Eng Temp Limits

[MightyGem]

All engines have their temperature limits. Exceed them and you risk damaging the engine. So, can someone explain the following quote from the latest Defence Helicopter:

"Ingenuity has characterised efforts to keep the UK's helicopters running in Iraq. The Sea King HC Mk4s have had their engines upgraded to enable them to lift an extra 1360kgs at temperatures of up to 50 degC, by taking the engine temperature measurements from an alternative source to convince the engine that it is operating at a cooler temperature."

So, if the engine was temping out before the mod, how can it operate at the higher temperatures? Admittedly the TBO for the engines is now 150-200 hours.

[fkelly]

I think you've answered your own question. I'm sure there are plenty more qualified than me to comment but the stress on the turbine is a function of temp and time - if one goes up the other must come down to achieve the same life. The simplest example would be a high start transient with the emphasis on the transient. Reduce the TBO and the statistical chance of a turbine going walkies is less - but costs more in the long term

[ShyTorque]

This is my best stab:

The engines were obviously being limited by the engine control unit / computer "topping" them out (limiting them) at the programmed T4 / T5 limit.

The boys obviously needed a little more power to do the job and formally said so. Some boffin advised that at the expense of a lower Time Between Overhaul (TBO), a bit more power could be obtained from the same engines.

Presumably, the quickest way to do this in theatre was to take the T4/5 input from the alternative source which gives a lower output signal. This was probably easier than reprogramming the ECU; I suspect it was just a cable swap-over or a simple rewiring job. The signal readout to the engine control system now has a "fudge factor" or bias built in, thereby releasing the top end power a little. The ECU doesn't actually know the engine is running so hot, so it's "happy" and simply governs normally, albeit at a higher T4/5 limit.

The "TBO" life of a turbine is usually determined by hot end blade growth, causing casing rub. The hotter it is run, the more blade growth is likely per hour and the earlier the engine must be pulled for overhaul.

Hopefully someone (engineer) who really knows what they are talking about will correct me where required. :)

[[email protected]]

The main difference is that for a normal 1400 Sea King engine, any excursions to max contingency (2.5 minutes) are not cumulative and are reset as soon as you lower the lever. The 1400T engine in the RN SK 4s is knackered once you amass a total of 2.5 mins at max con.
It is a short term gain to allow the aircraft to have some useable perfromance in conditions it wasn't designed for.
And soon they are going to fit Carson blades and send it to Afghanistan!!!!

[SASless]

Admittedly the TBO for the engines is now 150-200 hours.

You reckon someone in MOD has shares of the engine manufacturer?

[ShyTorque]

Possibly a case of "desperate times = desperate measures".

Short term gain - perhaps they will be coming home sooner rather than later?

Let's hope so, it's time this half-baked campaign was ended.

[NickLappos]

Engine life is a function of the time at the temperatures, which cause hot section deformation - called "creep". The time at higher temps is doubly bad, because those temps make the metal softer, and occur when the gas producer rpm is higher, so the stresses are higher. Softer metal and higher stresses lead to reduced life.

The trick with the electric control to fool them into letting the engine run hotter will cost life, but the expense of an overhaul earlier could be a good bet if the payload carried or mission performed is a high value one.

The later model black hawks have a T4.5 bias built into the fuel controls to fool them into thinking the engine is about 75 degrees cooler, for the same reason, but those later engines were designed to run at the higher temp, and the fuel control trick was a way to tap that power without redesigning the control.

[helopat]

Perhaps an aside from the main thrust of the discussion is the paperclip analogy.

When you talk about running engines at high, high temperatures (as one of our learned colleagues has already mentioned) the metal bits start to creep. Now, some folks say that operating in the 2.5 minute range is not cumulative...I do NOT agree...thus the paperclip analogy, to wit, if a paperclip is flexed once, it is still good to go...twice, still good to go, but if you flex that bad boy enough times, you induce metal fatigue and 'boink' bits go flying.

A discussion with engineers at the US Naval Safety Centre many years ago yielded the idea that if you REALLY have to exceed limits, its better to just keep pulling power till you're out of danger rather than ducking in and out of the 2.5 minute range repeatedly (ie flexing the paperclip).

Anyway, just my two cents worth.

HP

[MightyGem]

I think you've answered your own question
Yes I thought that that might be the case. Thanks for all the input guys.

[[email protected]]

Helopat - it's not a case of 'some folks say the 2.5 minute range isn't cumulative' - it is what Rolls Royce say and since they make the engines, they should know.

[212man]

"Helopat - it's not a case of 'some folks say the 2.5 minute range isn't cumulative' - it is what Rolls Royce say and since they make the engines, they should know"

Well, it's not what Turbomecca, GE and P&W say! Typically they allow 10 minutes of cumulative time at Max Contingency power, between TBO periods. How else could you do topping checks?

[NickLappos]

crab, rolls is wrong, or the fellow who thinks rolls said that is wrong. As a test, turn an engine in and tell them you pulled 10 intervals of 2.5 minute power per hour, and see what the bill is.....

ALL engine operations are cumulative, and engine damage is accrued roughly proportional to the square of the temperature. In other words, you damage an engine each time you use it, but the amount of damage is calculated to allow overhaul and safe life according to the overhaul interval.

Pilots foolishly believe that no engine damage occurs if you fly 2.49 minutes into the 2.5 minute rating, lower the lever for a few seconds, you can then just launch into another 2.49 minute session. Legally true, all this does is accru the damage faster than if you did the routine once per flight, or once per month or once per year. The engine has just so many fatigue beans in its jar, and each hot/high power activity takes some out.

[Graviman]

Nick, would there be a case for justifying a future helo engine project with an "auxiliary" combustion chamber? I am thinking along the lines of fixed wing after burners being an effective way to temporarily boost thrust without exceeding turbine entry temp. The F22 supercruise is really just an admission that 1st turbine stages can now handle higher combustion temp, leading to higher efficiency at high power.

The helo version would have a second combustion chamber after the first turbine stage, which most of the time would do nothing. When the extra power was required the second chamber would light, allowing greater power for reduced efficiency (bsfc). For a small weight penalty the machine versatility would be much higher. Perhaps a sneaky way of effectively adding an extra engine...

Mart

[[email protected]]

Nick and 212man, it isn't something I made up or heard via rumour - it was published in a Rolls Royce newsletter that they sent to their customers. I am familiar with your beans in a jar analogy but this info did come straight from the manufacturer.

[NickLappos]

Crab,
The Product Support former master sergeants who write the newsletter couldn't know how to certify an engine. I promise you that the same engine parts, exactly, are often used with different top part numbers and different overhaul intervals when one changes to engine usage spectrum to allow more (or fewer) cycles at high power. In such cases the hot section is typically listed at a different dash number so it cannot be confused with any hot sections from other engines that have different usage, even when the parts of that hot section are identical in every way. Often, the engine data plate, maintenance manual chapter 5 (life limits) and dash number are different for the same (otherwise) engine when the useage changes. Those who do not believe this are those who don't understand it.

I have sat with the engine companies (Rolls included) and negotiated more cycles for less TBO as a way to get more aircraft performance.

[SASless]

212man,

One could use part power stops as P&W does on the PT-6 engine installed in the 212 for example. For aircraft working offshore....climbing to a zillion feet and pulling the guts out of an engine just to convince one's self that it is able to pull max power seems a bit much when a ground run with a part power stop and some ballast accomplishs the same thing.

If the aircraft is going to be working at altitude in the mountains, then I might consider a topping check in flight at alitiudes the aircraft is to be working.

How many Topping checks have been done with the Governor in Manual on a 212? That expecially stood out as being a daft thing to do.

But then what do I know about such things. I was utterly opposed to the Torque Monitor system (which removed the torque damping) installed on the 212 by the Bristow brain trust.

I would offer an analysis of "why" these needs to do something unusual would yield an answer those in control do not wish to hear. Thus as one wise fellow has said....."The harder one has to argue a problem does not exist indicates the magnitude of the problem." (....or words to that effect)

[Geoffersincornwall]

Once had a conversation with a guy at Leavesden (where they used to make Gnomes etc.) who told me that "Sea King engines were life-limited based on cycles", so I asked what a cycle was and he said "one start, once up to TO power back to idle and a shut down".


I expressed my concern that we actually do more than one excursion up to TO power in a typical flight.

"Oh!" he said, "how many times do you pull that in a flight?".

I told him that in a general handling training flight it could be 5 to 10 times, maybe more and doing the ASW 'dunking' job in nil wind in the tropics or even a warm European day it could be the same. He was dumbstruck - but nothing changed. That was 35 years ago - has anything changed since ?


G

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