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Sikorsky S-76: Ask Nick Lappos

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Sikorsky S-76: Ask Nick Lappos

Old 18th Jun 2001, 10:01
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Heliport
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Post Sikorsky S-76: Ask Nick Lappos

Sikorsky Press Release
In a move that underscores Sikorsky's commitment to the civil marketplace, go-ahead has been has been announced on future product improvements for the S-76.

"The S-76 has represented Sikorsky's entry in the civil marketplace for the last 20 years and has maintained its reputation as the best helicopter in its class through constant improvement," said Tommy Thomason, Sikorsky Vice President of Civil Programs. "In parallel with S-92 certification, Sikorsky is committed to maintaining the technical and operational leadership of the S-76 by bringing in practical and proven technology and innovative support concepts to be fielded with the S-92.

"The strengths of the S-76 have been its unparalleled safety record, balanced performance, reliability, low operating cost and large cabin that excels for so many applications. Our plan is to insert the latest technology available from our other product lines in a way that enhances these strengths and maintains the S-76 as the customers' clear choice for its size class," Thomason said.

Paul Martin, Sikorsky's Vice President of Engineering, pointed to the technologies involved. "With the technology being incorporated on the S-92 and COMANCHE, and on new international offerings of the BLACK HAWK and its derivatives, Sikorsky has demonstrated a portfolio of improvements that can be applied to the S-76. Those chosen are the ones that bring the most value to our customers," he said.

The improvements taking place were selected by analyzing potential changes in the context of how they would most cost effectively satisfy customer requested features. Among the candidate improvements, the following were considered the most attractive and form the basis for the planned program:

Uprated Engine - Certification of the Turbomeca Arriel 2S2 with approximately 6% more power than the current 2S1. Thomason noted that the 2S2's increased power will be used not only for performance enhancement, but to assure the engine remains in service until the TBO limit is reached: "We are working closely with Turbomeca to see that the 2S2 is designed and demonstrated to achieve maximum durability in the S-76," Martin said.
Integrated Cockpit -- The new cockpit will have flat panel liquid crystal displays, digital data bus architecture and rack adaptability to incorporation of optional equipment. The BFG HUMS will be an integral feature of this new design and the Sikorsky Integrated Customer Support System. Differential GPS navigation with automatic approach to hover will be an option.
Quiet Tail Rotor -- This design incorporates an advanced airfoil design with swept tip and flush fastener attachments. It has been tested and flown to demonstrate its performance and acoustic characteristics. The advanced airfoil provides improved aerodynamic efficiency permitting the rotational speed to be slowed to further enhance its acoustics. This feature will be retrofitable to S-76 aircraft currently in service.
Improved Comfort Interior -- Improved cabin comfort is to be achieved by a to-be-selected combination of passive noise suppression, Active Vibration Control (AVC), Active Noise Control (ANC) and a low noise main transmission. AVC has been developed and successfully tested on the S-92 and is directly applicable to the S-76. It consists of vibration sensors, a computer, power amplifier, and shaker assemblies. The basic approach is to create an out-of-phase vibration that cancels the vibration caused by the helicopter dynamic systems. A production design Active Noise Control system will be flown this year for evaluation. The improved low noise transmission would employ advanced COMANCHE gear designs that have proven their noise reduction capability.

The selected product improvements on the S-76 are tentatively planned for certification and introduction in 2004. Trade studies continue for definition of an affordable and retrofittable de-ice system based on the S-92 design.

More than 500 S-76 helicopters have been produced and are in operation worldwide with customers in more than 40 countries.
 
Old 19th Jun 2001, 10:47
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"The S-76 has represented Sikorsky’s entry in the civil marketplace for the last 20 years and has maintained its reputation as the best helicopter in its class .........."


Is it still "the best helicopter in its class"?
If so, what makes it the best?
If not, what is?

What other changes would you like to have seen introduced?
 
Old 19th Jun 2001, 11:48
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My favorite mods are the HUMS and the increased engine power. The one thing I really don't like about the S76 is it's low altitude hover out of ground effect (1800 ft at gross weight, ISA). The S-76 seems to operate at near gross all the time, so this has restricted the helicopter to low altitude operations. Hopefully the Arriel 2S2s will help this.

------------------
Safe flying to you...
 
Old 13th Jul 2001, 09:38
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IHL
 
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Question Another Question for Nick Lappos

Nick you almost need your own forum, here is my question.

Are heated blades an option on the SK76 C+
and if so is Sikorsky doing any engineering into developing a main rotor anti-ice system for older model 76 aircraft.

Thanks IHL.
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Old 15th Jul 2001, 11:39
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collective bias
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Question S76C (yep! more for Nick)

I wonder if Nick Lappos would answer a few more queries.....
Nick, it has been a while since I flew the C model. I wonder if you could quickly run through the OEI Bias system and the what is shown on the IIDS regarding hashed and un-hashed boxes and what it all means.
Also, define transient and static droop as it applies to the 76.
Finally, A few years ago I read an article which had a 76 on fixed skids and some other mod's courtesy of the russians. Has this progressed at all?
Greatly appreciate your time and input to these threads Nick, and if anyone else has time to reply
 
Old 17th Jul 2001, 15:45
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as i understand it, when you are oei, the N1 box will be hashed, if it is not hashed then you must check what N1 you can pull via the placard.
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Old 29th Jul 2001, 17:58
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IHL asked:
Are heated blades an option on the SK76 C+ and if so is Sikorsky doing any engineering into developing a main rotor anti-ice system for older model 76 aircraft.

Nick answered:
Sorry for the long delayed response, I just stumbled acoss your request!
There are plans to de-ice the blades on the S-76 family in the next few years, but I am not directly connected to those who are planning it, so I have no definite timetable. The baldes are all the same, so it should be easy enough to make it universal to all types of S-76, although the details of markets and approval agencies may change that logic.
 
Old 29th Jul 2001, 18:16
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Collective Bias asked:
I wonder if you could quickly run through the OEI Bias system and the what is shown on the IIDS regarding hashed and un-hashed boxes and what it all means.
Also, define transient and static droop as it applies to the 76.
Finally, A few years ago I read an article which had a 76 on fixed skids and some other mod's courtesy of the russians. Has this progressed at all?

Nick answered:
The OEI bias on the C lets the pilot need to recall the N1 limit as a constant, regardless of the fact that the N1 limit varies with ambient conditions. On other Turbomeca Ariel equipped helos, the pilot must look up the limit on a chart while flying out after an engine failure (yea, right!) We Sikorsky pilots asked for a simple calculator to fudge the indicator to always read a constant number, so only one number had to be remembered by the pilot, and no chart had to be consulted.

If the bias computer fails, the indicator tells you that the chart must be consulted. I cannot recall which way is hatched.

For all engines, the reduction in Nr/N2/Np with power application is called droop. If the engine takes a bit of time to come to power, and the Nr droops but then recovers, we call that transient droop. For many engines with hydro-mechanical or hydro-pneumatic controls (not FADEC's), the long term Nr may not be a constant, as the combination of N2 setting thru the beeper, the collective bias and the ambient conditions might all work out to a number somewhat off from the nominal setting. Most engines must reduce rpm as power is applied for engine control stability, so the natural engine speed trend with increased power is a down droop, usually about 1/2 to 1% for each 10% power. This is cancelled by the collective bias, or by a computer governor, in most cases. The permanent droop with power is called static droop. FADECs make all this go away, as they allow very sophisticated control techniques to keep the output rpm very constant.

S-76 fixed skids were considered a few years back, but I don't think it went too far, as the market seemed to like where we were with retractable. The weight of the retract mechanism is about 90 pounds, and the fuel saved by the drag reduction actually saves about 100 pounds per full fuel tank!

[ 29 July 2001: Message edited by: Nick Lappos ]
 
Old 31st Jul 2001, 14:10
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Cool S76 T/R Jammed Pedals/Cable Failures

This might be a good topic for Nick.

I was at FSI a couple of months ago and for the first time since I started flying the 76, I was shown a very simple method that can be applied to either a "Rotten Right" or a "Lucky Left" pedal jam/cable failure.

Apparently this procedure is based on Sikorsky Flight test data from the original 76 program, but for some strange reason, it was never incorporated in the RFM.

Basically all you have to do is once you level the A/C and assess the condition, reduce A/S to 80 KIAS and set your Q to 40%. This will provide a wings level, ball centered condition. When setting up for your approach (run on landing) set 60 KIAS and 30% Q, this too provides a ball centered, stabilized descent for approach at about 350 fpm down. After crossing the threshold, set 40 KIAS and 20% Q (results in about 100-150 fpm) ball centered and stable.

If rotten right was the culprit, brief the PNF, on your command after touch down to slowly retard both throttles to maintain center line. Lucky left, I found you could slow to about 30 KIAS and run on NO PROBLEM, however you can still retard the throttles to keep on C/L.

My question to Nick is, why do we have to get this procedure from FSI and why isn't it incorporated in the RFM? Surely that is better than having a different procedure for every condition.

Hope to get some input from those who are interested.

Cheers OffshoreIgor
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Old 31st Jul 2001, 18:00
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Cool S76 Servo positions....

On the third page of the 'Ultimate Arbitration' thread, Nick mentioned the position of the servos on the 76 were incorrect.
I have had engineers mention this before. As I understand the casting positions on the GB are wrong and some poor bloke had the unenviable job of figuring the linkage assembles in the mixing section to get the Gamma right. Correct?
One good thing about the arbitration thread is all the new terms. ie. gamma.
Never heard that before....
PS. I promise to listen and learn.
 
Old 31st Jul 2001, 18:10
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collective bias
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Nick,
Message read and understood. I have trouble remembering the Hashed box indication also but I think (?) hashed is biased. At any rate a refresh on the manuals will reinstall that info. Thanks again
 
Old 31st Jul 2001, 18:14
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To: collective bias

What you say may very well be true. When I posed the question to Nick regarding the positions of the servos I was asking if they were in the same place as the older Sikorsky models, (fore and aft servo at 45-degrees ahead of the longitudinal axis and the lateral servos disposed 90-degrees to either side of the fore and aft servo). He said I was wrong but he didn’t say why. With that answer I contacted the Sikorsky Service Department. I asked them about both the S-76 and the Blackhawk relative to the position of the servos. I was told that the Blackhawk servos do not connect to the swashplate but are mounted in the control run for ballistic tolerance. With that knowledge I amended my initial info request and asked about the positioning of the control linkage on the stationary swash plate asking if they were the same as the older models. I am awaiting their response.

If you remember in one of Nicks' posts he indicated that he did a lot of test flying where they were experimenting with different bellcranks in the mixing unit to get Gamma right. This adds credence to what you heard.

I also asked if when the cyclic was moved forward if only the fore and aft servo moved or, if several servos moved resulting in a vector to compensate for shifting of Gamma.

[ 31 July 2001: Message edited by: Lu Zuckerman ]
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Old 31st Jul 2001, 18:32
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Morning Lu, Have a great day... I am off to bed. Currently 1am,
regards
 
Old 1st Aug 2001, 00:51
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I'm not going to try to catch up with the Ultimate....I looked at page three and just knew most of it went straight past me, particularly the bits where one guy slags off another (doesn't that just get SOOO tedious??.)
However (comma), when I did the S76A course with the good people of American Airlines in Florida in 1981, the instructor's explanation of the slightly odd servo positions was that Igor's norm of 45deg offset would have produced bulges in the fairings.....since first & foremost this was to be a sexy looking machine the decision was to twiddle the jacks around a tad....result - smoooooth fairing and that nightmare of a mixing unit just to keep your wrist movement parallel with the required A/C direction.
Lacking both the intellect and the desire to prove myself superior to anyone, I humbly pass on this snippet, which even if incorrect, is hopefully less 'in your face ' than some of the aggro we've seen....(on the other hand Igor's 76 has paid a great share of my mortgage over the last 20 years, so for that, thanks.... :o
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Old 1st Aug 2001, 02:02
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Based on what I have heard instructors say in both factory and military schools I would say your instructor didn’t know of the engineering screw up mentioned above or he was repeating what he heard when he went through the school.

Here are a few examples.

An instructor was covering the rotor system on the Sikorsky H-37. A student asked what would happen if a blade broke off. These blades weighed around 260 pounds and the centrifugal loading was around 72,000 pounds. The instructor responded that the blades being able to lead and lag would reposition themselves to close up the gap left by the departing blade.

A Bell instructor was conducting an orientation for some Iranian mechanics and he told them that most of their helicopters were Agusta Bell and that they were constructed in Augusta, Georgia.

Having been a technical instructor my best advice is to challenge your instructors and make them dig for the answers. They are not always right and as a result the students don’t always get it right and cause a lot of damage.

I was on a tour of Army bases checking on the students that had passed through my classes at Fort Eustis, Virginia. At Fort Knox I visited an H-34 group and while there I passed through the hanger. A mechanic was getting ready to rig an H-34 and he was connecting his hydraulic power pack to the aux servo system. I cautioned him that it could cause problems and he should connect to the primary system. About that time his sergeant came out and jumped me for telling his mechanic what to do. I told him it would cause problems and by that time a major came out asking what was going on. Both the sergeant and I explained our positions and the Major asked the sergeant if he was sure. He said yes and the mechanic turned on the power. There was lot of screeching and cracking. When the aux servo centered itself it jammed the rigging pins in the servo support casting. The pins were supposed to be Monel but the sergeant made them out of steel so they had to be cut out. I told the Major that he had just bought a Cadillac and I walked away. I don’t know what happened to the sergeant
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Old 1st Aug 2001, 07:04
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Always good posts,

I would suspect that the settings you describe probably work for many stuck conditions but not all.

And I would worry about the torque sharing system on final. If you slipped a throttle or power lever slighly off from the other torque compensating might make the last bit of your ride pretty interesting.

I'm going to try the idea when next at FSI, thanks for the thread.

At a hover with stuck left have you tried reducingthe rpm?

The ship will initially spin faster left but with the reduction of the lift equation over the t/r blades it then slows down quite a bit. Have done it to a stop before.

fly safe
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Old 1st Aug 2001, 09:14
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I've done much sim work and instruction on Sea Kings around the world and have found that with any stuck position for the tail rotor setting min power speed (70 kts ish) and putting the ball in the middle by raising or lowering the collective will give you a stable wings level and no yaw situation BUT with either climb or descent! In a climb the secret is to use airspeed to change the power requirement in your favour without changing the collective which is balancing tail thrust.

If the a/c is climbing try reducing air speed and/or rotor speed to get a comfortable ROD and run on speed and set that on finals for a constant speed landing. Further reduction of rotor speed may be needed to settle through the ground cushion and to keep straight once on the ground but as run on should be in the region of 20kts or so should not be a problem.

The descent situation is a bit more of a challenge. Min power speed will give you lowest ROD but high ground speed, there may be 10 kts or so that can be reduced without the ROD increasing too much and this may be sufficient if you've got a runway or big field. A slight increase in collective will also improve things but will cause a yaw, counter this with a little wing low aircraft maintains heading but is sideslipping, if a cross wind runway is available a cross wind landing may be tried using the wind effect to counter the sideslip.

These procedures work in the sim across a wide range of stuck yaw settings and give options to try in varying amounts. Always try the approach profile at height giving room for adjustment/recovery if you start to lose it.

The above is the condensed version of an article I've written for military flight magazines after some useful info from pruners amongst others, any one interested in a copy give me an e-mail address at [[email protected]]
& I'll send one


[ 01 August 2001: Message edited by: Harry Peacock ]
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Old 1st Aug 2001, 09:22
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This snippet ( thanks for the word coriolis)
is a little off topic but here goes.

The S-76 Flight Safety training manuals show a check valve in the pressure line from the hydraulic pump/s. If there is such a check valve, then why do the main rotor blades spin backwards, if external hydraulic power is applied with-out the rotor brake on?
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Old 1st Aug 2001, 14:03
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collective bias
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mmm, leaking check valve?
Must be driving back through the pumps into the GB. Why??? dunno without schematics to study but I bet it does the pump no good. Perhaps the return system is responsible.
 
Old 2nd Aug 2001, 01:10
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Guys,
This is a great thread, with many good thoughts. We don't discuss stuck pedal in the flight manual because it is really pretty rare (can't recall any in any S-76 in 4 million hours), but it makes good training talk because it exercises the brain about anti-torque and stuff.

I must say that practicing it in a Bell is easy because the Bell has no collective-to-yaw mixing, so if the pilot freezes his feet, it flys like a stuck tail rotor system. In a Sikorsky (Sea King and S-76) the collective will automatically retrim the tail rotor with the pedal stuck, so the pilot cannot simulate a stuck tail rotor control system in flight. If you freeze the pedals and fly around working the collective, the aircraft flys very well, because the mechanical mixing is nearly spot on.

Generally, stuck controls are bad things, and a chance to either become a hero or a goat. I have had several, but not in yaw. I believe that they will come at a bad time, and that the pilot must figure out what he must do to get down. Left pedal is generally good, because if slowed down properly, the aircraft can be brought to slower and slower speeds, but care must be taken because if the left pedal setting is near the limit, a reduced collective deceleration might turn into a nightmare as the left yaw goes out farther, and the aircraft could depart and spin.

The closer to full right pedal the failure is, the more the answer looks like an autorotation.

With either, the pilot can practice trimming in a steady heading sideslip, which is more difficult than it seems when the trim ball is somewhere in Kansas. The big problem in flying at such large tail force conditions is that the airspeed system may not indicate at those angles, and if the speed truly drops, the aircraft will spin. Rule of thumb is Nose Left, raise collective, Nose Right drop it.
 

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