Sikorsky S-76: Ask Nick Lappos
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Rotor RPM droop AND stability
To: Nick Lappos, Shawn Coyle, et'al
Although I guess I've fairly understood Static & Transient Droop in rotors (essentialy from John Fay's Book) but for
(i) Its effect on RPM stability with regard to various possible combinations of Static or Transient droop Being more vis-a-vis less.
(ii) specifically what does RPM stability mean in this case.?
(iii)any graphs depicting these two types of droop clarifying the variables along each of the co-ordinate (x- & y-axis).
please enlighten on subject.
Although I guess I've fairly understood Static & Transient Droop in rotors (essentialy from John Fay's Book) but for
(i) Its effect on RPM stability with regard to various possible combinations of Static or Transient droop Being more vis-a-vis less.
(ii) specifically what does RPM stability mean in this case.?
(iii)any graphs depicting these two types of droop clarifying the variables along each of the co-ordinate (x- & y-axis).
please enlighten on subject.
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pcpahari_IAF,
You mix the two effects a bit, they are quite different.
Transient droop of the engine/rotor is due to the lag of the engine in responding to collective pitch. It is measured by noting the minimum rotor speed as experienced when a power demand is made in a specific period of time. For example, a pull of collective from 0 torque to 95% torque is made in 2 seconds, and one sees a minimum rotor rpm of 5% below the normal (a pretty fast response). The collective pull is made by carefully trimming the collective pitch at 95% torque and noting its position, then lowering the lever to get to zero torque, then raising it smoothly and steadily to the previous position in the agreed upon time.
The main ingredients in transient torque response are the engine's maximum acceleration rate (usually described as the N1 rate, which should be perhaps 10% per second or more) and the rotor inertia (which resists the power demand and damps the minimum rotor rpm.) The engine is protected from stall by slowing its N1 acceleration, so stall protection balances against engine responsiveness.
Static Droop or Permanent Droop is the long term reduction in rotor rpm with increased power, usually about 1/2% per 10% power. This is put into the old-style governors as the best way to make the engine stable because it does not allow the engine to oscillate about the set rpm. That rpm "hunting" or oscillating about the set point is what we call rpm instability. Think of a thermostat in a cheap house, one that lets the house cool down to 18 degrees before it turns on the furnace, and then allows the house to go the 20 degrees before it shuts the furnace off. This house temperature constantly oscillates between the two temperatures because the thermostat is not stable. If one makes the thermostat shut off at a lower temperature on the heat cycle, the temperature stays closer to the set point, and is stable. In a similar manner, a permanent droop was necessary in old governors for stability. Usually, the collective bias would reset the selected rotor rpm upward with increasing collective pitch, which would reset the rpm upward to cancel the permanent droop - making it invisible to the pilot.
In the previous paragraph I say "old style" governors because modern digital governors have better control logic that generally removes the need for permanent droop - these modern governors are isochronous, constant speed.
Here is a fine discussion of droop from Woodward Controls that has plots of rpm vs power for transient and static droop (they make engine fuel controls). Note that it describes electric generator controls, so they use 60Hz as a speed reference. Same theory, of course:
http://www.canadiancontrols.com/documents/technical/Speed Droop and Power Generation.pdf
Here is a larger, more general discussion of how the fuel controls actually work, it is fantastic (although dated, as most FADECs use computers, not springs and things!):
http://www.woodward.com/pdf/ic/26260.pdf
You mix the two effects a bit, they are quite different.
Transient droop of the engine/rotor is due to the lag of the engine in responding to collective pitch. It is measured by noting the minimum rotor speed as experienced when a power demand is made in a specific period of time. For example, a pull of collective from 0 torque to 95% torque is made in 2 seconds, and one sees a minimum rotor rpm of 5% below the normal (a pretty fast response). The collective pull is made by carefully trimming the collective pitch at 95% torque and noting its position, then lowering the lever to get to zero torque, then raising it smoothly and steadily to the previous position in the agreed upon time.
The main ingredients in transient torque response are the engine's maximum acceleration rate (usually described as the N1 rate, which should be perhaps 10% per second or more) and the rotor inertia (which resists the power demand and damps the minimum rotor rpm.) The engine is protected from stall by slowing its N1 acceleration, so stall protection balances against engine responsiveness.
Static Droop or Permanent Droop is the long term reduction in rotor rpm with increased power, usually about 1/2% per 10% power. This is put into the old-style governors as the best way to make the engine stable because it does not allow the engine to oscillate about the set rpm. That rpm "hunting" or oscillating about the set point is what we call rpm instability. Think of a thermostat in a cheap house, one that lets the house cool down to 18 degrees before it turns on the furnace, and then allows the house to go the 20 degrees before it shuts the furnace off. This house temperature constantly oscillates between the two temperatures because the thermostat is not stable. If one makes the thermostat shut off at a lower temperature on the heat cycle, the temperature stays closer to the set point, and is stable. In a similar manner, a permanent droop was necessary in old governors for stability. Usually, the collective bias would reset the selected rotor rpm upward with increasing collective pitch, which would reset the rpm upward to cancel the permanent droop - making it invisible to the pilot.
In the previous paragraph I say "old style" governors because modern digital governors have better control logic that generally removes the need for permanent droop - these modern governors are isochronous, constant speed.
Here is a fine discussion of droop from Woodward Controls that has plots of rpm vs power for transient and static droop (they make engine fuel controls). Note that it describes electric generator controls, so they use 60Hz as a speed reference. Same theory, of course:
http://www.canadiancontrols.com/documents/technical/Speed Droop and Power Generation.pdf
Here is a larger, more general discussion of how the fuel controls actually work, it is fantastic (although dated, as most FADECs use computers, not springs and things!):
http://www.woodward.com/pdf/ic/26260.pdf
Last edited by NickLappos; 8th Jul 2009 at 01:58.
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Static and Transient Droop
TO: Nick
Thankyou Sir for the elaborate detail and referenced links. Amply clear and Informative as these are, I still have (even at the cost of repetition BUT in the interest of clearer/correct understanding) a peculiar query, (essentially in two parts) from one of my recent Grd Subjects tests:
1. RPM Stability: Normally Stability is the tendency to revert to original after being left independent from the point following a disturbance(it may therefore be Unstable, neutrally stable or Stable) Ofcourse, this Stability has Static and Dynamic aspects. Apparently our case of RPM stability is at cross-roads with this 'tendency' because here, 'Steady' RPM (even though closer to the disturbance figure) is stable operation as against oscillating RPM which is unstable. With this backdrop, particularly as regards my doubt w.r.t rpm stability, do clarify once again, do we imply stable RPM as that RPM tendency to settle at a lower figure as stable OR will it be the RPM returning to original figure, (though practically it ought to be RPM oscillating around the Original figure with Reducing Amplitude)...???
2. Fill in the Blanks:
"If the Static Droop is more Rotor RPM Stability will be ______. (More/Less).
Thanking you in anticipation.
Thankyou Sir for the elaborate detail and referenced links. Amply clear and Informative as these are, I still have (even at the cost of repetition BUT in the interest of clearer/correct understanding) a peculiar query, (essentially in two parts) from one of my recent Grd Subjects tests:
1. RPM Stability: Normally Stability is the tendency to revert to original after being left independent from the point following a disturbance(it may therefore be Unstable, neutrally stable or Stable) Ofcourse, this Stability has Static and Dynamic aspects. Apparently our case of RPM stability is at cross-roads with this 'tendency' because here, 'Steady' RPM (even though closer to the disturbance figure) is stable operation as against oscillating RPM which is unstable. With this backdrop, particularly as regards my doubt w.r.t rpm stability, do clarify once again, do we imply stable RPM as that RPM tendency to settle at a lower figure as stable OR will it be the RPM returning to original figure, (though practically it ought to be RPM oscillating around the Original figure with Reducing Amplitude)...???
2. Fill in the Blanks:
"If the Static Droop is more Rotor RPM Stability will be ______. (More/Less).
Thanking you in anticipation.
Last edited by pcpahari_IAF; 9th Jul 2009 at 11:59. Reason: clarify
Complex? Where...................
Last edited by Senior Pilot; 14th Jul 2009 at 09:19. Reason: Embed YouTube link
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to pcpahari_IAF,
No problem, this is complex stuff, keep plugging away!
Basically, it is confusing to use the term "stability" to describe all three properties under discussion: 1)the transient droop and 2) the permanent droop and 3) the oscillation. I would suggest that for clarity in differentiating between the three properties we are discussing, let's use 'stability" in the classic sense, as only 3) the freedom from oscillation.
your question: do we imply stable RPM as that RPM tendency to settle at a lower figure as stable OR will it be the RPM returning to original figure. IN answer, I would say neither - as I would like to clarify, stability is the measure of making the engine/rotor response free of oscillations. In fact, permanent droop (where the rpm is allowed to reduce slightly) is used to make the response more stable (more free of oscillations).
"If the Static Droop is more, Rotor RPM Stability will be ______. (More/Less) more stable
If I could use an analogy for permanent droop, imagine a steel table upon which we drop a solid steel ball. The ball would bounce for a long time against the table, because the rigidity of the table returns to the ball all the energy that the ball gives to the table. The ball is almost unstable, and is quite a problem as it oscillates. Now cover the steel table with 1 cm of rubber. When dropped, the ball would dent the rubber, and quickly come to rest. At rest, the ball would be slightly lower than the surface of the rubber, due to the permanent droop of the weight of the ball. In effect, the surface "gives" a little bit, and in doing so, helps the ball find true dynamic stability.
No problem, this is complex stuff, keep plugging away!
Basically, it is confusing to use the term "stability" to describe all three properties under discussion: 1)the transient droop and 2) the permanent droop and 3) the oscillation. I would suggest that for clarity in differentiating between the three properties we are discussing, let's use 'stability" in the classic sense, as only 3) the freedom from oscillation.
your question: do we imply stable RPM as that RPM tendency to settle at a lower figure as stable OR will it be the RPM returning to original figure. IN answer, I would say neither - as I would like to clarify, stability is the measure of making the engine/rotor response free of oscillations. In fact, permanent droop (where the rpm is allowed to reduce slightly) is used to make the response more stable (more free of oscillations).
"If the Static Droop is more, Rotor RPM Stability will be ______. (More/Less) more stable
If I could use an analogy for permanent droop, imagine a steel table upon which we drop a solid steel ball. The ball would bounce for a long time against the table, because the rigidity of the table returns to the ball all the energy that the ball gives to the table. The ball is almost unstable, and is quite a problem as it oscillates. Now cover the steel table with 1 cm of rubber. When dropped, the ball would dent the rubber, and quickly come to rest. At rest, the ball would be slightly lower than the surface of the rubber, due to the permanent droop of the weight of the ball. In effect, the surface "gives" a little bit, and in doing so, helps the ball find true dynamic stability.
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Thankyou Nick
Sorry for the inordinate delay... was busy with my exams....I had seen your replies though.... Thankyou... guess i got the fundamentals of the issue pretty clear in my head... enough for a pilot I guess...
Will keep querrying in whenever in doubt
... Its an excellent place n opportunity to learn while interacting with those amongst the best in the field of rotorcraft..
pcpahari
I think you might have drifted away from the topic - "Sikorsky S-76: Ask Nick Lappos.
Look for the new thread button.
Look for the new thread button.
Chief Bottle Washer
RVDT,
Thanks for the reminder: all the posts on VRS have been moved to Vortex Ring/Settling with power thread
Thanks for the reminder: all the posts on VRS have been moved to Vortex Ring/Settling with power thread
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Question for Nick....
Maybe you can answer this question for me:
Why did Sikorsky choose an "improved planetary" MRGB configuration over the theoretically superior "torque split" design (that they did extensive trade studies on) for the 53K MRGB?
Was it a customer decision? Or did Sikorsky's management lose their backbone?
TIA for your response.
Why did Sikorsky choose an "improved planetary" MRGB configuration over the theoretically superior "torque split" design (that they did extensive trade studies on) for the 53K MRGB?
Was it a customer decision? Or did Sikorsky's management lose their backbone?
TIA for your response.
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S76 A++ Question
Hi Nick,
Was sitting around with C & T and Flight Safety and a couple of other captains last night. We begin discussing why our 2 A++'s regularly, once one of the engines has been started and is in the Flight Gate, and you go to start the 2nd engine and it won't light off after approximaly 6 seconds. You then pull the lever back to cut off, wait the 30 secs and attempt restart and it then fires up just like the first. We have 1s1 engines fitted.
Our engineers have apparently spoken to Arriel in France and they claim its just us who has this problem. They then have spoken to engineers at ESSO in Longford, Vic who operate C+ machines with same engine and have the same problem occur.
We have been informed that what happens is as the 2nd engine is spinning faster on the initial start up than it would if it was the 1st engine started and he FCU is introducing too much fuel into the engine. Have you heard of this before and if so is there a fix.
Cheers & Beers
LT
Was sitting around with C & T and Flight Safety and a couple of other captains last night. We begin discussing why our 2 A++'s regularly, once one of the engines has been started and is in the Flight Gate, and you go to start the 2nd engine and it won't light off after approximaly 6 seconds. You then pull the lever back to cut off, wait the 30 secs and attempt restart and it then fires up just like the first. We have 1s1 engines fitted.
Our engineers have apparently spoken to Arriel in France and they claim its just us who has this problem. They then have spoken to engineers at ESSO in Longford, Vic who operate C+ machines with same engine and have the same problem occur.
We have been informed that what happens is as the 2nd engine is spinning faster on the initial start up than it would if it was the 1st engine started and he FCU is introducing too much fuel into the engine. Have you heard of this before and if so is there a fix.
Cheers & Beers
LT
LowTime
"Our engineers have apparently spoken to Arriel in France and they claim its just us who has this problem"
Wish I had £ for every time a service tech\ manufacturer has told me that, another one is" it only happens in the UK" (substitute the country of origin of complaint)
They really think users live in a bubble & don't have contact, complainlouder,
"Our engineers have apparently spoken to Arriel in France and they claim its just us who has this problem"
Wish I had £ for every time a service tech\ manufacturer has told me that, another one is" it only happens in the UK" (substitute the country of origin of complaint)
They really think users live in a bubble & don't have contact, complainlouder,
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Spot on there 500E, I wonder if we should say it hapens because we are 'Downunder' so to speak. 
Being that I am only new to the offshore flying, I am always asking questions about this and that. I'm sure our chief engineer will eventually get to the bottom of this strange and totally unique Aussie problem. 
But anyone will a another theory or fix about this please feel free to dip your oar in the water.
Cheers & Beers
LT
Being that I am only new to the offshore flying, I am always asking questions about this and that. I'm sure our chief engineer will eventually get to the bottom of this strange and totally unique Aussie problem. But anyone will a another theory or fix about this please feel free to dip your oar in the water.
Cheers & Beers
LT
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I don't recall any problems starting the second engine on an A++, and I've started a lot of them, often on post-maintenance runups. The idle gate is an imaginary location, and can be set anywhere you like. I like it set on the low side of the specs, and I don't use it for the start. The flight manual calls for pressing the trigger before moving the throttle, and that's the way I do it. I go to the idle gate, more or less, by feel, and may sometimes move the throttle just a little one way or the other to get the desired results. It takes a few starts to get a feel for how much to move the throttle, but it's not really that difficult. Putting the throttle past the gate before pulling the trigger seems to me to be asking for an overtemp.
It may be that the problem is caused by the aircraft being upside down, along with everything else down there.
It may be that the problem is caused by the aircraft being upside down, along with everything else down there.
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Thanks Gome Pylot, yes you are correct in your description in starting. Button In and then move the lever to Idle gate and if need be modulate slightly from there to keep T5 up or down.
We / I have found that after you start 1 engine and you have moved the lever into the Flight Gate and then you go to start the 2nd engine exactly the way you started the 1st, it won't light off. I have had the engine start to light off as I have been pulling the lever back, but because i have had the starter whining away for at least 6 secs and nothing happen i have discontinuted the start. No T5 indication of light off at all.
Wait your ususal 30 secs before attemting a restart and 'Bingo' that engine will fire up quite normally. Our A++ have 2 small 'Gates' and a trigger set up on the lever. There is a Gate for Idle and a Gate for Flight. Pulling the trigger in allows you to move the lever backwards until hitting the gate. (Bloody hard to describe the overhead Engine & Fuel Levers in words)
You may think that we are upside down, we prefer to think of the rest of ya are at the bottom the wrong way up.
Cheers & Beers
LT
We / I have found that after you start 1 engine and you have moved the lever into the Flight Gate and then you go to start the 2nd engine exactly the way you started the 1st, it won't light off. I have had the engine start to light off as I have been pulling the lever back, but because i have had the starter whining away for at least 6 secs and nothing happen i have discontinuted the start. No T5 indication of light off at all.
Wait your ususal 30 secs before attemting a restart and 'Bingo' that engine will fire up quite normally. Our A++ have 2 small 'Gates' and a trigger set up on the lever. There is a Gate for Idle and a Gate for Flight. Pulling the trigger in allows you to move the lever backwards until hitting the gate. (Bloody hard to describe the overhead Engine & Fuel Levers in words
)You may think that we are upside down, we prefer to think of the rest of ya are at the bottom the wrong way up.
Cheers & Beers
LT
but because i have had the starter whining away for at least 6 secs and nothing happen i have discontinuted the start
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LowTime, the fCU does not introduce the fuel on initial start but supplies the start electro valve on top of the engine which supplies the fuel injectors. If you start your arriel and look carefully you should see 2 phases to the start. The first is a t4 rise to about 300-350 degrees where she stops for a split second-this is the igniters and fuel injectors working like a lighter in getting heat and some flame going for the next stage. She will then climb for the second phase which is when the FCU supplies the injection wheel and the self sustainable flame is established and the acceleration on the 1S1 modulated if required and/or on a very good FCU controlled by the acceleration control unit and overspeed and drain valve within the FCU utilising P2 bellows and fuel differential pressures which will be related to N1 due to the fuel pressure variances in the pump speed. That is my best attempt to a very short explanation of the system so please maybe ask your engineer to show you his training manual with the fancy drawings and you will understand easily.
Are you sure it is too much fuel and maybe not to little? The 76 does not have airframe booster pumps like most other models(used to prime the engine before start for the Arriel 2 and Arriel 1 for priming and assiting the single engine driven pump) using the Arriel. They have the venturi on the supply line just below the engine above the deck assisting the engine to suck the fuel but to be effective it is critical that the check valves mentioned works as it needs fuel in the line to be effective and then obviously N1 so the pump can initiate the venturi effect. This is why you have the function of priming 1 engine that would not start if the other is online by using the online engines fuel p to prime the problem engine. Ask your engineer as he can tell you that if you disturbed a engine supply fuel line you have a manual bleeding process before the next start to ensure the lines are primed.
The C+ uses the 2S1 engine. The big difference is FCU hydromechanical vs HMU DECU controlled. However, the 2S1 will have the same symptoms if the airframe fuel system supplying it is not primed with fuel for whatever reason. If both engines does it regardless of which one you start first, maybe have a look at the priming line between the 2 engines as it might be allowing air into the system when you start the first engine.
Hope this helps but I am sure Nick can give a much better explanation and look forward to the solution to your problem-so we all shall learn.
Are you sure it is too much fuel and maybe not to little? The 76 does not have airframe booster pumps like most other models(used to prime the engine before start for the Arriel 2 and Arriel 1 for priming and assiting the single engine driven pump) using the Arriel. They have the venturi on the supply line just below the engine above the deck assisting the engine to suck the fuel but to be effective it is critical that the check valves mentioned works as it needs fuel in the line to be effective and then obviously N1 so the pump can initiate the venturi effect. This is why you have the function of priming 1 engine that would not start if the other is online by using the online engines fuel p to prime the problem engine. Ask your engineer as he can tell you that if you disturbed a engine supply fuel line you have a manual bleeding process before the next start to ensure the lines are primed.
The C+ uses the 2S1 engine. The big difference is FCU hydromechanical vs HMU DECU controlled. However, the 2S1 will have the same symptoms if the airframe fuel system supplying it is not primed with fuel for whatever reason. If both engines does it regardless of which one you start first, maybe have a look at the priming line between the 2 engines as it might be allowing air into the system when you start the first engine.
Hope this helps but I am sure Nick can give a much better explanation and look forward to the solution to your problem-so we all shall learn.
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Thanks for the replies guys.
BB In CA - I discontinue the start after 6 secs approximately as the engine does not give any indication that it is going to light off. The 6 secs I talk about is after the lever put forward to the Idle gate and you wait and watch T5 and the other gauges for the light off. I suppose I'm still a bit cautious when it comes to hung starts.
Industry Insider - Sir I stand corrected.
Victor Papa - Thanks for an excellent description of the system and how it works. I have re -read my original post and I think I need to clarify it doesn't matter which engine you start first either Number 1 or 2, it's just which ever one is up and running, we get the problem on the other engine. Got me a bit perplexed and I'm sure some my Captains are quietly watching the discussion here lol.
Cheers & Beers
LT
BB In CA - I discontinue the start after 6 secs approximately as the engine does not give any indication that it is going to light off. The 6 secs I talk about is after the lever put forward to the Idle gate and you wait and watch T5 and the other gauges for the light off. I suppose I'm still a bit cautious when it comes to hung starts.
Industry Insider - Sir I stand corrected.
Victor Papa - Thanks for an excellent description of the system and how it works. I have re -read my original post and I think I need to clarify it doesn't matter which engine you start first either Number 1 or 2, it's just which ever one is up and running, we get the problem on the other engine. Got me a bit perplexed and I'm sure some my Captains are quietly watching the discussion here lol.
Cheers & Beers
LT
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LowTime, did some 6,000 hours on those very aircraft and from the time they were new this problem would arise from time to time. Assumed by most to be a loss of prime and never a problem on the second start attempt by getting the other engine running (if not already) and putting fuel lever to "prime" to get the recalcitrant one going.
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Hi Brian and thank you for your info.
For others following this, i have just spoken to a mate of mine who is a coey on an A++ for CHC and they are having exactly the same probem.
General consensus is that it sems to happen more on the Number 2 engine than the Number 1.
Looking forward to reading what Nick has to say, but am enjoying the discussion.
Cheers & Beers
LT
For others following this, i have just spoken to a mate of mine who is a coey on an A++ for CHC and they are having exactly the same probem.
General consensus is that it sems to happen more on the Number 2 engine than the Number 1.
Looking forward to reading what Nick has to say, but am enjoying the discussion.
Cheers & Beers
LT