Sikorsky S-76: Ask Nick Lappos
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S76 take off Nr
Just a quick question,
The Sop where I fly is to set Nr at 104% for take off, and during flight for that matter. I have not seen this written in any publications, as far as I can you should use 100%.
What do other guys use? I know some companies use 102. and more to the point - why?
The Sop where I fly is to set Nr at 104% for take off, and during flight for that matter. I have not seen this written in any publications, as far as I can you should use 100%.
What do other guys use? I know some companies use 102. and more to the point - why?
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On the A+: Runway, 100% , offshore 107% presumably for a bit more inertia and T/R effectiveness in case of an engine failure. In cruise minimum vibration Nr, which can be anything...
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Some time ago I asked the same question of the forum, specifically regarding the A model in offshore operations, and received a very informative answer from Nick Lappos.
He told me that rotor rpm essentially makes no difference in takeoff performance as long as the engines aren't temperature limited. The engines do put more power into the rotor system at 107%, but the rotors lift more efficiently at 100%. The two effects are about equal.
If T5 is the limiting factor, then use 100% rotor rpm, which requires slightly less engine power. The difference in T5 at a hover (between 100% and 107%) can be as much as 25 degrees.
Nick didn't mention anything about coning and the resulting decrease in rotor disc area. If you're operating at heavy weights this may be a consideration.
Conversely, Nick told me that in cruise flight above 3000 feet, increasing rotor rpm to 107% reduces the size of the stalled region on the retreating side, decreasing drag. I've found that at 5000 feet the difference between 100% and 107% is worth 4-5 knots airspeed.
-Stan-
He told me that rotor rpm essentially makes no difference in takeoff performance as long as the engines aren't temperature limited. The engines do put more power into the rotor system at 107%, but the rotors lift more efficiently at 100%. The two effects are about equal.
If T5 is the limiting factor, then use 100% rotor rpm, which requires slightly less engine power. The difference in T5 at a hover (between 100% and 107%) can be as much as 25 degrees.
Nick didn't mention anything about coning and the resulting decrease in rotor disc area. If you're operating at heavy weights this may be a consideration.
Conversely, Nick told me that in cruise flight above 3000 feet, increasing rotor rpm to 107% reduces the size of the stalled region on the retreating side, decreasing drag. I've found that at 5000 feet the difference between 100% and 107% is worth 4-5 knots airspeed.
-Stan-
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Right on, Stan. The extra Nr in takeoff has an advantage if a landback is made, due to the extra rotor energy as S76heavy alludes (to be exact, the inertia is the same, but the stored energy is higher).
When trying to hover and engine temperature limited, it is better to be at lower Nr. If torque limited it is a wash, just as you say, Stan.
The later model 76's use the 107 Nr exclusively, mostly for high speed maneuver effectiveness, and also for tail rotor effectiveness at low speed.
When trying to hover and engine temperature limited, it is better to be at lower Nr. If torque limited it is a wash, just as you say, Stan.
The later model 76's use the 107 Nr exclusively, mostly for high speed maneuver effectiveness, and also for tail rotor effectiveness at low speed.
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I hope I'm not taking things too far off-course here, but I keep seeing "107%", "102%", "104%". How did this situation arise? Would it not make more sense to re-calibrate 107% to 100%? Were the engineers wrong to give the label "100%" to the actual power output or rpm that they did? And if so, is it that complicated a matter to do the recalibration?
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Hilico,
This is always an interesting point. The concept that a limit must be 100% is the issue, because you are thinking the gauge is showing percent of the maximum. The gauge is actually showing percent of an arbitrary number (what is 326 rpm, or 826 degrees, after all?) and you must keep the needle below another arbitrary number.
The rotor is allowed to go to 115% power off, and can slip to 125% before maintenance action is required. Where do we declare 100%? which one? Good question.
Redesign of all the gauges is expensive, and not particularly necessary, mostly.
This is always an interesting point. The concept that a limit must be 100% is the issue, because you are thinking the gauge is showing percent of the maximum. The gauge is actually showing percent of an arbitrary number (what is 326 rpm, or 826 degrees, after all?) and you must keep the needle below another arbitrary number.
The rotor is allowed to go to 115% power off, and can slip to 125% before maintenance action is required. Where do we declare 100%? which one? Good question.
Redesign of all the gauges is expensive, and not particularly necessary, mostly.
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Numbers is just numbers.
THe Polish W-3 'normal' rotor RPM is 85%, so unless it's a limit, it's not a problem.
But then again, why put a number there at all? Why not just marks on the gage???
Sorry, have to stop this line of thinking, must lie down and rest.
THe Polish W-3 'normal' rotor RPM is 85%, so unless it's a limit, it's not a problem.
But then again, why put a number there at all? Why not just marks on the gage???
Sorry, have to stop this line of thinking, must lie down and rest.
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Shawn: If there were just Red Lines, Yellow Lines and Green Lines Arcs. Chaos would rule.
Candidates preparing for check rides would not have to cram the night before a check ride in order to memmorize all the limits and therefore devote more time to understanding systems, airspace,
aerodynamics ect.
Transitions to other types would be less dificult with all the limits being standard.
For these reasons most Airworthiness Authorites would consider this completely unacceptable.
Candidates preparing for check rides would not have to cram the night before a check ride in order to memmorize all the limits and therefore devote more time to understanding systems, airspace,
aerodynamics ect.
Transitions to other types would be less dificult with all the limits being standard.
For these reasons most Airworthiness Authorites would consider this completely unacceptable.
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I may be off the mark (?) ....but I always thought the 100% value was related to the original design criterea limitation ...which then obviously changed with equipment/engineering upgrades but due to certification (STC) difficulties OR just being able relate the upgrade changes back to earlier model aircraft... this then allowed for the 102 or 107% type indications...... with the Bell 412EP for instance Cat A rig (if used) allows for 103% Nr/N2 for takeoff ... would this not also allow for Nr droop margin in an OEI situ on takeoff without having to 'Beep' the bejusus out of the sucker ??? ...dont know 'bout the S76. 
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spinningwings,
That's the whole point, 100% does not mean at 101% things go to worms, because the value used to ratio the 100% is an arbitrary number. the rotor on most Sikorsky helos has been taken to 125% in flight. Does that mean that we should reindex all gages so that we cruise at 80%, and max Nr is 100%? No it does not, because if we tell you to please not allow the oil pressure to get below 32 psi, you somehow learn to respect that odd number. Please treat the percent gauges as just markings of convenience, and don't allow previously held notions about 100% equaling some kind of limit to cloud your understanding. the red line is at a place, the needle is at another place. You are in charge of keeping them apart.
My daughters can easily spend 110% of their husband's salaries, so what's the problem?
That's the whole point, 100% does not mean at 101% things go to worms, because the value used to ratio the 100% is an arbitrary number. the rotor on most Sikorsky helos has been taken to 125% in flight. Does that mean that we should reindex all gages so that we cruise at 80%, and max Nr is 100%? No it does not, because if we tell you to please not allow the oil pressure to get below 32 psi, you somehow learn to respect that odd number. Please treat the percent gauges as just markings of convenience, and don't allow previously held notions about 100% equaling some kind of limit to cloud your understanding. the red line is at a place, the needle is at another place. You are in charge of keeping them apart.
My daughters can easily spend 110% of their husband's salaries, so what's the problem?
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Yikes...I guess my wife knows more about the S76 rotor system than I do (or your daughters) because she has flight tested my bank account to 125% on a regular basis...High side failure, High side failure, PAN PAN PAN...
LOL
LOL
Surely the reason for going to percentage scales in the first place is to simplify the numbers for key flight parameters. Do we agree that the specific values with units are only of interest to the engineers - what the pilot needs to know is: is it good/bad, go/nogo in as succinct a manner as possible.
The reason why some numerical information is required is for the case when only a partial range is shown on the gauge - i.e. 50-100%. If it were not necessary to do this then colours and marks would be better still.
CRAN
The reason why some numerical information is required is for the case when only a partial range is shown on the gauge - i.e. 50-100%. If it were not necessary to do this then colours and marks would be better still.
CRAN
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S76C drivers
I have never flown a 76C model but I have a question regarding the FMS #9. (I don't have the supplement myself)
Is the aircraft gross weight restricted and if it is by how much when performing a category "A" vertical landing?
During the landing profile they mention the "dot" positioned on the inside surface of the window and aligning it with the near edge of the helideck.
Is there something stuck to the windscreen that is used more as a target when performing this procedure?
Is the aircraft gross weight restricted and if it is by how much when performing a category "A" vertical landing?
During the landing profile they mention the "dot" positioned on the inside surface of the window and aligning it with the near edge of the helideck.
Is there something stuck to the windscreen that is used more as a target when performing this procedure?
Last edited by Xnr; 6th Apr 2003 at 13:07.
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I'll get back to you on the gross weight bit but yeah, there's a little dot that gets stuck to the inside of the windscreen and the idea is you come in straight and level at a constant airspeed and when that dot is seen to align with the near edge of the deck you're landing on, you start the approach down.
Our aircraft haven't had that dot for quite some time now, I guess they just fell off and no one ever bothered replacing them, which doesn't really bother me because I always thought that method would result in the approach being much too steep.
I prefer what Supp. 8 says to start the steep approach and that is using the pitot tube as a reference. If you have a chance, take a look at both Supp. 8 & 9 and compare them. Makes for some interesting reading in my opinion.
Our aircraft haven't had that dot for quite some time now, I guess they just fell off and no one ever bothered replacing them, which doesn't really bother me because I always thought that method would result in the approach being much too steep.
I prefer what Supp. 8 says to start the steep approach and that is using the pitot tube as a reference. If you have a chance, take a look at both Supp. 8 & 9 and compare them. Makes for some interesting reading in my opinion.
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Excuse me, non-pilot speaking here, but I have a question: wouldn't the effective position of that dot vary depending on how tall the pilot was and how far back or forward they were sitting? So for example wouldn't a short-a**e start the descent early, and Lanky start when the a/c was almost over the helideck?
With or without the dot....
Along the pitot to midway between the bottom of the window and the pitot. Good and steep with 500ft ROD, power in use, aim to crash onto the deck once beyond the decison point.
Hell of a lot better than shallow and into the crane at the bottom or worse still into the living quarters and a billion dollars of platform.
Along the pitot to midway between the bottom of the window and the pitot. Good and steep with 500ft ROD, power in use, aim to crash onto the deck once beyond the decison point.
Hell of a lot better than shallow and into the crane at the bottom or worse still into the living quarters and a billion dollars of platform.
Hey Nick!
This should be an interesting question for that erstwhile teetotaller.....
This should be an interesting question for that erstwhile teetotaller.....
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Hilico
Effective position of the dot should remain the same no matter the height of the pilot in the seat as I believe it's accepted the pilot will be sitting at what is called the Design Eye Reference Point.
This point requires the pilot to adjust the seat height so the eye is placed in approximately the same place and is explained in the Flight Manual.
When flown correctly, this particular approach profile is very comfortable and results in no real dramas at the bottom end should an engine fail after LDP.
Effective position of the dot should remain the same no matter the height of the pilot in the seat as I believe it's accepted the pilot will be sitting at what is called the Design Eye Reference Point.
This point requires the pilot to adjust the seat height so the eye is placed in approximately the same place and is explained in the Flight Manual.
When flown correctly, this particular approach profile is very comfortable and results in no real dramas at the bottom end should an engine fail after LDP.