Initial climb with a constant speed prop

jd21476

jd21476

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jd21476
I learned to fly a constant speed prop airplane about 18 months ago and at that time my instructor basically said you take off with the prop full forward and full power similar to a fixed prop. Then he explained when you about 1000’ up or to where you could glide back to the airport if you had to, go ahead and adjust it to 25/25 for the remainder of the climb and then 24/24 for cruise. I have an IO-360 in my plane.

Recently I was discussing this with others and we have mixed opinions. Some will climb all the way with prop forward and full throttle and others will do 25/25.

Opinions?
 
If it’s a certified airplane it should say in the POH what to do. If experimental do what ever you like as long as it is within the operating parameters of your engine. Some engines can only run so long at max power. I’m my plane I just leave it all full forward. It climbs fast enough the decreasing air density lowers the power enough.
 
Most instructors I talk to do their technique not because the POH says to or because they did their homework but because that's the way they were taught.

In my Bonanza, I climb full throttle, full prop. At cruise altitude, I pull the prop to what I want to cruise, leave the throttle full open and pull mixture to LOP.

If you haven't read up on the guys at Gami and what they've done with their internal sensors on the test stand, it's worth your time. There's some reports on Savy Aviator too.
 
Check your AFM and engine's manual, if available. Look for any limitations on engine power (MP) or RPM. Look for recommendations for power settings in the climb charts in the performance section. If there are any limitations or recommendations, use those.

The "square" 25/25, 24/24, etc. is a worthless rule-of-thumb. As someone else mentioned, when it's taught, it because that CFI's CFI taught it and that CFI taught it because his CFI taught it, etc., etc. It isn't because there's any science behind it.

In a normally aspirated engine, leave the throttle wide open, unless a limitation requires reduction. If you are going somewhere you'll probably climb high enough that the throttle will stay at Full until you're in your descent. If you are going to reduce power for climb, do it by reducing RPM. This will reduce noise, both inside and outside of the airplane, and reduce engine wear.

For cruise, pick the lowest RPM setting that gives you the desired power percentage (cruise speed), smooth engine operation, and is within any cruise limitations for the airplane or engine.

In the descent, leave Full throttle and Cruise RPM until power must be reduced to prevent excessive airspeed or to manage engine cooling.
 
jd21476 said:
Its a 1959 and on many older planes the PoH was a pamphlet.
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In lieu of anything, I’ll bring the prop back off redline (if appropriate, our club’s 182rg says to leave it full in) just a bit and leave the throttle all the way forward. God will take care of the manifold pressure as you climb (assuming normally aspirated).


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jd21476 said:
Its a 1959 and on many older planes the PoH was a pamphlet.
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The engine manufactureres have guidance on engine operation, much of it online and free. The sparseness of info in an old owner's manual is no excuse for folks to go making up stuff and spreading it.
 
As always, the POH and safety take precedence over anything else.
But, all airports have noise issues and I would suggest you pull the prop back some as soon as possible so you don't fly like an AH.
 
At our airport, noise abatement requires reduced RPM for departure, if the propeller tip speeds are near sonic. A 100 or 200 RPM reduction produces a dramatic reduction of noise for some planes, such as our Cessna 172 with a Franklin, red line RPM 2800. 2600 made the inside noise very much less too. That reduction did not impair the safety of climb out in normal weather, and we did use max on very hot days, full gross.

Some Bonanza's on the field also had excessive prop noise, and reduced RPM slightly for peace with the neighbors.

Noise abatement can be an annoyance, but it really pays off if it prevents the closure of the airport. The performance reduction is slight, as most of the increased HP is used to produce the near sonic shock waves at the tip, not thrust.

A side note on "square" RPM and MP that are universally recommended by instructors. They are safe for all engines, so they don't have to know anything about your specific engine.
 
I climb at 32", 2500 RPM until 2,000 agl then maintain 30"/2500 until I reach my cruising altitude (typically 10k-15k range) then adjust to 25"/2300.
 
I always did 25-25-Lean 13gph for climbing to altitude in the 177RG. IO-360.
 
Wide open throttle for me, but I'll pull the props back ~25 to 50 RPM at 500 AGL and synch them up for the rest of the climb. Naturally aspirated Lycoming IO-540s. Typical cruising altitudes 8,000 to 16,000 depending on which direction I'm headed.
 
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GRG55 said:
Wide open throttle for me, but I'll pull the props back ~25 to 50 RPM at 500 AGL and synch them up for the rest of the climb. Naturally aspirated Lycoming IO-540s. Typical cruising altitudes 8,000 to 16,000 depending on which direction I'm headed.
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I do the same but drop RPM to 2600 in the climb, it cuts way down on the noise. According to the GAMI advanced engine management class dropping 100-150 RPM at WOT is ok.
 
Rgbeard said:
I climb at 32", 2500 RPM until 2,000 agl then maintain 30"/2500 until I reach my cruising altitude (typically 10k-15k range) then adjust to 25"/2300.
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Wait wait... let me get this straight.... you fly over-square at 32/25 and then 30/25 all the way to cruise?? And your engine hasn't exploded into a million pieces????

But we were all told when MP > RPM the result is massive rifts in the space-time continuum and the release of satan's henchmen to claim the Earth as their own... How has this not happened????
 
58 Model with O-540. I keep it within engine limits. 2575 is max RPM. After clearing obstacles/airport environment I keep the throttle wide open, dial back to 2400 or 2300 RPM, and start leaning. Have an engine monitor that gives me all the info to know if cylinders are ever hurting...they never are.
 
Deelee said:
Wait wait... let me get this straight.... you fly over-square at 32/25 and then 30/25 all the way to cruise?? And your engine hasn't exploded into a million pieces????

But we were all told when MP > RPM the result is massive rifts in the space-time continuum and the release of satan's henchmen to claim the Earth as their own... How has this not happened????
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Funny thing, ain't it?

I guess you don't wanna know I'm lifting off at 36"/2575, and pull it back to 32/25
 
Deelee said:
Wait wait... let me get this straight.... you fly over-square at 32/25 and then 30/25 all the way to cruise?? And your engine hasn't exploded into a million pieces????
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Rgbeard said:
I guess you don't wanna know I'm lifting off at 36"/2575, and pull it back to 32/25
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The things some people will do to their engines to get to the "impossible turn" altitude in a hurry...:D
 
Unless he can show you where in the POH or engine manufacturer’s guidance it says to pull it back to 25/25, dump him and get a competent CFI.
 
Larry in TN said:
The "square" 25/25, 24/24, etc. is a worthless rule-of-thumb. As someone else mentioned, when it's taught, it because that CFI's CFI taught it and that CFI taught it because his CFI taught it, etc., etc.
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That is quite false, numerous aircraft POH's specify 25/25 for the climb. But let's not let facts get in the way of making fun of CFIs.
 
jd21476 said:
I learned to fly a constant speed prop airplane about 18 months ago and at that time my instructor basically said you take off with the prop full forward and full power similar to a fixed prop. Then he explained when you about 1000’ up or to where you could glide back to the airport if you had to, go ahead and adjust it to 25/25 for the remainder of the climb and then 24/24 for cruise. I have an IO-360 in my plane.

Recently I was discussing this with others and we have mixed opinions. Some will climb all the way with prop forward and full throttle and others will do 25/25.

Opinions?
Click to expand...

Some aircraft AFMs have climb power settings for maximum climb and normal climb. The normal climb promotes engine longevity. If I was opting for the later in your engine I would select 25/2500 = about 80% power.
 
Some of us have POHs that were created before the dead sea scrolls. With absolutely no mention of MP/RPM settings for climb.

The 1969 Arrow POH discusses such things as best rate of climb, angle of climb and even a handy cruise climb speed (all in MPH !!!).

But nowhere in that simply written single paragraph is any mention of MP or RPM settings.

So what to do? Go off the ol' standby of pulling back power to 25" and square it up with a nice 2500 RPM setting... thus pulling the power back somewhere around 800 or 1000' AGL. Why? Because... well...... save the engine. Won't somebody please think of the engine??

But seriously? Why? There is no mention of power settings until you get to the cruise power settings... at cruise, you can (sort of) rely on the POH for guidance. But until then? According to the POH you can run it WOT and max RPM until the locals close down the airport due to the noise generated by your bug smasher's prop turning at full speed.

So what's the harm in being nice and bringing that prop back to 2500 so Karen McKarenson doesn't call the county to complain? Aside from the fact that I'll be over-squared on MP vs RPM and probably induce pressures so great a small black hole will form and allow aliens to pass from their dimension to ours....
 
I roll the prop back a little bit while on the takeoff roll. It pulls better a little off of max rpm. If I'm actually trying to climb a few thousand feet I'll leave throttle full while pulling the prop back to 2300. My engine just really feels smooth at 2300rpm. Manifold pressure? we don't get that much up here, I take off at 24, but at cruise I'm at 20, that's firewalled.
 
dmspilot said:
That is quite false, numerous aircraft POH's specify 25/25 for the climb.
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The rule-of-thumb is worthless.

I was quite clear that a pilot should look for limitations or recommendations in the AFM and engine manual and use those when they exist. If the AFM recommends 25/25, or has a METO limitation of 25/25, then you aren't using a rule-of-thumb when you set it. You are following the AFM.
 
If no POH, look up and follow the engine mfg’s recommendation.

Some planes are tightly cowled, so you might want to be more conservative than the engine mfg’s rules. If you have an engine monitor, let CHTs be your guide.

My 182P with IO-470F (which is rated to climb at full power by the engine mfg), based on others with engine monitors (I don’t have one) - I pull back to 25”, 2500 rpm after 1k ft. I climb well enough and figure if it contributes to engine longevity due to lower CHTs then it’s ok by me. If not, no downside? Maybe a little distraction or workload?
 
On a NA engine you aren’t gonna hurt anything with the black lever, but you might hurt things with the red one. Unless your POH says otherwise, no reason not to run oversquare. “Oversquare” is pretty much just a myth that just happens to correlate to how we measure manifold pressure. If we measured it in hPa like most of the world the term wouldn’t even exist.

If you installed a manifold pressure gauge in most fixed pitch planes, you would find that you are running oversquare most of the time. The horror!
 
Deelee said:
Some of us have POHs that were created before the dead sea scrolls. With absolutely no mention of MP/RPM settings for climb.

The 1969 Arrow POH discusses such things as best rate of climb, angle of climb and even a handy cruise climb speed (all in MPH !!!).

But nowhere in that simply written single paragraph is any mention of MP or RPM settings.

So what to do? Go off the ol' standby of pulling back power to 25" and square it up with a nice 2500 RPM setting... thus pulling the power back somewhere around 800 or 1000' AGL. Why? Because... well...... save the engine. Won't somebody please think of the engine??

But seriously? Why? There is no mention of power settings until you get to the cruise power settings... at cruise, you can (sort of) rely on the POH for guidance. But until then? According to the POH you can run it WOT and max RPM until the locals close down the airport due to the noise generated by your bug smasher's prop turning at full speed.

So what's the harm in being nice and bringing that prop back to 2500 so Karen McKarenson doesn't call the county to complain? Aside from the fact that I'll be over-squared on MP vs RPM and probably induce pressures so great a small black hole will form and allow aliens to pass from their dimension to ours....
Click to expand...

You have good points and good questions in your post. "Think of saving the engine..." Climbing at WOT IS saving the engine! Here's why. The GAMI people have built the most sophisticated engine test stand in the world to determine what really works and what doesn't. They test engines in much the same way the Wright Brothers tested their own airfoils. They found that during the climb more stress is applied to the engine if the throttle is pulled back off the stop, independent of Manifold Pressure. This is because at WOT the mixture is kept richer than at other settings, and leaning the mixture at near maximum power puts a lot more stress on the engine, much more so than running high manifold pressure below 4500 feet. Above about 4500 feet the MP will be below 25 anyway, but the recommendation is to maintain WOT during the entire climb.

Why is stress less? Most of our engines have fixed ignition timing, so the spark occurs at the same place near the top of the compression stroke every firing cycle. But flame propagation, the actual time delay until the fire begins burning well, varies with mixture. At just slightly rich of peak exhaust temperature the flame propagation is very fast. This puts the timing of peak cylinder pressure too close to Top Dead Center, where the piston is barely moving and there is little angle on the crank throw and connecting rod, thus causing the pressures to increase sharply without actually pushing the crank. This multiplies the stress on the engine without increasing the output power at all. Running maximum rich delays flame propagation so that peak pressure occurs later after TDC, when the piston begins to move faster and the crank angle has more leverage to push the crank. Pulling off WOT leans the mixture closer to minimum propagation time and dramatically increases stress on the engine.

Last summer I took the advanced engine management class from GAMI. I believe it is the best $500 in training I have ever spent. I am a diesel and natural gas engine mechanic by trade and I have been to dozens of factory training classes. I learned more about internal engine stress and how to control it in this class than in all the others combined. I highly recommend it,
 
Now let's address prop RPM. There is no harm in reducing RPM from 2700 to 2550 (or even 2500) during takeoff and climb as long as the throttle is set wide open to ensure maximum rich mixture. Just do it smoothly as I am sure you do, but don't pull the throttle off the wide open stop. The old wives tale to reduce MP before adjusting the prop applies more stress to the engine not less.

After you get the gear and flaps up, note the exhaust temperature. Note it again after you reduce RPM below 2700. The idea is to get a baseline temperature for proper leaning during the climb. As you climb the MP and exhaust temperatures will both drop as you gain altitude. Adjust the mixture to bring the exhaust temperature back to the initial climb baseline temperature, that way you keep the same air fuel ratio as you gain altitude. Remember that number if you have to climb again after you have been cruising for awhile.
 
Now let's talk about cruise. You can run any RPM and manifold pressure you want (within published limits of course) so long as the mixture is set in such a way as to be outside of the Red Fin danger area. See the chart attached. You need to know the load in order to determine the safest mixture settings to use. Below 60% load any mixture setting is acceptable and cannot harm the engine. Above 60% the mixture must be rich enough or lean enough to keep engine stress out of the danger zone.

How do you tell if you are overstessing the engine? Cylinder head temperature closely follows engine stress, so monitor CHT to keep tabs on it. Aluminum loses half its strength at 400 degrees F. Keep it under 375F during the climb and under 350F at cruise. Getting hot during the climb? Richen the mixture 25-50-100 degrees and CHT will drop. Getting hot in cruise? If you are Rich of Peak (ROP) then richen the mixture more to cool it down. If you are Lean of Peak (LOP) then lean more to cool it, if you can. Can't get it cool? Reduce power by lowering RPM or/and MP and go full rich until it cools down. Then figure out what to do next.

I know this all seems complicated, and it is, but its not so bad once you get used to it. Hope this helps. One Miracle at a time.
 

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William Pete Hodges said:
You have good points and good questions in your post. "Think of saving the engine..." Climbing at WOT IS saving the engine!....
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Thanks for saving me a lot of typing!

I did some flying and training for a company that utilized Cirrus aircraft. Their checklist called for a reduction from 2,700 to 2,500 rpm at 1,000’. Their plane, so I did it their way. In a Cirrus the throttle and prop control are linked, but in such a way the first bit of “power lever” movement off full power reduces rpm while barely moving the throttle plate. So in effect all you’re doing is reducing rpm initially.

In my own SR22 it was full rpm throughout the climb. My plane seemed to have a “sweet spot” at about 2,550 rpm, so that’s where I cruised at. But not to “save the engine”.
 
DavidWhite said:
If you installed a manifold pressure gauge in most fixed pitch planes, you would find that you are running oversquare most of the time. The horror!
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Interesting. I never considered this. Might be an interesting and educational experiment to run.
Any takers?
 
Fabio said:
Rod Machado Has a very good article on constant speed propeller engine.
http://krepelka.com/fsweb/lessons/commercial/commerciallessons01.htm
for what is worth he always recommends “prop on top” when it comes to MP and RPM, for the longevity of the engine.
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While that is a general recommendation, there are many power charts that provide prop on bottom combinations and there is zero negative effects on the engine from operating with those settings.
 
William Pete Hodges said:
I do the same but drop RPM to 2600 in the climb, it cuts way down on the noise. According to the GAMI advanced engine management class dropping 100-150 RPM at WOT is ok.
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Quoting John Deakin,

Now, some will yell about this, based on the Old Wives' Tale that goes "Always reduce MP before reducing RPM." But look at the logic. On my 550, that 200 RPM drop amounts to a 15hp loss. If you watch the JPI (see chart), you will see the EGT drop, the CHT will remain about the same or a bit less, and the actual pressures inside the combustion chamber remain essentially the same. Please tell me how this can be harmful to the engine? Anyone?

In fact, TCM did exactly the above, by simply limiting the RPM to 2500 on the same engine, to satisfy the German noise requirements for Beech A‑36s delivered there! They just call it a 285 hp engine, instead of 300, modify the performance data to suit, with no other changes. ​


Fabio said:
Rod Machado Has a very good article on constant speed propeller engine.
http://krepelka.com/fsweb/lessons/commercial/commerciallessons01.htm
for what is worth he always recommends “prop on top” when it comes to MP and RPM, for the longevity of the engine.
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It's an unhappy coincidence that the manifold pressure values expressed in inches of mercury happen to resemble the rpm in hundreds. If our instruments were calibrated in kilopascals and mega-revs per fortnight, respectively, "oversquare" would not be an issue.

Rgbeard said:
@Pilawt has one in his 180-hp C172.
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Yeah, I have a manifold pressure gauge in my 180 hp C-172N with fixed-pitch prop, so I can see how oversquare it is in climb, and there's nothing I can do about it. Isn't that a scary thought? Just about every fixed-pitch-prop airplane climbs oversquare!!! :eek::rolleyes:

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simtech said:
In my bonanza I am WOT and full forward on the prop until cruise alt. I'll pull the prop back to 2500 if in a noise sensitive area. I do love though that those who choose to Iive next to an airport get upset by airplane noise.
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Especially when the airport has been around since the 50's and subdivision built 60 years later.
 
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