Shouldn’t the “red box” vary with rpm?

[Shadrach]

On 8/21/2019 at 9:24 PM, RobertE said:

I reran the GAMI test for the first time in a couple of years today which got me thinking about where, exactly, that red box resides.  I know that Mike Busch tends to express it as a % of power (basically no red box at 60 or 65% power, then it gets progressively bigger the higher the power setting).  I’m wondering if % of power isn’t just imprecise shorthand for where it actually resides?  I know if I set the timing too advanced on my old Corvair it used to knock like crazy when lugging along but would disappear at higher rpm.  And that engine advanced it’s timing with rpm which should have mitigated that effect.  In our fixed timing engines the problem should be greater, no?

Specifically, in my normally aspirated IO360 70% power is achieved at around 23 inches MP and 2500 RPM but at the same 23 inches and, say, 2300 RPM I’m down to somewhere between 60 & 65%.  Does that mean I’m generally safe to run at peak EGT at 2300 RPM but not 2500?  This seems wrong, at least if I’m going to trust my Corvair experience.  Anyone know the truth?  

Probably safe but may not be cooler. The slower piston speed at the lower RPM will result in higher peak cylinder pressure per combustion event and peak pressure will occur closer to TDC...but with 200 less combustion events per minute.

Edited August 23, 2019 by Shadrach

[carusoam]

Notes for ROP Climb mixture adjustment in a modern NA Mooney... also can apply to all NA Mooneys...

1) The EGT gauge is calibrated in °F.

2) T/O is performed at full rich at S/L...  max FF is displayed...

3) There is a blue box on the EGT gauge with a range of 2-300°F ROP...

4) As the plane climbs the mixture changes and gets more rich....

5) When the needle falls out the bottom of the blue box... lean the mixture to the top of the blue box....

6) Repeat...

 

7) typically, the first sign of needing to lean comes above 3k’ DA...

8) G1000 systems use a white box for the display, and the Ship’s EGT is actually a seventh probe that got accidentally labled TIT...

9) The seventh probe is in the confluence of three exhaust streams and averages a much higher temp than any single EGT...

10) The higher temp is solely because three temp peaks roll past the sensor in the same time period as a single EGT probe... 

PP thoughts only, not a CFI....

Best regards,

-a-

[moontownMooney]

I'd read some of Mike Busch's magazine articles in the past and understood/agreed in principle, but remained fuzzy on the details (e.g., what defines the boundaries of this box, do I need a map of the box vs power or other flight condition, etc.) as many here, including the OP are. I got to hear Mike talk about all this at Oshkosh this year and it was VERY helpful. He made clear that as far as his reference to the red box goes, it is literally a target/peak CHT that defines the boundary. So that boundary is a function of power first and foremost though there are other factors (cooling =f( density , airspeed)). So the box boundaries can vary a fair amount, even disappearing entirely, in the course of a flight, but the simplicity of CHT being the defining factor is that you dont have to know or even think about the red box or fin boundaries as a function of anything, you just fly your CHTs. If you are less than your target/boundary CHT then you are good, if you are not, then you are not. The idea behind CHT being the critical factor is related to (as a prior post mentioned) it being a good surrogate for internal combustion pressure (ICP) which is what can cause real and relatively rapid damage to your pistons/cylinders. In addition, there are about 6 different specific ways that elevated CHTs can lead to premature cylinder head assembly wear. His target/boundary CHTs also aim to avoid these. I will add that I'm new to the Mike Busch bandwagon, but after hearing him speak at Osh, I immediately bought his book and it is exceptional. Incredibly detailed and educational while also remarkably easy to read.

Sent from my LM-V405 using Tapatalk

[Shadrach]

13 minutes ago, moontownMooney said:

I'd read some of Mike Busch's magazine articles in the past and understood/agreed in principle, but remained fuzzy on the details (e.g., what defines the boundaries of this box, do I need a map of the box vs power or other flight condition, etc.) as many here, including the OP are. I got to hear Mike talk about all this at Oshkosh this year and it was VERY helpful. He made clear that as far as his reference to the red box goes, it is literally a target/peak CHT that defines the boundary. So that boundary is a function of power first and foremost though there are other factors (cooling =f( density , airspeed)). So the box boundaries can vary a fair amount, even disappearing entirely, in the course of a flight, but the simplicity of CHT being the defining factor is that you dont have to know or even think about the red box or fin boundaries as a function of anything, you just fly your CHTs. If you are less than your target/boundary CHT then you are good, if you are not, then you are not. The idea behind CHT being the critical factor is related to (as a prior post mentioned) it being a good surrogate for internal combustion pressure (ICP) which is what can cause real and relatively rapid damage to your pistons/cylinders. In addition, there are about 6 different specific ways that elevated CHTs can lead to premature cylinder head assembly wear. His target/boundary CHTs also aim to avoid these. I will add that I'm new to the Mike Busch bandwagon, but after hearing him speak at Osh, I immediately bought his book and it is exceptional. Incredibly detailed and educational while also remarkably easy to read.

Sent from my LM-V405 using Tapatalk
 

So then, what’s  your target CHT in July? What is it in February?

[carusoam]

Relying on CHTs comes with a bunch of other challenges...

from the single point of measurement, the type of sensor that is being used, to the depth of the well, to how close it is to the exhaust valve... how well your cooling is working, your baffle health, the OAT...

Much more helpful when somebody defines the red fin, for your engine, using these exact instruments...

Since the fin has fuzzy edges at best... how close to the fuzzy edge do you want to operate with fuzzy CHT measurements...

 

The cool thing about APS’s instrumented engine with IPC sensors... they were using IO550s for their research... which is great if you fly behind an IO550... 

Some translation may be required... compression ratio, ignition timing, the usual...

How about a knock sensor?

Best regards,

-a-

[Hank]

1 hour ago, carusoam said:

Notes for ROP Climb mixture adjustment in a modern NA Mooney... also can apply to all NA Mooneys...

1) The EGT gauge is calibrated in °F.

2) T/O is performed at full rich at S/L...  max FF is displayed...

3) There is a blue box on the EGT gauge with a range of 2-300°F ROP...

4) As the plane climbs the mixture changes and gets more rich....

5) When the needle falls out the bottom of the blue box... lean the mixture to the top of the blue box....

6) Repeat...

 

7) typically, the first sign of needing to lean comes above 3k’ DA...

8) G1000 systems use a white box for the display, and the Ship’s EGT is actually a seventh probe that got accidentally labled TIT...

9) The seventh probe is in the confluence of three exhaust streams and averages a much higher temp than any single EGT...

10) The higher temp is solely because three temp peaks roll past the sensor in the same time period as a single EGT probe... 

PP thoughts only, not a CFI....

Best regards,

-a-

For many of us flying NA Mooneys, there is no blue box on our EGT gage, thus the need to find, record and remember the Target EGT number . . . . .

[moontownMooney]

So then, what’s  your target CHT in July? What is it in February?

For legacy Lycomings he recommends in a range 400-420 for cold vs hot day. For legacy Continentals he recommends 380-400, again reflecting cold day vs hot day. For modern aircraft with highly efficient cooling designs, his recommendation reduces by 20 deg for all the above. Obviously these arent exact, the box boundaries are fuzzy, and no sensor (CHT or otherwise) is perfect, but there is presumably appropriate conservatism in these targets.

Sent from my LM-V405 using Tapatalk

[carusoam]

14 minutes ago, Hank said:

For many of us flying NA Mooneys, there is no blue box on our EGT gage, thus the need to find, record and remember the Target EGT number . . . . .

Hank,

I’m glad you asked... trying to put this in words was going to take another page...

+1 on Target EGT.  That is a more important piece of information.

With The EGT sensor you have.

Is it possible to Mark it with a grease pencil on the front to temporarily indicate Peak and the range of 2-300°F ROP...?

I can’t remember the full range of the EGT sensor in the C... I was happy to just have an EGT sensor in my M20C...

I get the feeling that the standard EGT instrument doesn’t have a wide enough range to draw a blue box on it...

 

Now I need to go back and refresh my memory of the Target EGT methodology... it’s been too long... 

Best regards,

-a-

[Shadrach]

11 minutes ago, moontownMooney said:

For legacy Lycomings he recommends in a range 400-420 for cold vs hot day. For legacy Continentals he recommends 380-400, again reflecting cold day vs hot day. For modern aircraft with highly efficient cooling designs, his recommendation reduces by 20 deg for all the above. Obviously these arent exact, the box boundaries are fuzzy, and no sensor (CHT or otherwise) is perfect, but there is presumably appropriate conservatism in these targets.

Sent from my LM-V405 using Tapatalk
 

I respect Mr, Bush a great deal but I don’t think those are good recommendations. Moreover, in many cases they are outside the realm of what’s possible with a well baffled, stock “Legacy Cowl”.  It just does not get that hot in level flight. I run a “legacy Lycoming” and I see no benefit in using 400-420. I can count the number of times my engine has been to 420 on one hand and it has always been in a slow, steep climb on a hot day with heavily loaded plane. I would need to pick the most abusive settings and configuration possible to attain those kinds of temps in the summer. In February? Even less likely,   I use CHT as a target for leaning but in my plane I shoot for <360 in the summer and <330 in winter.  My engine is and always has been timed to 25BTDC. I flew on Tuesday. OAT was 90 plus. At 4500msl (6500ish DA) peak EGT yielded a CHT of 338 on the hottest cylinder...should I have leaned to 40 ROP to try and heat it up a bit?

[Hank]

6 hours ago, carusoam said:

Hank,

I’m glad you asked... trying to put this in words was going to take another page...

+1 on Target EGT.  That is a more important piece of information.

With The EGT sensor you have.

Is it possible to Mark it with a grease pencil on the front to temporarily indicate Peak and the range of 2-300°F ROP...?

I can’t remember the full range of the EGT sensor in the C... I was happy to just have an EGT sensor in my M20C...

I get the feeling that the standard EGT instrument doesn’t have a wide enough range to draw a blue box on it...

 

Now I need to go back and refresh my memory of the Target EGT methodology... it’s been too long... 

Best regards,

-a-

Here's mine,  ropped from a panel shot. While there is a large enough temperature span, the unstrument itself is pretty small. It should be doable, it just wasn't a "thing" that far back . . . .

It's pointing at 1450°F, a common place for me to run. 

[carusoam]

Hank,

Your ship got a few upgrades that mine only wished for.  

I had the Alcor... when calibrated, There is enough room on the gauge to assume the equivalent of a blue box...

 

a pic of the Alcor... here...

https://www.aircraftspruce.com/catalog/inpages/alcor_replacement4.php?gclid=CjwKCAjwnf7qBRAtEiwAseBO_ElcpYk-U1Ao4zVarNMZQJKb1ctWfA4Aqkrj6hdL7-qqffYXfSLy9BoCDVUQAvD_BwE

 

Best regards,

-a-

[gsxrpilot]

11 hours ago, moontownMooney said:

I'd read some of Mike Busch's magazine articles in the past and understood/agreed in principle, but remained fuzzy on the details (e.g., what defines the boundaries of this box, do I need a map of the box vs power or other flight condition, etc.) as many here, including the OP are. I got to hear Mike talk about all this at Oshkosh this year and it was VERY helpful. He made clear that as far as his reference to the red box goes, it is literally a target/peak CHT that defines the boundary. So that boundary is a function of power first and foremost though there are other factors (cooling =f( density , airspeed)). So the box boundaries can vary a fair amount, even disappearing entirely, in the course of a flight, but the simplicity of CHT being the defining factor is that you dont have to know or even think about the red box or fin boundaries as a function of anything, you just fly your CHTs. If you are less than your target/boundary CHT then you are good, if you are not, then you are not. The idea behind CHT being the critical factor is related to (as a prior post mentioned) it being a good surrogate for internal combustion pressure (ICP) which is what can cause real and relatively rapid damage to your pistons/cylinders. In addition, there are about 6 different specific ways that elevated CHTs can lead to premature cylinder head assembly wear. His target/boundary CHTs also aim to avoid these. I will add that I'm new to the Mike Busch bandwagon, but after hearing him speak at Osh, I immediately bought his book and it is exceptional. Incredibly detailed and educational while also remarkably easy to read.

I attended the APS course in Ada, OK. It was interesting the conversation in that course about Mike's theory and process. They said that Mike had actually been to Ada and sat through the APS course three times. And that they are all good friend with Mike and communicate regularly. And while they agreed that high CHT's are problematic and should be avoided, they disagree with Mike that an engine can be safely and reliably flown off CHT's or TIT's for turbos. And they gave several reasons for this.

One thought is that Mike is trying, rightfully, to make these concepts as easy as possible so that everyone can easily understand and follow. But he might have "dumbed it down" a bit too much. And unnecessarily so. 

It's been a while since I've flow a NA engine, but with my turbo engine, I could easily fly it with low CHT's but still be burning through cylinders. The 252 cools so well that keeping CHT's down is easy. But "most" turbo engines don't make TBO without cylinders. I believe that by flying based on %HP and degrees LOP or ROP,  (staying outside the Red Box or Red Fin) I've been able to maintain a good running engine to near TBO without any cylinder work. Fingers crossed...

[Ned Gravel]

What about the characterisation of Internal Combustion Pressures (ICP) that determines the parameters within which it is hard on the pistons?

[Cyril Gibb]

10 hours ago, moontownMooney said:

For legacy Lycomings he recommends in a range 400-420 for cold vs hot day. For legacy Continentals he recommends 380-400, again reflecting cold day vs hot day. For modern aircraft with highly efficient cooling designs, his recommendation reduces by 20 deg for all the above.

To be clear, he isn't recommending that engines be run within that range.  He is warning that those are the upper limits.  He very clearly states in the Advanced Class video that CHT should never exceed 400F.

[jaylw314]

16 hours ago, Shadrach said:

I respect Mr, Bush a great deal but I don’t think those are good recommendations. Moreover, in many cases they are outside the realm of what’s possible with a well baffled, stock “Legacy Cowl”.  It just does not get that hot in level flight. I run a “legacy Lycoming” and I see no benefit in using 400-420. I can count the number of times my engine has been to 420 on one hand and it has always been in a slow, steep climb on a hot day with heavily loaded plane. I would need to pick the most abusive settings and configuration possible to attain those kinds of temps in the summer. In February? Even less likely,   I use CHT as a target for leaning but in my plane I shoot for <360 in the summer and <330 in winter.  My engine is and always has been timed to 25BTDC. I flew on Tuesday. OAT was 90 plus. At 4500msl (6500ish DA) peak EGT yielded a CHT of 338 on the hottest cylinder...should I have leaned to 40 ROP to try and heat it up a bit?

IIRC, he did mention having to adjust those target temps cooler for aircraft with good cooling.  He only mentioned Cirrus's specifically as shooting for temps 40-50F cooler, but I imagine that may apply to some degree with J's or earlier models with cowl mods.