I would think the purpose of flying a turbine better be a hella lot more than just an extra 10 knots.

The Commander runs the same as any 331 but are slow turn (thankfully).

Based on what I’ve gathered from Garrett school and Honeywell’s TPE rep at the time (an MU2 driver btw), blade elongation aka creep is a thing and the physics of 100% rpm operation are not conducive to longevity without sacrifice. Temp is the limiting factor, of course, but my takeaway was always to run lower RPM and higher temp within limits. Less blade stretch and more efficient.

Plus, I really can’t stand 100% from a frequency perspective. It just sounds wrong. On the other hand, some planes and prop combos do better than others. It’s definitely worth careful performance testing because what else are you going to do at altitude for hours at a time?

Of course that’s not how it works in the real world. But they had to draw the line somewhere, or so they thought.

As a former Garrett engineer (including a stint as the TPE331-14GR/HR/-15AW master designer), I can state that creep is a very real time at temperature and speed phenomenon. Normally, the cycle would not be set such that 100% spool speed would be a creep problem (speed limits are generally set by mechanical overload criteria at temperature). However, depending on the difference between the original design intent and field experience, materials technology, and casting processes, it is not unheard of for blades to begin venturing into the aft part of the secondary part of the creep curve (see below) at cycles below design intent.

The intent is that there will be a small amount of elongation during early hours of operation (primary region), followed by small increases during the majority of blade life (secondary region). Usually, the allowable amount of elongation is set such that the blade does not stretch to the point where blade contact with the shroud occurs (especially true for cooled turbine blades). Usually, this point is at or below the yield point for the blade design at the operating temperature (assuming the engine is operated within limits). If the blade gets into the tertiary zone (assuming it hadn't already failed due to tip rubbing), the elongation is rapid and catastrophic.

In the TPE design, the spool runs at 100% most of the time (that's where the design point is) with power being modulated via the conditioning lever. More power requested, more fuel added to the mix, but the spool speed remains relatively constant. With twin (or three) spool turbines, this is obviously not the case.

Definitely, the cooler you can run the engine, the better it is for longevity. In the turbine design world, there's a rule of thumb that "every 50 degree F reduction in operating temperature doubles blade life."

Can't tell you much about the frequency issues. I flew around in Garrett's fleet of TPE-powered aircraft for years (without ear protection) and just got used to the noise and vibration (which as often as not was the result of airframes design choices, not the engine)

Art

As a former Garrett engineer (including a stint as the TPE331-14GR/HR/-15AW master designer), I can state that creep is a very real time at temperature and speed phenomenon. Normally, the cycle would not be set such that 100% spool speed would be a creep problem (speed limits are generally set by mechanical overload criteria at temperature). However, depending on the difference between the original design intent and field experience, materials technology, and casting processes, it is not unheard of for blades to begin venturing into the aft part of the secondary part of the creep curve (see below) at cycles below design intent.

The intent is that there will be a small amount of elongation during early hours of operation (primary region), followed by small increases during the majority of blade life (secondary region). Usually, the allowable amount of elongation is set such that the blade does not stretch to the point where blade contact with the shroud occurs (especially true for cooled turbine blades). Usually, this point is at or below the yield point for the blade design at the operating temperature (assuming the engine is operated within limits). If the blade gets into the tertiary zone (assuming it hadn't already failed due to tip rubbing), the elongation is rapid and catastrophic.

In the TPE design, the spool runs at 100% most of the time (that's where the design point is) with power being modulated via the conditioning lever. More power requested, more fuel added to the mix, but the spool speed remains relatively constant. With twin (or three) spool turbines, this is obviously not the case.

Definitely, the cooler you can run the engine, the better it is for longevity. In the turbine design world, there's a rule of thumb that "every 50 degree F reduction in operating temperature doubles blade life."

Can't tell you much about the frequency issues. I flew around in Garrett's fleet of TPE-powered aircraft for years (without ear protection) and just got used to the noise and vibration (which as often as not was the result of airframes design choices, not the engine)

Art

Art,

In the TPE -10, how do they inspect blades and decide which ones need to be replaced or reused? When a turbine wheel gets replaced, is that blades and all or just the disk and reuse the blades?

50 deg F is 27 deg C. Running the engine cooler might make sense if it doubles blade life. However, if 650C was good for a really long time, doubling the blade life might not really help. Where along the line on the creep curve are we?

I have no plans operating aggressively. The book numbers are fantastic as is. The aerospace engineer in me wants to understand where the limits are or why/how they were set.

My uninformed guess would think it has less to do with creep and more to do with tip clearances and rubbing.

I’m 1500 hours from inspection, but curious how it all works. At that time, I’ll have to replace one timed out wheel.

Metal can have very different strength at different temperatures. the turbine blades are subject to very high tension due to centripetal acceleration. At 650C the material has enough strength to maintain its dimensions, at 670C it does not

You can see this if you try to weld aluminum. You heat it and nothing changes, then, suddenly, it totally melts

The nonlinear yield strength/temp concept is clear to me. But I’d iguess we’re quite a ways to the left on the creep/temp curve.

Overspeed governors limit max RPM. Sort of curious why no such limiting devices exist for temperature. Seems ?foolish? we are trusted to not push the levers forward and grenade the engines in 5 seconds.

I’ve also heard -10 engines make more power and handle hot sections better than the -1,-5, etc. What makes it more durable?

I truly have an endless pool of questions on this one!

Actually a higher rpm and resulting lower temperature is more conducive to increasing blade life (if you don’t add more fuel and bring the temperature back up). Increasing the rpm from 96% to 100% increases centripetal acceleration 8.5% but decreases EGT dramatically for the same power.

What Helmut (Garrett pilot advisor) was saying at the engine school was that the SRL computer adjusted the temperature based on RPM and that the actual ITT inside the engine at an indicated 650C was lower at 96% than at 100%. I had quite a long discussion with him on this. Not sure one can say this applies to a 331-10 without a SRL computer

It’s probably beneficial to reiterate that changes in blade life at different cruise settings are minuscule compared the hot starts, hung starts or exceedences. Tha data presented by Art shows a doubling of blade life with a decrease from 650C to 623C while an increase from 650C to 677C will destroy the blade almost immediately.

There was an ITT limiting system on MU2s. It was unreliable and was removed. Your MU2 probably has two plugged holes for the switches near where the ammeter/voltage gauges are located.

The -10 has holes in each first stage turbine blade through which bleed air is run cooling the blade. This allows higher ITT for more power. The lower -10 HSI costs,are,probably a function of more conservative limitations

Actually a higher rpm and resulting lower temperature is more conducive to increasing blade life (if you don’t add more fuel and bring the temperature back up). Increasing the rpm from 96% to 100% increases centripetal acceleration 8.5% but decreases EGT dramatically for the same power.

What Helmut (Garrett pilot advisor) was saying at the engine school was that the SRL computer adjusted the temperature based on RPM and that the actual ITT inside the engine at an indicated 650C was lower at 96% than at 100%. I had quite a long discussion with him on this. Not sure one can say this applies to a 331-10 without a SRL computer

It’s probably beneficial to reiterate that changes in blade life at different cruise settings are minuscule compared the hot starts, hung starts or exceedences. Tha data presented by Art shows a doubling of blade life with a decrease from 650C to 623C while an increase from 650C to 677C will destroy the blade almost immediately.

It’s been a while, so I may be conflating things.

As i recall, the small block TPEs (through the -12), all have forged impellers and cast, uncooled, turbine wheels. Typically, inspection looks at the condition of the blade tips and shrouds (gouging, scratches, etc) and diametral checks to ensure radial growth is within limits. If a blade (or blades) fail to meet limits, the entire turbine wheel (or impeller) is replaced. There was a lot of work back in the day to figure out ways of repairing the tips on these wheels (various types of welding technologies), but I don’t recall anything making its way into the field.

In larger engines (those with insertable compressor and turbine blades) they will replace (or repair) blades that are below limits. Cooled blades are repaired/restored due to their acquisition costs, while uncooled blades are typically replaced. Since a lot of uncooled blade designs have tip knives, weld repair is actually quite straightforward, but often it’s cost effective to replace with new hardware.

As an aside, a large contributor to shortening blade and disk lives is the start-stop cycle. Thermo-mechanical damage accrues every time a turbine goes from cold to hot and back. This is the rationale behind tracking engine cycles to determine life. If you never shut the engine down (or limit rotational excursions), you essentially have unlimited life disks (and blades too assuming no FOD, overtemps, etc). Consider industrial power plant turbines operate continuously for 50,000+ hours.

Good design practices target blade operating temperatures at a level where creep never enters the tertiary section of the curve. Normal inspection intervals will catch excursions into the second half of the secondary area of the curve and those blades should be removed from service. Tip rubs are mostly the result of either over-temperature operation and/or excessive G-maneuvers. To your observation, operating temps are set conservatively for these and other reasons.

If you ever get a chance to talk to shop guys at an airline, they’ll tell you something like “the disks are replaced (timed out) every 10,000 hours/cycles, but the blades have 35 to 40,000 hours on them and keep chugging along”.

The -10 engine has a cooled first stage turbine.

"In the Dash 10-converted engines, cooling from the shaft is tapped to cool the blades in the first stage."

https://www.aopa.org/news-and-media/all ... e-pilot-(6)

There are air holes in the blades which take air flow and help cool them internally.

I'd guess the -12 is the same, but don't know for sure.

The -10 engine also has segmented first stage blades, not a blisk.

Mike C.

The -10 engine has a cooled first stage turbine.

"In the Dash 10-converted engines, cooling from the shaft is tapped to cool the blades in the first stage."

https://www.aopa.org/news-and-media/all ... e-pilot-(6)

There are air holes in the blades which take air flow and help cool them internally.

I'd guess the -12 is the same, but don't know for sure.

The -10 engine also has segmented first stage blades, not a blisk.

Mike C.

My post was obviously not worded clearly. If you were flying 300kts at 96% and 650C you could increase RPM and temperature will drop because more air is being supplied by the compressor. Power will also drop because more energy is now being consumed by the compressor. If you add enough fuel to regain your 300kts your temp will be significantly less than 650C, probably 630C. So, to go 300kts you have a choice 96% 650C or 100% 630C.

Any turbine engine will be more “efficient “ (BSFC) as the EGT goes up. Your TPE331 will be VERY “efficient “ at 670C for the few seconds before it grenades. Efficiency and longevity are at odds with each other

My first MU2 was an N model and I only had it a year before Frank Borman convinced me to get a Marquise and I don’t recall the details of -6 engine operation. I never had a -10 converted airplane . My understanding is that that the SRL system was introduced to allow the temperature sensors to be installed in cooler conditions which extended their life. An operating ITT/EGT gauge for each engine is required for any flight so failed ITT sensors were a big deal. The same thing could have been accomplished by giving the pilot a table showing what temperatures could be used under which conditions. The SRL system reduces the pilots workload allowing automatic starting and a single temperature redline to be used under all conditions.

As you stated, the engine will go between HSI at 100% RPM and 100% ITT/EGT. However there is no room for operator error and the HSI might be more expensive. In my Solitaire, I run 96% and 640C. If find the noise less obnoxious, the speed adequate and there is some room for error or creep on the EGT. Periodically, I will run 96% 650C and verify that I’m getting book performance numbers.

Helmuth sometimes didn’t go into detail on whether he was referring to measured EGT, the “massaged” EGT on an SRL equipped aircraft, or the ITT those numbers actually represented. Obviously, the first stage turbine blades only care about the temperature they are being exposed to, not the temperature somewhere else. I don’t think all pilots understand how this all works.

My prior MU2 was a Marquise. Pre RVSM, I would fly long legs at FL290 which would require 100% 650C when heavy if the temp was ISA +10C or higher. The Marquise is a pig compared to the Solitaire. Same wing, same engines and 1000lb weight difference!