Sikorsky S-76: Ask Nick Lappos
Hi Nick,
Your comments on this discussion would be much appreciated please!
We operate S76A models on EMS/SAR work in an environment which is mostly at sea level and temps up to the high 20's.
Cat A style helipad takeoffs ( yes no Cat A in the RFM for elevated helipads ) as well as confined area departures and winching ops.
The discussion revolves around the use to the engine trim switches to alter the Nr when taking off or during winch operations.
The RFM only refers to using 100 % Nr.
Are there any advantages/disadvantages in using 107%Nr in these situations with regard to a single engine failure and subsequent actions.
Your comments will be much appreciated thank you!
Your comments on this discussion would be much appreciated please!
We operate S76A models on EMS/SAR work in an environment which is mostly at sea level and temps up to the high 20's.
Cat A style helipad takeoffs ( yes no Cat A in the RFM for elevated helipads ) as well as confined area departures and winching ops.
The discussion revolves around the use to the engine trim switches to alter the Nr when taking off or during winch operations.
The RFM only refers to using 100 % Nr.
Are there any advantages/disadvantages in using 107%Nr in these situations with regard to a single engine failure and subsequent actions.
Your comments will be much appreciated thank you!
I would say that it depends on whether the aircraft is cleared to operate at the higher Nr. If it's not mentioned in the RFM then it's possiblely not.
However there are advantages of increasing the Nr for take off:
https://www.linkedin.com/pulse/engin...sanjeev-kumar/
However there are advantages of increasing the Nr for take off:
- Power is directly proportional to product of torque and rotor RPM. Thus higher rotor RPM means lower torque for the same power requirement with all engines operating. Or more power for the same torque, whichever way you like to see it.
- With a higher rotor RPM, the angle of attack and thus blade pitch for the same power is slightly less. At high angles of attack, this improves the lift-drag ratio of the blades.
- More RPM means more energy stored in the rotor – always a good thing if you lose power!
- In the event of losing an engine, the higher rotor RPM gives greater reaction time to pilots before lower limit of rotor RPM is encountered. Going from our theory for single engine helicopter, greater reaction time is always money in the wallet!
- The higher rotor RPM also reduces the height loss in transitioning to safe OEI flight –something significant while operating to offshore decks where this ‘drop down height’ versus height available is crucial for a safe flyaway.
- Higher rotor RPM translates to higher tail rotor energy through its gear ratio thereby enhancing tail rotor authority.
https://www.linkedin.com/pulse/engin...sanjeev-kumar/
Not sure if we're going to see Mr L stop by here, but buried in this deep thread Nick talks of 100% being a more efficient rotor speed below Vy due to S76 wide chorded blades, which were designed for high speed flight. 107Nr is better for tail rotor effectiveness and low speed handling, if I remember correctly. Certainly in the 139 the bump up to 102Nr whips the tail end into line. The change in tail rotor effectiveness is quite noticeable.
Oh Nick, where are you? Higher rpm is an easy weight increase for certification, but it costs you. Into wind, tail-rotor is fine, S76 does not suffer from lack of authority. For Cat A OEI, the pulldown from 107 to 90 will slingshot you to Vtoss with the right technique, with the wrong technique you just bleed everything away and fall in the ocean. Been a long time from the old A-models, and even the much better A++, but I do recall starting out at 107 and N1 limiting, so then beeping down, matching beeps, all the way to 100, or maybe even a little less. Position on the edge of the deck, hover, bounce of the wheels and a mighty pull to launch it over the side dynamically, even a little sideways so you didn't catch the tailrotor on the edge of the deck falling down trying to get Vtoss. Reject in the case of an engine failure was into the water, why we had floats, no external rafts. Damn those were the days. A sentimental way of saying that no, you don't get more power at a higher rpm. You store potential energy, that's all. Anyone else want to try fill Nick's shoes?
I've been in touch with Nick and asked him to drop by, so hopefully some informed opinion may be coming soon!
I recall that the 76A was 100%Nr across the board until an advisory notice appeared advising of the added performance available in the cruise when using 107%Nr, but maintaining 100% for T/O and landings. There may also have been an altitude associated with the 107%, the grey cells are struggling but was it >5,000ft?
Having established a fair baseline of ops at 100% there was no doubt that 107% gave a cruise performance improvement of some 3-5KIAS and most of our ops were 7-9,000ft out of Essendon. All for the same fuel burn, too.
I recall that the 76A was 100%Nr across the board until an advisory notice appeared advising of the added performance available in the cruise when using 107%Nr, but maintaining 100% for T/O and landings. There may also have been an altitude associated with the 107%, the grey cells are struggling but was it >5,000ft?
Having established a fair baseline of ops at 100% there was no doubt that 107% gave a cruise performance improvement of some 3-5KIAS and most of our ops were 7-9,000ft out of Essendon. All for the same fuel burn, too.
As long you are not engine limited, you should have more power available with 107%. (The old horsepower formula.) But the blade profile might be more efficient at 100%.
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76 RPM
The original S76A had an rpm range in powered flight from 96% to 107%. This allow the pilot to select the RPM based on flight condition and the optimal performance. Some information was provided to show that the rotor was generally more efficient at providing hover lift for a given horsepower at the lower RPMs, and at high speed cruise efficiency gives better cruise speed the high RPM. Thus it was sometimes better to be at 100% RPM during the steady hover if there was not sufficient engine power to reach transmission limit. In this case then the more limited engine power provide maximum lift at 100% RPM. If however sufficient engine power existed to pull 100% torque, then there was no great advantage to staying at 100% Nr, And in fact the extra rotor energy at 107 percent can produce a better single engine land back if a OEI is experienced.
Note that when power limited in OEI, a droop down to 96% Nr was required, and the climbout acceleration to Vtoss was helped by the lower RPM.
The later model S76 helicopters raised the gross weight, so that generally the 107% RPM was better in cruise on component lives. For this reason on the B, C and D models, only one High RPM was provided. The reason why 107% RPM was favored as the gross weight was increased was because effects of retreating Blade stall create additional rotor loads that stress opponents a bit more and reduce their lives. Use of 107% RPM allow the lives to stay high even up to a 11,700 pounds.
If you fly an old A model, or in A + series, you can try this experiment to see how efficient the rotor is at 100% as compared to 107%. Just taxi out on a very still morning and establish a very low hover at 100% RPM. Adjust the aircraft to just barely bounce the left landing gear, and note the engine temperature required at that very low hover. Now beep the RPM up to 107% and repeat the wheel bounce. Note the engine temperature again. You should note that it takes about eight or 9°more temperature at 107. This is because when the rotor is spinning at 107 it uses more drag to spin the rotor Leaving a little less power to produce the thrust. FWIW, 3.5 degrees of Temp = 1% power.
Now take your aircraft into high-speed cruise, and set 694 degrees engine temperature at 100% rotor and some altitude above about 5000 feet. Take a very careful level flight point and note your cruise air speed. Now beep to the rotor RPM up to 107% and note your cruise speed. Generally at higher altitudes 107% RPM yields 5 to 7 knots more cruise speed, which at the same power means more range for the fuel. This is the because at the higher speed the beginnings of retreating blade stall begin to show and higher RPM has less blade stall effect and more rotor efficiency.
Note that when power limited in OEI, a droop down to 96% Nr was required, and the climbout acceleration to Vtoss was helped by the lower RPM.
The later model S76 helicopters raised the gross weight, so that generally the 107% RPM was better in cruise on component lives. For this reason on the B, C and D models, only one High RPM was provided. The reason why 107% RPM was favored as the gross weight was increased was because effects of retreating Blade stall create additional rotor loads that stress opponents a bit more and reduce their lives. Use of 107% RPM allow the lives to stay high even up to a 11,700 pounds.
If you fly an old A model, or in A + series, you can try this experiment to see how efficient the rotor is at 100% as compared to 107%. Just taxi out on a very still morning and establish a very low hover at 100% RPM. Adjust the aircraft to just barely bounce the left landing gear, and note the engine temperature required at that very low hover. Now beep the RPM up to 107% and repeat the wheel bounce. Note the engine temperature again. You should note that it takes about eight or 9°more temperature at 107. This is because when the rotor is spinning at 107 it uses more drag to spin the rotor Leaving a little less power to produce the thrust. FWIW, 3.5 degrees of Temp = 1% power.
Now take your aircraft into high-speed cruise, and set 694 degrees engine temperature at 100% rotor and some altitude above about 5000 feet. Take a very careful level flight point and note your cruise air speed. Now beep to the rotor RPM up to 107% and note your cruise speed. Generally at higher altitudes 107% RPM yields 5 to 7 knots more cruise speed, which at the same power means more range for the fuel. This is the because at the higher speed the beginnings of retreating blade stall begin to show and higher RPM has less blade stall effect and more rotor efficiency.
Last edited by NickLappos; 28th Jul 2018 at 18:13. Reason: clean up dictating typos - thanks MS Speech recognition!
When not TOT limited the adverse was true with a BK117 B2.
You could squeeze more speed out of the bird with reduceing the Nr.
While doing so, the TQ reduced, so you could add more collective and at the end you gained about 5 to 8 KIAS, compared to 100/102%Nr ;-)
We shuttled birds sometimes and even after taking off second, with this trick is was possible to arrive first ;-)
You could squeeze more speed out of the bird with reduceing the Nr.
While doing so, the TQ reduced, so you could add more collective and at the end you gained about 5 to 8 KIAS, compared to 100/102%Nr ;-)
We shuttled birds sometimes and even after taking off second, with this trick is was possible to arrive first ;-)
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The original S76A had an rpm arrange in powered flight from 96% to 107%. This allow the pilot to select the RPM based on flight condition and the optimal performance. Some information was provided to show that the rotor was generally more efficient at providing lift for a given horsepower at the lower RPMs, and at high speed cruise efficiency gives better cruise speed the high RPM. Thus it was sometimes better to be at 100% RPM during the steady hover if there was not sufficient engine power to reach transmission limit. In this case then the more limited engine power provide maximum lift at 100% RPM. If however sufficient engine power existed to pull 100% torque, then there was no great advantage to staying at 100%, And in fact the extra rotor energy at 107 percent can produce a better single engine land back if a OEI is experienced.
Note that when power limited in OEI< a droop down to 96% Nr was required, and the climbout acceleration to Vtoss was helped by the lower RPM.
The later model S76 helicopters raised the gross weight, so that generally the 107% RPM was better in cruise on component lives. For this reason on the B, C and D models, only one High RPM was provided. The reason why 107% RPM was favored as the gross weight was increased was because effects of retreating Blade stall create additional rotor loads that stress opponents a bit more and reduce their lives. Use of 107% RPM allow the lives to stay high even up to a 11,700 pounds.
If you fly an old A model, or in A + series, you can try this experiment to see how efficient the rotor is at 100% as compared to 107%. Just taxi out on a very still morning and establish a very low hover at 100% RPM. Adjust the aircraft to just barely bounce the left landing gear, and note the engine temperature required at that very low hover. Now beep the RPM up to 107% and repeat the wheel bounce. Note the engine temperature again. You should note that it takes about eight or 9°more temperature at 107. This is because when the rotor is spinning at 107 it uses more drag to spin the rotor Leaving a little less power to produce the thrust. FWIW, 3.5 degrees of Temp = 1% power.
Now take your aircraft into high-speed cruise, and set 694 degrees engine temperature at 100% rotor and some altitude above about 5000 feet. Take a very careful level flight point and note your cruise air speed. Now beep to the rotor RPM up to 107% and note your cruise speed. Generally at higher altitudes 107% RPM yields 5 to 7 knots more cruise speed, which at the same power means more range for the fuel. This is the because at the higher speed the beginnings of retreating blade stall begin to show and higher RPM has less blade stall effect and more rotor efficiency.
Note that when power limited in OEI< a droop down to 96% Nr was required, and the climbout acceleration to Vtoss was helped by the lower RPM.
The later model S76 helicopters raised the gross weight, so that generally the 107% RPM was better in cruise on component lives. For this reason on the B, C and D models, only one High RPM was provided. The reason why 107% RPM was favored as the gross weight was increased was because effects of retreating Blade stall create additional rotor loads that stress opponents a bit more and reduce their lives. Use of 107% RPM allow the lives to stay high even up to a 11,700 pounds.
If you fly an old A model, or in A + series, you can try this experiment to see how efficient the rotor is at 100% as compared to 107%. Just taxi out on a very still morning and establish a very low hover at 100% RPM. Adjust the aircraft to just barely bounce the left landing gear, and note the engine temperature required at that very low hover. Now beep the RPM up to 107% and repeat the wheel bounce. Note the engine temperature again. You should note that it takes about eight or 9°more temperature at 107. This is because when the rotor is spinning at 107 it uses more drag to spin the rotor Leaving a little less power to produce the thrust. FWIW, 3.5 degrees of Temp = 1% power.
Now take your aircraft into high-speed cruise, and set 694 degrees engine temperature at 100% rotor and some altitude above about 5000 feet. Take a very careful level flight point and note your cruise air speed. Now beep to the rotor RPM up to 107% and note your cruise speed. Generally at higher altitudes 107% RPM yields 5 to 7 knots more cruise speed, which at the same power means more range for the fuel. This is the because at the higher speed the beginnings of retreating blade stall begin to show and higher RPM has less blade stall effect and more rotor efficiency.
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Yes, all works as described. Have used the 107nr in cruise when remaining fuel was on the minimums. easily "made" 50-80 lbs on landing from long sectors. Nick, isn't there also a TAS gain in cruise........107nr for same power setting as 100nr yields "flatter" blade pitch....less drag????
DK
DK
This is the big compromise for helo designers, having a very efficient hover rotor (which favors heavily loaded blades and higher collective, near the optimum L?D) vs high speed cruise (which favors wide blades, lightly loaded and nowhere near stall.)
Hi Nick,
Not an S76 question but a S70 or UH60A question if I may?
Do you know why the UH60A prohibits the opening of the cargo door and pilots windows in flight, except when hovering?
And secondly why can't they be flown with the pilots doors off?
Thank you,
Bront
Not an S76 question but a S70 or UH60A question if I may?
Do you know why the UH60A prohibits the opening of the cargo door and pilots windows in flight, except when hovering?
And secondly why can't they be flown with the pilots doors off?
Thank you,
Bront
Hawk Doors/Windows
Bront, unless there have been a number of changes since I retired, the UH-60 restrictions you quoted are incorrect. In fact the flight which led to the original limits involved both Bob Zincone, who was the development chief for the UTTAS test program, and John Chapkovich, the head structural design engineer. One other note, if you ever get in the UH-60 cockpit and it has the sliding window: if you do open it in flight, don’t put your paper map anywhere close to it-the window has another name, “ Automatic Map Jettison “.
Last edited by JohnDixson; 20th Sep 2018 at 17:13. Reason: typo
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I've managed to find my files with limits for the S-70s I flew until 1998 (where did those twenty years go?).
These were copied from the RFM, unfortunately I don't hold a copy of the actual document.
Cabin Doors
May be fully open up to 100 kts with soundproofing installed aft of Station 379.
May be fully open up to 145 kts with soundproofing above removed or properly secured.
Doors not to be intentionally moved from full closed/open position in flt. Only in hover. If doors move in flt then they can be positioned open/closed
Gunner's window fully open to 170 kts.
Cockpit door sliding windows not to be opened/closed at any airspeed.
Flt with cockpit doors off prohibited.
I can vouch for the "vacuum cleaner effect" of the cockpit windows. My newly amended and fabloned local area map went out over the sea one dark night....it was ripped straight out of my tight fist before I could even blink!
These were copied from the RFM, unfortunately I don't hold a copy of the actual document.
Cabin Doors
May be fully open up to 100 kts with soundproofing installed aft of Station 379.
May be fully open up to 145 kts with soundproofing above removed or properly secured.
Doors not to be intentionally moved from full closed/open position in flt. Only in hover. If doors move in flt then they can be positioned open/closed
Gunner's window fully open to 170 kts.
Cockpit door sliding windows not to be opened/closed at any airspeed.
Flt with cockpit doors off prohibited.
I can vouch for the "vacuum cleaner effect" of the cockpit windows. My newly amended and fabloned local area map went out over the sea one dark night....it was ripped straight out of my tight fist before I could even blink!
My manual says exactly what Shy says.
I have a mate that regularly jumps out of the back of one in Saudi and has to freeze all the way up to drop altitude because they wont open the door inflight, so that's one air force using that restriction.
I'm really curious about this limitation because it seems like quite a restriction for a military helicopter.
Ha ha, I'll keep the 'vacuum cleaner effect' in mind, thanks for the warning.
I have a mate that regularly jumps out of the back of one in Saudi and has to freeze all the way up to drop altitude because they wont open the door inflight, so that's one air force using that restriction.
I'm really curious about this limitation because it seems like quite a restriction for a military helicopter.
Ha ha, I'll keep the 'vacuum cleaner effect' in mind, thanks for the warning.
Bront and Shy: The original limit for cabin door opening was 120 KIAS. We could have made it higher at the time, but just thought that speed would cover operations, and flight tested accordingly. There was no spec/US Army requirement in that area as I recall. Not being able to do this at all imposes unreasonable restriction, operationally speaking. There was discussion about how to handle the cockpit door on/off configuration, and quite honestly, the restriction had no basis with regard to structural intergrity etc*,but rather a judgment regarding the logistics about “ whose got, or where are “ my doors, once you take them off. You have probably seen a number of pics ( all USAF In my viewing experience ), of doorless UH-60’s from the Middle East of late. I know someone who might shed some light on the door transition restriction in forward flight. I’ll inquire.
* Anecdotal support: when we did the 12 ft/sec landings on a 9 degree concrete pad for the SH-60, I had to take the door off, as the USN had a budget issue and only built a 50 ft pad. In order to get the vertical speed nailed at 12 ft/sec, it was necessary to start at 100 ft and seeing the pad became a door problem.
The restriction on opening the sliding window is new to me, and (other than the map disposal functionality considerations ), baffles me. I can say that the original door design drawing did not have an openable window in it, which, after a few animated discussions, was rectified.
Rev 1 ( 2125 hrs lol ). Good friend said that door open/close restriction due to cracks in original aluminum tracks. However the USN MH-60S had a test done to provide a 120 KIAS open/close approval but Army wasn’t interested in the data. Going to have to look into this further.
* Anecdotal support: when we did the 12 ft/sec landings on a 9 degree concrete pad for the SH-60, I had to take the door off, as the USN had a budget issue and only built a 50 ft pad. In order to get the vertical speed nailed at 12 ft/sec, it was necessary to start at 100 ft and seeing the pad became a door problem.
The restriction on opening the sliding window is new to me, and (other than the map disposal functionality considerations ), baffles me. I can say that the original door design drawing did not have an openable window in it, which, after a few animated discussions, was rectified.
Rev 1 ( 2125 hrs lol ). Good friend said that door open/close restriction due to cracks in original aluminum tracks. However the USN MH-60S had a test done to provide a 120 KIAS open/close approval but Army wasn’t interested in the data. Going to have to look into this further.
Last edited by JohnDixson; 22nd Sep 2018 at 01:29. Reason: Additional info
