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# Sikorsky S-76: Ask Nick Lappos

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# Sikorsky S-76: Ask Nick Lappos

7th Aug 2001, 17:45

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'Cos we have to do it at the end of every flight.........Oh and we all believe it won't happen to us!!

I still suspect that increasing rpm has an exponential increase in required torque (I'm sure there's a 'V squared' in there somewhere)

(Spelling again...Or too much gin .....I was very...very drunk!!!!!!)

[ 07 August 2001: Message edited by: Harry Peacock ]
7th Aug 2001, 22:20
Nick Lappos
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Harry peacock said;
I still suspect that increasing rpm has an exponential increase in required torque (I'm sure there's a 'V squared' in there somewhere)

Nick sez:
Your instinct is correct, the thrust increases with the square of the rpm, and the torque varies with the rpm directly (first power)if the collective is lowered to maintain height, or with the square of the rpm if the collective is not moved. A 5% increase in rpm makes a 10% increase in thrust and torque if the collective is not changed.
8th Aug 2001, 04:10

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To tie in Nick's comments and the Vsquared perspective, I would suggest that, in the example - excessive power pedal (left pedal in American helos) - the increasing torque requirement from a reduction in RRPM is caused by Vsquared.

As Rotor RPM is bled off, collective is increased to maintain a constant hover height. The reason this has to occur is because of Vsquared in the lift formula:
Lift = Coefficient of lift X 0.5roe X Vsquared X S.
Therefore, as RRPM decreases, Velocity over the blade decreases, and this effects the lift exponentially because V is squared.

As you need to produce the same amount of lift to stay at a constant hover height, the only variable to change is the coeffecient of lift - ie pull more pitch to increase Angle of Attack (AoA) thus uncreasing drag as well, hence requiring more torque. Clear as mud?
As Nick says, the extra torque would require more left pedal, thus helping your situation. But this is not all that helps. As the tail rotor spins at a fixed ratio to the main rotor, but at much greater RPM, a decrease in main rotor RPM will result in a proportionately larger decrease in tail rotor RPM, and hence V over the tail rotor. As V is squared, lots of lift is lost from the tail rotor, and becuase the pitch is fixed, total tail rotor thrust is reduced thus helping you even more. Going back to the UH-1H, the tail rotor is nearly always at it's peak in the hover so any reduction in main rotor RPM really reduces tail rotor thrust quickly, and small main RRPM decreases are very effective in stuck left pedal forward cases. Incidently, this quirk has caused/contributed to many limited power accidents in the UH-1H, and I seem to recall some mention in the FM about it. Can any one recall?

I guess it gets more complex on aircraft equipped with mixing units because the effectiveness of rotor bleeding depends on whether the pedal jam is before or after the mixing unit. I am guessing that is why it is a hard procedure to practice on these aircraft. Nick?
8th Aug 2001, 05:36
Nick Lappos
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Helmet Fire,
The reason why the torque rises when the rpm is bled off is actually quite simple:
It is not because the velocity is lower (which it is) because the collective rises to make more alpha for the lower air velocity, and the lift is a constant (as measured by the fact that the aircraft neither accelerates upward nor downward). In actuality, the rotor is almost exactly as efficient at lower rpm as at higher (same lift for same power) in fact, the rotor on most helos gets a tad better at lift for power as the rpm is reduced a small amount.

The simple fact is that the rotor is eating power, not torque, and at lower rpm, more torque is needed to generate the same power.

Torque is how much twisting force the shaft needs, and is measured in foot-pounds (a unit of torque is the twist exerted by a force of 1 pound exerted one foot from the center of the shaft (like a torque wrench). If the shaft is not turning, you can still exert a torque on it, but it takes no power to do so. If you exert a constant torque on the shaft, and it is turning at a constant rpm, you are generating a fixed amount of power. Turn the shaft twice as fast while exerting the same torque, and you are now generating twice the power.

So power is torque (twisting force) times the rpm of the shaft. At a lower rpm, for the same power you must increase the torque. Remember the rotor needs constant power for constant lift across a small rpm band.

Try this out, hover carefully and precisely in low wind and note the exact torque and engine temperature (TGT, T5, T4.5, etc). It is easy to be precise if you just barely bounce a wheel or skid a little to get the hover precision down to an inch or so. Then beep the rpm down by 3 or 4 % and stabilize at the same height. Note the torque, and engine temp again. The torque should be about 3 or 4% higher, but the engine temp should be the same (because the power is the same, although the torque is different!)

On some model helos, the lower rpm actually takes less power than the higher rpm, and you can measure it this way, note that 3.5 degrees C of T5 or T4.5 equals 1% power on most gas turbines.
8th Aug 2001, 07:32

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Roger that Nick, thanks. My helmet is now well and truly alight and getting worse! I am currently flying an AS350 so I will not be able to try your experiment safely just yet. I think I remember the Huey was specifically operated at 6600N2 so that when rotor bleed occured you were initially getting an increased effeciency from your rotor system, maxing out at about 6400N2, before getting less efficient. Any Huey drivers to clarify?

Nick, what happens to N1 in the situation you described. I had always thought it must increase in a bleed situation (along with resultant TGT increase) or are you simply refering to beep down rather than bleed due topping with N1, TGT, Fuel flow, etc?

How does the power/torque relationship effect the aircraft when the tail rotor slows? Is there any applicability of the Vsquared situation I was on about before?
8th Aug 2001, 08:52

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Helmet,

I think you were spot on in your really well done description above.

I goofed by simplifying too much and using the term "torque value" instead of thrust value compared to torque value.

As we get back to a five foot hover the M/R thrust is the same and the torque is slightly higher then when we started. (thanks nick!)

So that relationship has changed linnearily and at first glance, not in our favor.

But the gains we have made in reducing tailrotor thrust have followed an exponential path thanks to the lift equation as you stated.

So the resultant works in our favor.

Much as this professional interaction,

Thanks everyone !

8th Aug 2001, 09:03

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Sorry, goofed fourth line down in above post,

Going to happy nappy now

Night
19th Aug 2001, 19:58

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S76 Vne question

With the NR less than 100% the OEI VNE on the S76 is limited to the best rate of climb speed. When conducting the appropriate procedure for a low side speed trim failure you can fly at a speed greater than Vbroc with the NR below 100%.

My question is : why the VNE restriction when OEI.
20th Aug 2001, 21:23
Nick Lappos
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The S-76A uses a slight reduction in rotor rpm to gain single engine climb rate. The 96% Nr is worth about 75 feet per minute climb increase when the engine is temperature limited. The use of 96% rotor in effect creates the need for a new structural envelope, and could even call for a new Vne chart and the whole structural qualification (in addition to the full 100 to 107% rpm envelope already provided). Since the lower rpm is only useful for establishing single engine climbout, we conveniently controlled it by limiting to flight below best rate of climb speed (one can argue that if you can accelerate above Vy you don't need the lower rpm anyway). This way we give a convenient way to have our cake and eat it too.
21st Aug 2001, 04:02

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Thanks Nick:
But,How does that apply to NR below 100% because of a low side speed trim malfunction? To parphrase the flight manual " flight with the N2 below 100% when conducting the appropriate procedure for a speed trim malfunction is approved with-out restriction" or something like that.

Thank-you.
21st Aug 2001, 08:37
Nick Lappos
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IHL,
The issue is one of frequency of use. We flew at 96% Nr up to Vne and in various maneuvers to prove that everything was safe, but we didn't want to take the stresses of that speed and Nr as part of the component lives calculations. The failure case when N2 runs down is rare, so we didn't have to assume that below 100% Nr was a typical case. With OEI practice as a common thing, we would have had a tougher time convincing ourselves and the FAA that those occurrences were rare.

I was the Chief S-76 pilot back then, and part of the decisioning. I wrote the emergency procedures section of the manual (along with Dave Wright, who is the current S-76 Chief Pilot).
21st Aug 2001, 09:47

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If someone hasn't said this for a while, thanks for taking the time to share this kind of information, Nick.

It's one thing to talk in the crew room about why we do things a certain way, but to hear the real reasons given by those who were there doing the initial set-ups is fantastic.

Much appreciated.
21st Aug 2001, 09:57
Nick Lappos
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Arm Out the Window:

It is fun to share thoughts with folks from all over the world. I get a real sense of what we do when we all communicate. I get every bit as much out of this forum as I hope I can give. It is the real power of the web, I think.
21st Aug 2001, 11:40
collective bias
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Amen
21st Aug 2001, 17:51

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That explains it perfectly, thanks Nick.
IHL

[ 21 August 2001: Message edited by: IHL ]
21st Aug 2001, 21:47

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It seems to me that limitations of modern helicopters are economically based vice structurally or aerdynamically based. Ie, Vne is a product of what component life customers are willing to accept and how much test flying the company is willing to do.

If this is true, could an operator pay for further test flying and more maintenance to increase a limit?

Matthew.
22nd Aug 2001, 05:39
Nick Lappos
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heedm,
Almost all certification/qualification decisions are based on economics, one way or another. If a part falls short of expectations, it will be redesigned and retested to meet the original business plan, and if a limit penalizes the operator it will similarly be retested. The operator's needs are a big part of the decision process, so most likely your desire is almost automatically met. Exceptions might be for those who operate in a corner of the envelope (maybe a pure hover lift operation in a non-crane type helo, for example) where the appeal of a re-do might not be very universal.

Your basic premise is right on, Matt. The economics of component life costs and maintenance made the S-92 team decide to design and qualify all components with 30,000 hour minimum lives, and at least 6,000 hour TBO's on the gearboxes.

The cost to recertify most components is eye-watering. It is not too hard to spend 1 million dollars on aeronautical engineering tasks, and I doubt that any individual operator wants to foot that kind of bill. Mostly, customers vote with their purchasing dollars!
22nd Aug 2001, 16:16

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Well said Nick, and great postings.

The price the customer will bear is a serious design driver, and can often be the source of frustration for the specialized operator to which you refer. They are forced to make do with something designed for the broadest customer base possible, or to modify their operation to suit the aircraft.

The "Budget priced, 160 Kt, fuel mizer, all weather, on-condition, full mission OEI, zero exposure Cat A, pull stumps out of the side of a hill at 12,000'" aircraft hasn't been worked out yet......unless you guys have one
4th Sep 2001, 12:44

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S76 Vne

One for Nick...

I understand that the Vne on the 76 is imposed for handling reasons rather than structural/aerodynamic ones. Namely, it has a negative pitch trim gradient. I am curious to know what speeds have been reached in testing, and how the handling qualities change at these speeds.
5th Sep 2001, 20:20

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While we're discussing higher speeds in the 76, I'd like to know if there's any truth to the notion that the 76, if it suffers a dual engine failure in cruise (say 135-145 kts), will roll at such a rate as to be un-recoverable.

As I understand it, this is something passed along by a well respected training facility in south Florida

Nick?