The 25/25 myth

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JC150

Pre-takeoff checklist
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JC150
I've owned a normally aspirated piper arrow iii for about a year now and was taught by a flight instructor when i bought the plane to reduce mp and prop to 25/25 at around 500ft AGL on climb. About a month ago I discovered John Deakin's article on avweb. After reading the article, I browsed other articles online that contradict what I learned from this cfi and also state that it can be dangerous to operate in this fashion. After checking my POH, I noticed there is nothing in there that mentions reducing manifold pressure or rpm during the climb, and the performance charts clearly state full power and 2700RPM.

I've gone around and asked at least 15 flight instructors at my local airport, and found out they all teach the 25/25 method because that's also how they were taught.

I'm curious if anyone on this forum runs full power and 2700rpm, or instead uses the 25/25 method on climbout. I feel hesitant to run max power because everyone else seems to use the 25/25 method.
 
Not sure what engine you're talking, but on a 550 we climbed at the top of the green MP & RPM.
 
My arrow has the Lycoming IO-360, 200hp. I read online when you pull the MP to 25", the automatic controller reduces fuel flow which could overheat the engine since it relies on higher fuel flows to keep the engine cool. I'm not sure if this is true, interested in everyone's opinions.
 
I was always taught to use 25/25 in the C172RG with a 180hp O-360. The PIM states that's the proper setting for a normal climb, but a maximum performance climb to use full throttle and 2700RPM. Seeing as there are no limitations related to that, I'd say it's acceptable to use indefinitely. I think perhaps most people use the 25/25 because it's a bit easier on the engine, and you'll normally get plenty of climb performance.
 
JC150 said:
My arrow has the Lycoming IO-360, 200hp. I read online when you pull the MP to 25", the automatic controller reduces fuel flow which could overheat the engine since it relies on higher fuel flows to keep the engine cool. I'm not sure if this is true, interested in everyone's opinions.
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Two ways to keep the engine cool, add excess fuel, reduce fuel (run LOP), take your pick. One will climb you a bit faster, the other will save you money on fuel and maintenance.
 
Henning said:
Two ways to keep the engine cool, add excess fuel, reduce fuel (run LOP), take your pick. One will climb you a bit faster, the other will save you money on fuel and maintenance.
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Depends how hard you climb, I'm still on board with fuel being cheaper than jugs.

In cruse, if equipped for it, LOP is cool, but for a aggressive climb no thanks.

Ask the big DZs, they climb for a living, won't find one running LOP, even the ones with nice JPIs.

I'd consult with your POH or the engine manufacturer, most CFIs are like dogs, one pukes up something interesting, the other eats it, pukes and so on.
 
I believe it is taught to reduce workload on training engines and noise abitment procedures. I teach it because that is what our in-house mechanic wants to have happen on the engines.

Now, that being said, I also think it is a myth to always have the MAP under the RPM at all times. Your engine isn't going to break if you run at 26"/2400 RPM. Sure it doesn't look good, but the engine can handle it.

Read some of these articles, specifically the ones near the bottom for your question.

http://www.avweb.com/news/pelican/182544-1.html?redirected=1
 
James331 said:
Depends how hard you climb, I'm still on board with fuel being cheaper than jugs.

In cruse, if equipped for it, LOP is cool, but for a aggressive climb no thanks.

Ask the big DZs, they climb for a living, won't find one running LOP, even the ones with nice JPIs.

I'd consult with your POH or the engine manufacturer, most CFIs are like dogs, one pukes up something interesting, the other eats it, pukes and so on.
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You are under the misconception that more fuel saves jugs. This is incorrect, excess fuel costs jugs. It is the cause of burnt valves, sticking valves, worn valve guides, premature ring wear, and ring seal washout.

Combine carbon with lead and you get a well stuck hard abrasive coating that blocks heat displacement from the valve face to the seat.

The only thing you are preventing with going rich is detonation. There are three ways to get out of, or prevent detonation, add fuel, subtract fuel, subtract load.

You hit upon the key issue though, "how hard you climb". How hard do you really need to climb? Do you need to climb so hard that you are running into detonation risk? BTW, with most .5:1 HP:CU light aircraft engines, you don't have a lot of risk, especially when 8.5:1 or lower compression pistons are involved. If you want to climb hard, leave the RPM at max, that will keep the load down and allow you to still reduce fuel. You also have to remember with a naturally aspirated engine, you reduce both power and load as you climb.

Unless you're trying to out climb a mountain ridgeline though, it's a lot more economical to use a reduced power, lean climb and it's much kinder on your engine.
 
It is definitely easier on the engine to bring it back a bit. Monitor the CHT's and make sure you are staying cool. If it is not restricted in the manual, then you can run it anywhere you want. Usually there are sample RPM/Manifold Pressure examples in the manual. If in doubt use those. I turn mine back at 500' AGL too. It is commonly done that way.
 
When I want to reduce power in a climb I leave everything forward and lean back 10° LOP and watch the CHT and fuel flow. When operating LOP fuel flow is a direct indicator of % power and low CHT verifies a good burn timing and by maintaining maximum RPM I am reducing my load and Inner Cylinder Pressure from what they would be to achieve the same performance at reduced RPM.

If you want to reduce power, you are most efficient, and kindest to your engine, to do so by removing fuel. You can cool the cylinders using fuel or air. Air is a hell of a lot cheaper with no destructive by products.
 
This is all very interesting information. I also heard somewhere that you are running the risk of detonation if you reduce MP on climb. Does anyone know if this is true? I read online that the 25/25 idea came about in the "old days" when engines were restricted in how long they could be run at full power. But the Arrow can run full power according to the manual.
 
Reducing power in the climb is hard on the engine. It just prolongs the time in the climb.

I also have lycoming 360's. I leave props full forward and throttle wide open. Reduce rpm when leveling off for cruise. Don't touch the throttles, they stay wide open until partway through the descent at the end of the flight.
 
JC150 said:
This is all very interesting information. I also heard somewhere that you are running the risk of detonation if you reduce MP on climb. Does anyone know if this is true? I read online that the 25/25 idea came about in the "old days" when engines were restricted in how long they could be run at full power. But the Arrow can run full power according to the manual.
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The only potential for detonation on reduction of throttle would be a very brief transitory one as you just crack off full throttle and the enrichment circuit closes but you are at high load. The Arrow will run 2000hrs at WOT Redline RPM flowing 10.5gph; if you fly like that twice a week every week and give you very few problems. You can reduce RPM, especially at altitude, and increase your efficiency at 2400rpm and bring your fuel flow back around 7.7-8gph without giving up a bunch of speed.
 
JHW said:
Don't touch the throttles, they stay wide open until partway through the descent at the end of the flight.
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+1

I do back the prop off of redline just a bit at 500' AGL...when I remember. ;)
 
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A lycoming should always be climbed at WOT. Both the carb'd and the injected ones are designed to increase the cooling a bit at this setting. If the max continuous is not WOT/HI RPM, then reduce the RPMs after takeoff. Do what the manufacturer says not some old wives tale your instructor gave you.
 
whom I do not know but I'm using 25/25 method. Because I have taught my dad and he at one time also taught this method
 
JHW said:
I also have lycoming 360's. I leave props full forward and throttle wide open. Reduce rpm when leveling off for cruise.
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I keep climb power for 30-45 seconds after leveling off to allow the airframe to accelerate to cruise speed, then as you, reduce rpm and then go LOP. I leave the throttle full forward during climb, cruise and part of decent.
 
There is one other way to cool the engine: drop the nose and speed up.
 
Bringing the power back to 25/25 is a pure myth in the arrow. I fly my Bonanza the same way I fly a C152, full throttle. I stopped pulling the power back after I installed a JPI and noticed that the engine got hotter when I pulled the power back. Pulling the power back is harder on the engine, not easier. The only adjustment I make is to retard the prop somewhere during the climb, but I am in no hurry, so it is usually pulled back from 2700 RPM to 2500 RPM by the time I reach 3000 feet and rarely will I pull it back before 1000 AGL.

If you notice what is going on as you retard the throttle, you have to move it a significant amount before it starts to exhibit a reduction in MP. This is because during the early portion of the throttle reduction, all you are doing is leaning the engine. The Arrow has much less horsepower and pulling the throttle back reduces the climb rate substantially and makes it run hotter.
 
Bill Jennings said:
I keep climb power for 30-45 seconds after leveling off to allow the airframe to accelerate to cruise speed, then as you, reduce rpm and then go LOP. I leave the throttle full forward during climb, cruise and part of decent.
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A Mooney driver
 
HighFlyingA380 said:
I think perhaps most people use the 25/25 because it's a bit easier on the engine...
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No, it's not. Did you even read the article(s)?

To everyone else who says it's easier on the engine, did you even read the article(s)? This has got the be the biggest display of ignorance I've seen on here. It's not April 1st, is it?

:nonod:

I'm WOT until the pattern entry on the destination airfield. I do pull the props from 2800 rpm to 2600 for noise. Those things are loud. Props come back to the bottom of the green arc for descent. I'm in the yellow until it gets bumpy.
 
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Jaybird180 said:
Nothing more than: it is recommended for Mooneys per the MAPA articles.

Don't read anything into it:D
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OK, but yeah, everything I've read on Mooneys with 360 engines is full power all the way up! Our plane loves it, I get 120kts at 500fpm climb and good temps.
 
By the way, "square" is a myth. It's a convenient power setting purely by coincidence on certain engines. On my old gopher engine, I never got anywhere near square. Cruise power was something 23" and 3000 RPM.
 
The anything-squared rule is urban legend, rooted in some high compression radial engines. There has been plenty of research and documentation covering this. I basically teach to do a 1st power reduction at either 500 or 1000 feet. What this consists of depends on the airplane, but there is never any consideration given to whether RPM and MP are "squared".
 
My 182 is limited to 2400 RPM, I normally climb out at full throttle and 2400 RPM until I reach my cruise altitude, then it's 2300/22 or 23 inches of MP or full throttle. Seems to work fine, our old 182 had the higher RPM engine and I remember pulling the prop back slightly at 500 or 1000 ft. I don't remember the exact setting, but I think it went from 2650 to 2500?? I could be wrong, it's been over 20 years and I am not real sure what I had for breakfast yesterday. :D
 
write-stuff said:
The anything-squared rule is urban legend, rooted in some high compression radial engines. There has been plenty of research and documentation covering this. I basically teach to do a 1st power reduction at either 500 or 1000 feet. What this consists of depends on the airplane, but there is never any consideration given to whether RPM and MP are "squared".
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I wasn't comfortable doing this when I flew my 1st blue-knob airplane. I commented to the CFI that I'd prefer to at least get to TPA before making a power change. He was fine with that.

I want to get to safe altitude ASAP before making any changes. If it's working and I change something, I don't have time to troubleshoot how to undo the not-working condition.
 
When I was doing my Commercial stuff in the Arrow III, my instructor taught me 25 squared at 500-700 AGL pitch for 90 kts all the way up to cruising altitude. Then lower the nose and build up some airspeed. Then 23 squared for cruise.
 
The POH in my Arrow II clearly states to pull back 25/25 after obstacle clearance. This is wrong??????
 
My 68 Arrows Operators manual states...

" After take off has proceeded to a safe altitude, recommended climb power is 25 inches of manifold pressure and 2500 RPM."

Section III page 25

I am now confused...but that's not hard to do these days...
 
Sac Arrow said:
The POH in my Arrow II clearly states to pull back 25/25 after obstacle clearance. This is wrong??????
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If the manual says it, do it. No one is suggesting that squaring is wrong in some cases, just that it's not a mandatory rule.
 
Cpt_Kirk said:
No, it's not. Did you even read the article(s)?
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Umm, no, they were posted after my post, so that would have been difficult... But I am now. As I said, that's just what I was always taught, and it sounds like that's the norm at flight schools.
 
Jaybird180 said:
I wasn't comfortable doing this when I flew my 1st blue-knob airplane. I commented to the CFI that I'd prefer to at least get to TPA before making a power change. He was fine with that.

I want to get to safe altitude ASAP before making any changes. If it's working and I change something, I don't have time to troubleshoot how to undo the not-working condition.
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That's fine. 1000 feet is generally TPA for piston engines. And also, depending on your make/model, the 1st power reduction may be very small.
 
John Deakin has this to say about the silly CFI (and some POH's, apparently) regurgitated 25/25 thing -

Remember that this is the chart of operating parameters at 25" and 2,500 RPM. Does that setting sound somehow familiar? Yes, it's the setting many have used for years right after takeoff, for climb power! The high temperatures are the direct result of pulling the MP back to 25", which cuts out the power enrichment feature of the fuel controller, leaning the engine dramatically!

This practice (pulling the throttle back after takeoff) may be the single most damaging thing many people do — with the best of intentions, to "make it easier on the engine!" Forget it, folks. Leave that throttle fully in, unless you need to make a substantial reduction in MP, maybe to 22", or so. (Remember, we're talking about normally-aspirated engines here.

I have never understood why people think you need to pull MP back right after takeoff. Pull RPM back a little if you want to reduce noise, but no reason to pull MP. Those flying fixed pitch props don't pull power back after takeoff - full power is used. A common climb RPM is in the 2300-2500 range with fixed props. This is no different than using full power (max MP) and adjusting the CS prop to those same RPM numbers. If it doesn't hurt anything with a fixed pitch prop, it sure won't hurt anything with a CS prop. It's the same thing really.

It's amazing how many pilots and CFIs advocate and use techniques only because they were "taught that way". There's so much good information out there for those who really want to educate themselves. Break the cycle of regurgitated OWT's.
 
RoscoeT said:
John Deakin has this to say about the silly CFI (and some POH's, apparently) regurgitated 25/25 thing -

Remember that this is the chart of operating parameters at 25" and 2,500 RPM. Does that setting sound somehow familiar? Yes, it's the setting many have used for years right after takeoff, for climb power! The high temperatures are the direct result of pulling the MP back to 25", which cuts out the power enrichment feature of the fuel controller, leaning the engine dramatically!

This practice (pulling the throttle back after takeoff) may be the single most damaging thing many people do — with the best of intentions, to "make it easier on the engine!" Forget it, folks. Leave that throttle fully in, unless you need to make a substantial reduction in MP, maybe to 22", or so. (Remember, we're talking about normally-aspirated engines here.

I have never understood why people think you need to pull MP back right after takeoff. Pull RPM back a little if you want to reduce noise, but no reason to pull MP. Those flying fixed pitch props don't pull power back after takeoff - full power is used. A common climb RPM is in the 2300-2500 range with fixed props. This is no different than using full power (max MP) and adjusting the CS prop to those same RPM numbers. If it doesn't hurt anything with a fixed pitch prop, it sure won't hurt anything with a CS prop. It's the same thing really.

It's amazing how many pilots and CFIs advocate and use techniques only because they were "taught that way". There's so much good information out there for those who really want to educate themselves. Break the cycle of regurgitated OWT's.
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Concur with all of this.

I prefer to have as much power as I can get my hands on during climb. In the 182, I'll climb WOT. After I level off, if the MP is still above the green, I'll pull it back there (or more for economy), but depending on the altitude, that's not necessarily an issue.
 
Would 25/25 be recommended if we used SI units for manifold pressure?

I think not.
 
JimNtexas said:
Would 25/25 be recommended if we used SI units for manifold pressure?

I think not.
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Yeah, 847 mb / 25 RPM just doesn't have the same ring. Not to mention that it's purely coincidental that two totally unrelated units of measure (inches of MP and RPM x 100), if "squared", happen to fall within an acceptable power/RPM combination range. There is zero magic to these two numbers being equal, or "squared".
 
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