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Sikorsky S-76: Ask Nick Lappos

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Sikorsky S-76: Ask Nick Lappos

Old 2nd Jun 2017, 01:52
  #1241 (permalink)  
 
Join Date: Sep 2007
Location: USA
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Originally Posted by NickLappos View Post
Regarding the TR overspeeding, the engine overspeed should catch a runaway before the TR gets to 120%, and the TR has been tested many times to well above that, I have personally seen 125% Nr/NrTR in flight.
Just to help clarify things, I'm assuming the S-76 drivetrain is similar to most other helicopters where the TR is coupled to the MR at some fixed speed ratio, right? And there is an overrun clutch at each engine input to the main rotor gearbox, which allows the gearbox side of the clutch to overrun the engine side of the clutch, but obviously does not allow the engine side to overrun the gearbox side. This basically means the engines themselves can't slow the rotor system by providing a braking load. All the engines can do is reduce fuel flow to the point where no engine torque is transferred to the gearbox.

This also brings up a relevant issue regarding turbine engine operating conditions. One thing turbine engines don't like is a rapid reduction in speed at very low load. This makes maintaining stable combustion very difficult, and turbine engines typically have control laws limiting the rate of speed change at low load for this reason.
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Old 2nd Jun 2017, 18:45
  #1242 (permalink)  
 
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riff_raff is right, there is a minimum fuel flow for an engine that dictates how quickly it can slow down in normal circumstances. This min fuel flow is what we see when we drop collective rapidly, or when we cut the throttle.


But when a rapid acceleration due to some failure is sensed, the engine has a means to cut its fuel flow very sharply, to contain the max rpm and prevent a burst. In the case where a large load is being shouldered by the engine and is suddenly cut, such as where an engine drive shaft fails, or a sudden internal transmission failure occurs and the engine is now unburdened but still producing gobs of power. The overspeed protection system (designed because the FAR/JAR requires that the engine handle a sudden cut like this) then senses the rapid upspeed of the engine, far faster than the normal fuel control. This sensitive system is virtually always electrical and triggers a cut signal that dumps fuel at a very high rate, forcing the engine to momentarily "fail" and stopping the upspeed before it becomes destructive to the engine.
The cut shaft is the worst case, since the engine sees virtually no load to absorb its power. Usually the overspeed sensing circuit has a way to judge the acceleration so that it is not just an absolute rpm protection, it drops the triggering rpm as the rpm rate increases. Most overspeed systems return the fuel to normal levels as the momentary event is recovered.


Here is an EASA document that describes the need for the test: https://www.easa.europa.eu/system/fi...ilure_PUBL.pdf
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Old 18th Aug 2017, 20:05
  #1243 (permalink)  
 
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During my FlightSafety 76 initial course, I was told that the VNE on the 76 is set more from a tail boom fatigue issue rather than retreating blade stall. It was mentioned that flying past VNE, as well as prolonged flight at high power settings, will put undue stress on the tail boom and vertical fin. The first indication of this would be loose screws on the panel behind the left main gear.

Just wondering if this is factual.

Thanks
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Old 18th Aug 2017, 21:25
  #1244 (permalink)  
 
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TIMTS
That doesn't have a grain of truth to it! The Vne is a complex limit, because once it is established, then many calculations about dynamic system life are made from the flight loads at Vne (and reducing multiples of it, based on the fatigue spectrum). The static structure (airframe) does certainly not limit speed on any helicopter I know of. Above Vne, the rotating components are under more stress and their lives can be shorter as a result.
The Vne of the S-76 was initially set by three simple factors:
1) the maximum speed needed to meet the business plan goals for a practical high speed cruise of 145 knots with some margin for gusts.
2) a level flight speed that allowed productive flight tests where the Vne could be achieved in level flight at full power, so that no hairy chested dives were needed during the structural testing, since dives consume lots of time climbing back to the test altitude and then accelerating back to Vne.
3) The need to meet 29.173 longitudinal static stick stability, which involved pushing to 1.1 Vne in a screaming dive as a "level flight" point. The FAR was since corrected, and is much more sane now.


I have seen some comparisons of aircraft where the Vne was published as a "maximum speed" even if it is a dive point far above anything achievable with normal flight performance. These are usually fallouts of military Vdive points, and relatively meaningless for comparison, where actual flight performance is the real index. The Black Hawk has a 193 Vne, but the S-76 has just the same speed in level flight.
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Old 18th Aug 2017, 21:53
  #1245 (permalink)  
 
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Thank you!
That's what I suspected.
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Old 19th Aug 2017, 00:12
  #1246 (permalink)  
 
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15 years ago I was flying an S-76 at 145kt with a 30-kt tailwind (groundspeed 175kt) when I flew into a Southerly Buster making its way up the coast at 40kt. It was like hitting a brick wall, airspeed leapt to 180kt, 25kt above Vne, and we zoomed up in the sky, even as I lowered the lever and slowed down.

No bad flying effects were displayed by the machine, and an email to Nick confirmed that this would have no effect on the aircraft at all - thanks Nick!
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Old 19th Aug 2017, 13:37
  #1247 (permalink)  
 
Join Date: Dec 2003
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Nick,

I just came across this thread thought I would add a slightly different, very dated, recollection of the events in posts 27 and 87. I was involved in both incidents, which happened a couple of months apart in 81.

The first incident was a collective to yaw lockup that resulted from the loss of the #2 hydraulic system. There I was at 6500 feet when the #2 system failed and the collective felt frozen. I was able to reduce the collective about 10-15% by forcing it down (maybe my feet moved but I wasn't feeling it at first.) Once I got lined up with the runway, I just drove it on with the cyclic. Interesting side note - once on the ground, I was so relieved, I reached up and yanked both throttles to idle. Big mistake. The nose jerked to the right and I instinctively stomped left pedal and I was flying again. The aircraft jumped about 5 feet into the air before plopping back down on the runway.

The second incident involved the transmission. Once again, there I was, when there was a big clunk that came from the #1 engine side. Split torque, with the #1 engine showing Hi temp and low N1 along with a pronounced grinding noise. We thought we had lost the #1 engine, so we shut it down and flew back to base. The next morning when I came to work, our lead mechanic told us, we were lucky - very lucky. The night before, they had done a run up using the #2 engine when the tail rotor failed.

I recall a phone conversation with you about the second incident and probably filled you in on the first one. I recall you issued a procedure that in the event of a partial #1 failure to beep it back but not shut it down.

Sikorsky sent a tech rep and a transmission guy to our base to look at the transmission. We all stood around as our mechanics opened the case. At first, nothing looked out of the ordinary, when your transmission guy pointed to the #1 side gears. As I recall what was later revealed, the bull gear at manufacture is pressed onto a spline that fits on a shaft that is then held in place with a nut. In this case the gear was not pressed all the way onto the spline, and after about 900 hours finally seated itself which allowed the retaining nut to work loose and the gear slid out on its shaft and started rubbing on the inside of the case but still had enough contact to drive the tail rotor.

Oh - and keeping with the theory that things happen in threes, I was the one that was flying the S92 that lost tail rotor control back in 2011.
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Old 19th Aug 2017, 15:21
  #1248 (permalink)  
cpt
 
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Hi js,

Just being curious, did you know the reason of your collective to yaw lockup after the loss your Hyd 2 system ? Hyd 1 wasn't powering the tail rotor servos anymore ?
..... and normally after landing, when pulling both engine levers back to idle, shouldn't you have yawed left instead of right ?
.... and in a situation of collective to yaw lockup, shouldn't an action on the left pedal, force your collective down instead of up ?
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Old 19th Aug 2017, 16:58
  #1249 (permalink)  
 
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According to Nick back in post 27, he said there was a notch worn in the servo. It's been a long time, but I seem to remember the right pedal pushed pretty much all the way in after landing with the collective all the way down. Now that I think about it, I do believe the nose jerked left. The collective to yaw system still think they are at 60% torque and 130 knots. I may have instinctively tried to center pedals when the nose jerked, but I do know the collective popped up and the aircraft jumped in the air.
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Old 20th Aug 2017, 21:36
  #1250 (permalink)  
 
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If I dig deep into the bowels of my memory back, I recollect a conversation in the Sikorsky pilot’s office discussing the Vh/Vne limits of the S-76A. Initially the S-76A was certified with the Detroit Diesel Allison 250-C30 engines. At sea level the S-76A was capable of a Vh of 155 KIAS. This was also used as the Vne. There was no reason to push the aircraft to a faster Vne. At altitude, with the drop off in engine power the S-76A’s Vh was similarly reduced. This is evident on the cockpit Vne placards for the S-76A. There was no effort to push any faster than these published Vh limitations. As a result, the level flight performance of the S-76A at altitude was not competitive with competitors i.e. AS-365 of A-109. With the development of the S-76B/C/C+ there was no effort to recertify and increase the Vh or Vne at altitude with its higher power engines. I for one always wondered if there was an underlying structural or aerodynamic issue that restricted the growth in speed performance with the increase in installed power.
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Old 22nd Aug 2017, 14:04
  #1251 (permalink)  
cpt
 
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...Now, a question about navigation equipment ...

1 aircraft is fitted with an SPZ 7600 DAFCS (AHRS/EFIS)
2 LNAV coupled to FMS (UNS1)
3 heading is not synchronized with the flux valve (in DG or because of any other disturbance) so that the EHSI set in "route mode" displays a false heading, showing therefore a uncorrectly oriented route.

Untill now, my experience (based on Garmin 530W and not on a true FMS) tells me that the FD will follow the coupled source, in this case the GPS great circle segment between 2 WPs, and this, whatever the indicated heading ( indicated wind and GS will be wrong in this case)
But a colleague, more familiar with the UNS1 is sure of the opposite i.e the FD will follow the displayed route wich in this case is wrongly oriented resulting in an actual route deviation .

Any return of experience or idea ?
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Old 22nd Aug 2017, 14:43
  #1252 (permalink)  
 
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Originally Posted by cpt View Post
...Now, a question about navigation equipment ...

1 aircraft is fitted with an SPZ 7600 DAFCS (AHRS/EFIS)
2 LNAV coupled to FMS (UNS1)
3 heading is not synchronized with the flux valve (in DG or because of any other disturbance) so that the EHSI set in "route mode" displays a false heading, showing therefore a uncorrectly oriented route.

Untill now, my experience (based on Garmin 530W and not on a true FMS) tells me that the FD will follow the coupled source, in this case the GPS great circle segment between 2 WPs, and this, whatever the indicated heading ( indicated wind and GS will be wrong in this case)
But a colleague, more familiar with the UNS1 is sure of the opposite i.e the FD will follow the displayed route wich in this case is wrongly oriented resulting in an actual route deviation .

Any return of experience or idea ?
My experience of the UNS-1 on other types is that the FD/AP will fly the track regardless of the heading displayed.

indicated wind and GS will be wrong.....
GS won't be wrong - just the wind.
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Old 22nd Aug 2017, 18:05
  #1253 (permalink)  
cpt
 
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Thanks 212. As a naive simple mind that's what I also believe, but these plastic brains are so wicked !
.... and yes that's right, the GS indication is not affected.
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Old 23rd Aug 2017, 04:58
  #1254 (permalink)  
 
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Departing from the the east side heliport in Manhattan, NY after running for some time waiting on your IFR clearance and you'll see all sorts of weird things. Heading and ground speed will be erroneous. However, once you couple the FD to the flightplan in the UNS1, despite the incorrect heading, track, etc. the correct path (as programmed in the UNS1) will be flown.
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Old 23rd Aug 2017, 10:15
  #1255 (permalink)  
 
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Curious whether a S76 which is immersed in salt water to about windscreen level would be a write-off. We have two 76's and a 92 floating in the hangar.

Last edited by gulliBell; 24th Aug 2017 at 01:00.
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Old 23rd Aug 2017, 15:36
  #1256 (permalink)  
cpt
 
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So the effect of melting sea ice on sea level rise is not a joke !

I've heard some cases, although not sure of the types, of helicopters reconditioned and become airworthy again after a sea ditching. Certainly the shortest immersion time, the better.
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Old 24th Aug 2017, 00:57
  #1257 (permalink)  
 
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Originally Posted by cpt View Post
...I've heard some cases, although not sure of the types, of helicopters reconditioned and become airworthy again after a sea ditching..
Not in China. Hardly ever recondition or buy used anything. Always buy new.

If you had a S76C cabin, could you rebuild it from all the good parts from a C++?

Last edited by gulliBell; 24th Aug 2017 at 01:22.
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Old 24th Aug 2017, 03:02
  #1258 (permalink)  
 
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Originally Posted by gulliBell View Post
Curious whether a S76 which is immersed in salt water to about windscreen level would be a write-off. We have two 76's and a 92 floating in the hangar.
WOW - can you show us a picture of that??!!
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Old 24th Aug 2017, 03:03
  #1259 (permalink)  
 
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Originally Posted by Kulwin Park View Post
WOW - can you show us a picture of that??!!
I do have pictures, but it's a state of turmoil at the moment and I'm not game to post them just yet. 2 Super Puma and a S76 at a China Rescue base in submarine mode also. So that's at least 6 aircraft that were playing battleship in their hangars during Typhoon Hato. Expensive. Has to be USD $100 million-ish +.

Last edited by gulliBell; 24th Aug 2017 at 11:28.
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Old 24th Aug 2017, 12:17
  #1260 (permalink)  
 
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https://www.facebook.com/jim.song.35...91?pnref=story
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