At altitude, which throttle setting?

mandm

mandm

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On a NA piston, once you reach a certain altitude you do not achieve full power. So the throttle essentially whether it is 100% open or 70% (the %’s are a guesstimate) open produces the same manifold pressure. So does it matter if the throttle is 70% or 100% open at altitude when they achieve the same manifold pressure? I’m wondering if more fuel is being used (even though the fuel flow reads the same once full power is achieved whether the throttle is open 70 or 100%). The drawback I see is when you descend for whatever reason if you are leaned that you need to richen the mixture otherwise the engine may start to act rough or lose performance.

Also I notice that foreflight calculations assume maximum available power at all altitudes? Anyway to adjust this?
 
When you're seeing 100%, 75%, 65% etc power referenced it's talking about engine output not throttle position. Look in your POH, there should be a chart or table for power settings. For a fixed pitch prop you'd set it by RPM. SO maybe in the chart you look up a pressure altitude of 6,000' and it says for 65% power you run 2300RPM(I'm making that up but it will be something like that). On fixed pitch your cruise power settings are done by RPM.

On constant speed props we have a similar chart but you'll set the prop RPM and adjust throttle for manifold pressure. If you're not flying something that has a blue knob/lever between the throttle and mixture don't worry about that too much yet.

In either case you're not really worried about specifically how far open your throttle is, you're moving it to wherever it needs to be to get the RPM or MP for the power setting you want to use.
 
So typically for takeoff we have (talking the position of the levels):
Full throttle open + Full prop + Full rich

I find that takeoff positions give good performance (such as POH climb performance tables), I don’t see any limitation on my POH for time in takeoff power, however the checklist says for climb to pull back throttle to 25” and prop to 2500RPM.

When I do this 25”/2500RPM, my climb performance sucks (maybe 300-400fpm if lucky). Yes, the summer.

Now if I climb at the en-route climb recommended speed (110mph), I will get 0-100fpm. So I need to pitch for 80-90mph to get any climb performance. Now this is at 25”/2500RPM.

Now let’s go back to my original post, I takeoff with full performance, and upon safe altitude (pattern altitude 500-1000’AGL). Then I reduce to 25”MP/2500RPM. Let’s say I’m trying to climb to 7500’, I set the 25/2500 after takeoff let’s say at 2000’, then around 5000-6000’, without touching the throttle, the MP reduces. As expected, as performance for NA piston decreases with altitude. But my throttle level was not full power, it was reduced at sea level to achieve 25”MP. So my question is about the throttle position at higher altitude, if MP drops automatically to 22”MP due to DA, then the throttle level can be pushed forward more, but its still at 22”MP. So my question is does it matter if the throttle level is pushed full forward or left at the 25”MP position set at sea level?
 
I would like to understand full throttle enrichment features on some aircraft engines better.

My experience suggests that it is usually used on fixed pitch props, but not always.

Of course if your engine has one, you can set full throttle and then lean, but it will go quite lean when you pull the throttle back a bit.


Watching the EGT temps of full throttle vs pulled back a bit will usually tell you if the airplane is equipped with such a feature.

Brian
 
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mandm said:
...I set the 25/2500 after takeoff let’s say at 2000’, then around 5000-6000’, without touching the throttle, the MP reduces.
Click to expand...
I think an NA reduces continuously unless it's turbocharged.
 
I leave full throttle full prop until cruise altitude. Do you have a reason why you do not do that, noise, max rpm limit?? Once at cruise I leave it WOT adjust prop and lean. I’m in a NA IO-520 bonanza. I know this doesn’t answer your question but wondering why you go 25 square and accept poor climb performance.
 
Full throttle, 2600+ RPMs till cruising altitude (6500’ or more), then drop RPMs to 2400-2500 and then LOP, IIRC at 6500’ that’s 65% HP burning ~9gph.
 
182-S (IO-540), per POH... enroute normal climb- pull MP to 23/mixture to 15gph/prop 2400 (I pull at 500-700' agl). In cruise above 6800' I am at the firewall with the throttle for 23" and MP drops from there as I climb (as the OP is observing). If I pull the throttle until the MP starts to drop and set it just above that drop the fuel flow stays the same and CHT/EGT does not alter. I just leave it forward at those altitudes. Nothing in the POH addresses throttle position once at altitudes where MP is no longer throttle position limited.
 
I might be misreading the question, but if you're at any altitude (in a non-turbo plane) with the throttle pulled back, you have two things reducing your MP: the reduced atmospheric pressure at altitude and the partially closed position of the throttle plate. If the combined effect of both of those is giving you the % power that you want then do nothing. But if you push the throttle forward you'll see an increase in MP. If not I'd suspect something is wrong with the plane.

When I got my complex endorsement a long time ago I was taught this dance of pulling power back to 25"/2500 at 1000' and then increasing MP another inch with every 1000' of climb until the throttle was full forward. But if there is no full power limitation on the plane, I'm not sure what the point of it is. In my plane I might pull RPM for noise, but if I'm trying to get somewhere it's usually full throttle from takeoff to TOD.
 
OP starts with the incorrect statment that full throttle and 70% throttle will give same MP. NO. Go fly and try it out.

25"/2500 RPM climb is old school. Modern engine management is full throttle, max RPM for climb. If you engine has a 5 minute rating, you will need to set max continuous power for the climb.

If you climb high enough, full throttle will be cruise power, so you for fixed pitch, leave the throttle forward and lean. For constant speed prop, leave the throttle forward, adjust RPM to desired cruise RPM and then lean.
 
I cruise with the throttle wide open, unless I'm very low and want to go slow for sightseeing.

Many people pay extra for a turbocharger or turbonormalizer to increase the manifold pressure. The reduced atmospheric pressure at altitude is performance-limiting for my normally aspirated engine at cruise, so why would I close the throttle and make things even worse?

Another way of looking at it: Would you knowingly fly an airplane with a partially clogged induction air filter? Probably not, right? So how is closing the throttle any different? Either way, the effect is a reduction in manifold pressure.

- Martin
 
mandm said:
On a NA piston, once you reach a certain altitude you do not achieve full power. So the throttle essentially whether it is 100% open or 70% (the %’s are a guesstimate) open produces the same manifold pressure. So does it matter if the throttle is 70% or 100% open at altitude when they achieve the same manifold pressure? I’m wondering if more fuel is being used (even though the fuel flow reads the same once full power is achieved whether the throttle is open 70 or 100%). The drawback I see is when you descend for whatever reason if you are leaned that you need to richen the mixture otherwise the engine may start to act rough or lose performance.

Also I notice that foreflight calculations assume maximum available power at all altitudes? Anyway to adjust this?
Click to expand...

The throttle is a butterfly valve which either lets more, or less, air into the engine causing it to produce more or less power. It doesn't set a percentage of rated power. Since your premise is flawed, one cannot answer the question.
 
If you have X mp and the throttle lever is not all the way in, then when you push the throttle lever in, you should get X+.

In other words, engine vacuum should decrease when you create a bigger hole on the intake side of that big ass air pump.
 
simtech said:
I leave full throttle full prop until cruise altitude. Do you have a reason why you do not do that, noise, max rpm limit?? Once at cruise I leave it WOT adjust prop and lean. I’m in a NA IO-520 bonanza. I know this doesn’t answer your question but wondering why you go 25 square and accept poor climb performance.
Click to expand...

25 squared is how I was trained and in the checklist.
 
Squared is how complex is being taught by CFI’s.

25squared = 25” MP + 2500RPM

I’m not wording my question correctly but one post seemed to understand it.

If I’m at 8000’ and throttle is fully open, if I pull back the throttle a little bit, I’m still getting the same MP and fuel flow on my gauge, I’m not going slower. If I push it back in, same thing, same MP and fuel flow reading and same IAS and GS. So I’m wondering about this “area” in which the throttle change has no effect.

Now at lower altitudes (<4000’), this is not the case. Any movement in the throttle has a change in MP, fuel flow and airspeed.
 
Squared is a bit of an OWT, or at least over square. Change MP to millibars and it’s no longer a thing. As to the question, small movement of the throttle near WOT don’t do much. That’s because the butterfly has little impact on airflow when it is near vertical. There is a theory that the turbulence from being less streamlined helps if you are running lean.
 
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mandm said:
25 squared is how I was trained and in the checklist.
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25 squared is in a number of manuals in one form or another and as a result often gets used even airplanes where the manual says differently.
It is also easy to remember.

Is your checklist the Manufacturers checklist, or a 3rd party checklist?
What does the manual actually say.

I like 25 squared climbs in a lot of airplanes especially if I haven't had a chance to fully read the manual yet.

Brian
 
Take your shop vac and restrict the intake 65%, 70%, and 75% and answer your own question.
 
Martin Pauly said:
I cruise with the throttle wide open, unless I'm very low and want to go slow for sightseeing.

Many people pay extra for a turbocharger or turbonormalizer to increase the manifold pressure. The reduced atmospheric pressure at altitude is performance-limiting for my normally aspirated engine at cruise, so why would I close the throttle and make things even worse?

Another way of looking at it: Would you knowingly fly an airplane with a partially clogged induction air filter? Probably not, right? So how is closing the throttle any different? Either way, the effect is a reduction in manifold pressure.

- Martin
Click to expand...

Maybe that works for you but it can depend a lot on how your enrichment circuit is setup.

Admittedly, I haven’t flown behind a flat engine in several years now, but in my airplanes (radials) full throttle opens the enrichment circuit pumping a whole lot of extra fuel into the engine. If I tried to operate the way you do, my engine would be running rough as all get out.
 
I only recently started flying complex HP aircraft as I bought a Comanche. Since then, I've been playing around with the throttle at different altitudes and it definitely isn't correct that you get the same MP if you pull the throttle back at higher altitudes. If I'm at, let's say 7,500 ft, I have the throttle wide open and get maybe 21 MP (just making up the numbers). If I pull the throttle, MP goes down to 20, pull further, it'll go down to 19 and so forth.

As for procedure, takeoff is at full throttle and prop full forward. My instructor who has owned his Comanche for a long time said to not keep full power for more than five minutes so once at 1,000 ft AGL, I set the power to 24 MP and 2,400 RPM. That still gives me a climb of over 500 ft/m even right now in the middle of summer. As I keep on climbing, I push the throttle in further. Once in cruise (usually somewhere between 7,500 and 9,500), the throttle is full open, giving me somewhere around 20 - 22 MP, and the prop is at 2,300 or 2,400 RPM.
 
25 squared makes little sense, unless the engine or airplane manufacturer has it as a limitation. When you’re climbing with a fixed pitch prop, you’re a few inches “oversquare”, and it’s never caused a problem. Reducing power to climb doesn’t make sense, especially at 500 AGL which is where a lot of go-to-the-freezer-get-the-box CFIs teach it. You need that precious altitude between terra firma and aluminum ASAP.

Where did you checklist come from? Is it in accord with the manufacturer’s current procedures and designed for efficiency, or is it the typical jumbled, multiple redundant, human factors are for wimps, hand-me-down checklist?
 
Pinecone said:
OP starts with the incorrect statment that full throttle and 70% throttle will give same MP. NO. Go fly and try it out.

25"/2500 RPM climb is old school. Modern engine management is full throttle, max RPM for climb. If you engine has a 5 minute rating, you will need to set max continuous power for the climb.

.
Click to expand...
If you climb high enough, full throttle will be cruise power, so you for fixed pitch, leave the throttle forward and lean. For constant speed prop, leave the throttle forward, adjust RPM to desired cruise RPM and then lean.
This !
 
texasclouds said:
In this thread we find that none of us know what we are talking about…
Click to expand...

LOL, you may be right. I just checked the POH and it doesn't say anything about not leaving it at full power for more than 5 minutes. It does mention that one CAN reduce RPM and/or MP for a reduced power climb if desired, but it's neither mandatory, nor recommended. I think I'll still keep doing it just because the noise level is MUCH lower and I still get over 500 ft/m at the reduced power setting, plus by the time I reduce power I'm high enough to give me time for emergency procedures if necessary.
 
brcase said:
25 squared is in a number of manuals in one form or another and as a result often gets used even airplanes where the manual says differently.
It is also easy to remember.

Is your checklist the Manufacturers checklist, or a 3rd party checklist?
What does the manual actually say.

I like 25 squared climbs in a lot of airplanes especially if I haven't had a chance to fully read the manual yet.

Brian
Click to expand...
It’s a third party checklist, I have two different checklists that say that IIRC.

The actual POH says:
Avoid continuous operation 2000-2200RPM
For all operations (limit) 2700RPM, 180hp
Nothing else

Another checklist says, limit full operation below 2400RPM.
 
mandm said:
Also I notice that foreflight calculations assume maximum available power at all altitudes? Anyway to adjust this?
Click to expand...
Get into your performance profile settings. You can make multiple settings per tail #. I made a normal and an econ cruise option by changing the TAS and gph.
 
I do 25"MP, 2600RPM climb out once I'm 1k AGL (500 AGL is too low to throttle back on the arrow, it can't glide :oops:). As I climb I'm constantly checking MP and adding add'l throttle where necessary to maintain 25"MP.
At cruising altitude I obey the table here. Always at 2400RPM column and hardly ever operate beyond 65% in cruise (probably a single digit # of times entire time I've flown this thing).
1689656567544.png
mandm said:
When I do this 25”/2500RPM, my climb performance sucks (maybe 300-400fpm if lucky). Yes, the summer.
Now if I climb at the en-route climb recommended speed (110mph), I will get 0-100fpm. So I need to pitch for 80-90mph to get any climb performance. Now this is at 25”/2500RPM.
Click to expand...
Your climb numbers seem concerning if accurately measured. Mine is only +20hp and I will routinely get 700fpm using my climb configuration above (25"MP/2600RPM). I also fly the plane w/just me (250lbs), full fuel usually (+300lbs), and about 50lbs of crap/tools/spare-oil and what not. If I was getting any sustained vertical of 0-100fpm in a cruise climb (pitched for 110mph) I'd be a little concerned.

EDIT: Also, I called 110mph cruise climb, but that's not a POH number or anything. That's just how I was taught...
 
mandm said:
It’s a third party checklist, I have two different checklists that say that IIRC.

The actual POH says:
Avoid continuous operation 2000-2200RPM
For all operations (limit) 2700RPM, 180hp
Nothing else

Another checklist says, limit full operation below 2400RPM.
Click to expand...
A POH for an Arrow states recommended climb power is 25"/2500RPM.

Screenshot 2023-07-18 at 1.27.05 AM.png
 
Martin Pauly said:
I cruise with the throttle wide open... Would you knowingly fly an airplane with a partially clogged induction air filter? Probably not, right? So how is closing the throttle any different? Either way, the effect is a reduction in manifold pressure.

- Martin
Click to expand...
I'm old school I guess. I don't even run my lawnmower that way and it specifically says to in the manual. I wouldn't fly with a partially clogged air filter because I wouldn't take off with partial throttle. Closing the throttle in cruise extends the range, saves gas and is quieter. It HAS to be better on the engine — think about it. When you need to wear the rings into brand new cylinder walls you use wide open throttle. Less wear, lower temps, increased volumetric efficiency, less piston-miles per flight. Now, I'm sure someone is chafing at the bit to set me straight, so I might as well say that "squared" power settings are a nice and convenient way to operate an NA airplane during climb.
 
Martin Pauly said:
I cruise with the throttle wide open, unless I'm very low and want to go slow for sightseeing…


Another way of looking at it: Would you knowingly fly an airplane with a partially clogged induction air filter? Probably not, right? So how is closing the throttle any different? Either way, the effect is a reduction in manifold pressure.
Click to expand...
Good analogy. I remember it from my Cirrus days.

There’s an acronym many Cirrus pilots use for cruise settings - WOTLOP (Wide Open Throttle Lean Of Peak). Maximum efficiency by not impeding the flow of air to the engine, and controlling power with the mixture. This, of course, pretty much requires fuel injection with reasonably balanced injectors.
 
FancyG said:
Get into your performance profile settings. You can make multiple settings per tail #. I made a normal and an econ cruise option by changing the TAS and gph.
Click to expand...
I did this, I have different profiles basically for different altitudes, and I’m still perfecting it.

The reason I don’t like this is when you flight plan, you click on the altitude and it shows the tailwind/headwind, time and fuel used, it’s not accurate across the board, only at altitudes that achieve the power for the indicated profile.
 
mandm said:
25 squared is how I was trained and in the checklist.
Click to expand...
Many people were trained in ways to do things that have been found to be NOT the best way to do things. And the POHs reflect the thinking at the time the aircraft was built.

I was taught 25 squared also. But have learned and modified my behavior.

Another point being, I recall some research that found that the most likely point for an engine to have an issue is at the first power reduction. So climbing WOT delays that possiblity to an altitude where you can deal with the issue and have some time.
 
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You might have a sluggish MP gauge, or something off in the throttle subgroup of parts?

I’m not sure if/how extremes of mixture would affect how linear or non-linear the throttle position is to MP.

Are you 100% that’s what’s happening?
 
ArrowFlyer86 said:
I do 25"MP, 2600RPM climb out once I'm 1k AGL (500 AGL is too low to throttle back on the arrow, it can't glide :oops:). As I climb I'm constantly checking MP and adding add'l throttle where necessary to maintain 25"MP.
At cruising altitude I obey the table here. Always at 2400RPM column and hardly ever operate beyond 65% in cruise (probably a single digit # of times entire time I've flown this thing).
View attachment 119088

Your climb numbers seem concerning if accurately measured. Mine is only +20hp and I will routinely get 700fpm using my climb configuration above (25"MP/2600RPM). I also fly the plane w/just me (250lbs), full fuel usually (+300lbs), and about 50lbs of crap/tools/spare-oil and what not. If I was getting any sustained vertical of 0-100fpm in a cruise climb (pitched for 110mph) I'd be a little concerned.

EDIT: Also, I called 110mph cruise climb, but that's not a POH number or anything. That's just how I was taught...
Click to expand...
I will try 25”MP + 2600RPM next time for climb to see how she performs. I’m thinking maybe my MP gauge is off by 1”MP or so too. Also with it being located under the yoke and I have a GPS on the yoke it is also difficult for me to see what it’s at precisely as I don’t want to stare at it (the gauges) too long as my priority is to fly the airplane.

En-route climb at max MP/2500RPM at/above 7000’ was difficult, weight was definitely close to useful load and DA was probably +1500’. Mind you, also flying in mountainous terrain with up/downdrafts not helping the situation either.

As for the initial climb, my airplane definitely has the performance and climb with throttle and prop full forward, the issue I’m running into is when I throttle back / reduce prop, I’m not getting what I think I should be getting. I don’t see a limitation for maximum continuous power and I’m hesitant to do something to mess up the engine thus why following the tails of 25”/2500RPM.

This is the table that I follow in flight.

IMG_3312.jpeg
 
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