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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. |
"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? |
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... |
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. |
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 :D |
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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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. |
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 ] |
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 :eek: OffshoreIgor :eek: |
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. :D |
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 |
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 ] |
Morning Lu, Have a great day... I am off to bed. Currently 1am,
regards :cool: |
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 |
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 |
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 |
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. :confused: 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 :D :D [ 01 August 2001: Message edited by: Harry Peacock ] |
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? :confused: |
mmm, leaking check valve? :D
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. :confused: |
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. |
Nick,
Please may I emphasise that the good advice given in the main thrust of your last post applies ONLY to helicopters with the main rotors turning in the "American" direction i.e. from right to left as the pilot looks out the front. It is vital for pilots to remember that for aircraft with the "French" main rotor direction i.e. left to right from the pilot's perspective, the OPPOSITE is true with regard to the "lucky" side. "Lucky left / Rotten right only applies to "American" rotor direction of rotation. I speak as someone who has had spells alternately flying Sikorsky then Aerospatiale / Eurocopter designs for some years (almost every other job!). For a while I was heavily involved in the RAF Puma Sim project and flew it from the front as well as instructing from the rear of the box, so I have seen how confused some pilots can get during tail rotor malfunctions. I have also since carried out the FSI S-76 sim initial and recurrent courses and similarly the US Army UH-60 Blackhawk sim courses. From my own experiences, and I've said this before on another recent thread on this forum about tail rotor malfunctions, but I do think it is worth repeating, - the "lucky" side appropriate to any helicopter is the RETREATING BLADE side. All the pilot then has to remember is which side is the retreating blade side - and hopefully he should remember that from the rotor start! The "lucky" side is the safe side to keep the nose cocked off towards during flight towards a suitable landing area and on the approach. Also it helps to keep any crosswind coming from that same retreating blade side during final approach with a tail rotor control malfunction, if there is a choice of landing direction. Increasing power by raising the collective then brings the nose around to point straight ahead for a landing, by increasing the main rotor torque reaction. Once the nose comes around towards the straight ahead position you have found the power/airspeed combination at which the aircraft should be best to land. If this lies within the normal landing parameters for the aircraft, then the pilot will have an excellent chance of landing without breaking anything. Two more points: I strongly advocate any pilot suffering a tail rotor malfunction to carry out a "dummy approach" at height if circumstances allow and personally I am not a great fan of messing around with engine controls to control yaw, as this can complicate the issue (I've seen quite a few pilots get it horribly wrong in the sim)although I would agree that it may be the good thing to try if you are on the ground and about to yaw off the edge of the landing area. Cheers, keep up the good work. Edited for typos; it's well past my bedtime. ShyT [ 02 August 2001: Message edited by: ShyTorque ] |
The RAAF had a pretty good way of dealing with stuck pedals that was developed for the Iroquois and adapted for the Squirrel when we got them. The Squirrels had been modded with a twist-grip throttle (not all that satisfactory in some ways) so the technique was easily transferable.
Your point about the possibility of stuffing things up when engine controls are manipulated is a good one, Shy Torque, but with the luxury of practice most people found they could cope with pedals stuck in a wide range of positions. The basic concept was a 60 kt skid ball analysis for a heads-up, followed by a long, low finals decelerating in 10 kt increments until the aircraft nose was pointing a certain limiting number of degrees off the direction of travel. Then, depending on which way the yaw was, you would (for an American helicopter): a) For too much left pedal, keep going to a rotating hover, then reduce Nr in small increments which would initially speed up the rotation, but then slow it down as the tail rotor slowed until you could land, or b) For too much right pedal, slow down until the nose got to be 20 degrees (I think it was) off the direction of travel, then maintain that speed until you were over your big flat grassy landing area, then quickly chop the throttle and run on to an autorotative landing. With a bit of forethought, you could also steer a bit with judicious (!) amounts of throttle as you ran along, although this was a bit of a mind-blower for some. Widens your options a bit, I guess, although to coordinate 2 pilots to do it to cope with lever-type throttles would be pretty tricky, I'd imagine. |
Arm,
You are right about the difficulty of co-ordinating two pilots during the mainipulation of engine controls; we also used to operate the Puma single pilot which meant the pilot didn't have enough hands, with the ECLs being in the roof panel! On many larger aircraft in the civilian role it just isn't practical to go messing about with ECLs for practice. On many it isn't allowed, even with an instructor on board, so it can only be practiced in a simulator which means it doesn't get done on a regular enough basis for crews to be really good at it. Some will no doubt argue this one, but I maintain that if it happens for real then there will be some element of the experimental for most line crews. Personally I would prefer an area of hard surface because if a run-on landing is required and the aircraft does touch down with some yaw it is is a little more forgiving i.e. there is less chance of a wheel or skid digging in and the aircraft rolling over. Of course in UK our grassy areas are usually very soft and muddy which makes our problem worse! It is a really good idea to keep on discussing tail rotor problems. In the past they were seldom covered in sufficient detail (well done, Sikorsky and FSI for having more recently put some detailed thought into it and come up with useful advice). Those who have never seen one for real or in a simulator (like myself a few years back) need to have the basic principles in the back of their mind. Also, on a selfish basis I like to keep my own dull brain refreshed because I see it as a very cheap life insurance policy. Two engines is great but there is still only one tail rotor drive shaft and control system! ShyT |
The S-76 servos are not in the "correct" place for simple unmixed controls to be used. We have a mixer on the top deck to take roll control (for example) and split it out to the two servos that must tilt the swashplate properly to affect a pure roll.
The servos would have been in the perfect place, but we decided to put in 17 degrees of delta 3 in the rotorhead after we had designed the main gearbox casting. To redesign the casting would have delayed the program (the boxes were already being poured) so we just fixed the angle with the mixer on the upper deck. It is interesting to hear all the reasons given here, from all the sources. This explains why Lu is so often misdirected. For many esoteric pieces of aerodynamic and programatic trivia, the answers are not found by calling someone at the "factory" or elsewhere! |
Thanks Nick.
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I also remember the article about the american 76 driver who after having landed safely had difficulty steering through biased braking on the pedals.
He complained that the FM directions were irrelevant to the actual event. I think (and I would've) he closed the throttles to bring the aircraft to a halt. |
Zimmerframe and Collective Bias,
I recall one yaw control problem like this in the long ago past on S-76's (maybe 1980), but it was an oddball, not a typical stuck control/cable/pedal type situation. The tail servo wore a notch in itself after a long time, and the servo just sort of locked itself in the cruise tail pitch position. The pilot (Gulf of Mexico) and I spoke the afternoon of the occurrance. He did a great job of flying home and made a very high speed running landing (about 80 knots as I recall. It might have been possible for him to slow down to the other side of the power required curve and get to 40 knots or so if he had done a near autorotation, but never argue with success! When he bottomed collective on landing, the turn was a mighty one, and he had to chop throttles and stand on one brake to stop the machine. We modified the servo to prevent any recurrence. Some observations on all this: 1) He really never figured out that it was a yaw problem, because with the yaw/collective mixing as he pressed hard on the pedals, he stopped the collective from going full down. He wasn't sure, but thought he had a stuck collective! 2) When he made his approach, he did what good pilots always do, he sized up what he could do, and made it all come together near the ground. He touched down with no yaw, but at high speed. 3) When he and I spoke, he was not fully convinced that it was a tail servo problem, but they checked the servo and confirmed the guess. 4) The next morning, he told me that his leg really hurt from all the pushing he was doing all the way home, done inadvertantly and instinctively, based on thousands of hours of automatic pedal motions! I know of no other stuck control problems on the 76 family. Does this one sound like the one you were referring to? A comment on simulator training - most training sims are not to be believed if you leave the normal flight envelope. Several sims I have flown are quite benign in emergencies that might very well be eye watering. They are excellent procedural guides, but not tools to polish technique or study the effects of large, improbable failures. |
Great replies!
Before we digress into an unrelated topic I should mention now that I left out the troubleshooting portion because I was trying to get a feel for what other operators do in their procedures. I think I should add a little background. WRT the mixing unit in the 76 as it pertains to Collective to Yaw coupling, we all know that in a dead foot or jammed pedal situation, if you were to apply full Right pedal input, the collective would rise, conversly if you were to apply full left pedal, the collective would drop, (maybe). This known reaction can be applied in the troubleshooting phase as pertains to determining whether you have a jam or a control cable failure. Remember, there are no 76's left without a self-centering device! Now wrt the comment about manipulating the throttles, first I want to reiterate that we tried every combination of jam/cable/mixing unit failure and the results were the same, without exception. In a well coordinated crew, the most important factor is to pre-brief the actions to be taken in handling an emergency. I wasn't suggesting that the PNF simply retard the throttles, but that the PF call for "reduce throttles gently Now" followed by "STOP" as the situation demands. The 76 Crew that went off the runway actually helped in the post accident analysis that developed this procedure. Keep the posts coming, it's always great to read your ideas and thoughts. Cheers :eek: OffshoreIgor :eek: |
Good thread !
Three comments; 1. Depending on landing site, area, gross weight, DA etc I might consider leaving one engine at idle to lock torque compensating out. (right stuck,US type) 2. I had the good fortune to train with an ex vietnam, ex air america pilot (the real company)in the early 80s on stuck pedals. We did hours of them to paved runway and sod. While holding the collective (twist throttle) if you point your index finger down and keep it there it gives you an easy visual reference for which way the nose will turn as you monkey with the throttle.(US type) 3. Landing downwind is not good. At all. Fly safe |
Nick Lappos, I seem to recall that Scotia had an incident in the recent past regarding a tail rotor control failure in a 76C. Can't remember the exact details, I will have to look them up, but the aircraft landed safely at Lossiemouth without incident.
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Zimmer,
What Nick refers to is known as "off model" responses. This is what happens if the simulator programme does not include sufficient (or incorrect) data to accurately represent what happens for real and it is a well-known issue. A simulator is, after all, a big boys sophisticated computer game built for a particular purpose. The sim "aircraft" does not actually have the properties of a real aircraft unless those properties are accurately included in the programming. I can assure you that the simulator company does not go out in a real aircraft and fail the tail rotor in order to gain data (obvious really, innit). Therefore, what is programmed in is a representation of what is assumed to happen. Yaw rates and other effects following on from those rates may have to be mathematically derived and are a "best guess". A few years ago, I worked alongside a simulator software guru who worked on site at a simulator we were using, as a major customer. He had written the programme into the simulator software. We had some particular problems because it was a new setup and we were using the sim for critical failures that no one had tried much before. The simulator response was definitely different to what the real aircraft did. On a number of occasions I explained the differences and stood next to him as he plugged in his keyboard, typed in raw machine code to change the programme and he had modified the response of the aircraft just like that. Just bear in mind that a simulator is great for procedures and working out techniques but it may not always be completely representative of the actual aircraft in its response. For example, the helicopter simulator I instructed on would fly a full fixed-wing aerobatics sequence, including inverted flight and slow and four-point rolls! I used to demo it for fun and a few pilots came out both shaken and stirred. I always warned everyone that under NO circumstances should the manoeuvres be tried in the real aircraft because they probably wouldn't have got through the first manoeuvre in one piece. However, simulators are still the best medium we have for training for unusual emergencies but these possible limitations of the system must always be borne in mind. A "flying by numbers" approach to any particular scenario in a particular simulator may or may not be 100% valid. I have seen that using two different manufacturer's simulators of the same aircraft type may give slightly different ideas of how best to cope with any one scenario in detail! ShyT |
Pitchlink says:
Nick Lappos, I seem to recall that Scotia had an incident in the recent past regarding a tail rotor control failure in a 76C. Can't remember the exact details, I will have to look them up, but the aircraft landed safely at Lossiemouth without incident. Nick sez: I am not sure, but I think that was a pedal trim actuator/damper failure, which gave problems to the crew (did it tend to run away in one direction?) The crew handled it well, and all was fine, I believe. It did not require the techniques we are discussing here, I think, but I will defer to someone who is more familiar with the event. Regarding Zimmerframe's point about simulators, I fully agree with you in their value. My only point is that sims are often only half right, and not particularly representative when improbable failures are being practiced. I do agree that as long as the training is reasonable preparation, they are quite valuable anyway. Regarding the necessity for two crewmen in a given aircraft, the need to manipulate throttles in a very remotely probable event does not justify the second person, in my opinion. If this were true, we would be spending thousands of dollars a year "just in case" when that cash might be well spent preventing more common occurrences. If one looks at helicopter accident statistics, you can see what drives our game, and it is an eye opener. The mishaps are almost always operational in nature (about 2/3 to 3/4). By operational, I mean what are sometimes called "pilot error" (a term I use with great care) accidents. Without extending the thread way off base, I feel that much of what happens to cause accidents is related to the whole operational scenario, from training and equippage, to weather, procedures, displays and the like, usually (and perhaps improperly) all lumped into "pilot error". I think we all need to work on fixing what actually happens to cause mishaps, and not sweat to much what might happen in the "extremely remote" probabilities, such as tail rotor failures. I was the program manager on the joint Honeywell/Sikorsky/FAA program which certified the EGPWS system that is now available on the S-76. This might help reduce CFIT accidents to a memory, and this would cut about 30% of all helo accidents. Here is a web site that has a great PDF file report on helo accidents offshore, based on 1400 aircraft, 1.4 million hours and 2 years. I believe the data is quite representative of helo operations anywhere, even military: http://www.ogp.org.uk/pubs/300.pdf Great stuff, and fodder for a bunch of comments from PPRUNERs. |
Great thread.
Nick makes some great points. I fully agree with his comments about simulators. They are fantastic for procedural training but, like all things, rubbish in equals rubbish out. In other words, the representative ability of the sim is dependant upon the quality of programming. My understanding (Nick will be able to help here) is that an actual aircraft is extensively fitted out with accelerometers, control position indicators, and other data sensors and taken through its paces. The data thus collected is used for the sim program. Accordingly, the quality (like everything else) is dependent upon the budget of the data-gathering phase. Unusual flight conditions are programmed by mathematical extension of collected data and supposition. Therefore, you cannot really "prove" specific reactions to emergencies in the simulator, but the simulation will provide preparation. Nick said something in his article on testing the S92 that is applicable: when faced with actual observed data disagreeing with computed prediction of behavior, the observed data always wins. Also, the lack of vestibular stimulation generated in simulators make them ideal for IFR/Night, Airline type flying (gentle attitude changes and turn rates) and less applicable for low flying, turn and burn stuff, such as autos, tail problems etc, where the pilot includes vetibular clues in his/her effort to control the aircraft. In response to Arm out the window's excellent post, I would like to add that the practice of bleeding off tail rotor RPM for excessive power pedal problems is, as you say, extremely effective for the UH-1H. However, I have found it to be far less effective on types that actually have a tail rotor that works! For example, in the UH 60 you have to bleed main rotor right off before you see a good reaction (to the point that generators may trip off line). In the B212 (and I presume the B205A1 and maybe the B412), the tail rotor is so effective (at low DAs)that with a decent left pedal stuck forward case, you are unlikely to bleed enough RPM off to help your tail problem before you lose so much main rotor RPM that you risk losing control. This is more so in the IFR B212 that has a form of collective to yaw coupling non-existent in the VFR version. Forgive me being slightly off the thread topic of the S76. [ 05 August 2001: Message edited by: helmet fire ] |
Back in the very early 80's when I was with Okanagan, we had a fatal S76 accident off Thailand (IIRC), which led to the development and installation of the mousetrap to centre the t/r following a cable failure. I'm well out of currency on type, is this mod. still around, and what (if any) effect would it have on the circumstances under discussion?
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John Eacott:The "bear trap "or tail rotor centreing spring, sets a negative 2 degree setting for the tail-rotor blades in the event of a double cable failure, allowing balanced flight at 135 and 40 knots . So one could do a run on landing at 40 with no or few problems.
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Concur Nick and Helmets comments on the fidelity of simulators V aircraft and the some times very poor first simulations of some types. In my limited experience caused by commercial pressures of one manufacturer not wanting to give accurate flight data to another. I know of test pilots going straight from the aircraft to the sim to 'fine tune' the sim responses.
BUT having used them for years and instructed in them for too many they do have a very valid use particularly with regards to the big mind bu$$£*!^$ faults such as Tail rotor snags. If you don't try it in the sim and experience 'best guess' symptoms your first try will be when it happens for real :( Helmet.. Having spoken to pilots, read reports on tail rotor failures and experienced loss of t/r effectiveness in a Gazelle (Fenestron stall......Oh hush my mouth!!) the rates of yaw experienced (150deg/sec+) would not be duplicated by any serviceable sim. More to the point would not be properly processed by the human vestibular apparatus except as a drastic disorientation where the visual apparatus would be required to try to make sense of the world. And another comment.... To my understanding the drooping of rotor speed is going to have very little effect on tail rotor thrust if stuck BUT (Again) reducing main rotor speed will reduce total rotor thrust! to maintain the flight path the collective will need to be raised to maintain lift. If collective is increased rotor drag increases and requires more torque to maintain the same flight path thus 'using up' some of the excessive tail rotor thrust. :confused: Right that's it for the mo. Time to give some poor stude a beasting in the Sim!! :D :D (Most of my victims have actually appreciated a couple of hours playing with different Tail Rotor faults.....or so they said at the time!!) ;) |
Main rotor rpm nudges can be effective at the tail end of a procedure, if the aircraft is almost lines up and just a tweek is needed to get the machine aligned with its direction of travel (a very good idea, skids or wheels, since a roll over can be noisy, expensive and painful).
(All directions are consistent with American and British rotor direction, front blades going to the left) The rpm is a double whammy, increasing the rpm will act like a boost of left pedal and decreasing the rpm will act like right pedal. Here is why. If the main rotor thrust is held constant as rpm is changed (for example, collective is lowered slightly with up rpm), then a raise in rpm will lower the main rotor torque exactly as the percent rpm increase. This is because the power needed by the rotor is almost constant, and the power is the torque times the rpm. More rpm means that less torque is needed for constant power. An rpm increase makes the current tail rotor pitch setting (which is stuck) produce somewhat more anti-torque, which produces a left nose movement. Also, the rpm increase will further increase the tail rotor thrust for even more left pedal equivilent. If the main rotor rpm is reduced, the main torque will increase, requiring more anti-torque, and absorbing some of the anti-torque the fixed-pitch tail rotor is putting out. This will cause a right nose swing. As above, the rpm change changes the tail thrust, reducing it in this case and making the right swing even more pronounced. The tail rotor thrust changes by the square of the rpm change, and the main torque changes linearly. |
As I understand it increasing rotor speed to increase tail rotor thrust for a low power problem (Low power balances stuck tail) will only have a limited benefit. Too much increase in rotor speed will exponentially increase rotor drag for the lift gained requiring more torque to keep rotor speed....but you are trying to keep torque down so a small + but too much and you'll be into the -'s.
Using a little more power and opposite cyclic to counter the yaw and using a cross wind to counter the resultant drift may get a reasonable run on speed and ROD if not I'd go for an engine off (My auto's are probably survivable) :) ps. The Sea King has had several snapped and jammed cables as well as a drive failure or two |
I think another way to say what Nick stated is so;
, Imagine the aircraft at a five foot hover with a stuck left pedal. While maintaining that height above the ground the mainrotor is developing a value of torque. Call it 30. If you lower the rotor rpm (say 5 percent) and want to maintain that five feet you must increase the collective. So you've increased the pitch in the main rotor blades (brought the torque value back to 30)and left the tailrotor blade pitch at it's smaller setting and happily, travelling slower. The lift equation has as it's largest factor airspeed. In fact it's airspeed squared. Same output from mainrotor, Less output from tailrotor. :) |
Tgrendl,
I like your way of illustrating the issue, so let me try again: If while hovering at 100% rpm at torque of 80%, your tail rotor becomes stuck at a fixed pitch setting. If you creep the rpm down to 95%, while holding hover height, the torque will rise to about 84% (1.05 x 80), because the power is a constant, and power is torque times rpm (84 torque times 95 rpm = 80 torque times 100 rpm). Because the torque is now higher (and the tail rotor has less rpm for the stuck pitch position), the tail rotor thrust is insufficient, so the aircraft will rotate to the right. In the same circumstance, if the pilot sneaks the rpm up to 105% and holds altitude with reduced collective pitch, the torque will go down to about 75%, and the tail rotor will now be producing excess anti-torque, so the aircraft will rotate to the left. In other words, the rpm can be used as a yaw control if well handled! The effects are used in the Huey stuck pedal procedure we have all practiced at one time or another, mostly because the bicycle chain tail rotor control on a Huey was more prone to failure at one time. I do not believe this is true any longer, but I defer to those who have more knowledge. One comment on all this stuff - we tend to focus on the emergencies that we can practice, so I have seen people do 5 of these stuck pedal procedures in one flight, and 5 or ten engine failures for the remainder. If one studies accident statistics, CFIT and the like are more likely to bite us. Why don't we practice not hitting the ground! |
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