black and White!
Miracles do happen on that dusty road.
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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
Thanks for the infos.
JD
Cheers Fidor, fly safe
Sasless - Ah dun see the light massah, ah dun see the light
Sasless - Ah dun see the light massah, ah dun see the light
Ran across this video a while ago....
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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
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
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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
JD
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
JD
Last edited by fijdor; 9th May 2012 at 18:32.
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.
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.
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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.
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.