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


RobertE

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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?  

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I tend to agree with you.  I'm going to say a few things that would probably result in rotten tomatoes being thrown at me if others were close enough to do so.

1.  The concept of the red box was born out of the idea of keeping the CHT below 400F.  It may not have been called the red box but it was implied (even graphically) in this John Deakin Pelican's Perch article:

Mixture Magic

2.  While I cannot quickly find the article, I've read at least one article by either Deakin or Busch that says the closest thing you or I have to measuring internal cylinder pressure is CHT.  The higher the pressure, the higher the CHT, all other factors being equal.

3.  So if the goal is to keep the CHT's below 400F (some prefer 380F), then however we do that should theoretically be outside the red box.

4.  My airplane has a -D engine.  I'm pretty sure that those of us with those engines tend to see higher CHT's than those the two separate magnetos.  That's because our engines are timed to 25 BTDC while many if not most of the non-D engines are generally timed at 20 BTDC.  That pushes our peak pressure closer to TDC, which in turn increases max pressure, which increases our CHT.

5.  In my personal experience with our airplane (others may have different experiences) I used to struggle to keep #3 CHT below 400F at higher altitudes, say 10,000 or higher.  That's because of fewer air molecules to carry the heat away.  When I used to cruise at 2500 RPM and full throttle, I would have to go deeper LOP than I wanted, with a resulting loss of speed in order to keep that CHT lower.  I could watch the CHT vary from about 385 to 399 as I flew through updrafts and downdrafts that caused my IAS to vary while I maintained altitude.  For quite some time now I've been cruising at 2600 RPM and my CHT runs about 10F lower.  I keep the CHT's lower, make more power, and cruise faster.

Is that what you were implying?

Edited by Bob - S50
Correct spelling of Busch
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Yes... it should.... but...

The avoid the red box is a bit of a simplification....

And the OWT of don’t run over square isn’t completely debunked...

Lowering the rpm, or increasing the time it takes to reach TDC... can aid the burning of fuel to be too soon.

Ever drive a stick? And start off in third gear?

It is similar to high power/low rpm... with really high ICPs... and the engine doesn’t like it very much... and the clutch won’t like it either...

So the POH gives us useable configurations of MP and rpm, and the related %bhp...

The higher rpm will be more ICP friendly than the lower rpm...

We have the choice to memorize the graph of the red fin, to really apply it... or use the guidance that being less than 65% is clear of the red box altogether...

Then we are relying on our engines to match the work done by the author of the graph...

Once thought it would be good to climb LOP... until, you see what %bhp you get while climbing 200°F LOP...

Increasing timing BTDC has a similar effect... more time to burn the fuel...

It would be interesting to get a torque curve overlaid on top of the HP curve... it probably just increases over the range of useable rpm...

Our operating range of rpm is not very wide compared to many other gasoline engines...

PP thoughts only, not a mechanic ...

Best regards,

-a-

 

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Very interesting discussion.  The red box/fin concept is what keeps me from using the power settings and altitudes that get the best performance.  My lowly C model will cruise >150kt easily at 5000-6000 AGL, 2500rpm, WOT (~24.5-24.8).  This is 75% power, where the POH and Lycoming manual say its ok to lean. Doing so allows me to run ~10gph with 3 CHTs <360 and my problem cylinder #2 in the 380s.  It is very appealing to operate this way. 

But the red fin/red box graphs say this is the worst possible thing to do, so I avoid except to show off once in a while how fast a C can go.  I’d love for this concern to be debunked....my engine seems happier here than at 24squared from a CHT perspective despite 24 squared being only 70% power - which may support the OPs observation regarding higher rpm.

 

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14 hours ago, 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?  

Higher RPM has the effect of delaying the peak cylinder pressure, since the cylinder has moved farther than at lower RPM, so generally this would be expected to lower the risk of detonation by some degree.  However, since you are producing proportionately less power, this benefit is only relevant if you keep the same overall power setting.

Someone can find this link for me, but one of the Deakin articles claims that when leaner than 25-50 ROP, the air fuel mixture ignites more slowly, which has the effect of lowering and delaying the peak cylinder pressure, and spreading it out over a longer interval of time. This effect increases as the mixtures is progressively leaned.  This would have a similar effect to retarding the ignition timing and protecting against detonation.

He also claimed that when ROP, the same thing occurs, with the maximum effect being about 125-150F ROP, but the effect is smaller than you can get very LOP

 

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50 minutes ago, DXB said:

Very interesting discussion.  The red box/fin concept is what keeps me from using the power settings and altitudes that get the best performance.  My lowly C model will cruise >150kt easily at 5000-6000 AGL, 2500rpm, WOT (~24.5-24.8).  This is 75% power, where the POH and Lycoming manual say its ok to lean. Doing so allows me to run ~10gph with 3 CHTs <360 and my problem cylinder #2 in the 380s.  It is very appealing to operate this way. 

But the red fin/red box graphs say this is the worst possible thing to do, so I avoid except to show off once in a while how fast a C can go.  I’d love for this concern to be debunked....my engine seems happier here than at 24squared from a CHT perspective despite 24 squared being only 70% power - which may support the OPs observation regarding higher rpm.

 

I have not seen a Red Box/Red Fin presentation which argues that 75 percent  power is "the worst thing to do". Just that the old POH recommendations of running 50 degrees ROP at 75 percent power is the worst thing to do. 

13 hours ago, Bob - S50 said:

2.  While I cannot quickly find the article, I've read at least one article by either Deakin or Bushe that says the closest thing you or I have to measuring internal cylinder pressure is CHT.  The higher the pressure, the higher the CHT, all other factors being equal.

That is discussed somewhere along the way in Mike Busch's publicly-available videos. Leaning Basics and Leaning - The Advanced Class.

 

Edit: It is in the first 11-12 minutes of the Advanced Class video.

Edited by midlifeflyer
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12 hours ago, RobertE said:

Well, I don’t happen to have the heat problem ( yet, legal or not, my A3B6 engine is timed at 25 BTDC) but my overall suspicion is that the % of HP is a practical but flawed approach to figuring out how to avoid detonation.  

25 degrees timing is approved for the IO-360-A3B6. To make it legal you just need to set the timing to 25 degrees, change the lag angle on the impulse-coupled mag, and change the data plate. 

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1 hour ago, DXB said:

Very interesting discussion.  The red box/fin concept is what keeps me from using the power settings and altitudes that get the best performance.  My lowly C model will cruise >150kt easily at 5000-6000 AGL, 2500rpm, WOT (~24.5-24.8).  This is 75% power, where the POH and Lycoming manual say its ok to lean. Doing so allows me to run ~10gph with 3 CHTs <360 and my problem cylinder #2 in the 380s.  It is very appealing to operate this way. 

But the red fin/red box graphs say this is the worst possible thing to do, so I avoid except to show off once in a while how fast a C can go.  I’d love for this concern to be debunked....my engine seems happier here than at 24squared from a CHT perspective despite 24 squared being only 70% power - which may support the OPs observation regarding higher rpm.

 

I follow the MAPA recommendation of MP + RPM = 46, so at 5-6K I'm usually at 22"/2400. Speed is usually pretty good, I'm often indicating ~145 mph [160 mph / 139 KTAS]. Higher is faster.

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2 hours ago, DXB said:

Very interesting discussion.  The red box/fin concept is what keeps me from using the power settings and altitudes that get the best performance.  My lowly C model will cruise >150kt easily at 5000-6000 AGL, 2500rpm, WOT (~24.5-24.8).  This is 75% power, where the POH and Lycoming manual say its ok to lean. Doing so allows me to run ~10gph with 3 CHTs <360 and my problem cylinder #2 in the 380s.  It is very appealing to operate this way. 

But the red fin/red box graphs say this is the worst possible thing to do, so I avoid except to show off once in a while how fast a C can go.  I’d love for this concern to be debunked....my engine seems happier here than at 24squared from a CHT perspective despite 24 squared being only 70% power - which may support the OPs observation regarding higher rpm.

 

It is your decision, but as I imply in my posting, I don't worry about the red box.  I just make sure my CHT's stay below 400F.  And since higher MP and lower RPM both move peak pressure closer to TDC, at lower altitudes I tend to run higher RPM and lower MP, generally 22-23" and 2600 RPM.  At higher altitudes I run WOT and 2600 RPM.  Since I always cruise LOP, I'm fine with anything below 10 gph which would be 75% from my engine.  I'm usually in the 8.7 to 9.5 gph range depending on altitude.

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@Hank how do you lean when climbing? The free advice I scoured on MS a while ago was to gather CHT/EGT data for each cylinder just after takeoff on a 59 degree day. Use the data as a reference when mildly leaning during a climb and never exceed the temps noted. @gsxrpilot did you lean while climbing when you had your C?

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7 minutes ago, tigers2007 said:

@Hank how do you lean when climbing? The free advice I scoured on MS a while ago was to gather CHT/EGT data for each cylinder just after takeoff on a 59 degree day. Use the data as a reference when mildly leaning during a climb and never exceed the temps noted. @gsxrpilot did you lean while climbing when you had your C?

Yes I did. But I would try to err on the side of too rich during the climb. I would often have a hot cylinder or two during the climb, so I'd drop the RPM to about 2600 and run a little richer in the climb. But still leaning during the climb.

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2 hours ago, Hank said:

But @Bob - S50, you're forgetting that @DXB and I fly carbureted C models, and LOP is really dicey and often not practical for us.

Thus my preference for the MAPA Key Number approach. So far, so good . . . . since 2007.

Hank.  The same thing applies to ROP, you just can't use fuel flow to determine power.  You have to use 'the numbers' as you said.  I don't know the C numbers, but my J numbers are 47 = 65% and 50 = 75%.  70% would be in between those numbers.  But to get to 47 I can run 24" and 2300 RPM, 23" and 2400 RPM, 22" and 2500 RPM, etc.  Higher MP and lower RPM will still result in higher peak pressures and higher CHT's even when running ROP.  Personally, were I to run ROP, I'd still prefer 2600 RPM and 21" to get 65% rather than something like 2400 RPM and 23".

In addition to keeping my CHT's lower, my personal experience is that the further I run from square, the less oil blow by I get.

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1 hour ago, tigers2007 said:

@Hank how do you lean when climbing? The free advice I scoured on MS a while ago was to gather CHT/EGT data for each cylinder just after takeoff on a 59 degree day. Use the data as a reference when mildly leaning during a climb and never exceed the temps noted. @gsxrpilot did you lean while climbing when you had your C?

Search here for Target EGT. That's what I do when I remember:

Takeoff on a standard day, as xlose to 29.92" and 59°F as you can get, and record your EGT as soon after rotation as you have brain power to do. Then shoot for that same EGT on every takeoff, leaning as needed ti maintain that value all the way from runway to cruise, when you will set power and lean anyway. 

Approximately. Best I can remember. YMMV. 

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31 minutes ago, thinwing said:

Your hopefully not doing that with your 252?

Nope. When I attended the APS class in Ada, OK a few years ago... they were going through the whole process on the whiteboard from takeoff, climb, level off, and cruise. It included setting takeoff power, leaning in the climb, setting cruise power and finally, going to LOP. It was all for NA engines. 

I put up my hand and asked if they'd go over the same for turbo engines. George just went to the whiteboard and erased everything between takeoff power and setting LOP at cruise altitude. He said, "that's all do you with the turbo". 

At the end of the runway, push everything forward. Sit back and relax for the next 20 or 30 minutes, put on the O2, and wait to level off somewhere in the flight levels. Then pull back to LOP and settle in for the trip.

 

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1 minute ago, Hank said:

Search here for Target EGT. That's what I do when I remember:

Takeoff on a standard day, as xlose to 29.92" and 59°F as you can get, and record your EGT as soon after rotation as you have brain power to do. Then shoot for that same EGT on every takeoff, leaning as needed ti maintain that value all the way from runway to cruise, when you will set power and lean anyway. 

Approximately. Best I can remember. YMMV. 

I wouldn't bother being exact, just have that number in your head and take a couple swipes at the mixture knob every thousand feet or so to keep the EGT a little below that number.

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10 minutes ago, jaylw314 said:

I wouldn't bother being exact, just have that number in your head and take a couple swipes at the mixture knob every thousand feet or so to keep the EGT a little below that number.

With a mixture lever on the quadrant, it's a very inexact process . . . . But I still need a good Target value to aim for.

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2 minutes ago, Hank said:

With a mixture lever on the quadrant, it's a very inexact process . . . . But I still need a good Target value to aim for.

Good point, I hadn't thought of that--I guess I've been spoiled with the Vernier cable :rolleyes:

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2 minutes ago, larrynimmo said:

in the summer...when I am not in a hurry....I fly at 2,200RPM, 25" 7GPH, and get 125KIAS at about 2,000 ft. lean of peak, 1490 EGT, 380 highest head temperature... Effective air speed about 128K and cruise at above 20 statute miles per gallon.

Why do you fly so low??? Altitude is good for performance, and it reduces summer heating of the pilot.

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