Sikorsky S-76: Ask Nick Lappos
Join Date: Apr 2003
Location: 18 Degrees North
Posts: 699
Likes: 0
Received 0 Likes
on
0 Posts
we fly anywhere from 100-107 Nr, in practice nearer 107 than 100. and in the case of single engine failure shortly after TDP raise the lever to droop the NR TO 96% as part of the profile.
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Xnr,
You have to watch those FSI instructors, they tend to color the stories with semi-half baked warstories that have only a glimmer of truth.
The S-76A uses 96% Nr because it allows a better climb rate at fixed horsepower (that is when the engine is temperature limited and you are not on the torque limits of the transmission). The rotor is more power efficient at low speeds when it is at low Nr.
Almost every helicopter is that way, it has to do with the fact that the rotor is designed to allow high speed flight without stalling. The blades are wider that needed to hover, so they need somewhat more power than if they have less chord. One way to make the rotor behave like it has less chord is to slow the rotor down. This drives the collective to a higher angle (for the same lift) and that gets you optimal lift to drag. Thus the power is lower.
We used the rotor rpm at 96% to gain performance (we were not in trouble with too little performance, we uised it to get more than we had planned.) To qualify the rotor at the lower rpm, we tested all the way to Vne that way, at 96%.
One reason why we could use Nr that way was that the vibration absorbers were effective axcross a wide rpm range (the VTA and Bifilar are great that way).
You have to watch those FSI instructors, they tend to color the stories with semi-half baked warstories that have only a glimmer of truth.
The S-76A uses 96% Nr because it allows a better climb rate at fixed horsepower (that is when the engine is temperature limited and you are not on the torque limits of the transmission). The rotor is more power efficient at low speeds when it is at low Nr.
Almost every helicopter is that way, it has to do with the fact that the rotor is designed to allow high speed flight without stalling. The blades are wider that needed to hover, so they need somewhat more power than if they have less chord. One way to make the rotor behave like it has less chord is to slow the rotor down. This drives the collective to a higher angle (for the same lift) and that gets you optimal lift to drag. Thus the power is lower.
We used the rotor rpm at 96% to gain performance (we were not in trouble with too little performance, we uised it to get more than we had planned.) To qualify the rotor at the lower rpm, we tested all the way to Vne that way, at 96%.
One reason why we could use Nr that way was that the vibration absorbers were effective axcross a wide rpm range (the VTA and Bifilar are great that way).
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
X,
Not fully. We did a sweep of low speed climb at 94% and up, as I recall. In the Cat B landings, and in the auto landings, we got as low as about 80 or 82% as I recall.
Not fully. We did a sweep of low speed climb at 94% and up, as I recall. In the Cat B landings, and in the auto landings, we got as low as about 80 or 82% as I recall.
Hi Nick, Interesting that you mention the VTA and bifilar in the equasion as my company is in the throws of removing the VTAs from all its 76a+, and as you know they have already one less bifilar than an A. What effect would this have?
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Pitchlink,
I have been away from the 76 for a while now, and don't know the latest info. As long as its factory samctioned, you can bet it is safe to do. Sometimes the vibration absorbers affect the vibration at critical pieces of gear, such as engines, so the effects of removing or changing them must be measured by engineers to assess the wisdom.
I have been away from the 76 for a while now, and don't know the latest info. As long as its factory samctioned, you can bet it is safe to do. Sometimes the vibration absorbers affect the vibration at critical pieces of gear, such as engines, so the effects of removing or changing them must be measured by engineers to assess the wisdom.
Join Date: Dec 2001
Location: Gold Coast, Queensland
Posts: 943
Likes: 0
Received 0 Likes
on
0 Posts
S76 Hydraulics/Nick?
Hi Nick
Interesting to read your comments about hardovers.
About 25 years ago, when I was a young pilot, two of us were flying a Mk 1 S76, I think it was serial number 3, with barely 30 hours on type between us.
At 1000 ft downwind our Chief Pilot asked us to switch off the hydraulics in turn as a visiting Esso manager said he had felt a restriction in the controls. So we switched hyd 1 off, wriggled all the controls, felt normal, switched back on. The hyd 1 light went out. This got us thinking as it should have remained on but with 3 weeks on type we thought the a/c knew better.
By now on long finals at 500 feet, switched hyd 2 off. The right pedal shot forward, the a/c yawed 30-40 degrees to the right, the collective dropped, the cyclic went to the right slightly and we were heading for an early landing. Between us we could not move any control and decided to switched hyd 2 back on somewhat speedily. At 200 feet all was normal and we carried out a gentle running landing and the hyd light went off in the correct manner.
On back tracking down the runway, we reported this and were told to do it again. So we did. Nothing unusual happened. Put in some right pedal to turn onto the taxiway and the a/c jumped about 3 feet into the air, turned 40 degrees to the right and headed for the grass.
Reached the hangar and closed down. All the companies brains trust and the Sikorsky engineering rep, a terrific bloke, got their heads into the manuals and decided to connect a hyd rig. Nothing happened. Started up and switched the hyd 2 off at idle, nothing happened. Informed by all that I had a vivid imagination, true but not in this case! Then they asked us two to try again.
I suggested we move away from the hangar doors, just in case. Told not to be stupid. Okay says I, here we go. They all climbed in the back, I started up, 100% rrpm, switched hyd 2 off. Nothing happened, so I said watch this and put in some right pedal. The a/c jumped out of the chocks, turned 40 degrees to the right and headed for the hangar. After listening to the screams in the back, we switched hyd 2 back on. All normal again.
To make a long story longer, the machine was grounded and the Sikorsky rep suggested the tail rotor hyd should be removed for inspection at the factory.
I never got the full follow up but I believe that someone had left out some valves inside the hyd jack which allowed me to turn off the good one even though the other one did not work, thus basically giving us a full hyd failure which makes a 76 pretty difficult to fly. After this the 2 of us always did a tail rotor pedal check at idle in case another 76 had the same problem. Interestly Sikorsky also came up with the requirements for pedal checks for the next few years.
So I guess the moral is that no matter how well you know your a/c and RFM, an unusual problem can still occur that the makers have not forecast. Unfortunately I have been on the receiving end of several. One of the joys of flying a new type of helicopter!
Nick,
Do you remember this incident?
Interesting to read your comments about hardovers.
About 25 years ago, when I was a young pilot, two of us were flying a Mk 1 S76, I think it was serial number 3, with barely 30 hours on type between us.
At 1000 ft downwind our Chief Pilot asked us to switch off the hydraulics in turn as a visiting Esso manager said he had felt a restriction in the controls. So we switched hyd 1 off, wriggled all the controls, felt normal, switched back on. The hyd 1 light went out. This got us thinking as it should have remained on but with 3 weeks on type we thought the a/c knew better.
By now on long finals at 500 feet, switched hyd 2 off. The right pedal shot forward, the a/c yawed 30-40 degrees to the right, the collective dropped, the cyclic went to the right slightly and we were heading for an early landing. Between us we could not move any control and decided to switched hyd 2 back on somewhat speedily. At 200 feet all was normal and we carried out a gentle running landing and the hyd light went off in the correct manner.
On back tracking down the runway, we reported this and were told to do it again. So we did. Nothing unusual happened. Put in some right pedal to turn onto the taxiway and the a/c jumped about 3 feet into the air, turned 40 degrees to the right and headed for the grass.
Reached the hangar and closed down. All the companies brains trust and the Sikorsky engineering rep, a terrific bloke, got their heads into the manuals and decided to connect a hyd rig. Nothing happened. Started up and switched the hyd 2 off at idle, nothing happened. Informed by all that I had a vivid imagination, true but not in this case! Then they asked us two to try again.
I suggested we move away from the hangar doors, just in case. Told not to be stupid. Okay says I, here we go. They all climbed in the back, I started up, 100% rrpm, switched hyd 2 off. Nothing happened, so I said watch this and put in some right pedal. The a/c jumped out of the chocks, turned 40 degrees to the right and headed for the hangar. After listening to the screams in the back, we switched hyd 2 back on. All normal again.
To make a long story longer, the machine was grounded and the Sikorsky rep suggested the tail rotor hyd should be removed for inspection at the factory.
I never got the full follow up but I believe that someone had left out some valves inside the hyd jack which allowed me to turn off the good one even though the other one did not work, thus basically giving us a full hyd failure which makes a 76 pretty difficult to fly. After this the 2 of us always did a tail rotor pedal check at idle in case another 76 had the same problem. Interestly Sikorsky also came up with the requirements for pedal checks for the next few years.
So I guess the moral is that no matter how well you know your a/c and RFM, an unusual problem can still occur that the makers have not forecast. Unfortunately I have been on the receiving end of several. One of the joys of flying a new type of helicopter!
Nick,
Do you remember this incident?
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
Deja Vu all over again
To: Nigel Osborn
Nigel let me make a slight change to your statement above.
So I guess the moral is that no matter how well you DESIGN your a/c and RFM, an unusual problem can still occur that the makers have not forecast. Unfortunately I have been on the receiving end of several. One of the joys of flying a new type of helicopter!
As I had stated to Nick in my post above no matter how much care is taken in the design of an aircraft and no matter the level of quality control things will go wrong.
What strikes me is that problem had not been detected earlier. It should have been detected during the functional test of the servo unit at the manufacturer. It should have been detected at Sikorsky during receiving inspection. It should have been detected when the helicopter underwent flight-testing. You were able to repeat the problem three times yet it was never detected by anyone previous to the time it happened to you. If parts were left out of the servo the problem should have manifested itself at the factory and the servo should have been rejected and sent in for remanufacture.
I'll bet that the servo manufacturer had a bunch of flags outside the entrance of his plant attesting to the fact that they were ISO approved
So I guess the moral is that no matter how well you know your a/c and RFM, an unusual problem can still occur that the makers have not forecast. Unfortunately I have been on the receiving end of several. One of the joys of flying a new type of helicopter!
So I guess the moral is that no matter how well you DESIGN your a/c and RFM, an unusual problem can still occur that the makers have not forecast. Unfortunately I have been on the receiving end of several. One of the joys of flying a new type of helicopter!
As I had stated to Nick in my post above no matter how much care is taken in the design of an aircraft and no matter the level of quality control things will go wrong.
What strikes me is that problem had not been detected earlier. It should have been detected during the functional test of the servo unit at the manufacturer. It should have been detected at Sikorsky during receiving inspection. It should have been detected when the helicopter underwent flight-testing. You were able to repeat the problem three times yet it was never detected by anyone previous to the time it happened to you. If parts were left out of the servo the problem should have manifested itself at the factory and the servo should have been rejected and sent in for remanufacture.
I'll bet that the servo manufacturer had a bunch of flags outside the entrance of his plant attesting to the fact that they were ISO approved
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Nigel,
I do recall that issue we faced. It was the reason why we used a pedal check during run-up to look for the problem until we could get all the bad servos out of the field. It was not caused by turning off the hydraulics, it was uncovered that way.
The problem was that one of the tail servo sections had seals that wore very quickly (I can't recall if this was a material or an assembly problem) and allowed too much blow-by, a condition like badly worn piston rings. In this condition, a single worn servo would not develop enough force by itself to control the tail rotor. Since we have a dual tandem servo, and since either piston can fully control the tail rotor, the condition was completely masked in normal dual operations.
That problem was found and fixed a long time ago, but I was on the other end of the S-76A system, working to help identify it, find a way to locate the problem, and of course, fix it. Small world, Nigel! It was all fixed with a change to the tail servo seals, and has not been an issue since.
When you turn off one servo, you get the other as the sole force provider. If that servo was worn, you can get the pedals to drive because the tail rotor is going to a pitch setting it likes, and the only thing stopping it (the good half of the tail servo) was shut off.
It actually was not a servo hardover, it was a lazy servo, but that let the tail rotor walk back to the neutral position (sort of the zero thrust position).
Why does the tail rotor need a servo? Because the blades are all producing a pitching moment as they are moved to new collective pitch angles. At high thrust, the pitching moment is quite appreciable, and the servo is your way to muscle the blades to the position you want.
Why did the collective drive? Recall that the collective and yaw are interconnected, and if the force is very high on the pedals, and you push on them with a bunch of leg force, that push will be reflected off the mixer and come out on the collective. Yes, it was you who helped drive the collective pitch, because the yaw was frozen, and your massive leg force had nowhere else to go. There are corners where the collective will drive by itself, but the collective will have lots of range in the middle flight position.
Did the cyclic freeze? No, but when the collective drove and the pedals leaped, who can blame a pilot for not doing a staff study while he leaps to move the switch back?
Why was it not seen when the rotor is not turning? because it is not a hardover, jus a lazy servo, so it needed some appreciable tail rotor force to show itself.
I do recall that issue we faced. It was the reason why we used a pedal check during run-up to look for the problem until we could get all the bad servos out of the field. It was not caused by turning off the hydraulics, it was uncovered that way.
The problem was that one of the tail servo sections had seals that wore very quickly (I can't recall if this was a material or an assembly problem) and allowed too much blow-by, a condition like badly worn piston rings. In this condition, a single worn servo would not develop enough force by itself to control the tail rotor. Since we have a dual tandem servo, and since either piston can fully control the tail rotor, the condition was completely masked in normal dual operations.
That problem was found and fixed a long time ago, but I was on the other end of the S-76A system, working to help identify it, find a way to locate the problem, and of course, fix it. Small world, Nigel! It was all fixed with a change to the tail servo seals, and has not been an issue since.
When you turn off one servo, you get the other as the sole force provider. If that servo was worn, you can get the pedals to drive because the tail rotor is going to a pitch setting it likes, and the only thing stopping it (the good half of the tail servo) was shut off.
It actually was not a servo hardover, it was a lazy servo, but that let the tail rotor walk back to the neutral position (sort of the zero thrust position).
Why does the tail rotor need a servo? Because the blades are all producing a pitching moment as they are moved to new collective pitch angles. At high thrust, the pitching moment is quite appreciable, and the servo is your way to muscle the blades to the position you want.
Why did the collective drive? Recall that the collective and yaw are interconnected, and if the force is very high on the pedals, and you push on them with a bunch of leg force, that push will be reflected off the mixer and come out on the collective. Yes, it was you who helped drive the collective pitch, because the yaw was frozen, and your massive leg force had nowhere else to go. There are corners where the collective will drive by itself, but the collective will have lots of range in the middle flight position.
Did the cyclic freeze? No, but when the collective drove and the pedals leaped, who can blame a pilot for not doing a staff study while he leaps to move the switch back?
Why was it not seen when the rotor is not turning? because it is not a hardover, jus a lazy servo, so it needed some appreciable tail rotor force to show itself.
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 418 Likes
on
221 Posts
Hence the saying:
"Never fly the 'A' model of ANYTHING!"
"Never fly the 'A' model of ANYTHING!"
Join Date: Sep 2001
Location: In the Haven of Peace
Age: 79
Posts: 600
Received 0 Likes
on
0 Posts
What criteria exactly are you looking for? If you're just looking for the differences between Cat A and Cat B, they're the same for every helicopter. If you want specific figures regarding take off weights etc., to which model of the S76 do you refer? Are you wanting to know ICAO definitions or national definitions? You'll have to be more precise to get any meaningful answer.
Join Date: Nov 2003
Location: Canada
Posts: 68
Likes: 0
Received 0 Likes
on
0 Posts
Sorry about that, I'm looking what are the differences, and what it takes to be part of one or the other categories. The model and the figures doesn't really matter it is more to a point of ICAO or FAA, on what they base their criteria ( rate of climb, weight, OEI performance). Some country use Cat 1 or cat 2 or cat 3, those ones for what I know are base on performance when you loose an engine, depending on the phase of the flight(CDP). I' m just looking if they have difference between those has well.
Cheers
FH
Cheers
FH
Join Date: Dec 2001
Location: Philadelphia PA
Age: 73
Posts: 1,835
Likes: 0
Received 2 Likes
on
2 Posts
ICAO does not, as far as I know, specify Category A peformance to the detail that the FAA or CAA or JAA would do it. ICAO says you have to set out criteria, and the FAA/CAA/JAA then sets out their own criteria.
If you want to know the criteria for the FAA, then look in Part 29 of 14CFR (the politically correct term for what we all in aviation know as the FARs). to get an idea of what paragraphs apply, look at Appendix D of Part 27, which lists all the appropriate paragraphs of Part 29 for Category A. There are quite a few, and most are based on performance following an engine failure. There are a few that deal with other redundancy issues.
There shouldn't be much difference between the FARs and JARs when it comes to Category A performance, but I haven't done a side-by-side comparison.
If you want to know the criteria for the FAA, then look in Part 29 of 14CFR (the politically correct term for what we all in aviation know as the FARs). to get an idea of what paragraphs apply, look at Appendix D of Part 27, which lists all the appropriate paragraphs of Part 29 for Category A. There are quite a few, and most are based on performance following an engine failure. There are a few that deal with other redundancy issues.
There shouldn't be much difference between the FARs and JARs when it comes to Category A performance, but I haven't done a side-by-side comparison.
Join Date: Sep 2001
Location: In the Haven of Peace
Age: 79
Posts: 600
Received 0 Likes
on
0 Posts
FH,
Normally for Cat A in Europe (broadly CAA/JAA) the broad outline is that in the event of the failure of a critical power unit (usually one engine failure, but they write the rules to cover helicopters with more than 2 engines) the aircraft must be able to either safely land back on to a suitable surface or continue the flight to a place where a safe landing may be made. For helicopters where the CDP is at greater than zero knots, the helicopter must be rejected back on to a sujitgable prepared surface before it reaches CDP. At CDP the pilot has the option of continuing or rejecting and after CDP he will normally continue to climb away on one engine. (I say he has the option, because in the event of an engine failure after CDP on a large airfield it may be preferable to reject the take off). The aircraft must then be capable of climbing to set criteria, depending on altitude, temperature and weight, at Vtoss to 200 feet and there it normally accelerates to Vy and has to be capable of maintaining a rate of climb at Vy of 150 feet per minute on 30 minute/MCP to either 1000 feet above the take off elevation, or to MSA if IMC. At 1000 feet above the surface (or MSA if IMC), it must be capable of climbing at 50 feet per minute on one engine at Vy at 30 minute power/MCP (some helicopter performance charts cover only one or the other of these power ratings). When approaching to land, in the event of the failure of a critical power unit it is committed to land after the LDP. When carrying out its landing it must be able to do so safely, so a twin engine helicopter, with a wheeled undercarriage, rejecting on to a waterlogged grass surface and rolling over as one wheel digs in, is not operating to Category A.
Category B is a relaxation of the Cat A rules and says that at times there may be a period during which the helicopter may be at risk during the take off procedure. During this period the aircraft may not be able to land safely, but if it does have to land it must be able to do so without damage to its occupants or third parties outside. When it reaches a 'Defined Point' it must then have Category A performance. The defined point varies in different countries and organisations. In many countries this will be when the helicopter attains Vy.
The rules vary a bit in different countries and are more detailed than this, but that's a basic outline. I'm sure there will be many other contributors to this forum far better qualified and more knowledgeable than myself who can considerably expand on what I've said and who will correct any errors I have made.....but I hope this helps a bit
Soggy
Normally for Cat A in Europe (broadly CAA/JAA) the broad outline is that in the event of the failure of a critical power unit (usually one engine failure, but they write the rules to cover helicopters with more than 2 engines) the aircraft must be able to either safely land back on to a suitable surface or continue the flight to a place where a safe landing may be made. For helicopters where the CDP is at greater than zero knots, the helicopter must be rejected back on to a sujitgable prepared surface before it reaches CDP. At CDP the pilot has the option of continuing or rejecting and after CDP he will normally continue to climb away on one engine. (I say he has the option, because in the event of an engine failure after CDP on a large airfield it may be preferable to reject the take off). The aircraft must then be capable of climbing to set criteria, depending on altitude, temperature and weight, at Vtoss to 200 feet and there it normally accelerates to Vy and has to be capable of maintaining a rate of climb at Vy of 150 feet per minute on 30 minute/MCP to either 1000 feet above the take off elevation, or to MSA if IMC. At 1000 feet above the surface (or MSA if IMC), it must be capable of climbing at 50 feet per minute on one engine at Vy at 30 minute power/MCP (some helicopter performance charts cover only one or the other of these power ratings). When approaching to land, in the event of the failure of a critical power unit it is committed to land after the LDP. When carrying out its landing it must be able to do so safely, so a twin engine helicopter, with a wheeled undercarriage, rejecting on to a waterlogged grass surface and rolling over as one wheel digs in, is not operating to Category A.
Category B is a relaxation of the Cat A rules and says that at times there may be a period during which the helicopter may be at risk during the take off procedure. During this period the aircraft may not be able to land safely, but if it does have to land it must be able to do so without damage to its occupants or third parties outside. When it reaches a 'Defined Point' it must then have Category A performance. The defined point varies in different countries and organisations. In many countries this will be when the helicopter attains Vy.
The rules vary a bit in different countries and are more detailed than this, but that's a basic outline. I'm sure there will be many other contributors to this forum far better qualified and more knowledgeable than myself who can considerably expand on what I've said and who will correct any errors I have made.....but I hope this helps a bit
Soggy
