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Author Topic: SRLs  (Read 1043 times)


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« on: January 28, 2019, 12:45:15 am »
Interesting article on SRLs in the 980 and 1000:

I didn't realize that the SRL automatically reduces the available power when operating below 100% RPM:

Quote from: Flight Levels

The SRL system also is designed to provide an automatic reduction in TIT for continuous climb/cruise operations at the same fixed SRL (650 degrees C) limit. This feature, visible on the cockpit EGT indicator when RPM is reduced, is intended to provide a reasonable balance between maximum thermodynamic performance at 100 percent RPM, and a more conservative value for continuous operation at less than 100 percent RPM.

I have noticed that when you pull the condition levers back to reduce RPM at cruise, the temperatures increase, but I didn't realize that was simply a function of the temperature the SRL was indicating. I always reduce power to about 600 degrees before bringing the condition levers back.

I guess I can't remember what the 690 did when you reduced RPM...

Anyway, interesting article!

Adam Frisch

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Re: SRLs
« Reply #1 on: January 28, 2019, 07:15:38 am »
The temp gauges in all Commanders are a bit smoke and mirrors. It's one of the things I dislike about it. Mine has EGT compensators that don't really correspond to reality and can be set to show pretty much anything. They don't show true temperature - they show a modified and adjusted temperature depending on a lot of external factors. So, it can be hard to figure out exactly how an engine's health is doing, because the temp gauges move the goal posts.

Now, I don't want to hijack this SRL thread as the -1's are completely different, but I would like for posterity just add this info here. It might or might not also be accurate for -5's and -10 with or without SRL's. Because I'd forgotten what Stan Perkins told me about this very subject (it's a shame he doesn't post here, as he's a wealth of knowledge), I decided to ask him in an email to clarify. Here's his response:

Our EGT gauges have OAT and airspeed compensation modules. What we really want to know when operating our engines is ITT, but ITT is very hot, and thermocouples donít last long at really high temperatures. EGT is cooler than ITT, so as a practical matter the designers measure EGT as a proxy for ITT. There is a calculatable offset between EGT and ITT, but that changes with OAT and airspeed. As airspeed increases, the EGT reads lower than it should, and as OAT increases, the EGT reads higher than it should. Our compensation boxes automatically change the EGT reading with changes in OAT and airspeed so all we have to worry about is a single temperature.
This was really important on the old pre-century engines that were run right at their limits, but since we run our -1 engines so far below their limits (roughly 90 degrees C below red line even at takeoff power), thereís significant margin for us and this compensation, which usually only amounts to around 20 degrees or so, can effectively be ignored. The compensators, particularly the airspeed transducer component, can get flaky, so in our case, one can just bypass the compensators and be done with it.

Don't know how this info applies to the later TC models.
The other thing you asked aboutóthe temperature change at startóis this: the TPE331 is tested on a Lebow stand after hot section, overhaul, etc. and the actual performance is accurately measured. The engineers decided that an EGT of 576 degrees C should correspond to 575 HP output. Most engines perform much better than this. For example, letís say an engine puts out 575 HP at 546 degrees EGT. The shop installs an EGT compensator module right on the engine (different from the airspeed/OAT compensator above) that offsets the EGT reading by +30 degrees C. That way, at 575 HP power, the EGT will show an adjusted reading of 576 degrees C. Each engine will have a different compensator installed, depending on how much EGT offset is needed. When you switch on your master, the EGT gauges should read the ambient air temperature, but when you turn the engine switch to start/run, the engine compensator is switched on , and youíll see the EGT reading instantaneously change to reflect the compensatorís offset value. If the reading goes up, thatís good, and that means that your engine is generating rated power at cooler temperatures. The amount of offset shows how much better than book your engine performs. My engines are offset by +35  and +42 degrees.

This is very useful info, as the temp drop in the EGT's on startup is the only clue on a -1 to find out if the engines are getting "tired" or not longer outputting what they used to. If you'd been able to read ITT directly, it would have been much easier to see trend development. Which is why I said earlier it's not a favorite feature of mine. When automation masks vital info, it's not good in my opinion.
« Last Edit: January 28, 2019, 07:21:48 pm by Adam Frisch »
Slumming it in the turboprop world - so you don't have to.


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Re: SRLs
« Reply #2 on: January 28, 2019, 02:41:46 pm »
Adam, that is really interesting. Sounds like the -1s had an analog precursor to the SRLs. The SRL does what you describe digitally.

The -5 has an ITT gauge and a compensating resistor, but none of the airspeed and temperature compensation that you are describing.