M20K climb settings

[marooneypilot]

I was taking a look at the POH recently for climb performance and noticed that while the "normal" climb suggests 110KIAS and reduced power (34"/2600), there are no performance charts for this configuration. As far as I can tell the time/fuel/distance to climb chart is for the best-rate climb (40"/2700/96KIAS)

This strikes me as odd. Does anyone know where one would find performance charts for a normal climb? How do y'all normally configure to climb?

David

[Jsavage3]

I have a 252, but it’s certainly close enough to be relevant. Before, I always climbed out with everything fire walled (36” MP, 2700 RPM and mixture rich for 23 GPH) all the way up. Recently I’ve started climbing as the POH calls for, which is 32” MP and 2500 RPM with mixture still full rich...this profile gives me just under 17 GPH...all the way up. What I’m seeing is more fuel on board at TOC and the time-to-climb penalty is essentially negligible. This cruise climb profile costs me 100 FPM, but I’m still climbing at the same 120 KIAS that I always have. Seems like a good trade off to me...IMHO that is.

For vertical speeds, I see the following at 120 KIAS:
Max power climb (23 GPH)...800-1000 FPM (cold vs hot weather)
Cruise power climb (17 GPH)...700-900 FPM

I wish to point out that I climb at 120 KIAS no matter what I do with the power...whether I max climb or cruise climb.  I do pay close attention to my CHTs in the climb, but they always stay very cool...around 330-350...maybe 350-380 in the summer time...and that’s with my cowl flap in the trail position.

These vertical speeds are dialed into the A/P, so these numbers are pretty accurate b/c it holds them alot steadier than I can...

Edited January 24, 2018 by Jsavage3

[marooneypilot]

What vertical speeds are you seeing in each of these configurations?

[jrwilson]

5 hours ago, djm181 said:

I was taking a look at the POH recently for climb performance and noticed that while the "normal" climb suggests 110KIAS and reduced power (34"/2600), there are no performance charts for this configuration. As far as I can tell the time/fuel/distance to climb chart is for the best-rate climb (40"/2700/96KIAS)

This strikes me as odd. Does anyone know where one would find performance charts for a normal climb? How do y'all normally configure to climb?

David

do you have intercooler or anything?  231 or 252? 

[marooneypilot]

I have a 231 with an intercooler. The numbers I put up here are the book numbers, so obviously MP would be reduced in my plane, but even so it strikes me as odd that the POH lists reduced power for a "Normal" climb in the procedures section, yet only provides performance data for a Best Rate climb.

[kortopates]

I don't personally recommend using the cruise climb power setting but do recommend a cruise climb speed well over Vy. Instead I prefer to climb at WOT to my desired altitude yet may pull back the RPM for noise, but not MAP. The reason being is desire to be as rich as possible to keep ICP and CHTs as low as possible. Using the extra power to climb at a higher IAS helps further to reduce CHT and as @Jsavage3 says above, the loss of climb rate from the faster IAS in climb is pretty negligible.

My advice is if you want to reduce power, do so with RPM only. Full rich at WOT will provide the lowest EGT/TIT's which is an indication of how rich your mixture is. 32" and 2500 rpm, being only 81% power might indeed be cool enough to climb at full rich since its just a bit more than cruise, but I prefer a faster IAS in climb with a small reduction in rate yet while also keeping CHT down.  I would suggest watching out for any reduced MAP cruise climb power setting that pushes TITs to 1400F or above as I would really refrain from operating too lean like that. Not everyone has there max fuel flow set up quite high enough to begin with.

Edited January 24, 2018 by kortopates

[marooneypilot]

Interesting reasoning. I've never heard that reducing power (specifically MP) results in a leaner mixture - reduced fuel-flows, sure, but mixture is about ratio. Is there further reading you could point me to on that topic?

[kortopates]

1 hour ago, djm181 said:

Interesting reasoning. I've never heard that reducing power (specifically MP) results in a leaner mixture - reduced fuel-flows, sure, but mixture is about ratio. Is there further reading you could point me to on that topic?

It does from the standpoint that all our fuel systems are designed to run richest at full WOT redline. Consequently mixture is not exactly linear with MAP and should be richest at redline MAP.  Further, if one is reducing MAP because out of concern for higher CHTs, they'll see better cooling by keeping the the MAP up and reducing the RPM.

Here are a couple articles that will go into more detail:

First, https://www.avweb.com/news/pelican/182104-1.html this is probably the best example I can think of because it discusses what happens when you make any one change in isolation - discussing mixture, RPM and MAP one at a time and how they change the effective timing of peak ICP. It all adds up to make the point that greatest efficiency comes from WOT MAP and to use RPM to reduce power or speed if need be, not MAP. 

Another article that speaks to the efficiency argument directly is this one by Mike Busch. : https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-10_flying-efficiently.pdf 

I somewhat stated the MAP effect on mixture backwards since virtually everything you'll read will say it the other way around, that the engine is designed to be most rich at WOT.  So you might also look at your own EGT/TIT data from reducing MAP from WOT maintaining full rich and see EGT go up with MAP reductions. But both of the first two articles above make the point quite well. But perhaps efficiency make a more compelling reason to keep MAP high and just reduce RPM in climb out since the second article clearly shows by contrasting two different identical power levels with the same FF that the higher MAP results in lower EGT and CHT which equates to larger detonation reserves and lower and ICP - all good things. (although we got into this topic just on the basis of reducing power from 100%, my corollary point was to only reduce RPM is wanting to reduce power)    

You are probably aware of Bob Kromers article years ago about climbing all Mooney's at WOT and inn this article he specifically talks about climbing WOT in the 252  all the way to top of your climb; although no real science here but largely good advice from a past well respected Mooney test pilot. http://www.mooneypilots.com/mapalog/M20K252_evaluation_report.htm I don't agree with all that is said but he has the right concept about how the engine was designed to run coolest and most efficiently. 

[MIm20c]

@kortopates

The first article talks about 25 squared and how it can be damaging to the engine. I noticed in the example given all the pressures look good on the pulled back curve with his only complaint being high EGT readings. However, the data was given for pulling throttle only not T/P to 25 squared. From the other graphs it looks like reducing the RPM should move the peak towards the 18 degree ideal and lower EGT’s and CHT’s.  To me the data reinforces the advantage of climbing at 25/2500*. 

*My plane is probably a special case because I can’t keep CHT’s under 380 at WOT/2700 with my fixed jet and my turbo stc specifies 25 squared as an acceptable power setting up to 25k. 

[kortopates]

59 minutes ago, MIm20c said:

@kortopates

The first article talks about 25 squared and how it can be damaging to the engine. I noticed in the example given all the pressures look good on the pulled back curve with his only complaint being high EGT readings. However, the data was given for pulling throttle only not T/P to 25 squared. From the other graphs it looks like reducing the RPM should move the peak towards the 18 degree ideal and lower EGT’s and CHT’s.  To me the data reinforces the advantage of climbing at 25/2500*. 

*My plane is probably a special case because I can’t keep CHT’s under 380 at WOT/2700 with my fixed jet and my turbo stc specifies 25 squared as an acceptable power setting up to 25k. 

Carbureted Turbo's are indeed an added challenge. Frankly the carbureted O-360 can be challenging as it is to keep CHTs in check in climb.  Have you tried or considered running at 29" and a lesser rpm. 29" and 2100 rpm should be very close to your 25 squared but cooler. Or the max MAP you can maintain with a lower RPM for equivalent power to make the comparison?

[milotron]

3 hours ago, kortopates said:

It does from the standpoint that all our fuel systems are designed to run richest at full WOT redline. Consequently mixture is not exactly linear with MAP and should be richest at redline MAP.  Further, if one is reducing MAP because out of concern for higher CHTs, they'll see better cooling by keeping the the MAP up and reducing the RPM.

 

Are you suggesting to run WOT ( 36 or 39" on the M20K ) at reduced RPM say 2500? Conceivably maximum enrichment as well as full rich on the mixture?

I had considered this in the past but assumed cylinder pressures would be excessive. There is no setting in the POH corresponding to this either.

 

iain

[rainman]

I refer you to Norman Howell's masters degree thesis on cruise climb Vz. It includes getting furthest distance by cruise altitude. I found it on google.

[marooneypilot]

Thanks @kortopates - fascinating stuff!

[kortopates]

11 hours ago, milotron said:

Are you suggesting to run WOT ( 36 or 39" on the M20K ) at reduced RPM say 2500? Conceivably maximum enrichment as well as full rich on the mixture?

I had considered this in the past but assumed cylinder pressures would be excessive. There is no setting in the POH corresponding to this either.

 

iain

Yes, with your -MB climb at 36" WOT, but preferably at full RPM, yet if you feel the need to reduce power I am advocating reducing RPM only, not both. Your engine is designed and certified to run at 100% power all day long. However, if I feel compelled to reduce power in climb, such as for noise abatement purposes, I'll just pull back RPM to 2400 maintaining redline MAP for about 92-93% power while maintaining Vy+ ~15-20 kts. Not quite Carson speed or Vz but still at a good climb of rate. Although @rainman comments about Vz, its an important consideration, but my climb speed is much more biased to safety and other factor discussed in Deakins article Vx & Vy Debunked with less concern for optimal fuel efficiency. and perhaps more concern for engine efficiency.

As for the POH's of the K's, all give a max performance climb with full power and a cruise climb setting, but the 231's performance tables only included data for the max performance climb at full power to the flight levels.  Not till the 252's did the performance section also provide performance data for their cruise climb setting which equates to 81% power. There is nothing magical about the RPM and MAP combination that Mooney publishes for cruise climb nor the many more various equivalent cruise power combinations of MAP & RPM they list the cruise tables. The key difference is that any ROP power setting above the max cruise power setting should be done full rich, never leaned. 

[milotron]

Thanks @kortopates, this makes sense. I am a little uncomfortable allowing full boost pressure at too low an RPM and don't really know where that point is. The whole concept of 'lugging' the engine at high MP and low RPM is magnified in the turbo in that it can make full MP for most of the RPM range.

I am reading through the Vz thesis as well as re-reading Deakins article. All very interesting stuff. Can't wait to get the plane out of annual and get some flying in.

 

iain

[kortopates]

Thanks [mention=7862]kortopates[/mention], this makes sense. I am a little uncomfortable allowing full boost pressure at too low an RPM and don't really know where that point is. The whole concept of 'lugging' the engine at high MP and low RPM is magnified in the turbo in that it can make full MP for most of the RPM range.
I am reading through the Vz thesis as well as re-reading Deakins article. All very interesting stuff. Can't wait to get the plane out of annual and get some flying in.
 
iain

At least in cruise, over square is good! Please check this article out. https://www.avweb.com/news/airman/184483-1.html
But in climb, we should be at higher power settings between; preferably 90-100%.


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[milotron]

10 hours ago, kortopates said:

At least in cruise, over square is good! Please check this article out. https://www.avweb.com/news/airman/184483-1.html
But in climb, we should be at higher power settings between; preferably 90-100%.


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I have always taken the comments in the Bob Komer MAPA reviews for the 252 indicating that 2500 RPM was most efficient for the prop and intake system and tried to keep around that point. Logically, yes there is less windage losses, frictional losses, etc at lower RPM and that is evident in the fuel flows at different RPM for the same power setting.

I'm going to give the 2200 RPM ranges a try. Certainly much quieter. If I can go LOP and run at 65% with the same airspeed, this would be the way to go.

 

Somewhat on topic, but one of the guys at my flying club runs a Columbia 350/400 and can run around 2200 at way LOP and gets all the way from Victoria to Toronto without a fuel stop. ( east bound only of course ) 1800nm, and in the high teens, but impressive none the less. He is also an engineer and over analyzes things too, coming to similar conclusions as noted here.

 

Good conversation!

[jetdriven]

These props aren’t loud at 2700 RPM. Not like a Bonanza or a 185. Pulling the prop back to 2500 RPm for noise abatement actually makes the airplane climb slower, which translates to lower altitudes along your flight path departing the airport. So which is louder a Mooney flying overhead at 1500’ at 2700 RPM,  or one flying overhead at 1200’ at 2500 RPM.  I just don’t see convincing evidence pulling the prop back saves anything, time, fuel, noise pollution, engine life, or anything else. 

[jlunseth]

I do the same thing Paul does.  A couple of things.  First, there is an interlink between the throttle and fuel flow in the 231, so if you change one, the other changes also.  The interlink has a “bump up” as the engine nears full power, in other words, the mixture becomes quickly richer.  If you lean out a couple of inches from full power, the interlink brings the fuel down faster than it does in the cruise power range, in other words the mixture becomes leaner.  Don’t ask me how this works exactly, I just know from experience that it does.  So to produce full rich mixture for takeoff and climb it is important to run the engine at full power.

Also, Paul is right that most 231 engines are not set rich enough, particularly those with an intercooler.  The intercooler reduces the MP at which the engine makes full power.  So if the engine is set up to make 22.5-24 GPH at 40” of manifold pressure and 2700 RPMs (that’s the factory engine full power setting) and the mechanic uses the SID for the factory engine to set the mixture, and then you actually use 36 or 37” on takeoff because that produces 100% power with the intercooler installed, the engine is now running at full power but leaner than 22.5-24 GPH.  There are instructions to modify the standard factory engine settings that are in the STC, almost no mechanic uses them unless you provide them yourself.  Footnote: its not called a SID anymore, Paul knows the name, I just don’t remember.

I just climb at 500 fpm and descend at 500 fpm, makes it easier on the passengers’ ears, and the math is easy.  I might fudge the climb to around 700 fpm.  This lets the engine stay cool.

Cruise climb settings work ok in many weather conditions, but if you have ever done a takeoff and climb in “high, hot” conditions, such as departing Phoenix when its one hundred and teens, or departing high plains airport during the summer, you need all the fuel flow you can get to keep the cylinders cool.

Here in MN, I can climb at whatever power setting I want to in the winter, the problem is to get the engine warm enough, not to keep it cool.  Summer climb out of Rapid City SD to the flight levels is a different deal entirely.