Retreating Blade Stall No 2
Hey Crab I guess I am not making myself clear,
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crab: just being politely self deprecating actually .... fairly un-officer-like to jump on that I feel.
.... in fact I am amazed at your inability to concede anything ever - when it is clear you are wrong you don't debate the point and try and attack something diversionary.
.... I think you must (if only to prove you can be a fair minded and reasonable individual) concede the point that you were wrong about peak AoA and that I was not contradicting myself as you charge ..................... (you won't !)
.... in fact I am amazed at your inability to concede anything ever - when it is clear you are wrong you don't debate the point and try and attack something diversionary.
.... I think you must (if only to prove you can be a fair minded and reasonable individual) concede the point that you were wrong about peak AoA and that I was not contradicting myself as you charge ..................... (you won't !)
A point that AnFi and others are missing is that the swash plate is continually changing the pitch angle during the orbit.
Sure, the retreating blade is at its max pitch at the 9 o'clock position, and the blade is at its greatest rate of flapping up as well, but from then on the swash plate is driving its pitch angle downwards - but the disc still reaches its highest point in the orbit at the 6 o'clock position due to its inertia.
So, even if a segment of the blade reaches a stall condition, it will rapidly come back out of it due to the combined effect of the decreasing pitch angle, the fact that the blade is still on its way UP (not flapping down), and the changing direction of the airflow going from behind the blade at 9 to along the blade at 6.
People get stuck on the idea of flapping to equality with the disc high at the front and low at the back, but to fly forwards, the swash plate has feathered the blades and it is flapping down to the front and up to the back. Equality of lift is forced by the swash plate.
Sure, the retreating blade is at its max pitch at the 9 o'clock position, and the blade is at its greatest rate of flapping up as well, but from then on the swash plate is driving its pitch angle downwards - but the disc still reaches its highest point in the orbit at the 6 o'clock position due to its inertia.
So, even if a segment of the blade reaches a stall condition, it will rapidly come back out of it due to the combined effect of the decreasing pitch angle, the fact that the blade is still on its way UP (not flapping down), and the changing direction of the airflow going from behind the blade at 9 to along the blade at 6.
People get stuck on the idea of flapping to equality with the disc high at the front and low at the back, but to fly forwards, the swash plate has feathered the blades and it is flapping down to the front and up to the back. Equality of lift is forced by the swash plate.
Ascend Charlie - most of what you are saying is correct when you are just referring to how the control orbit changes pitch but you are forgetting that it is the aerodynamic effects that control what happens to the blade.
It is not enough just to consider pitch angles - you must consider the relative airflow as that is what determines the AoA experienced by the blade for a specific pitch setting.
Flapback overcomes the swashplate demand - that is why you have to move the cyclic progressively further forward to go faster.
AnFI - you just can't get over the fact you said peek when you meant peak and that you have said repeatedly that the blade stalls when it reaches peak AoA which isn't true.
It is not enough just to consider pitch angles - you must consider the relative airflow as that is what determines the AoA experienced by the blade for a specific pitch setting.
Flapback overcomes the swashplate demand - that is why you have to move the cyclic progressively further forward to go faster.
AnFI - you just can't get over the fact you said peek when you meant peak and that you have said repeatedly that the blade stalls when it reaches peak AoA which isn't true.
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AC; correct, except about the point I'm missing (thank you) hence:
please re-read.
Crab I can easily admit my spelling is not all ways perfect YOU on the otherhand cannot concede a single point of your many errors wrt aerodynamic understanding or English comprehension.
Crab what does this mean: "you have said repeatedly that the blade stalls when it reaches peak AoA which isn't true." pls explain
that's not really how helicopters work ('pitch dis-symmetry' counters 'dis-symmetry of lift') - but that's another story...
Crab I can easily admit my spelling is not all ways perfect YOU on the otherhand cannot concede a single point of your many errors wrt aerodynamic understanding or English comprehension.
Crab what does this mean: "you have said repeatedly that the blade stalls when it reaches peak AoA which isn't true." pls explain
AnFi....Au Contrare!
Why just the other day Crab publically admitted quoting Yank Army sources for one of his explanations....and perhaps in a fit of silliness....casting the 100 colored pencils issued him by CFS aside for use in the future. No greater humility than that Sir!
Why just the other day Crab publically admitted quoting Yank Army sources for one of his explanations....and perhaps in a fit of silliness....casting the 100 colored pencils issued him by CFS aside for use in the future. No greater humility than that Sir!
Perhaps because, with a few minor terminological exceptions (blowback vs flapback, transverse flow vs inflow roll etc) and one major explanation issue (precession vs aerodynamic forces) all the teaching on both sides of the Atlantic is the same and I used the US Army notes (particularly because they included a 205) when I was trying to discuss RBS with Fidor who I believe is from your side of the pond.
An FI - if a blade stalls at say 12 degrees AoA and then descends it will experience an increase in AoA so that the peak AoA the blade sees is not when it stalls. That clear enough?
CFS, ETPS, US Army, Wagtendonk, Prouty, Coyle - they all have something to say and I am always listening (but not to you AnFI).
An FI - if a blade stalls at say 12 degrees AoA and then descends it will experience an increase in AoA so that the peak AoA the blade sees is not when it stalls. That clear enough?
CFS, ETPS, US Army, Wagtendonk, Prouty, Coyle - they all have something to say and I am always listening (but not to you AnFI).
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Crab I don't want to suggest you are a bit dim so I'll just try again to explain it to you again - I do understand why you are mis-understanding me (it has to do with whether you read "peek AoA before stall" and "peek AoA after stall" with the "before" and "after" referring to a place in the cycle, or referring to; before the helicopter reaches an airspeed at which stall occurs or after it has reached an airspeed at which blades now stall...
Try to re-read this now:
as for your response:
As a helicopter flys faster the AoA on the retreating side will get bigger. The place where it will be biggest is 9 o'clock (Peak AoA before stall). When the helicopter reaches a speed which requires that peak AoA to equal 12degrees (in your case) stall will commence from that place in the cycle. Further around the cycle, once the helicopter is experiencing RBS, greater AoAs will be achieved, so in that state peek AoA will no longer be at 9 o'clock but further around the cycle.
As for right roll RBS at high DA and heavy AUM it seems obvious that if the whole disc is relatively close to stall , and the difference in AoA between Advancing and Retreating halves is not very great due to very low Vne (hence low speed difference, and less cyclic dissymmetry required) then once a stall has been triggered (at the 9 o'clock position!) the stalled region could easily extend past 6 o'clock - if that happens the pitch up would be accompanied by a Right Roll
factors:
1 flap down, which does not normally occur, will occur to a stalled blade and will further increase it's AoA (exceeding the AoA which initiated the stall at 9o'clock) and perpetuating the stall around the cycle.
2 conditions need to improve to significantly better than where/when the stall started
3 the AoA does not change as much around the cycle at lower speeds (compared to higher speeds), and so if part of the disc is close to stall then the rest of it is relatively closer to stall.
4 there is not much improvement to be had approaching 6 o'clock if the airspeed is not very high when RBS initiates.
Speed seems to be the key factor:
I think it is fair to assume that this is not so much about high DA per-se but more about the speed being particularly low which might also occur for other reasons.
So if you were VERY heavy such that RBS initiated at VERY low speeds then right roll would be expected.
Suggested name LOW SPEED RBS
Crab there is another point you may want to check about how helicopters work, indicated by your incorrect statement previously:
I think you believe that in the cruise blades flap down during the retreating half and that is how their AoA is increased - which is obviously not the case - in fact the swash plate commands a greater pitch on the retreating side to create bigger AoAs to compensate for reduced relative airspeed. 'Flapping to Equality' IS NOT how 'Disymmetry of Lift' is countered in steady state cruise - but that is another topic!
Crab you never concede anything ever - and generally just become sarcastic and rude - and if you won't take it from me I think you will find it in your CFS, ETPS, US Army, Wagtendonk, Prouty, Coyle etc if you re-read it... (probably not in the CFS/ETPS notes actually - I think they still dubiously state that re-circulation at cliffs can draw you in closer don't they?) Why don't you just honourably concede you are wrong and apologise? You won't.
Try to re-read this now:
I think where you are mis-understanding me is in the distinction between the pre-stalled cruise peak AoA (at about 9 o'clock) and what then happens after stall develops when the response to that (flapping down) will create an increased AoA further around the cycle.
So there's:
peek AoA before stall
and
peak AoA after stall
So there's:
peek AoA before stall
and
peak AoA after stall
An FI - if a blade stalls at say 12 degrees AoA and then descends it will experience an increase in AoA so that the peak AoA the blade sees is not when it stalls. That clear enough?
As for right roll RBS at high DA and heavy AUM it seems obvious that if the whole disc is relatively close to stall , and the difference in AoA between Advancing and Retreating halves is not very great due to very low Vne (hence low speed difference, and less cyclic dissymmetry required) then once a stall has been triggered (at the 9 o'clock position!) the stalled region could easily extend past 6 o'clock - if that happens the pitch up would be accompanied by a Right Roll
factors:
1 flap down, which does not normally occur, will occur to a stalled blade and will further increase it's AoA (exceeding the AoA which initiated the stall at 9o'clock) and perpetuating the stall around the cycle.
2 conditions need to improve to significantly better than where/when the stall started
3 the AoA does not change as much around the cycle at lower speeds (compared to higher speeds), and so if part of the disc is close to stall then the rest of it is relatively closer to stall.
4 there is not much improvement to be had approaching 6 o'clock if the airspeed is not very high when RBS initiates.
Speed seems to be the key factor:
I think it is fair to assume that this is not so much about high DA per-se but more about the speed being particularly low which might also occur for other reasons.
So if you were VERY heavy such that RBS initiated at VERY low speeds then right roll would be expected.
Suggested name LOW SPEED RBS
Crab there is another point you may want to check about how helicopters work, indicated by your incorrect statement previously:
AnFI - as previously posted, the blade starts to flap down as soon as it passes the 12 o'clock and sees a reduction in TAS so although the min speed over the blade is a 9 o'clock, the increase in AoA has already started due to the flapping.
Crab you never concede anything ever - and generally just become sarcastic and rude - and if you won't take it from me I think you will find it in your CFS, ETPS, US Army, Wagtendonk, Prouty, Coyle etc if you re-read it... (probably not in the CFS/ETPS notes actually - I think they still dubiously state that re-circulation at cliffs can draw you in closer don't they?) Why don't you just honourably concede you are wrong and apologise? You won't.
Methinks the lady doth protest too much
Just for you AnFI a concession - the blade pitch does increase on the retreating side due to the swashplate demand - but, that in itself is not enough to create the large AoA required to stall the blade. The blade must flap down due to the loss of V squared - that combined with the increase in pitch does give the required AoA.
The critical angle (where the blade stalls) is normally reached before the 9 o'clock position - this can vary with tip speed because the ratio of forward speed to tip speed (called advance ratio IIRC) is a limiting factor in helicopter design.
Whilst we are on the subject of speed, do not confuse the low VNE (IAS based and measured by the pitot) with what the blades are experiencing which is TAS - the difference at 20,000 to 25,000 feet DA (where Fidor was operating) is appreciable.
Every diagram made by those who have either calculated AoA or measured AoA (or both) will show you the distribution of AoA around the disc and both critical angle and peak angle occur before the 9 o'clock position and recover by the 7 to 8 o'clock. This is due to the maximum rate of increase of pitch (swashplate) and the maximum rate of flapping down (aerodynamics). Because the rate of change of pitch with a swashplate is sinusoidal, the greatest rate of change of pitch (note not the maximum degree of pitch) is before the 9 o'clock.
At higher DAs rotor thrust must still equal AUM so with thinner air the AoA across the whole disc will be larger. There is still a large difference between the TAS on the advancing and retreating side so remember it is V squared not just V that is important.
I only suggested the change in Lock number might be a cause for the syptoms Fidor experienced not that it actually was the case.
The critical angle (where the blade stalls) is normally reached before the 9 o'clock position - this can vary with tip speed because the ratio of forward speed to tip speed (called advance ratio IIRC) is a limiting factor in helicopter design.
Whilst we are on the subject of speed, do not confuse the low VNE (IAS based and measured by the pitot) with what the blades are experiencing which is TAS - the difference at 20,000 to 25,000 feet DA (where Fidor was operating) is appreciable.
Every diagram made by those who have either calculated AoA or measured AoA (or both) will show you the distribution of AoA around the disc and both critical angle and peak angle occur before the 9 o'clock position and recover by the 7 to 8 o'clock. This is due to the maximum rate of increase of pitch (swashplate) and the maximum rate of flapping down (aerodynamics). Because the rate of change of pitch with a swashplate is sinusoidal, the greatest rate of change of pitch (note not the maximum degree of pitch) is before the 9 o'clock.
At higher DAs rotor thrust must still equal AUM so with thinner air the AoA across the whole disc will be larger. There is still a large difference between the TAS on the advancing and retreating side so remember it is V squared not just V that is important.
I only suggested the change in Lock number might be a cause for the syptoms Fidor experienced not that it actually was the case.
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Thanks for the concession.
You've now changed the other point you are making to a different point (which happens to be true ish) - so I presume you now understand the point about stall commencing at peek AoA (about 9'oclock) and subsequently (in phase) reaching higher AoA's perpetuating stall around the cycle?
I think you are still wrong (now) about flapping:
No! if it did flap down prior to any stall developing then the attitude will change in pitch up direction - which will lead to the helicopter slowing down - if the pilot wants to go faster and make RBS he must apply forward cyclic to negate the need for any 'flapping to equality'. So big AoA on the Retreating side is because the pilot holds the stick forward - ok?
as opposed to: Once stall commences then the blades will flap down - pilot reaction cyclic (mostly) forward will aggravate the stall and advance the phase of onset as well as delay the phase position of recovery.... if not recovered by 6 o'clock then there'll be a Right Roll component - still it must more significantly Pitch Up.
If the whole disc is close to stall even at low speeds due high weight (or extreme DA) then it is perfectly understandable that stall continues around the cycle beyond 6 o'clock.
[One thing which screws up these discussions and causes mis-understanding between folk here is the reference for flapping - there are at least four references - against which it could be measured - the most useful for 'us' is probably the Tip Path Plane (as opposed to the Zero Pitch Axis or the Drive Axis)]
[ and Crab I was just offended by your rudely over confident post "Sorry fidor but that is complete horse**** and I can only assume you are trying to wind me up." I have tried to suggest to you that your over-confidence it your training and experience has not taught you to listen with an open mind to other pilots, particularly in the case of VRF, this is a clear cut case. ]
Admit - you have now learnt something interesting (cue sarcastic dismissal
)
You've now changed the other point you are making to a different point (which happens to be true ish) - so I presume you now understand the point about stall commencing at peek AoA (about 9'oclock) and subsequently (in phase) reaching higher AoA's perpetuating stall around the cycle?
I think you are still wrong (now) about flapping:
but, that in itself is not enough to create the large AoA required to stall the blade. The blade must flap down due to the loss of V squared
as opposed to: Once stall commences then the blades will flap down - pilot reaction cyclic (mostly) forward will aggravate the stall and advance the phase of onset as well as delay the phase position of recovery.... if not recovered by 6 o'clock then there'll be a Right Roll component - still it must more significantly Pitch Up.
If the whole disc is close to stall even at low speeds due high weight (or extreme DA) then it is perfectly understandable that stall continues around the cycle beyond 6 o'clock.
[One thing which screws up these discussions and causes mis-understanding between folk here is the reference for flapping - there are at least four references - against which it could be measured - the most useful for 'us' is probably the Tip Path Plane (as opposed to the Zero Pitch Axis or the Drive Axis)]
[ and Crab I was just offended by your rudely over confident post "Sorry fidor but that is complete horse**** and I can only assume you are trying to wind me up." I have tried to suggest to you that your over-confidence it your training and experience has not taught you to listen with an open mind to other pilots, particularly in the case of VRF, this is a clear cut case. ]
Admit - you have now learnt something interesting (cue sarcastic dismissal
)
Yeah - me and the rest of the world are wrong about flapping
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Very interesting (and entertaining) conversation. I do not have a video footage of an actual RBS but I do have a couple of videos showing the aircraft on the verge of LTE and one instance of it, to bad the bird operator stop filming when it happened.
It seems on the first video that the aircraft is close to the ground at one point but the Radar Alt is showing 200 ft straight down, it is just the steepness of the hill that makes it look really close.
JD
On the second video the actual altitude was higher than what you can see on the Altimeter due to the fact that the altimeter setting used was the one at the DGPS fixed station down at sea level and we were working in a very high pressure zone.
It seems on the first video that the aircraft is close to the ground at one point but the Radar Alt is showing 200 ft straight down, it is just the steepness of the hill that makes it look really close.
JD
On the second video the actual altitude was higher than what you can see on the Altimeter due to the fact that the altimeter setting used was the one at the DGPS fixed station down at sea level and we were working in a very high pressure zone.
Last edited by fijdor; 21st May 2012 at 13:09.
What Torque were you pulling?
What Torque was available?
I notice the IAS dropped right off....what was the Yaw Pedal position....well forward left pedal?
Did the Nr droop any at all?
Ever hit the collective up stop?
What Torque was available?
I notice the IAS dropped right off....what was the Yaw Pedal position....well forward left pedal?
Did the Nr droop any at all?
Ever hit the collective up stop?
Last edited by SASless; 21st May 2012 at 14:28.
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What Torque were you pulling?
On a climb I tried to maintain Max Continuous, you could not pull max torque, not enough pedal. It was always a trade-off.
What Torque was available?
On the torque gauge, plenty. Never had any problem power wise. T53-17
It was more like pull till you hit the left pedal stop.
I notice the IAS dropped right off....what was the Yaw Pedal position....well forward left pedal?
I wasn't watching my airspeed and let it fall to zero, left pedal is right on the stop, responses to inputs are too slow, by the time it wants to respond to the full left pedal the nose is already gone and has to much momentum. At that point you need to move away from the hill, pick up speed and when the timing is right you reapply full left pedal while reducing collective and it should stop spinning within a turn, you need patience.
Did the Nr droop any at all?
No not at all, problem was trying not to overspeed anything.
Ever hit the collective up stop?
No never. The easy part was the climb up, the hardest part was coming back down.
JD
On a climb I tried to maintain Max Continuous, you could not pull max torque, not enough pedal. It was always a trade-off.
What Torque was available?
On the torque gauge, plenty. Never had any problem power wise. T53-17
It was more like pull till you hit the left pedal stop.
I notice the IAS dropped right off....what was the Yaw Pedal position....well forward left pedal?
I wasn't watching my airspeed and let it fall to zero, left pedal is right on the stop, responses to inputs are too slow, by the time it wants to respond to the full left pedal the nose is already gone and has to much momentum. At that point you need to move away from the hill, pick up speed and when the timing is right you reapply full left pedal while reducing collective and it should stop spinning within a turn, you need patience.
Did the Nr droop any at all?
No not at all, problem was trying not to overspeed anything.
Ever hit the collective up stop?
No never. The easy part was the climb up, the hardest part was coming back down.
JD
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That's the worse video I've ever seen-
A Bell 205 with an empty seat up front what a waste.
Used to do power checks past 13000 ft . You don't have the BLR kit? It adds to power avalible & helps off load the tail rotor.
A Bell 205 with an empty seat up front what a waste. Used to do power checks past 13000 ft . You don't have the BLR kit? It adds to power avalible & helps off load the tail rotor.
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Crab:
Yes I am afraid you are wrong - but it's not you and all of the rest of the world, just most of it.
Proffessor J Gordon Leishaman
in his book
Principles of Helicopter Aerodynamics
(Cambridge Aerospace Series)
Is another person who is not wrong about this.
(page 208, line 14 from the bottom)
You should stop - you'll only hurt your head more
Yeah - me and the rest of the world are wrong about flapping
Proffessor J Gordon Leishaman
in his book
Principles of Helicopter Aerodynamics
(Cambridge Aerospace Series)
Is another person who is not wrong about this.
(page 208, line 14 from the bottom)
You should stop
- you'll only hurt your head more