I’m curious to know what temperatures others are running their engines at during cruise.

I was taught to operate 20–30 degrees cooler than the maximum EGT compensated for OAT, and that’s what I’ve been doing. However I am wondering if this is necessary, can I just run at max EGT, or on the other hand is 30-40 degrees colder than max even better.

On a recent trip running Max EGT at 535 degrees I saw 295 TAS, whereas running 505 EGT gave 285kts TAS, roughly a 10kt difference.

So with that said, what temperature do or did you run your Garrett at, and why?

Hilgard

I have always run 0-20C below the redline, generally 10C. I’ve had SRL computers so 640C (10C below the 650C redline. Changes in OAT will result in small EGT changes which is why I don’t run the full 650C. Turbine engines are more and more inefficient as you get further from max temp. I do anticipate the slow EGT rise after turning off engine bleed heat and will reduce to 620C so that it doesn’t exceed 650C if I’m not paying close enough attentions. You do need to avoid exceedences. 5000hrs of running 20C below max is not going to compensate for 10 seconds at 20C above redline

I fly a -10K MU-2. I run just under max EGT on takeoff (on hot days that are temp limited) climb at 100% 20deg cool of max EGT. At top of climb I let the plane accelerate and pull down to 97% still 20deg cool of max once the plane accelerates past 300kts.
As to why, both the 97% and 20deg cool of max EGT was based on discussions I had with Honeywell engineers shortly after buying my plane. Heat kills, so less heat is likely better. Components in the TPE-331 are designed to make it to inspection limits at max allowable EGT continuous, but they may or may not make it based on lots of variables and running a little cooler hedges bets and is just easier on the hot section. For my purposes, the plane is plenty fast at conservative cruise settings. On low altitude ( FL180) short hops, you can’t run flat out anyway as you run up against the max IAS airframe limits.

Mike, I assume the redline on the SRL equipped -10's is different than the converted -10's? For instance on my -10 P model temp adjusted redline EGT is around 535 at cruise. Is this because the temp probes are in different locations between the two engines?


What about on start up, where the redline/limit is 770C. Generally in the summer I see the temp going up to ~630C with some starts. I assume there is less air moving through the engine at start up and thus the total temp transferred to the hot section components is less during start up, hence the higher limits for start up?

Hilgard

Thanks, I generally climb at 20 degrees below max as well, and then in cruise around 20-30 below max. The speed drops off in the 4 bladed -10's if the rpm is set at 96% in cruise, but I might play around at 98/97% in cruise to see if I can keep it about the magic 300kts TAS.



I am curious what the variables are, I would guess:
- Temp gauge inaccurate or not calibrated.
- Faulty temp probe or wire.
- I also remember Mike C mentioning that the engine shop sets up the engine during HOT/OH to display a certain temp at certain power, which can be adjusted.




Yes, it's like crack cocaine hitting redline on the ASI in cruise...

I think I understand that, but it seems the aerodynamics of the MU2 might make this less relevant. I actually get better fuel efficiency pulling the power back on my -10, running cooler and going F model speed. If you compare the two pictures below you can see that at 260kts TAS, Fuel flow is 59.1GPH giving 4.3nm/gal. Engines are running very cool at ~430C.

In the second picture I went back to my normal setting, TAS 297kts, FF 71.7GPH, lower efficiency at 4.0nm/gal and temps are ~500C.

This is something to consider if we want to extend range...

Hilgard

RPM is critical. Blade elongation forces are exponential. Did they go over that with you?

Mike, I assume the redline on the SRL equipped -10's is different than the converted -10's? For instance on my -10 P model temp adjusted redline EGT is around 535 at cruise. Is this because the temp probes are in different locations between the two engines?


What about on start up, where the redline/limit is 770C. Generally in the summer I see the temp going up to ~630C with some starts. I assume there is less air moving through the engine at start up and thus the total temp transferred to the hot section components is less during start up, hence the higher limits for start up?

Hilgard

The MU2 airframe is no different than any other. The comparison you showed is not the right one. Power required to overcome drag increases as the cube of the velocity so it would take a huge reduction in engine efficiency to reduce MPG. Change the altitude to get the higher temperatures, not just the fuel flow. Fly at FL250 at .whatever power you chose. Then descend to a Lower altitude and use the same fuel,flow. Basically, at the higher altitude, less power is required to spin the compressor, leaving more to produce thrust

The power required to spin the compressor is huge in dense low altitude air. Look at your fuel flow stopped on the ground at zero thrust at sea level with the condition levers set to TAKEOFF. Then look at the flight manual and see what that same fuel flow will get you at FL310. It’s about 270kts. All the power to go that 270kts was being consumed just to spin the compressor at sea level. Pretty wild. That’s why the latest engines employ all kinds of strategies to allow higher temperatures. The latest use transpirational cooling where the turbine blades have a film of bleed air enveloping them. the combusted gases never touch the turbine blade. This allows a lot more of the air to be combusted instead of just compressed and used for cooling

Also, looking at your pictures, you might want to note the limits of RPM in the Maitenance Manual I posted. RPM over 100% is supposed to be adjusted back to 100% prior to the next flight. As you can see from the limitations, allowance for RPM exceedences are very limited. You definitely want to be able to get 100% but you don’t want to be running at almost 101%

The inlets are designed for max efficiency at 96% according to Garrett.

I have always concluded running max EGT on a -10 at 96-97% is my sweet spot and so we’ve recommended that for years. Climb, cruise and approach rpm the same until final. Why buy a turbine to go slow or save the next owner $12 on a hot section? Countless -10 hot sections run like this have proven it works. With -5’s, 885 degrees
Max. Biggest killer of Garretts seems to be hot start errors (preventable) and dirty fuel nozzles. Nozzles lead to streaking and carbon buildup that erodes t-wheels.

The rest of hot section expense you really can’t control and running around 10 knots+ slow seems to defeat the purpose of flying a turbine

If you’re upon a hot section or have higher time engines, do a compressor refurb. That can be an instant 10 knots and many skip it at the hot section time.

[quote="Hilgard Goosen"] Mike, I assume the redline on the SRL equipped -10's is different than the converted -10's? For instance on my -10 P model temp adjusted redline EGT is around 535 at cruise. Is this because the temp probes are in different locations between the two engines?[quote]

FWIW, I've operated 2 MU-2 -10 conversions (an M and a G). They both had SRL systems and 650C redlines.

George

[quote="Hilgard Goosen"] Mike, I assume the redline on the SRL equipped -10's is different than the converted -10's? For instance on my -10 P model temp adjusted redline EGT is around 535 at cruise. Is this because the temp probes are in different locations between the two engines?[quote]

FWIW, I've operated 2 MU-2 -10 conversions (an M and a G). They both had SRL systems and 650C redlines.


Yes the EGT number *as depicted on the gauge* is different. I believe the probes are the same. The ITT , which is the actual limitation should be the same. The SRL computer takes a lot of factors into account and gives one redline for all conditions of altitude, airspeed, temperature etc.

Really?

My -10 converted M model used the EGT tabulated card method, much like an F model.

I was pretty sure SRL is NOT available for an MU2 M model. If you have operated an M model with SRL, that is one unique bird.

To my knowledge, only the Solitaire and Marquise have SRL on MU2s and there is no retrofit to earlier models, even if you -10 upgrade.

I also wasn't aware you could put a -10 on a G model. There are precisely zero G models on the FAA registry. That's not surprising since it was a wheezy -1 engine on a long body airframe.

Mike C.

I am curious, what rpm does the commander run at?

On my F model with the 3 bladed prop (turning 2000rpm @100%) I notice a small change in TAS going to 96%, perhaps 1-2 kts.
In contrast, the P model with the 4 bladed prop (turning 1591 rpm at 100%) the TAS seems to decrease with about 5kts when slowing down to 96%. Admittedly I usually just run the P model at 100% but plan to experiment with this a bit more.


Thanks for the tip it seems logical to ensure the compressor is pushing as much air into the hot section as possible. Our hot sections are due in about 200 hours on mid-time engines. I will discuss this with then engine shop when that comes up.

Hilgard

Its very interesting that 20 degrees can be the difference between 5400 hours and 5 seconds. That seems hard to fathom. I’d love to know more about the best way to operate these engines for the long term since I own two -10s on my solitaire!

I've heard conflicting opinions. One suggests the -10 is super durable and can run 100% all day at 650 until hot section with no issue. Another suggests you can, but will pay a hefty price during hot section inspection. Still a third argues cost per mile, not hour, says faster is still cheaper in the end.

With the Cirrus and 58P Baron, the analysis returned an answer pretty quickly with not many variables. The optimum strategy in the solitaire seems more nuanced.