Retreating Blade Stall No 2
"Brother Dixson says his experience testing Sikorsky aircraft.....RBS causes the aircraft to roll in the same direction as the blades rotate. American....left, French/Russian....right.
John - what is confusing is that you start talking about S-55 which will be an anti-clockwise rotation with stall giving a roll left and then talk about a clockwise rotation regarding the aft cyclic demand - is that a typo?
Answers?
Crab, my recollection is that the coupling is an angular rate, not angular acceleration term. Could be wrong.
SAS, after getting out of flight school, I wangled an assignment to the Army Avn Test Board at FT Rucker ( 1963 ) and immediately started getting a CH-47 checkout from two Boeing pilots, Jim Campbell and Bill Fraser, both of whom were Air America KIA's a few years later. I did the most flying with Jim Campbell and in a conversation in flight one day he was talking about the CH-47A stall characteristics, having done that flying at the factory. Jim related that because the aft rotor was flying in a disadvantageous aerodynamic inflow situation, the control loads/stall entry occurred first on the aft rotor and the aircraft behavior was a nose up pitch, which, to some extent was in the self-correcting direction. Lowering the collective was the real solution, Jim said.
As to the stall vs altitude inquiry, the answer is a mix. Most of the dynamic stall events typically occur during the conduct of military structural demonstrations, and those take place at the lower altitudes, where the higher engine power/speed makes it possible to obtain higher Nz.
Most, but not all, of course. For example, the Portuguese were interested in a Coast Guard type S-92 for a European offshore SAR program. The additional fuel drove the Gross Weight to 31,000 lbs and we did a flight loads survey program up to 12K I think. I do recall quite clearly that at 31K and at maximum speeds, it was easy to achieve deep stall at the higher bank angles. Hard to make any comment re rolling tendency in those situations, as when the ship is driven into deep stall and you are over at 45-60 deg, the stall results in the rotor flat loosing lift and the ship descends, throwing the sideslip out the window and the lateral stick position with it. ( that was an interesting test program: at lower altitudes, that rotor would get us to just 150 KIAS at 31K , without stall of any kind ).
Now to the steady state comment background from my former post. \\When the USAF got their HH-53C ( not the B-we only made six of those ), with the larger -7 engines, the USAF needed a flight loads survey covering 42000 lbs up to 15K ft, to cover the refueling envelope, among other things.
My predecessor Frank Tefft was the 53 Project Pilot and came back from a flight saying that the rotor was obviously into stall but the Cruise Guide wasn't showing stall.* Aside re SA Cruise Guide: the cruise guide cockpit indicator is fed by an LVDT which measures main rotor servo load. Flight testing on S-61 and S-65 rotors had showed that this parameter was an accurate indicator for the edge of the stall boundary. Previously, at lower altitudes and with less available engine power, it had been impossible to drive the rotor into stall in level one "G" flight, and the cruise guide was therefore a very useful manuvering flight tool.
* I should mention that since this was just an ordinary flight loads test program, we weren't using telemetry at the time.
With the 53C/-7 engine, and at 42,000 lbs at 15,000 ft it was now possible to drive the rotor into stall at speeds at/near Vh. The problem with the cruise guide was that the servo vibratory load signal, normally a 6P ( six blades on this puppy ) trace, would, under stall conditions, break down into a 12P signal, and with the top of each control load maximum now "chopped off" so to speak control load, meanwhile, as the rotor proceeded into deep stall, the push rod loads would continue to increase, and rather dramatically.
This started what was to be a side test program, and since Frank had a few other things on his plate, I was assigned and did most/all of the successive load flying. The trim points, in definite stall, are the ones where one could observe a definite ( not large though ) right stick migration with stall ( we now had a cockpit push rod load indicator as well.
In retrospect, though, "sometimes pilots have to do what they gotta do". An HH-53C USAF pilot named Don Carty came to SA a couple of years later ( Vietnam still going ) to fly a tanker load survey on a 53C ( I'd never tanked up until then ) and Don had just returned from Thailand and points north, where he had been attacked by a MIG. I asked Don what he did to evade the MIG. He told me that there were some clouds around and he dove as fast as he could to enter the clouds. I asked him how fast he went ( thinking about this test program that had enlightened us about the cruise guide-BTW, the flight manual had a rev that treated that issue ), and he said that he didn't know, but the ship was vibrating like hell and very scary. He tells the rest of the story with some black humor, starting with, " OK we have to get out of the cloud and start home, now what's the sneakiest way to do this?" Etc. Don is a USN Test Pilot School grad by the way. One fine aviator.
Sorry to get side-tracked.
Thanks,
John Dixson
SAS, after getting out of flight school, I wangled an assignment to the Army Avn Test Board at FT Rucker ( 1963 ) and immediately started getting a CH-47 checkout from two Boeing pilots, Jim Campbell and Bill Fraser, both of whom were Air America KIA's a few years later. I did the most flying with Jim Campbell and in a conversation in flight one day he was talking about the CH-47A stall characteristics, having done that flying at the factory. Jim related that because the aft rotor was flying in a disadvantageous aerodynamic inflow situation, the control loads/stall entry occurred first on the aft rotor and the aircraft behavior was a nose up pitch, which, to some extent was in the self-correcting direction. Lowering the collective was the real solution, Jim said.
As to the stall vs altitude inquiry, the answer is a mix. Most of the dynamic stall events typically occur during the conduct of military structural demonstrations, and those take place at the lower altitudes, where the higher engine power/speed makes it possible to obtain higher Nz.
Most, but not all, of course. For example, the Portuguese were interested in a Coast Guard type S-92 for a European offshore SAR program. The additional fuel drove the Gross Weight to 31,000 lbs and we did a flight loads survey program up to 12K I think. I do recall quite clearly that at 31K and at maximum speeds, it was easy to achieve deep stall at the higher bank angles. Hard to make any comment re rolling tendency in those situations, as when the ship is driven into deep stall and you are over at 45-60 deg, the stall results in the rotor flat loosing lift and the ship descends, throwing the sideslip out the window and the lateral stick position with it. ( that was an interesting test program: at lower altitudes, that rotor would get us to just 150 KIAS at 31K , without stall of any kind ).
Now to the steady state comment background from my former post. \\When the USAF got their HH-53C ( not the B-we only made six of those ), with the larger -7 engines, the USAF needed a flight loads survey covering 42000 lbs up to 15K ft, to cover the refueling envelope, among other things.
My predecessor Frank Tefft was the 53 Project Pilot and came back from a flight saying that the rotor was obviously into stall but the Cruise Guide wasn't showing stall.* Aside re SA Cruise Guide: the cruise guide cockpit indicator is fed by an LVDT which measures main rotor servo load. Flight testing on S-61 and S-65 rotors had showed that this parameter was an accurate indicator for the edge of the stall boundary. Previously, at lower altitudes and with less available engine power, it had been impossible to drive the rotor into stall in level one "G" flight, and the cruise guide was therefore a very useful manuvering flight tool.
* I should mention that since this was just an ordinary flight loads test program, we weren't using telemetry at the time.
With the 53C/-7 engine, and at 42,000 lbs at 15,000 ft it was now possible to drive the rotor into stall at speeds at/near Vh. The problem with the cruise guide was that the servo vibratory load signal, normally a 6P ( six blades on this puppy ) trace, would, under stall conditions, break down into a 12P signal, and with the top of each control load maximum now "chopped off" so to speak control load, meanwhile, as the rotor proceeded into deep stall, the push rod loads would continue to increase, and rather dramatically.
This started what was to be a side test program, and since Frank had a few other things on his plate, I was assigned and did most/all of the successive load flying. The trim points, in definite stall, are the ones where one could observe a definite ( not large though ) right stick migration with stall ( we now had a cockpit push rod load indicator as well.
In retrospect, though, "sometimes pilots have to do what they gotta do". An HH-53C USAF pilot named Don Carty came to SA a couple of years later ( Vietnam still going ) to fly a tanker load survey on a 53C ( I'd never tanked up until then ) and Don had just returned from Thailand and points north, where he had been attacked by a MIG. I asked Don what he did to evade the MIG. He told me that there were some clouds around and he dove as fast as he could to enter the clouds. I asked him how fast he went ( thinking about this test program that had enlightened us about the cruise guide-BTW, the flight manual had a rev that treated that issue ), and he said that he didn't know, but the ship was vibrating like hell and very scary. He tells the rest of the story with some black humor, starting with, " OK we have to get out of the cloud and start home, now what's the sneakiest way to do this?" Etc. Don is a USN Test Pilot School grad by the way. One fine aviator.
Sorry to get side-tracked.
Thanks,
John Dixson
The increasing number of rotor blades from the 3-bladed S-55 has softened the roll rate for the cases where the rotor is driven into stall, straight ahead, one " G ". In those instances, one notices an increase in N/rev vibration and a measurable tendency for the lateral stick position at the trim points to migrate toward the right. That agrees with the consistent results we have seen in dynamic cases as well, where the rolling tendency is always left.
What I took from your statement above....is the old S-55 (Whirlwind), with three blades would pitch up and roll left during RBS....and as improvements in the design and increase in number of Rotor Blades per head increased....there was less rolling moment and more stick displacement to the right in balanced flight.
What I took from that is the control input required for balanced level straight flight (three different flight maneuvers in my book) necessitated cyclic input to the right to counter a rolling moment to the left.....despite the aircraft no longer pitching up due to the increase in blade numbers and other design features improving rotor blade performance.
Am I right in my perception of what you said?
Crab,
My comment was based upon level balanced flight not maneuvering or dynamic inputs.....just the regular ol' fashioned "going to fast" for the rotors to handle it" situation. Thus....meaning no cyclic input other than that required for the airspeed being sought that puts one into the RBS situation.
When one adds a cyclic control movements.....I can see where things can be much different than in the straight and level...fast cruise scenario I was talking about.
Last edited by SASless; 4th Jun 2012 at 19:11.
Understanding
SAS, I believe that you used a few different words but came up with the same underlying thought.
Joining SA in 1966, the pilot who did the structural demonstration on the S-55, Jim Chudars, was still there, and relayed that one of those maneuvers resulted in a stall that flipped him left and fully inverted*.Thankfully, the modern rotors are a bit more forgiving.
*All of the load factor maneuvers are specified to be done at maximum weight and most adverse CG, not a good place to be upside down in a UH-19!
Thanks,
John Dixson
Joining SA in 1966, the pilot who did the structural demonstration on the S-55, Jim Chudars, was still there, and relayed that one of those maneuvers resulted in a stall that flipped him left and fully inverted*.Thankfully, the modern rotors are a bit more forgiving.
*All of the load factor maneuvers are specified to be done at maximum weight and most adverse CG, not a good place to be upside down in a UH-19!
Thanks,
John Dixson
Brother Dixson is a very valuable resource of information as he had a very interesting career flying helicopters. He is the E.F. Hutton of the helicopter industry for me.....he talks....I listen!
Agreed SAS, I didn't mean to quibble but I just wanted to clarify the difference between roll caused by RBS in the fast cruise and roll caused by aft cyclic input.
Gyroscopic Coupling
Crab, it is possible that I haven't used quite the right words here, so let me add a bit.
After the design team gets the rotor blade data, weight, blade CG etc etc, the rotors and controls people can design the control system so that for the given rotor properties, Lock number etc, they can position the servos such that the cyclic controls are as purely orthogonal as possible ( pure forward cyclic results in pure forward rotor tilt etc ).
The gyroscopic coupling I referred to results from the rotor being in fact a gyroscope, and so for a US convention rotor, one gets:
Aft cyclic: pitch up, AND, as a function of how much cyclic used, and therefore how much nose up pitch rate results, there is a right rolling rate tendency.
It works the same in the other axes of control input. Nose down results in left roll, right roll results in nose down, and left roll results in a nose up tendency.
But the " gain " or magnitude of this coupling is fairly low*. In articulated rotors, one has to generate a decent nose up pitch rate before the right roll coupling becomes apparent. That is why, in responding to Army's recollection of events, it seemed difficult to envision his right roll as resulting from gyroscopic coupling due to the nose coming up.
*Lets see if I can quantify that with an example from experience. There are several video clips of the S-67 and CH-53 doing rolls ( not me ) on You Tube. If one looks at them closely, you will see that they are initiated with the nose up about 15 degrees and they end up with the nose somewhere around level. Both are done with pure right lateral cyclic and the roll rates are nominally 90-100 degrees/second, plus or minus. Just to be entirely accurate, if one looks even closer at just the end of the roll, one can see the nose down tendency correcting, and that is because the pilot is adding probably 10% aft control at that point.
For rigid rotors with higher effective offset, the coupling is quite a bit stronger. In fact, the Boeing UTTAS prototypes cross fed the output from the pitch and roll rate gyros to damp this tendency, i.e., they had positive pitch rate fed into a left roll SAS input etc. Comanche did the same. In my recollection, SA hasn't done this sort of " anti gyroscopic coupling " on any of the articulated rotors. Done a lot of other pretty neat tricks, but never thought that area was worth the effort.
Bottom line: it will be very interesting to see what Army can find out in talking to the Bell test people.
Thanks,
John Dixson
After the design team gets the rotor blade data, weight, blade CG etc etc, the rotors and controls people can design the control system so that for the given rotor properties, Lock number etc, they can position the servos such that the cyclic controls are as purely orthogonal as possible ( pure forward cyclic results in pure forward rotor tilt etc ).
The gyroscopic coupling I referred to results from the rotor being in fact a gyroscope, and so for a US convention rotor, one gets:
Aft cyclic: pitch up, AND, as a function of how much cyclic used, and therefore how much nose up pitch rate results, there is a right rolling rate tendency.
It works the same in the other axes of control input. Nose down results in left roll, right roll results in nose down, and left roll results in a nose up tendency.
But the " gain " or magnitude of this coupling is fairly low*. In articulated rotors, one has to generate a decent nose up pitch rate before the right roll coupling becomes apparent. That is why, in responding to Army's recollection of events, it seemed difficult to envision his right roll as resulting from gyroscopic coupling due to the nose coming up.
*Lets see if I can quantify that with an example from experience. There are several video clips of the S-67 and CH-53 doing rolls ( not me ) on You Tube. If one looks at them closely, you will see that they are initiated with the nose up about 15 degrees and they end up with the nose somewhere around level. Both are done with pure right lateral cyclic and the roll rates are nominally 90-100 degrees/second, plus or minus. Just to be entirely accurate, if one looks even closer at just the end of the roll, one can see the nose down tendency correcting, and that is because the pilot is adding probably 10% aft control at that point.
For rigid rotors with higher effective offset, the coupling is quite a bit stronger. In fact, the Boeing UTTAS prototypes cross fed the output from the pitch and roll rate gyros to damp this tendency, i.e., they had positive pitch rate fed into a left roll SAS input etc. Comanche did the same. In my recollection, SA hasn't done this sort of " anti gyroscopic coupling " on any of the articulated rotors. Done a lot of other pretty neat tricks, but never thought that area was worth the effort.
Bottom line: it will be very interesting to see what Army can find out in talking to the Bell test people.
Thanks,
John Dixson
The BO-105 and BK-117 aircraft have a very noticeable Pitch/Rpll Coupling effect in steep banked right turns and in the conversion training done by the factory pilots...the recovery technique taught is the application of left pedal rather than pure cyclic movement.
That gets the nose of the aircraft pointed uphill.....which at low level might mean the difference in surviving the situation. Nothing to do with RBS but does add to the understanding of effect of control inputs that operate in a different axis than normally thought.
That gets the nose of the aircraft pointed uphill.....which at low level might mean the difference in surviving the situation. Nothing to do with RBS but does add to the understanding of effect of control inputs that operate in a different axis than normally thought.
Last edited by SASless; 5th Jun 2012 at 15:17.
John, thanks for the clarification, it agrees with my experience instructing on the Lynx which, as I am sure you know, has a very high effective hinge offset. All the display manoeuvres are flown without the stab, partly to give maximum responsiveness and partly because the gyros would topple anyway.
When performing loops or backflips where a maximum cyclic displacement and high rate of application is required, the amount of left cyclic required to keep the 'wings' level is quite noticeable.
However, I have heard arguments from both sides about whether or not the rotor is a gyro since it has little mass (compared to a similar sized gyro) and a very low rotational speed.
I don't see any reason why there shouldn't be some precessing forces when dealing with rapid applications of cyclic but the waters are muddied by the teaching in the US that phase lag is a function of precession rather than aerodynamic forces.
Equally, I can see that because the highest AoA is always on the left side of the disc, increasing rotor thrust using aft cyclic will give an overall increase of AoA and the higher angles on the left side could well produce that same right roll. This would agree with experience in steep turns to the right where, once the desired AoB has been reached in steady state, an application of collective again makes the aircraft roll further right and more left cyclic is required to compensate.
When performing loops or backflips where a maximum cyclic displacement and high rate of application is required, the amount of left cyclic required to keep the 'wings' level is quite noticeable.
However, I have heard arguments from both sides about whether or not the rotor is a gyro since it has little mass (compared to a similar sized gyro) and a very low rotational speed.
I don't see any reason why there shouldn't be some precessing forces when dealing with rapid applications of cyclic but the waters are muddied by the teaching in the US that phase lag is a function of precession rather than aerodynamic forces.
Equally, I can see that because the highest AoA is always on the left side of the disc, increasing rotor thrust using aft cyclic will give an overall increase of AoA and the higher angles on the left side could well produce that same right roll. This would agree with experience in steep turns to the right where, once the desired AoB has been reached in steady state, an application of collective again makes the aircraft roll further right and more left cyclic is required to compensate.
Retreating Blade Stall
Hi everyone, Im teaching an aerodynamics class in the near future and have always included RBS in these classes. The problem is that i have no real world experience on what the aircraft does when it enters RBS. I know what the textbooks say. But I'm looking for more detailed accounts of RBS, such as pitch up rate, roll rate, effects of positive and negative pilot inputs. and such. any flight testing video would be nice too. I have dug deep on the PPRuNe, but haven't found this sort of answer. Any chance some of you experts can help me out! Thanks.
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This is from a friend of mine who had it demonstrated in the OH-58 (Bell 206) in the army:
“At VNE, the nose will gradually try to climb, and when you push the cyclic forward more to maintain level flight, it will continue to do so. Just before the max forward cyclic, the fuselage panels begin to oil-can, at which point the nose will come up and force the machine into a vertical climb, before breaking sharply, similar to a hammerhead stall.”
Will see if I can find more
phil
“At VNE, the nose will gradually try to climb, and when you push the cyclic forward more to maintain level flight, it will continue to do so. Just before the max forward cyclic, the fuselage panels begin to oil-can, at which point the nose will come up and force the machine into a vertical climb, before breaking sharply, similar to a hammerhead stall.”
Will see if I can find more
phil
When I taught Gazelles in the late 80's early 90's we used to teach RTB as a once only demo for ab-initio's to show them what happens.
Put the nose into a steep dive, achieve Vne (about 167kts from memory) and pull hard on the stick.
Now I don't know if this was to induce RTB, or RTB just happened but it certainly upset the poor student who didn't know what to expect!
The hydraulics locked up, the cab pitched up and towards the retreating blade side (I think) - all quite violently I seem to recall and came out of it by itself. But for a fleeting second or so - you were out of control of that aircraft and became a passenger.
I can see why they put a stop to that and so many other tortuous manouevres shortly thereafter! Poor airframe!
[Feel free to correct me - all those with better memories!].
Put the nose into a steep dive, achieve Vne (about 167kts from memory) and pull hard on the stick.
Now I don't know if this was to induce RTB, or RTB just happened but it certainly upset the poor student who didn't know what to expect!
The hydraulics locked up, the cab pitched up and towards the retreating blade side (I think) - all quite violently I seem to recall and came out of it by itself. But for a fleeting second or so - you were out of control of that aircraft and became a passenger.
I can see why they put a stop to that and so many other tortuous manouevres shortly thereafter! Poor airframe!
[Feel free to correct me - all those with better memories!].
Last edited by Thomas coupling; 4th Sep 2013 at 22:12.
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Visual Aids
When delivering your class please think about some unique visual aids as these can sometimes help students to better understand a concept and can be very useful in exams when trying to recall a particular principal.
This is a standard illustration used to explain the principal of Dissymmetry of Lift:
But as well as that how about using something a little innovative, such as this image below?
This image shows the path of a model helicopter at night where the rotors are illuminated by LEDs.
The variation between R and r = the difference between the rotor's Radius of Curvature and when I first saw this example I immediately thought to myself "Pretzels" even if they are not very common where I come from but in the US they have lots of pretzels. The point being, this is a great graphic way to illustrate the principal of Dissymmetry of Lift.
The video explaining the photo above is here:
I would also like to see real life footage of a helicopter in RBS so if you find it maybe you (or one of your students) can put it on You Tube.
Good luck with the lecture.
This is a standard illustration used to explain the principal of Dissymmetry of Lift:
But as well as that how about using something a little innovative, such as this image below?
This image shows the path of a model helicopter at night where the rotors are illuminated by LEDs.
The variation between R and r = the difference between the rotor's Radius of Curvature and when I first saw this example I immediately thought to myself "Pretzels" even if they are not very common where I come from but in the US they have lots of pretzels. The point being, this is a great graphic way to illustrate the principal of Dissymmetry of Lift.
The video explaining the photo above is here:
I would also like to see real life footage of a helicopter in RBS so if you find it maybe you (or one of your students) can put it on You Tube.
Good luck with the lecture.
Apart from the bit where he says that gyroscopic precession is what recovers the aircraft from RBS!
