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Sikorsky S-76: Ask Nick Lappos

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Sikorsky S-76: Ask Nick Lappos

Old 1st Nov 2001, 04:40
  #81 (permalink)  
Nick Lappos
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Steve76 asks:
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.


Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.

Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)

In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)

Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.

The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
 
Old 1st Nov 2001, 14:48
  #82 (permalink)  
 
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This is probably the wrong thread to add this little story into, but it may have some relevence to any one who drives the Robbie.

The topic reminded me of an overspeed situation that a pilot had in the N.T of Oz around 6 years ago.

Early morning, fat dumb and happy on the way to a job. I hear "oh **** " over the company radio. Not the usual frazeology. Several calls made to said pilot, only to hear a garbled chuckle.

The engine on the R22 rotates a set of belts on a sheave set up. The upper sheave is connected to the drive train and houses the free wheel unit. Just forward of the upper sheave there is a flex plate that allows flexing of the drive shaft as the upper sheave moves away from the lower sheave when the belts are tensioned. Forward of this flex plate the drive shaft goes through another flex plate and then into the gear box. Aft of the upper sheave the drive shaft goes rearward to the tail rotor GB etc.

The yoke that attached the short shaft to this aft flex plate broke. The result of this was a very unsubtle scream from the engine as it oversped. The Main Rotor RPM bled off as the fuselage went in all sorts of directions as the pilot inputed corrections for what he perceived was going wrong.

I often think of this story, trying to picture what the reactions would be. Drooping RPM, screaming engine, exaggerated tail rotor response as the engine drive went directly to the tail rotor.

Have a think about it.

P.S He did a text book auto and put it into a clearing the size of a backyard swimming pool surrounded by sand stone rocks 4 metres tall. .......He was our chief pilot after all.

Cheers.
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Old 1st Nov 2001, 16:04
  #83 (permalink)  

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STEVE76:

I would have to agree with Nick. This condition would not lead to a catostrophic failure, although the load to #1 Eng would be decreased, keep in mind there would still be a restricting load (T/R).

Another point to be consider is that this type of failure, as far as I know, has never occurred and is a precautionary carry over from the S61 days when a TTO (Tail Take-Off) failure was a real possibility.

In fact most companies have eliminated the emergency procedure from their SOP's.

Cheers, OffshoreIgor
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Old 1st Nov 2001, 22:42
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Nick; has this occurence ever been documented on the 76 ??
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Old 1st Nov 2001, 23:29
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On the 'A' model C30 engines we drive, the O/S system has been removed due to random failures which generated their own particular emergency.
That was info I should've mentioned in the original thread.
So, that is why I enquired about the O/S to the TR. I expect the engine govenor will control most of the problem with or without the O/S system.
Igor,
I quite agree that the emergency proceedure is a little dated but I was just curious to hear Nicks opinion, as everyone else round here just shrugs and is a little uncertain.
Cheers CB
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Old 2nd Nov 2001, 09:42
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rotorque :

I heard a similar story about an EMS 412 in NSW which suffered a combiner shaft failure - increasing engine RPM, decreasing RRPM, and so on.

The pilot ( Pete Cook ? ) put it down on a ridge. The previous flight had been a night EMS over similar terrain. And I heard that on the sim check he'd done some months ago he'd specifically asked to go through that specific emergency. Luck or what !
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Old 2nd Nov 2001, 15:09
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for Steve76:
The procedure is based on an actual failure that occurred about 15 years ago, where the input gear attachment bolts lost torque and the separation that you describe actually occurred. The gear was redesigned, and no repeat failure occurred. The flight crew noted the problem as noise and rumbling, a momentary upspeed of #1 engine, a swing to the left (extra tail thrust) and then back to normal, with very low #1 torque and high #2 torque.
After a bit of discussion, the crew left well enough alone, and flew home without shutting down #1 (what a good pair of guys! If it works, leave it alone!).

When they landed, they noted that the failure, in that the tail rotor was not connected to the main rotor.
We id'd the problem and fixed it asap, of course, and no repeat has occurred. We inspect all boxes on overhaul for signs of lost torque on that gear to see if any recurrence is creeping back, and everything is fine now.

For Nr Fairy
The above is not like a more common failure that NR fairy notes, where the engine shaft going to the transmission can fail, and N2/Np can go up while torque goes down. In that case, you have an engine power loss, but a healthy rotor drive train otherwise. That can be confusing because the engine rpm on the triple tach goes up, but the rotor goes down. The rotor is your closest friend, so it is wise to make it happy first, of course. Crews can get confused when those needles, always stuck together before, start to disagree.

Also, I am surprised that any A's are flogging around with disconnected electric overspeed systems. They were a pain in the early days (1979) but should be healthy now. The normal governor will catch these failures we describe, I think, so it is not essential to the failures on this thread, but the electric overspeed is helpful for internal engine failures where the power section can unbutton from the compressor, the internal engine overspeed can get very high and engine rupture is possible.
 
Old 2nd Nov 2001, 21:01
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Nick ; I don't think that there is an "A" model flying that has the electronic overspeed connected. I think they were removed because of a service bulletin or AD.

I think it is removed from Flight Safety's A model simulator as well.
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Old 2nd Nov 2001, 23:32
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Tech,
You are right, the system was deactivated in 1992. My bad.
 
Old 3rd Nov 2001, 00:44
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Thanks Nick for your informative comments.
Interestingly, I just watched an Icing Briefing from FS that you delivered. Very interesting for a guy who has never flown in snow before. Thanks for the tips regarding constant collective/power settings if suspected ice around. I will be certain to implement them if ever silly/unfortunate enough to get iced.
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Old 3rd Nov 2001, 01:26
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Steve 76,
That tape has gotten around! The trick of watching torque to detect ice is something I noticed in the spray rig at Ottawa.
 
Old 3rd Nov 2001, 09:51
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Steve 76, for early detection of ice keep an eye on the wiper blades, they're easy to check at night with a flash light. For some reason signs of icing first show up on the wiper blade side closest to the centre post between the windshields.i.e if in the right seat look across at the left wiper blade and vice-a-versa. A part from torque going up airspeed also goes down accompanied by roughness and shaking, at about this time you've got to do something about it i.e. exit the area.
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Old 4th Nov 2001, 05:15
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Tech,
The problem with using the wipers as ice indicators (I especially like the small hose that provides the washer fluid) is that it may not reliably tell when the rotor is experiencing ice. The issue is that the rotor and the airframe are in two different environments aerodynamically, so the rotor might not be experiencing ice when the wipers show some, and vice versa. It has to do with the fact that the blades are doing near Mach 1 while the wiper is poking along at Mach .2.

I have had ice so heavy that I was descending at full power, with no ice on any part of the airframe that I could see, and I have had 1/4 inch of clear ice on the entire nose with absolutely no ice on the rotor.

The only way to tell if rotor ice is around is to watch for a torque rise with fixed collective and fixed nose attitude. Of course, if ice forms on the wipers, that is a good sign that it is time to be somewhere else.

Of small comfort, but true nonetheless, the engines will behave wonderfully in ice that will otherwise reduce performance to nil. The inlets on the various S-76 models are well suited to ice conditions by design and test.
 
Old 4th Nov 2001, 05:54
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Nick you are correct, I use the wipers as a guide and if they are collecting ice I am planning altitude or heading changes to exit the area.

My experience with ice is only operational and from what I've seen it usually starts at the blade root and travels out.I've read somewhere, though I can't recall where that at 375 knots the temperature rise is appx 16 degrees. That would explain why it starts at the root.

From an operational point of view if you're in cloud below 0 degrees C, you're going to be picking up some ice.

In the good old days we use to transit icing areas but know flight in visible moisture below 0 degrees C is VERBOTEN and flying is much easier.

[ 04 November 2001: Message edited by: tech ]
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Old 4th Nov 2001, 06:06
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tech,
You hit the nail on the head, cold + clouds = ice. If you try to get cute, you'll get caught.

I think the tip heating is about 5 degrees C, from what I have been told. Where do you fly?
 
Old 4th Nov 2001, 20:47
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Nick, I fly EMS in North Western Ontario.
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Old 5th Nov 2001, 05:15
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Quick question, If the slower moving parts of the rotor will ice up easier does this include the swashplate? I would think that if you got enough ice on the plate it would severly hamper the controlability of the ship. Any one ever experienced this? If this is a silly question it's the inexperience talking.
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Old 5th Nov 2001, 09:06
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Baranfin,

The mechanicals are pretty much immune, because the ice is much weaker than the forces these parts handle normally, but this must be proven in certification. One concern is the servo input valves, which need only an ounce or two to create big problems.
 
Old 10th Nov 2001, 01:22
  #99 (permalink)  

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Question S-76 Nr Limits OEI

This is a question aimed primarily at Nick Lappos if he's around. (Thanks in advance for your support Nick)!

Whilst OEI, the Nr lower limit for the C+ is 93% then 100% whilst accelerating up to Vy but 107% above Vy.

Please could you give a definitive answer why this is so? How critical is this (apart from possibly failing a checkride by busting a limit by accelerating 1 knot over Vy)?

Thanks.
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Old 10th Nov 2001, 06:43
  #100 (permalink)  
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I helped instigate this in the S-76A back in the early days, and we still use it. The OEI climb rate is better at low Nr (the rotor is more efficient at lower speeds), also you extract maximum rotor energy during the critical low speed fly away and finally you can be sure the engine is producing maximum power if you pull against the limiter, so the procedure is written that way. The climb difference at 93% Nr is small, but worth it.

When you achieve Vy, you are able to transition to normal flight, and the lower Nr isn't needed. To prevent the need for a special Vne and chart at an oddball Nr, the procedure asks that you not go above Vy at a low Nr. If we permitted you to fly to a higher speed, we'd have to publish a Vne for it. It is more convenient for all of us to simply remember to get to 107 when above Vy.

In test, we flew the A to 150 knots at 96%Nr at weights up to 10300 lbs, so you will do no great harm to the rotor to slip up above Vy by a few knots.
 

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