Sikorsky S-76: Ask Nick Lappos

[Flight Safety]

Just fyi...from the NOAA website, their definition of wind chill...

1. What is Wind Chill Temperature?

The wind chill temperature is the temperature that it feels like outside to people and animals. Wind chill is based on the rate of heat loss from exposed skin caused by combined effects of wind and cold. As the wind increases, heat is carried away from the body at an accelerated rate, driving down both the skin temperature and eventually the internal body temperature. Therefore, the wind makes it feel much colder. If the temperature is 0 degrees Fahrenheit and the wind is blowing at 15 mph, the wind chill is -19 degrees Fahrenheit. At this wind chill temperature, exposed skin can freeze in 30 minutes.

Estimated time to frostbite from exposure due to unprotected skin, seems to be the main reason for the existence of the wind chill factor.

[Steve76]

Igor!. .Londons warmer than Moosenee!. .Great to be here. Thanks for your help.. .I didn't know that the 212 has heated fuel, so that explains that.. .Could you send me another email. I have misplaced your address.. [email protected]. .Met any aussies I know lately...?. .

[Cyclic Hotline]

When I'm running around naked outside my sauna here, I like to quickly refer to the windchill chart to ensure I do not "overdo" it! <img src="wink.gif" border="0">

From previous experience, I would not recommend a sauna and roll in the snow if there are any female observers in the immediate vicinity - it might be accurately described as a withering experience! <img src="eek.gif" border="0">

Just to ensure that we keep up with the times, always ensure you keep up with the <a href="http://205.156.54.206/om/windchill/index.shtml" target="_blank">new improved windchill!</a> <img src="smile.gif" border="0">

[LAFalot]

S76 question for Nick Lappos

Just wondered what the reasoning was behind the 12 second spot turn limitation for the '76? The aircraft would certainly appear capable of a higher rate of turn in the hover whilst still fully controllable, not that one's tried it of course....... And just to dig a little further, should it be taken therefore that if you only turn through 180 degrees then not less than 6 seconds is required, etc,etc? . .If so, perhaps a better wording in the Flight Manual might be along the lines of 'hover turn rate not to exceed 30 degrees per second...'. .Thanks. LAFalot.

[RoamingCyclic]

The Limitation is due to stress on the tail boom.. .Yes you can assume 30 degree per second as the limitation.

[RoamingCyclic]

More Strange S76 Q's for Nick Or Anyone!

A couple of strange electrical occurrences on the A++, would appreciate any info or direction on where to start.

1. Running 100% Nr, battery off, #1 DC gen off. .When #1 DC gen turned on, power is removed from #1 DC Pri bus for about 3 seconds (#1 Fuel flow to zero). .This is not the case on #2 and I have never seen this before.. .Possible clues: - Problem with #1 generator contactor (slow switching from #2 gen to #1 gen)?. . - Problem with bus tie contactor (should stay closed for 2 seconds after no generator power - maybe it's open with no warning light?

2. AC gen fails when either engine goes below 65% N1, as per the Flight Manual. However if the snow blankets are on, the AC gen will stay on line. Question is: if the blankets are off when the engine fails, can you turn on the remaining engine snow blanket and restore the AC gen? (Haven't tried this yet)

3. #2 DC gen will intermittently not come on line until N1 is increased above 65% N1. Doesn't make any difference which engine is started first or if the other generator is switched off to "shock" #2 on. Only increasing the N1 will bring it online. When it is working normally (60% of the time), it comes on line as soon as the starter button is released at 40%N1.

4. Aircraft shut down on ramp. Turn on external power.. .When inverter is turned on the pedals move 2" (to the right) quite forcefully. Vertical gyro (ADI) is just jumping around as it gets powered up.. .AFCS is off. Subsequent attempts to duplicate produced less and less pedal movement until after 4 tries produced no pedal movement. This aircraft has a basic Phase 2 AFCS, but a very strange looking pedal damper (no electrical connections, one hydraulic supply line)

Weird ones aye? Thanks in advance!

[IHL]

2. AC gen fails when either engine goes below 65% N1, as per the Flight Manual. However if the snow blankets are on, the AC gen will stay on line. Question is: if the blankets are off when the engine fails, can you turn on the remaining engine snow blanket and restore the AC gen? (Haven't tried this yet)

Rep&gt;; I don't understand why the AC generator drops off line when either engine goes below 65% N1. It's been awhile since I've flown an A+ or ++ but on the A model the snow blankets are powered by the DC generators under normal operations and in the event of a Generator failure both snow blankets are powered By the AC Generator through the monitor bus phase A for #1 engine and phase B for the # 2 engine.

3. #2 DC gen will intermittently not come on line until N1 is increased above 65% N1. Doesn't make any difference which engine is started first or if the other generator is switched off to "shock" #2 on. Only increasing the N1 will bring it online. When it is working normally (60% of the time), it comes on line as soon as the starter button is released at 40%N1.

Rep&gt;; Sounds like the generator voltage outputs need to be " parallelled".

4. Aircraft shut down on ramp. Turn on external power.. .When inverter is turned on the pedals move 2" (to the right) quite forcefully. Vertical gyro (ADI) is just jumping around as it gets powered up.. .AFCS is off. Subsequent attempts to duplicate produced less and less pedal movement until after 4 tries produced no pedal movement.

Rep&gt;; This makes absolutely NO SENSE!. . . .This aircraft has a basic Phase 2 AFCS, but a very strange looking pedal damper (no electrical connections, one hydraulic supply line). .Weird ones aye? Thanks in advance!

Rep&gt;; The phase 2 has only 1 supply line , it is connected to the #2 hydraulic systems return line and not pressure line , the return line only provides operational fluid to the pedal damper to restrict pedal movement, it is not affected by operation of the priority valve. The phase 3 has a pedal damper/ yaw trim actuator and therefore electrical connections, it is affected by operation of the priority valve.. . . .Hope this may be of some assistance.

[ 28 February 2002: Message edited by: IHL ]</p>

[Nick Lappos]

I feel like a Doctor at a cocktail party. take two aspirin and call me in the morning!. .. .Comments in turn:. .. .1) does sound like a slow bus tie contactor.. .. .2) ouch! You have me there, I don't know. IHL has it right, the DC gens are the normal source for the blankets, AC is backup.. .. .3) This sounds like the gen controller does not recognize the voltage from the gen, so it won't let it on line sometimes. If the gen has lower voltage than the one already powering the bus, it will lose the argumant. You can get it on by turning the other off first, then the controller has nothing to compare it with. Could be controller or gen, a quick voltage check of the gen at 50%N1 will tell. Unlike other DC systems, a bus never connects to two generators at the same time.. .. .4) IHL has it exactly right about the damper, it is purely a mechanical device, getting its fluid from the hydraulics. If all SAS switches are off, the SAS shouldn't jump when power is put on, but they will if any switch has stuck in the on position (or failed that way). If so with a yaw switch on, the gyro flopping will tickle the sas servo on the yaw quadrant, and the pedals might jump a bit.

[Nick Lappos]

Have to disagree with Roamingcyclic.. .. .The limits were set by the turns we did for original certification structural substantiation. We took data while turning, and labled the data "12 second turn". I did most of that flying, along with Dave Wright, the current S-76 Chief Pilot. That set the limit. We have since turned at blithering rates without concern, but never submitted the data to the FAA for increase of the limit. The S-76 was used as a demonstrator for LHX maneuverability, and the FANTAIL demonstrator had no tail cone mods necessary to do 90 degree snap turns in 2 seconds at 120 knots.. .. .But the law is clear, you are stuck with the limit.. .. .As a general comment, few helicopter limits are set by the strength of the sheet metal, most are rotor stresses due to maneuvering. This is not true of airplanes, where the loading of the metal tails or wings directly affects the maneuver envelope. That is why most helo pilots think that pulling too much G or yawing too fast will wrinkle the metal. In reality, the rotors will hit limits, and bearings will wear out early, or blades will get wrinkled if you maneuver too fast or too hard. Seldom will the tailboom or fuselage structure be the problem.

[Steve76]

Thanks guys, I just learned something. <img border="0" title="" alt="[Smile]" src="smile.gif" />

[Av8r]

S76A++ Question for Nick....or anybody else.

I fly an S76A++.. .Two questions have been bugging me for a while, can’t seem to get an answer I’m comfortable with.. .. .1. Why 107% Nr for all ops?. .. .2. N1 Over speed limitation. A little explanation for those of you who don’t fly the A++. 2 X Arrial 1s1 engines. The engine over speed protection works like this.. .2 X N2 overspeed probes 1 X N1 overspeed probe per engine. (A) If N2 hits 122%, it shuts down the engine and disables the overspeed in the remaining engine.. .(B) If both N2 probes have failed for whatever reason on an engine, the N1 probe takes over. If the N1 is above 82.3%, it shuts down the engine. (!) . .How and who worked that out?. .If I voluntarily shut an engine down for whatever reason, does that disable the over speed on the remaining engine or is there potential for a double engine failure?. .. .Regards. .. .Av8r

[MaxNg]

AV8r. .. .I have no 76 exp but consider this!!. .. .1st case ......drive shaft failure. .. .If you have a driveline failure between the engine and MGB the power turbine will spin to distruction hence N2 overspeed limit at 122% to prevent this. This failure is more likley and hence the need for redundancy ( 2xN2 probes). .. .2nd Case........ .. .possible power turbine failure ie loss of blades on power turbine would reduce the downstream airflow resistance and would result in the compressor overspeeding.. .. .regarding the figure of 82.3% sorry I can't help you there but the limit may not be set by the design of the power turbine but more to do with ancillaries such as starter/Gen etc.. .. .Incomming !!!!!!!!!. . . . <small>[ 22 March 2002, 10:27: Message edited by: MaxNg ]</small>

[Red Wine]

Considering that the ++ is really an "A" model with 1S1 Arriels rather than the RR C30.....with an increased MTOW........... .. .Something has to give to keep the coning angel under control........if the MTOW increases whilst the Nr remains the same.......guess what happens to the coning angle...???. .. .Increase the Nr, whilst increasing the MTOW....and the coning angle remains acceptable [maybe even the same]..... .. .The 2nd question.........let me get the ++ books out.......... .. .The 3rd question........who certified and flew the first A++'s........??. .. .Keep your revs up...!!!!!

[leading edge]

Max Ng,I don't think that a failure between the engine and the MGB could necessarily as the "more likely" failure. Failure of an N2 overspeed sensor (Phonic wheel type I seem to remember)is more likely.. .. .Deadcentre. .. .Point taken on the coning angle but what about the single engine case where you are still allowed to droop the Nr down to 96%? Surely if coning angle was a problem with increasing Gross weight, you wouldn't be allowed to droop any more in the single engine continued take off case.. .. .Also, the TOGW of the A+ and the ++ are the same and the A+ doesn't require 107%NR all the time.. .. .LE

[Steve76]

Righto..... .I was waiting to hear what NIck had to say but here is what I have dredged from the archives. Forgive me if its wrong its been 3 years since flying the C model.. .All the info on the overspeed system is available in the venerable Flight Safety manual. . .. .1. 107% has been addressed on this forum before. To my knowledge it is simply with the retrofit 1S1 the NR needed to be run at 107 rather than retest and all that stuff test pilots do. Ask Nick or go searching around late last year. The aircraft flys really well at 107 due to the reduced pitch angle. Those with the old 'A' commonly increase to 107 for vertical departures and when the blades are not tracking so well, as the reduced pitch reduces the verticals.. .. .2. If I recall correctly, the overspeed system is NOT disabled if one engine is offline. . .. .Many a check pilot will mention that you could consider pulling the O/S CB to prevent a shutdown during OEI. Remember... you didn't hear that from me. <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .. .The reason for 2 probes is to prevent random electrical signals from accidently engaging the OS system ie. magnetic emmissions while flying over high tension power line. The 82.3% N1 limit is extra insurance. . .. .Both probes run at seperate frequencies to avoid this because the entire purpose of the OS is to prevent a condition of excessive turbine RPM launching the blades out of the engine and through the other donk and associated hardware in that area. <img border="0" title="" alt="[Frown]" src="frown.gif" /> Happy thought.... . .. .Additionally on the 1S1 you will see a very heavy steel ring around the turbine area as a practical preventative to this problem. OS may not only be from shaft breaks etc... but also from govenor or FCU runaways.. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[Heliport]

I've heard Nick is on vacation in the UK at the moment.. . . . <small>[ 23 March 2002, 10:18: Message edited by: Heliport ]</small>

[paulgibson]

not wanting to digress from the original question but from memory earlier Arriel's also have the turbine retaining ring fitted and I believe it came about after a AS350 in PNG crashed, killing some Chevron Oil employees in the process after the turbine wheels 'exited stage left' and took out a M/R blade....... .. .Didnt endear Squirrel's to Chevron for awhile...

[Xnr]

Question for Nick Lappos

Nick....as we all know the collective to yaw coupling in the S76 is there to reduce the pilot workload.....but in tail rotor emergencies things can get confusing...

What is the best method of getting back to terra firma with a FULL right pitch setting?

We have tried a few options including autorotation (75 knots) but the nose of the A/C never really aligns itself with the intended landing area and we end up doing an uncomfortable sideways (yawed right) flare.

If we try a high speed run-on we are faced with speeds of 100 knots + and it doesn't look like the result will be too successful.

What do you think?

[Nick Lappos]

Xnr,

The biggest reason why it is hard to practice the classic stuck pitch tail rotor emergencies in a Sikorsky is that there is no way to shut off the yaw to collective linkage. This means that you always will get increased tail thrust when you raise the collective pitch, even when you hold the pedals "stuck".

You actually have to sneak in right pedal as you raise the collective to wash out the coupling, if you are to try to fly as though a failure had stuck the tail rotor at a fixed setting.

The simulator at Flight Safety is pretty effective at these emergencies, and the flight manual procedures work well (I wrote them, actually!).

Have you tried the sim?

As a general statement, practicing the standard tail rotor emergencies is good coordination excercise, and good for general orientation as to the classic pitch-power-antitorque relationships, but the probabilities of having one for real are virtually non-existant.

With a stuck tail rotor pitch setting, you must find the flight speed and main torque that comes closest to satisfying the tail rotor thrust. Remember that an auto trims almost perfectly at zero tail rotor thrust, and an S-76 can fly at 50 knots sideward (lots of right thrust) with full right pedal, so there will be a mismatch in auto. The best way to correct that is to touchdown at lower rpm, which reduces the tail thrust considerably. Arrange it so that in auto just at touchdown, pull down to 90% rotor at touchdown, and the tail thrust will be almost perfect for a full right tail setting. That will happen in a typical touchdown auto, actually. Again try the simulator.

[Xnr]

Thanx for the quick response.

Have been on the sim at flight safety but cant honestly say that I had the opportunity to play with this situation.

The question I had was for a fixed pitch setting or you could say a failure aft of the mixing unit that causes a full right T/R pitch setting.

As you raise the collective there will not be any change in T/R pitch.

Since the T/R has so much authority I agree that the best course of action is to lower the Nr.

Will the A/C come back into trim in an auto at 90% Nr with a full right pitch setting?

Do you recommend executing the auto at 90% Nr to align the A/C with the landing area or come down out of trim (100-105% Nr)hoping that it will align as you raise the collective in the PULL.

Next time on the sim I'll ask to play with a few of these.

Thanx again.