The 25/25 myth

There are no magic numbers valid for all aircraft/engine/prop combinations. Read the book for yours, then fly it as the book says. And if you get an instructor who wants you to use some "standard" power settings that aren't based on the book for the airplane you're flying, find another instructor.
 
The POH in my Arrow II clearly states to pull back 25/25 after obstacle clearance. This is wrong??????

The only portion of the POH which you must follow is contained in section 2.0 Limitations. If it is not there, then it is merely a recommendation and many of the recommendations are no longer the best advice.
 
Why reduce climb rate, especially at low altitude? I have never bought the idea that MP needs to be cut back, especially then the power setting charts in the POH show that full power is okay.
 
I also agree with John Deakin [and Bob Kromer]. At the beginning of the runway, everything goes fully forward and stays there until I level off at cruise altitude and accelerate enough that the ASI is steady--call it a minute or so. Then I set desired power.

Nearing destination, I push for 500 fpm, trim to hold it, and as I come down I walk the throttle back and the mixture forward to hold whatever my cruise settings were for MP and EGT.

But I don't fly an Arrow . . . and there's no "I" at the front of my O-360.

Pulling throttle back during climb only makes the engine warmer and the climb take longer. Try it yourself and see the difference.
 
Our plane loves it, I get 120kts at 500fpm climb and good temps.

Interestingly enough, the pitch setting I have dialed in right now on my ground-adjustable Sensenich prop also gives me about 2500 rpm at 25" MP when I pitch for climb at about 120 kts at wide open throttle (after takeoff from 1000MSL field elevation home airport.) Climb rate in the RV at that pitch and power setting is just a wee tiny bit better however :wink2: ;) :thumbsup:
 
Flying a variable pitch (air adjustable pitch) prop has taken a lot of the mystery about out of the "blue knob" for me. For the most part, I have three "not a good idea" areas:
- must pull the prop back to 2050 after 1 minute (POH HP limitation)
- Stay below 2500 RPM - prop is made of unobtainium and 3000 RPM is bad
- Don't get the RMP too low and the MP too high - basically reset the prop before landing (PUFFs)

That's pretty much it. You can "chase the RPM" trying to "stick" 2050 (e.g. like a prop with a governor), but it really isn't necessary. Get it close and check back from time to time ...
 
Arrow II (angled valved 200) owner here. Yep, I am a WOT/full-RPM in the climb operator as well. Pulling back the black lever in order to get 25" throws off the controller and robs you of the enriching feature, plus the climb takes longer. It's not conjecture, the engine runs hotter in that reduced setting.You're not doing the engine any favors climbing with such restriction. I otherwise operate the aircraft exclusively <65% WOT in cruise and lean aggressively. So far so good.

And yeah, the square thing is just coincidental numerical convenience that got passed over as gospel from the days of engines with a different anatomy. That needs to stopped. CFIs are admittedly lazy about power/engine management particulars when otherwise overwhelmed with the task of PRIMARY flight instruction. That has been known for ages. One ground lesson should be devoted to instilling a more intellectually ambitious operating knowledge of power management and piston engine thermodynamics on primary students, which would pay dividends for life. Full rich everywhere you go and mickey mouse climb power OWTs are not helping anybody, mechanics' wallets maybe.
 
"Hotter" is relative. Keeping the cylinders under 375° by dumping a bunch of raw fuel into them is doing you no favors either. You're better off going 5°LOP to keep your CHT down and taking some extra airspeed and lower rate of climb. You'll save yourself some valve and guide work at minimum.
 
See pilots have trouble remembering numbers. So they made the numbers the same. 25 and 25. That way the pilot doesnt have to remember which is which.

So are you supposed to pull the rpm's back first, or pull the manifold pressure back first?????
 
The POH in my Arrow II clearly states to pull back 25/25 after obstacle clearance. This is wrong??????

It is not wrong. Neither is running it at full bore. Both are fine, and on a Lycoming I doubt will ever result in an observed reduction in cylinder life.

What's funny is everyone arguing that one is better than the other, do whichever floats your boat.
 
So are you supposed to pull the rpm's back first, or pull the manifold pressure back first?????

In general:

Power up, work the knobs right to left

Power down, work the knobs left to right
 
The POH in my Arrow II clearly states to pull back 25/25 after obstacle clearance. This is wrong??????
If that's what it says, then doing that is fine. If you choose not to pull back to that setting, make sure you do stay within any limitations on full power operation which might happen to be in Section 2 of your AFM/POH (e.g., some planes have a 5-minute limit on "balls to the wall" operation -- don't remember if an Arrow II has anything like that).
 
I'm sure it is. I'd like a ride in an rV-8 someday.

If you ever find your way to Wichita Falls TX, I could probably arrange that. I helped a good friend build one with 192hp ECI Titan IO-360 and Hartzell blended airfoil prop. Climb rates of ~2000 FPM with two aboard are common, and on a cold day solo I've seen as much as 3000 FPM in it. Makes my RV-6 feel almost like my old Cherokee in comparison :redface:.

I got to fly this on its maiden pilgrimage to Oshkosh right after it was completed and the Phase-1 hours flown off.
img3327r.jpg


And yes, In this beast I normally used all levers forward for takeoff and initial climb, and as soon as I reached 500-800' AGL (basically pattern altitude), reduced power to 2500/25" for the climb to cruising altitude since that's what the engine builder advised.
 
Last edited:
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...
If your manual says do that, then doing that is fine. Just remember that what it says in your plane's manual may not be the same as in some other make/model airplane's manual, so if you fly something else, fly that something else the way that something else's manual says to fly it.
 
Why reduce climb rate, especially at low altitude? I have never bought the idea that MP needs to be cut back, especially then the power setting charts in the POH show that full power is okay.
In some cases, it's because of cooling problems requiring a power reduction to keep from cooking the cylinders (e.g., many aircraft have a 5-minute limit on full MP/full-high RPM operation). In other cases, it's a noise abatement procedure (particularly pulling back RPM if the tips really sing at max RPM). The key is to read the book for what you're flying, and then fly it the way its book says to fly it unless you have a really good reason to deviate from the book (e.g., slowing a 172 below the recommended 65-70 KIAS on final with full flaps, especially with only two people aboard).
 
"Hotter" is relative. Keeping the cylinders under 375° by dumping a bunch of raw fuel into them is doing you no favors either. You're better off going 5°LOP to keep your CHT down and taking some extra airspeed and lower rate of climb. You'll save yourself some valve and guide work at minimum.
These numbers may be applicable to the TCM engines in his 310, but they may not be applicable to what you're flying. Read the book for your plane, fly the book for your plane.
 
See pilots have trouble remembering numbers. So they made the numbers the same. 25 and 25. That way the pilot doesnt have to remember which is which.
Sounds like Senator Iselin in "The Manchurian Candidate" trying to remember how many Communists he was claiming there were in the Defense Department:
Mrs. Iselin: [at meal time] I'm sorry, hon'. Would it really make it easier for you if we settled on just one number?
Sen. John Yerkes Iselin: Yeah. Just one, real, simple number that'd be easy for me to remember.
[Mrs. Iselin watches her husband thump a bottle of Heinz Tomato Ketchup onto his plate]
heinz_ketchup_bottle.png

[cut to Sen. Iselin addressing the Senate:]
Sen. John Yerkes Iselin: There are exactly 57 card-carrying members of the Communist Party in the Department of Defense at this time!
It sounded stupid from him, and it's equally stupid for us to "settle on just one number...that'd be easy to remember".
So are you supposed to pull the rpm's back first, or pull the manifold pressure back first?????
Depends on what you're flying, so READ THE BOOK ON YOUR PLANE AND SEE!
 
Last edited:
In my 182 P-Ponk, I follow the procedure given to me by the CFI's that taught me HP and checked me out on the plane: at 500-700 ft, go 24"/2400 RPM, lower the nose to normal climb, and then keep that until leveling off. I tend to then go to 23/2300. You're saying that damages the engine?
 
In my 182 P-Ponk, I follow the procedure given to me by the CFI's that taught me HP and checked me out on the plane: at 500-700 ft, go 24"/2400 RPM, lower the nose to normal climb, and then keep that until leveling off. I tend to then go to 23/2300. You're saying that damages the engine?
I'm certainly not saying that unless your POH says not to do it. So without addressing what your CFI's told you, what's the book on that engine installation say to do? And if you don't know that, you should do some research.
 
Well I HAVE noticed that if I pull the manifold pressure back first, it then goes up when I pull the prop back. Anyone care to splain that one?
 
Well I HAVE noticed that if I pull the manifold pressure back first, it then goes up when I pull the prop back. Anyone care to splain that one?

The articles linked in post #38 explain everything.
 
Well I HAVE noticed that if I pull the manifold pressure back first, it then goes up when I pull the prop back. Anyone care to splain that one?

Sure, but I'd butcher it. John Deakin explains it nicely.

It has to do with the filter, obstructions, and the natural path of the air versus the speed and effort to which the engine (or vacuum pump, if you want to call it that) works.

Edit: Looks like I'm a little slow. See above.
 
Deakin wrote nice stuff for AvWeb back when they had real articles.

Around here, we're already struggling on a hot day to make 65% power at takeoff, so it's WOT and only pull the prop back if the airport neighbors are particularly annoyed by anything that sounds like an airplane.

Whoever asked, yes the Continental powered 182 up until the U model is 2650 and no limitations on time. If you're struggling to climb, you can just flog it.

CHT is almost always the real limiting factor up here, and there's only one way to fix that... Cowl flaps stay open maybe even in cruise on a hot day, and the nose comes down during the wimpy climb. ;)

On a really hot day up here you might even have to decide which you want... Altitude or a cool engine.
 
"Hotter" is relative. Keeping the cylinders under 375° by dumping a bunch of raw fuel into them is doing you no favors either. You're better off going 5°LOP to keep your CHT down and taking some extra airspeed and lower rate of climb. You'll save yourself some valve and guide work at minimum.
Per Deakins, I've been leaning during the climb but not doing LOP climbs. Rather I've been noting my EGTs at 500' or higher, then leaning every 1,000' or so to maintain the same EGTs. As I understand it, this is cooling with extra fuel but doing so without dumping even greater amounts of extra fuel as higher altitudes naturally richen the mixture. It's conservative in the sense that at any point, the engine is 'parked' in a safe spot. That is, if I stop leaning nothing bad will happen.

The thing that concerns me about LOP climbs is that engine is not 'parked' in a safe spot for climbing. As the climb naturally enrichens the mixture, pressures and temps will climb as the mixture moves from LOP to Peak or even slightly ROP, aka the 'red box'. This especially would concern me going only 5deg LOP.

Of course the remedy is to monitor the EGTs during a LOP climb and if 5deg LOP is the target, one would need to continue to lean as one climbed. But neglecting this fine tuning could put you right in the red box, especially early in the climb.

If climbing LOP, do you continue to lean as you climb?
 
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.

I have a turbonorm'd O-540 and I use the AFM's "normal climb" setting of 25"/24 for optimal cooling once I get above TPA. If I go 31"/2400, which is the approved max performance climb, the oil gets hot, although CHT seems OK. Hell, the oil temp sits near redline even at 25/25 most summer days. I do not have an engine monitor (yet).

I will add that "oversquare" is a huge OWT. My TR182 manual says I can run the engine at any combination of 17" - 25" and 2100 to 2400 RPM in cruise. It says not a thing about that old oversquare nonsense, which I have seen refuted by many people in many places.
 
Thank you all for the responses, this has been very informative. I did a quick test flight after work with my arrow and noticed a higher climb rate and cooler temperatures using WOT. I feel more comfortable using this technique now. Thanks!
 
Thank you all for the responses, this has been very informative. I did a quick test flight after work with my arrow and noticed a higher climb rate and cooler temperatures using WOT. I feel more comfortable using this technique now. Thanks!

You came to the correct conclusion, and nothing beats seeing it with your own eyes in your own plane. Now that you've learned that, read up on the "Target EGT" method to lean properly in the climb to maintain optimal power without wasting fuel. (Bill W just mentioned it above.)
 
Per Deakins, I've been leaning during the climb but not doing LOP climbs. Rather I've been noting my EGTs at 500' or higher, then leaning every 1,000' or so to maintain the same EGTs. As I understand it, this is cooling with extra fuel but doing so without dumping even greater amounts of extra fuel as higher altitudes naturally richen the mixture. It's conservative in the sense that at any point, the engine is 'parked' in a safe spot. That is, if I stop leaning nothing bad will happen.

The thing that concerns me about LOP climbs is that engine is not 'parked' in a safe spot for climbing. As the climb naturally enrichens the mixture, pressures and temps will climb as the mixture moves from LOP to Peak or even slightly ROP, aka the 'red box'. This especially would concern me going only 5deg LOP.

Of course the remedy is to monitor the EGTs during a LOP climb and if 5deg LOP is the target, one would need to continue to lean as one climbed. But neglecting this fine tuning could put you right in the red box, especially early in the climb.

If climbing LOP, do you continue to lean as you climb?
This is why aviation needs computerized fuel metering.
 
This is why aviation needs computerized fuel metering.
I think the reason we don't have it is related to the reason we been slow on electronic ignition systems.

Our dual magneto based ignition systems run independent of our electrical systems and they fail hot.

Our fuel injection systems rely on engine driven pumps and pneumatic analog fuel controllers that run independent of our electrical systems. Short of physical contamination, if the engine can suck air, it gets fuel.

We would seem to be on the verge of giving our digital electronic gadgets the same confidence we give metal and plastic gadgets. Turning the control of ignition sparks over to a computer and making our electrical systems redundant and robust enough to depend on is a first step. Combining that with electronic fuel control looks like a logical next step and I guess that's what Cirrus's FADEC system offers (?).

But there is still a certain elegance and comfort in a mechanical gadget that use air pressure to modulate fuel flow just as there is comfort in an ignition system that creates and times the spark based on engine rotation.

Of course we still value hand flying over George's work....
 
"Hotter" is relative. Keeping the cylinders under 375° by dumping a bunch of raw fuel into them is doing you no favors either. You're better off going 5°LOP to keep your CHT down and taking some extra airspeed and lower rate of climb. You'll save yourself some valve and guide work at minimum.

Isn't the excess fuel or the rich mixture resulting from it slowing down the burn, which effectively retards the timing? That's how I had it explained sometime years ago. BTW, I'm asking this as a question, and not trying to make a statement in the form of a question. I'm just wondering if you can confirm or shoot holes in this? I've read a lot of your posts over the years and find you well read on most things aviation.

Anyway I climb FT and 2,700 and keep leaning as altitude builds targeting 1250-1260f on the EGT's. After level off I build speed and do the big mixture pull to 12.4 gph on the JPI which is right at 65% on my engine. After settling into cruise I may or may not richen slowly to find peak and then lean again for the desired setting LOP.

I envy you guys that climb LOP. I know it is doable, and is advantageous but I just don't have the stones to do it. Walk proudly!
 
Last edited:
I have one of the planes that has a reduced power setting after full power. I have to reduce to 2300 after 5 minutes. Of course, I can leave the MP full forward, but the pressure carb has an enrichment circuit that puts too much fuel in for my taste. So, I cut back to 2300 and about 23-25"MP depending on altitude. I can run full MP to altitude but that seems to burn more gas than I like, and it runs a little worse than if I cut back some. So, I can do 23/23 and use a lot less gas, and take a bit longer to climb, or I can leave it full pressure and climb faster.

25/25 isn't a bad starting point for a NA plane with a constant speed prop. Can't do much damage there, and it does make a lot less racket for the folks on the ground. There's little to no chance of running too lean with the mixture all the way forward. Now, if you start leaning in climb, things begin to get more delicate. If I had a NA plane with a 2700 red line, I'd prolly be using 25/25 unless it was prohibited by the POH.
 
I got to fly this on its maiden pilgrimage to Oshkosh right after it was completed and the Phase-1 hours flown off.
img3327r.jpg

Looks nice and is something to think about. When the daughter is off the payroll, we would only need two seats, and an RV-8 would be a lot of fun.
 
read up on the "Target EGT" method to lean properly in the climb to maintain optimal power without wasting fuel. (Bill W just mentioned it above.)

Hmm, first I've seen of that one, I'll try it. What I have been doing is full rich to 5000msl, then leaning to ~1350 for the balance of the climb. Why 1350? From experience my engine peaks at 1450-1500 depending on conditions, and best power is normally ~100ROP, so leaning to 1350 on climb puts me at 100-150ROP.
 
25/25 isn't a bad starting point for a NA plane with a constant speed prop. Can't do much damage there, and it does make a lot less racket for the folks on the ground.

Why does there need to be talk of a "starting point"? You're perpetuating the squared nonsense. The "starting point" is to leave full power and reduce RPM slightly only if you want. And you reduce noise by reducing RPM, not MP. You are not going to hurt anything by leaving full power and reducing RPM to 24-2500 if you want to reduce noise. If this hurts things, then engines with fixed pitch props would be blowing up left and right. Doesn't happen.

If I had a NA plane with a 2700 red line, I'd prolly be using 25/25 unless it was prohibited by the POH.

Again, why pull the throttle lever back unless specifically advised to do so in the POH? The reason why there's no reason to do this (or even why there are negatives) have already been covered here.
 
Lol. You can't even get 25" out here on the ground...
 
Back
Top