Core RPM max 94%?

[Nealius]

The manual indicates normal maximum power is 98%, but I am unable to get over 94% indicated on the gauges with my physical throttle firewalled and the controls indicator is also showing throttles firewalled. The info bar RPM varies between 100% and +2% over what my gauges indicate (e.g. gauges indicate 94% but info bar indicates 96%), but the gauges consistently top out at 94% for me. Temp, baro pressure, and altitude don't affect it. It happens from ramp start 26C at Incirlik and it happens on air-start winter at altitude in the Caucasus. Is this normal?

 

Track attached for reference.

TankKillerRPM.trk

Edited March 25, 2021 by Nealius

[Caldera]

Nealius,

 

Check the outside air temperature (OAT).  If it is high enough that could activate the high exhaust gas temperature (EGT) fuel limiter (simulated) and reduce maximum power. 

 

I don't remember the temperature this occurs at.  I should pay attention to that myself and find out...

 

I had this problem really bad using the CREATE FAST MISSION option.  It was confusing as hell.  The engines would not complete the start sequence when the OAT was 50C.  If they did start the fan speed was limited to like 72%.

 

Caldera

[Nealius]

11 hours ago, Caldera said:

Nealius,

 

Check the outside air temperature (OAT).  If it is high enough that could activate the high exhaust gas temperature (EGT) fuel limiter (simulated) and reduce maximum power. 

 

I don't remember the temperature this occurs at.  I should pay attention to that myself and find out...

 

 

-13C at 12,000ft. Core RPM maxes out at 94% per engine, fuel flow 2650pph per engine. Setting the fuel switches into override have no effect. 

 

I also can't find the EGT fuel limiter mentioned in the manual anywhere.

[zinhawk]

What you are referring to is the T5 Amp and that is ITT based and physically set by the engine specialists to achieve a certain fan speed. It limits the fuel control (analog) and while the fan speed (not core) target is based on OAT, the IEPU/fuel control/T5 system is not FADEC. It only monitors OAT, compares it to expected fan speed changes and throws a code for engine troops to adjust when it doesn't like what it sees. So, changes in OAT doesn't "turn on" the T5, it is on the whole time. The fuel override switch turns off the T5 and you should see maximum, engine melting numbers after a long period. DCS does not model the override switch currently. DCS trim is 799 on the ITT guage which is a brand new engine kinda trim.

 

In response to the OP, core RPM is typically somewheres around 96-97ish. BUT depending on how good your compressor is they can run slower while putting out ridiculous discharge pressure and can achieve target fan speed the same as a crappy compressor running in the 98s. The core is managed by the fuel control so, yes seeing fairly consistent numbers across differing locations is normal. In terms of raw numbers I gave the core a meh.

 

What you should see change based on altitude, OAT, yadayada is the fan speed. DCS actually models this change exceedingly well. When you compare the calculated fan speed curve (which is our target curve) DCS has the same slope. However, this curve is about 4% below target. No engine on the line is below 1-1.5% of the target.

 

So, a low ITT number, sagging rpms, I think Caldera is thinking in the right track. The T5 is set too low if I were to diagnose this as a real engine. Normal is 820-830s.

 

[Nealius]

So is this an issue up to ED to fix, or fine as it is?

[zinhawk]

To be "accurate to what you would find", yeah it ought be fixed. Is it flyable? yes. The engine topic is a dead horse flogged many times over. ITT issues have been reported and taken in so who knows, maybe changes are in the works. There is an awful lot going on and frankly I would agree this is bottom barrel from a total game perspective.

[Nealius]

It doesn't seem to affect my flying, but then again I never take a massive load of ordnance/fuel. I could have sworn years ago when I last flew the OG A-10C the RPMs maxed at 98% like the manual stated, so getting 94% with the A-10C II really threw me off when I glanced down at the gauges. 

Edited March 26, 2021 by Nealius

[Caldera]

Nealius,

 

Check your (Inter stage Turbine Temperure (ITT).  If that is maxed that is all the power you get.  In function, the turbine has reached an operational ITT limit then a fuel limiter will activate to ensure that the ITT is not exceeded.  The effect is reducing power output.  The fan speed is a reflection of that power level.  Like I said, I have also experienced this in DCS to the point where the engine would not even complete the start sequence as the run up to idle hit the ITT limiter. 

 

The engine is designed to produce maximum power under design conditions.  Basically I think in your case, it is a little to too warm outside and the temperature is above design.  And while it does not seem like a lot of difference it can be a factor.   If you add 50F to the OAT then the ITT goes up by 50F.  If that additional temperature is above the setting of the ITT limiter then it reduces fuel until it is not above the limiter.  As Zin implies, if that maximum temperature is too high the metal parts in the engine may start to bend and possibly the high rpm moving parts might crash into non moving parts etc.  That is a bad thing...  I would call it "Corn Cobbing".

 

For more information you likely don't care about.  The are other defueling limiters.  Typically, high and low speed rotor rpm and maybe lubricating oil temperature those can also function in the very same way by limiting fuel.

 

For land based turbines that produce electricity, OAT is a significant factor.  For example, for a turbine unit capable of producing 100 MW of electricity if the OAT is 40F.  If the OAT is 110F it may only be able to produce 80 MW.  That is allot.  So what is typically installed are inlet evaporative cooling systems.  So instead if dealing with OAT you deal with Inlet Air Temperature (IAT).  So another example, 110F OAT with 25F of inlet cooling then the IAT is 85F and the maximum power is now 95 MW.  Congratulations, you just made allot more money.

 

This is impossible for aircraft... 

 

Fly higher, earlier in the day or deal with the loss of power.   Or like Zin says, override the controls and compromise your ride home.

 

Caldera

[Nealius]

10 hours ago, Caldera said:

Nealius,

 

Check your (Inter stage Turbine Temperure (ITT).  If that is maxed that is all the power you get.  In function, the turbine has reached an operational ITT limit then a fuel limiter will activate to ensure that the ITT is not exceeded.  The effect is reducing power output.  The fan speed is a reflection of that power level.  Like I said, I have also experienced this in DCS to the point where the engine would not even complete the start sequence as the run up to idle hit the ITT limiter. 

 

The engine is designed to produce maximum power under design conditions.  Basically I think in your case, it is a little to too warm outside and the temperature is above design.  And while it does not seem like a lot of difference it can be a factor.   If you add 50F to the OAT then the ITT goes up by 50F.  If that additional temperature is above the setting of the ITT limiter then it reduces fuel until it is not above the limiter.  As Zin implies, if that maximum temperature is too high the metal parts in the engine may start to bend and possibly the high rpm moving parts might crash into non moving parts etc.  That is a bad thing...  I would call it "Corn Cobbing".

 

For more information you likely don't care about.  The are other defueling limiters.  Typically, high and low speed rotor rpm and maybe lubricating oil temperature those can also function in the very same way by limiting fuel.

 

For land based turbines that produce electricity, OAT is a significant factor.  For example, for a turbine unit capable of producing 100 MW of electricity if the OAT is 40F.  If the OAT is 110F it may only be able to produce 80 MW.  That is allot.  So what is typically installed are inlet evaporative cooling systems.  So instead if dealing with OAT you deal with Inlet Air Temperature (IAT).  So another example, 110F OAT with 25F of inlet cooling then the IAT is 85F and the maximum power is now 95 MW.  Congratulations, you just made allot more money.

 

This is impossible for aircraft... 

 

Fly higher, earlier in the day or deal with the loss of power.   Or like Zin says, override the controls and compromise your ride home.

 

Caldera

 

 

ITT is not maxing out. Max is 865C according to the manual, and I am getting 780~795C with 93~94% RPM. Higher, earlier, colder, negative degrees celcius doesn't change anything. Override controls do not work. 

 

Test 1: -5C, night, 29.92, 8,000ft; ITT 790C core RPM 93%

Test 2: -13C, afternoon, 29.92, 12,000ft; ITT 780C core RPM 94%

 

 

[Caldera]

Nealius,

 

Then you have me....

 

I re-watched your track.

 

Sorry, this time I payed more attention to your fan speed than your ITT or RPM.  It looks to me like you were at full power.  Check your fan speed.  It is bouncing right at 83%.  I believe that this should be your best indication of engine power.

 

I am guessing your track is Free Flight Caucasus?  If that is true then everything looks pretty much identical to my aircraft when I fly it.  My fan speed is bouncing right at 83%.  Generally, I only really glance at fan speed and fuel flow while in flight if I am undamaged.  But my scan is mostly just fan speed especially when I change power.  The oil, ITT, hydraulics and RPM not so much.   And then mostly only during an engine start.  Probably a bad habit for sure but it has not killed me yet...

 

I have seen maximum fan speeds as low as 72% when I aborted take off.  At around 75-77% (memory), I could just barely get the wheels up before the end of the runway. 

 

How is your take off power?

 

Caldera

[zinhawk]

865 is max allowable for the T5 setting (done by ground crew) if you can't achieve fan speed without going hotter. After that you pull the engine out. Do not conflate max values with normal operation. The "manual" is chock full of misnomers and misunderstandings of systems. The T5 is your wall. On standard day and warmer temps you are hitting this ITT wall for sure. It is a single number no matter what. For DCS this seems to be 799ish.

 

Also you should test 1 parameter at a time to see what is changing what. In test 2 you dropped the temp and got a slight rise in speed which is expected but then you raised the altitude which should nerf it a bit. The core speed is kind of irrelevant though. It will do what ever it has to until it reaches the T5 limit, which should be fan speed target, which is the most critical for performance. 

 

What I do find interesting is ITT also decreasing. It should be the same mark (799ish) at full throttle. Perhaps the T5 system is a little dorked at the extremes or other factors are at play at those cold temps. We don't normally run that cold so I don't have a lot of real data to compare to down there. Now I want to test and pick brains.

[zinhawk]

5 minutes ago, Caldera said:

Nealius,

 

Then you have me....

 

I re-watched your track.

 

Sorry, this time I payed more attention to your fan speed than your ITT or RPM.  It looks to me like you were at full power.  Check your fan speed.  It is bouncing right at 83%.  I believe that this should be your best indication of engine power.

 

I am guessing your track is Free Flight Caucasus?  If that is true then everything looks pretty much identical to my aircraft when I fly it.  My fan speed is bouncing right at 83%.  Generally, I only really glance at fan speed and fuel flow while in flight if I am undamaged.  But my scan is mostly just fan speed especially when I change power.  The oil, ITT, hydraulics and RPM not so much.   And then mostly only during an engine start.  Probably a bad habit for sure but it has not killed me yet...

 

I have seen maximum fan speeds as low as 72% when I aborted take off.  At around 75-77% (memory), I could just barely get the wheels up before the end of the runway. 

 

How is your take off power?

 

Caldera

Where in the devil's hellscape are you getting 72% fan? 

[Nealius]

Takeoff power is fine for me so far. 90% MTOW takes off just fine on a warm-but-not-hot day; never loaded her up more than that. 

 

ITT 799C max in DCS makes sense. I've seen it hit that on occasion, but I'm still confused by being stuck at 93~94% RPM even at 760~780C. 

[Caldera]

Zin,

 

19 hours ago, zinhawk said:

What I do find interesting is ITT also decreasing. It should be the same mark (799ish) at full throttle. Perhaps the T5 system is a little dorked at the extremes or other factors are at play at those cold temps. We don't normally run that cold so I don't have a lot of real data to compare to down there. Now I want to test and pick brains.

 

Don't know... 

 

My only guess is that increasing altitude is decreasing OAT and that the Fan has a RPM limiter on it.  I have not looked at an engine diagram.  Is the fan coupled with a direct drive to the low speed rotor or does it use a fluid coupling (hot exhaust gas is a fluid) utilizing another power turbine?

 

I also notice the Fan speed bouncing at 83%.  That seems like to me like it would be hitting the limiter and defueling momentarily causing thr  RPM to drop below the limiter.  Then it catches back up overshoots and repeats the process.  This would be mostly due to the higher inertia of the fan blades.  Or it could be just a flaky transducer or bad tuning.  That is only a guess and this is a only a simulation that I think would mostly replicate something visual on the actual aircraft. 

 

Typically, any high (fuel) limiter that has is at set point or exceeding set point will generally function to limit maximum fuel rate.

 

T5?  Temperature at the fifth flange aft from the inlet?

 

 

19 hours ago, zinhawk said:

Where in the devil's hellscape are you getting 72% fan? 

 

LOL  

 

My "devil's hellscape" was CREATE INSTANT MISSION.  What happens is that you get allot of random variables including weather.  Not realizing it at the time because I had just started playing the game.  This was before I learned how to use the MISSION EDITOR.  I got some OAT's that were indeed created in a devil's hellscape.  Some really weird stuff happened to me then.

 

I only learned about it from the help of others.

 

Because:

"Like I said, I have also experienced this in DCS to the point where the engine would not even complete the (engine) start sequence as the run up to idle (would) hit the ITT limiter." 

 

And...  It would never reach idle speed.

 

I figured out that if that happened all I had to do was manually advance the throttle a bit.  Doing that cleared the start engine logic and allowed me to start the other engine.  And that, of course, allowed me to fall flat on my face trying to take off.  A few motivating crashes going through the boundary fence can be a hoot!

 

 

Nealis,

 

The only help that I can offer is don't worry about it.  If you are at 83% Fan speed then you are fine.  I think that the other engine values get tweaked by the developers from time to time. 

 

For my part, I check the mission brief before my flight.  I look at the ambient air temperature so I know what to expect.

 

Caldera

 

Edited March 28, 2021 by Caldera

[zinhawk]

8 hours ago, Caldera said:

I have not looked at an engine diagram.  Is the fan coupled with a direct drive to the low speed rotor or does it use a fluid coupling (hot exhaust gas is a fluid) utilizing another power turbine?

 

The Fan (single stage low pressure compressor) is physically coupled via fan drive shaft to the Low Pressure Turbine. Sandwiched in between is the High Pressure Compressor (Core on the guage) physically coupled to the High Pressure Turbine through the Combustion Chamber. The link between low and high pressure systems is core exhaust. Interstage Turbine Temperature (ITT) is read between the High Pressure Turbine and Low Pressure Turbine. This is partly why you dont have modern EGT readings instead. This is station (flange) 5 in answer to 

 

8 hours ago, Caldera said:

T5?  Temperature at the fifth flange aft 

 

T5 Amp is actually mounted on the forward portion of the engine but reads station 5 ITT data and limits it by (ding ding) fuel rate.

 

8 hours ago, Caldera said:

My only guess is that increasing altitude is decreasing OAT and that the Fan has a RPM limiter on it.

 

I also notice the Fan speed bouncing at 83%.  That seems like to me like it would be hitting the limiter and defueling momentarily causing thr  RPM to drop below the limiter.  Then it catches back up overshoots and repeats the process.

 

There are zero limiters besides the T5. Less dumpy fuel, less High Pressure exhausties, slower fan. If ED put one on besides that, that is in correct. Generally though I think they are faithful to original piece as much as can be. Except........

 

8 hours ago, Caldera said:

The only help that I can offer is don't worry about it.  If you are at 83% Fan speed then you are fine.

 

Compare to reality that is horrible. We never got below 84 in the 100 degree Nellis weather, 85-86 are normal days. Considering we can't be less than 1-1.5% off target and  DCS is on the average 4% less it is like smashing the defcon 1 button for an engine troop. But since I can't share the decades of engine data to change peoples minds it just is what it is.

 

ITT by itself isn't bad, we see new engines trim to that. Core RPM by itself isn't way off, we see both lower and higher but usually scales with the T5 trim. Low T5 limit and lower core jives. But when you put all three together: low fan, lowish core, low ITT, it spells a T5 limit set too low circling back to what I said earlier. Of I were king for a day I would change the T5 limit coding (however that is done) to something more average like mid 820s-30s and I bet the DCS TF34 will hum pretty close to realsies.

 

Sorry for hijacking a bit Nealius, but also I wouldn't be terrible concerned with core numbers by itself and thinking about the cold situation more it makes sense that you would run out of throttle range before you hit the T5 limit. Engines love cold dense air like me and a fat kid like milkshakes. I am still curious it figure out what the old timers say.

Edited March 28, 2021 by zinhawk

[Nealius]

22 hours ago, zinhawk said:

What I do find interesting is ITT also decreasing. It should be the same mark (799ish) at full throttle. Perhaps the T5 system is a little dorked at the extremes or other factors are at play at those cold temps. We don't normally run that cold so I don't have a lot of real data to compare to down there. Now I want to test and pick brains.

 

This is the part I'm curious about. My OAT at ground level in my missions is typically 20~26C, but where I'm spending most of my time at 15k~18k ft, temps are in the negative teens. When you say you "don't normally run that cold," do you mean when testing engines on the ground?

[zinhawk]

yes. That is my experience block when comparing apples to apples. We dont typically look at flight data unless a fault code is taken and 9 times out of 10 it is related to things other than core. There are reasonable inferences to be made looking at calculated fan speed charts for instance. colder, denser, more speed vise versa and mixed in between. 

 

Much of the test limits are designed to make sure stuff works at altitude.

[bkthunder]

Been reported and acknowledged by ED years ago and repeatedly thereafter. The current max ITT is too low and based on values from an UNINSTALLED engine. 

Several RL A-10 people (crew chiefs and pilots) have reported this but despite all the proof and efforts done by the community to inform ED and to provide substanital data, ED hasn't fixed it. I was hoping they'd fix it in A-10II, given they had agreed to correct the FM, but as usual it was rushed out without much of an afterthought for the this long-standing issue.

Same thing with the windmilling engines producing way too much HYD power...  

Edited March 28, 2021 by bkthunder

[zinhawk]

Especially frustrating since the real world fix is the equivalent of hitting backspace 3 times and typing in a new limit. Not a million dollar coding problem. 

[Caldera]

Hey Guys,

 

Interesting, great discussion!

 

Zin,

 

How does the engine prevent over speed or run away?  The physical capacity of the fuel delivery system?

 

From what I have experienced, the simulated ITT control is alive and well.

 

All,

 

Sounds like ED should have gotten its data from one of the below:

• A less worn out engine
• A more modern engine
• A freshly cleaned engine
• An engine running higher fuel quality
• A field tuned engine from Nellis
• An upgraded or modded rebuilt factory engine

But I do not know...

 

There may be a little bit of error between the data they used / use compared to the actual visuals in the cockpit that we experience.  If ED can accurately model flight dynamics then they could really accurately model engine characteristics.  If they wanted too.  I doubt this is done that way due to the extra processing load.  The data probably just comes off of a table and is displayed in the cockpit.  Easy enough to change.  

 

The RL engines that I have experienced were all digitally controlled.  All of the them operationally ran off tables (Fx curves).  And all that was easy enough to change.  Instant engine performance changes (possible catastrophic failures) with a key press.  No two of them would ever run exactly the same.

 

Considering the accuracy of the rest of the flight model I am also guessing that allot of research went into engine performance as we see it on the cockpit gauges.

 

But...

 

What we are seeing may not be what we are actually getting!

 

The actual virtual thrust is what is truly important.  That may indeed be correct or have bit of error as well.  Anyway to test that?

 

Caldera

Edited March 28, 2021 by Caldera

[zinhawk]

35 minutes ago, Caldera said:

Hey Guys,

 

Interesting, great discussion!

 

Zin,

 

How does the engine prevent over speed or run away?  The physical capacity of the fuel delivery system?

 

From what I have experienced, the simulated ITT control is alive and well.

 

The T5. At full throttle the fuel control is like an analog Scotty, just giving her all shes got until the T5 limits the fuel flow output. T5 ops checks are pucker time. Lots go into making sure that box works perfectly every time.

 

1 hour ago, Caldera said:

All,

 

Sounds like ED should have gotten its data from one of the below:

• A less worn out engine
• A more modern engine
• A freshly cleaned engine
• An engine running higher fuel quality
• A field tuned engine from Nellis
• An upgraded or modded rebuilt factory engine

But I do not know...

 

There may be a little bit of error between the data they used / use compared to the actual visuals in the cockpit that we experience.  If ED can accurately model flight dynamics then they could really accurately model engine characteristics.  If they wanted too.  I doubt this is done that way due to the extra processing load.  The data probably just comes off of a table and is displayed in the cockpit.  Easy enough to change.  

 

The RL engines that I have experienced were all digitally controlled.  All of the them operationally ran off tables (Fx curves).  And all that was easy enough to change.  Instant engine performance changes (possible catastrophic failures) with a key press.  No two of them would ever run exactly the same.

 

Considering the accuracy of the rest of the flight model I am also guessing that allot of research went into engine performance as we see it on the cockpit gauges.

 

As far as I am aware they have plenty of data on a normal worn in engine and when you compare things like the calculated fan speed curve at OAT vs DCS it has the same slope so their tables are probably really solid for a worn in engine. Two engines on the same aircraft will have slightly different core and pick an ITT within the limit range and they will have it. But what will be nearly the same between the two and the whole fleet within 1% is the Fan because we set them that way.

 

What appears to me is that they have a really good RPM data table for a typical mid life TF-34. It is just choked down way too low and not achieving fan speed limits.

 

1 hour ago, Caldera said:

But...

 

What we are seeing may not be what we are actually getting!

 

The actual virtual thrust is what is truly important.  That may indeed be correct or have bit of error as well.  Anyway to test that?

 

Caldera

 

 

That is a fair statement. Without a digital readout of pounds of thrust it is hard to say. No one has come out and said it is producing x pounds of thrust though and discussion always revolves around ITT and RPMs as if we were discussing the real thing. 

[Caldera]

Zin,

 

There was a one and only time I ever got to see and actual aircraft engine running on a stand.  It was for an F-15 and I don't know the model or anything else.  However, it was digitally controlled.

 

Full afterburner is a hoot!  When the run was over I felt like a groupy at a Stones concert. 

 

In my usual fashion, I asked a bunch of probably annoying questions.  The crew was happy and gracefully patient to answer them for me.  As this was a part of a post rebuild test, one question that I asked was "What was the engine thrust?".   Power output measurement for land based turbines is a crucial step.  It is typically how well or not how well the maintenance company makes or does not make its bogey for bonuses.  The crew all looked at each other and the chief said "We don't measure it.".  Frankly, that surprised me.  There was no load cell on the stand, nothing.  It was just bolted to the floor and the engine was just bolted to it.

 

I talked to him some more. The only thing that he seemed to be really concerned about in his testing, at least at that time, was the rate of change in power and that the engine parameters were correct at the various power levels he had tested.  He did not like the rate of change he was getting and planned for more tuning.  I already knew it was different turbine world with different priorities, but that drove the point home.  To me, the changes in power I had just witnessed seemed just about as instantaneous as a mechanical device of that type could be.  So I was completely off of my map.

 

As to the crew chief, he seemed the type to never be fully satisfied as to  how his engines were running.  That made me feel good.

 

Caldera

Edited March 29, 2021 by Caldera

[bkthunder]

Just to add to this, here's a video that provides some view of the gauges. 

Notice the higher fan speed as well as ITT above 800. 

In DCS max fan seems to go no higher than 80-82% and ITT no higher than ~790.

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