Retreating Blade Stall No 2
How do you go low "G" with a pitch up?
Likewise we are talking RBS in level flight....again...where is the negative G?
RBS is not an abrupt event.....during cruise flight.
Likewise we are talking RBS in level flight....again...where is the negative G?
RBS is not an abrupt event.....during cruise flight.
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Related trivia
Just back from Bell Training Academy B407 recurrent where I was told that Vne in the B407 is not limited by RBS but rather by maximum allowable forward cyclic input under FAR Part 27 certification standards. As I understand it, Part 27 requires that the helicopter meet the relevant certification standards using less than maximum physical cyclic inputs. With the maximum allowed fwd cyclic, the IAS turns out to be 140 KIAS, which becomes Vne. Presumably RBS occurs somewhere above 140 KIAS (for the DA's at which 140 KIAS Vne applies). Perhaps Shawn or others can add some color commentary.
Last edited by EN48; 16th May 2012 at 14:06.
The standards also require a positive stick gradient....the faster you go...the more forward the stick position has to be. That can cause designers some problems too.
Sasless - it is a fact that phase lag changes as DA increases due to lower aerodynamic damping (thinner air). It is another reason why the precession theory of rotor response is incorrect since a gyro would not be affected in the same way.
My conjecture is that it could affect the amount of flapping due to RBS even though the stall still occurs at the same place - I don't know if it is the case, I am just matching Fidor's experiences to a possible cause.
I have no doubt that Fidor's observations are accurate and that in the conditions he was in the aircraft rolled right - I am just trying to suggest a reason why. Got any better thoughts?
My conjecture is that it could affect the amount of flapping due to RBS even though the stall still occurs at the same place - I don't know if it is the case, I am just matching Fidor's experiences to a possible cause.
I have no doubt that Fidor's observations are accurate and that in the conditions he was in the aircraft rolled right - I am just trying to suggest a reason why. Got any better thoughts?
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well Crab - here's a possible way of looking at it:
the first point in the cycle where a stall might begin to occur is the place requiring the biggest AoA - ie the retreating side (9 o'clock if u like) once stalled: 1stly a wing will stay stalled until conditions are better than the conditions which started the stall AND 2ndly if a rotor stalls it will flap down which increases the AoA (this would normally compensate for the reduction in lift which caused the flapping rate) this will fail to improve the lift and 'deepen' the stall prolonging the stalled region of the cycle towards the aft position even though aerodynamic conditions are considerably improved...
.... increased vibration should be apparent to the pilot.
.... if the pilot responds to maintain attitude he will be increasing the pitch in the stalled region - which will worsten the stall and cause it to extend around the cycle (a little) ... if the stall extends beyond the aft (6 o'clock) position the new plane of rotation might alter slightly to the right ... but it can't be by much I would think ... probably chiefly a nose up pitching tendancy slightly left if mild and slightly right if more extreme - in any event if the pilot has sufficient cyclic authority it should be possible to maintain an attitude and effectively just experience a loss of TRThrust with vibration ....
If he does not maintain attitude and allows the disk to largely flap back (or blow-back) the increase in g will increase the AoAs and also extend the stalled region further around the disk and increase the rate
The natural response would be to back off the collective to accommodate slowing gently...
just a thought
the first point in the cycle where a stall might begin to occur is the place requiring the biggest AoA - ie the retreating side (9 o'clock if u like) once stalled: 1stly a wing will stay stalled until conditions are better than the conditions which started the stall AND 2ndly if a rotor stalls it will flap down which increases the AoA (this would normally compensate for the reduction in lift which caused the flapping rate) this will fail to improve the lift and 'deepen' the stall prolonging the stalled region of the cycle towards the aft position even though aerodynamic conditions are considerably improved...
.... increased vibration should be apparent to the pilot.
.... if the pilot responds to maintain attitude he will be increasing the pitch in the stalled region - which will worsten the stall and cause it to extend around the cycle (a little) ... if the stall extends beyond the aft (6 o'clock) position the new plane of rotation might alter slightly to the right ... but it can't be by much I would think ... probably chiefly a nose up pitching tendancy slightly left if mild and slightly right if more extreme - in any event if the pilot has sufficient cyclic authority it should be possible to maintain an attitude and effectively just experience a loss of TRThrust with vibration ....
If he does not maintain attitude and allows the disk to largely flap back (or blow-back) the increase in g will increase the AoAs and also extend the stalled region further around the disk and increase the rate
The natural response would be to back off the collective to accommodate slowing gently...
just a thought
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I would be very curious who at Bell said this. I have been told consistently, over the four years I have been flying a 407, and attending 407 recurrent at Bell, that the Vne in a 407 is a function not of retreating blade stall, but a durability/economy issue having to do with the useful life of components in the rotor system. An interesting feature of the 407GX is the red line moves with density altitude, and in bumpy air with airspeed fluctuations, you can get an overspeed warning like in a jet.
In the legacy 407, we used just under 140 knots TAS as a wag for Vne, but without the adjusting red line, I am sure folks momentarily exceed it in bumpy air.
It is not a coincidence that the Vne charts for the MD530F are mounted right in front of the pilot, and with any density altitude one might be surprised how slow Vne is. I learned this flying a 530F from the completion center to the factory, with a MD test pilot, when we were both surprised how "rough" the air was at 8,500 feet on a warm day -- until we finally looked at the Vne chart.
In the legacy 407, we used just under 140 knots TAS as a wag for Vne, but without the adjusting red line, I am sure folks momentarily exceed it in bumpy air.
It is not a coincidence that the Vne charts for the MD530F are mounted right in front of the pilot, and with any density altitude one might be surprised how slow Vne is. I learned this flying a 530F from the completion center to the factory, with a MD test pilot, when we were both surprised how "rough" the air was at 8,500 feet on a warm day -- until we finally looked at the Vne chart.
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George:
The Vne chart is supposed to be there in all helicopters! Your view of the 407 Vne is probably more correct than the previous poster. Forward cyclic might be an issue at some weight / cg combination, but isn't normally an issue.
But component life is!
I'll see if I can find out what the reason is from a better placed source.
The Vne chart is supposed to be there in all helicopters! Your view of the 407 Vne is probably more correct than the previous poster. Forward cyclic might be an issue at some weight / cg combination, but isn't normally an issue.
But component life is!
I'll see if I can find out what the reason is from a better placed source.
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The Vne chart is supposed to be there in all helicopters!
Last edited by EN48; 16th May 2012 at 13:51.
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Yes, the Vne chart sure is there on the legacy 407, and I would defy anyone with older than a teenager's eyes, or without an auto pilot, to be able to fly the 407 in anything but silky air and actually be able to read the chart placard on the ship. That is why I developed the wag on TAS, which kept me below Vne without constant reference to the chart.
I never figured out a wag for the 530F, but the Vne charts were quite readable.
I just exchanged email with my Bell contact and he again confirmed it was durability reacted to mast bending, but I will see what else I can turn up on this. EN48, my recollection is that the 2,000 fpm climb is related to an aerodynamic reason such as you were told holds for Vne?
I never figured out a wag for the 530F, but the Vne charts were quite readable.
I just exchanged email with my Bell contact and he again confirmed it was durability reacted to mast bending, but I will see what else I can turn up on this. EN48, my recollection is that the 2,000 fpm climb is related to an aerodynamic reason such as you were told holds for Vne?
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Just spoke to my friend at Bell, who, he said, learned this 16 years ago from the program test pilot, test engineer and supervisor for the 407 testing program. Vne on the 407 is a function of measured mast bending during the test program and how that relates to component durability. That is the reason gurney flaps were added to the horizontal during the development program, so the current Vne could be reached with less mast bending.
Retreating blade stall occurs at a higher speed.
The Vne also prevents some possible roughness that could occur in very cold temperatures at higher speeds.
The 2,000 fpm climb restriction relates to possible mild control reversal at slow airspeeds, and high climb rates, where the horizontal is less effective.
Retreating blade stall occurs at a higher speed.
The Vne also prevents some possible roughness that could occur in very cold temperatures at higher speeds.
The 2,000 fpm climb restriction relates to possible mild control reversal at slow airspeeds, and high climb rates, where the horizontal is less effective.
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my recollection is that the 2,000 fpm climb is related to an aerodynamic
reason such as you were told holds for Vne?
reason such as you were told holds for Vne?
Wondering if its possible that in both of these limitation cases the stated reasons are all partly correct?
I am definitely NOT trying to start an argument on these topics - just trying to learn something.
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.
In normal circumstances, the aircraft pitches up and rolls left but in Fidor's case his roll right might be explained by a reduction in aerodynamic damping at high DA allowing the blade to flap lower and take longer to recover to unstalled flight. It doesn't do this at 'normal' DAs.
Remember that from the 9 o'clock to the 6 o'clock the blade is seeing increasing TAS until at the 6 o'clock it has the same TAS as at the 12 o'clock so recovery (in normal DAs) is unlikely to be anywhere past the 6 o'clock.
As I said before, high DA affects the Lock number which affects the phase lag - I suspect Shawn or Nick can confirm this and whether or not it could be a causal factor in Fidor's observations of a right roll during RBS.
In normal circumstances, the aircraft pitches up and rolls left but in Fidor's case his roll right might be explained by a reduction in aerodynamic damping at high DA allowing the blade to flap lower and take longer to recover to unstalled flight. It doesn't do this at 'normal' DAs.
Remember that from the 9 o'clock to the 6 o'clock the blade is seeing increasing TAS until at the 6 o'clock it has the same TAS as at the 12 o'clock so recovery (in normal DAs) is unlikely to be anywhere past the 6 o'clock.
As I said before, high DA affects the Lock number which affects the phase lag - I suspect Shawn or Nick can confirm this and whether or not it could be a causal factor in Fidor's observations of a right roll during RBS.
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Crab:
... not really true tho' - that's not really how helicopters work ('pitch dis-symmetry' counters 'dis-symmetry of lift') - but that's another story... it doesn't matter where AoA starts increasing - what matters is where it is when it reaches the peek AoA - that will be where stall starts (unstall will be at a later phase )
"As previously posted":
the first point in the cycle where a stall might begin to occur is the place requiring the biggest AoA - ie the retreating side (9 o'clock if u like).
Once stalled:
1stly a wing will stay stalled until conditions are better than the conditions which started the stall
AND
2ndly if a rotor stalls it will flap down which increases the AoA (this would normally compensate for the reduction in lift which caused the flapping rate) this will fail to improve the lift and 'deepen' the stall prolonging the stalled region of the cycle towards the aft position even though aerodynamic conditions are considerably improved...
... you seem fairly 'attached' to your old CFS notes
... but I do agree with you that it is hard to imagine that stall will persit beyond 6 o'clock.
(anyway - mine is a fairly simplified version - and I don't think it really works like that either but it's slightly further along than your version - I beg for you to agree)
the blade starts to flap down as soon as it passes the 12 o'clock and sees a reduction in TAS
"As previously posted":
the first point in the cycle where a stall might begin to occur is the place requiring the biggest AoA - ie the retreating side (9 o'clock if u like).
Once stalled:
1stly a wing will stay stalled until conditions are better than the conditions which started the stall
AND
2ndly if a rotor stalls it will flap down which increases the AoA (this would normally compensate for the reduction in lift which caused the flapping rate) this will fail to improve the lift and 'deepen' the stall prolonging the stalled region of the cycle towards the aft position even though aerodynamic conditions are considerably improved...
... you seem fairly 'attached' to your old CFS notes
... but I do agree with you that it is hard to imagine that stall will persit beyond 6 o'clock.
(anyway - mine is a fairly simplified version - and I don't think it really works like that either but it's slightly further along than your version - I beg for you to agree)
AnFI you contradict yourself -
that is 'peak AoA' btw and not true
Then you say
so the peak AoA is after the blade stalls - do get your 'further along' theory right.
As for
do please explain what happens to a blade/wing at a certain AoA if the TAS reduces over that blade/wing - you believe it doesn't experience a loss of lift and then descend then
what matters is where it is when it reaches the peek AoA - that will be where stall starts
Then you say
2ndly if a rotor stalls it will flap down which increases the AoA
As for
not really true tho' - that's not really how helicopters work
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Thanks for the spelling lesson, here's one about helicopters:
I was pretty clear: stall starts at peak AoA ..... if that leads to stall then larger AoA's are achieved further around the disk
I thought that was fairly clear - hence no contradiction.
As for
- it's another story - but it's the reason helicopters used to have pilots, the feedback mechanism used to be; pilot looks out of the window and commands the attitude he desires - this results in "'pitch dis-symmetry' counters 'dis-symmetry of lift'" (using the cyclic pitch 'dis-symmetry' control)
I was pretty clear: stall starts at peak AoA ..... if that leads to stall then larger AoA's are achieved further around the disk
I thought that was fairly clear - hence no contradiction.
As for
('pitch dis-symmetry' counters 'dis-symmetry of lift') - but that's another story...
It seems pretty clear that if the blade stalls and then experiences greater AoA due to flapping that the peak AoA is after the stall and not at the point of stall or don't you understand your own musings.
The only time there is symmetry of pitch is in a free air hover in zero wind or at min pitch on the ground in nil wind.
Aerodynamic effects do not produce symmetry of pitch, only symmetry of lift.
The only time there is symmetry of pitch is in a free air hover in zero wind or at min pitch on the ground in nil wind.
Aerodynamic effects do not produce symmetry of pitch, only symmetry of lift.
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Hey Crab I guess I am not making myself clear, appologies:
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
not a contradiction i think you must agree (?) if you do agree you could possibly re-read the post to avoid repetition
- does that make it clear?
- that's true (ish) - but the cycle dis-symmetry is what I was talking about - that's why flapping to equality is not really the mechanism which counters dis-symmetry of lift (generally) - but that really is a different story ... perhaps for another thread?
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
not a contradiction i think you must agree (?) if you do agree you could possibly re-read the post to avoid repetition
- does that make it clear?
Aerodynamic effects do not produce symmetry of pitch, only symmetry of lift.
Last edited by AnFI; 18th May 2012 at 11:27. Reason: to add missing word "increased"
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The 2,000 fpm rate of climb limit is there because at light weight and maximum power, it was not possible to measure longitudinal static stability - not that there might not be some, but the rate of climb was so high that it was not possible to stabilize on the airspeeds long enough to get the necessary data. The limitation allowed that measurement to be made.
This would be one of the few times when I would say 'Be your own counsel' on a limitation.
This would be one of the few times when I would say 'Be your own counsel' on a limitation.
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More grist for the (gyro)mill
... I thought the B407 2000fpm climb limitation was imposed to ensure successful entry to autorotation in the event that the engine failed.
The theory/"received wisdom"/folklore being that the ship in 2000 fpm climb and AUW has sufficient momentum to continue on the upward trajectory after engine failure, and will allow unrecoverable RRPM decay even if lever is dropped immediately (due to inflows in the "wrong" direction).
Has not been an issue to date in the Schweizer .....
The theory/"received wisdom"/folklore being that the ship in 2000 fpm climb and AUW has sufficient momentum to continue on the upward trajectory after engine failure, and will allow unrecoverable RRPM decay even if lever is dropped immediately (due to inflows in the "wrong" direction).
Has not been an issue to date in the Schweizer .....