Combinations of RPM & Manifold Pressure

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whiskeycharlie

Well-Known Member
I've seen a few performance charts that give you different combinations of RPM & MP settings to obtain the same fuel burn & TAS. Is there any advantage/disadvantage to choosing a lower rpm/high manifold setting v. a higher rpm/lower manifold setting? Thanks!
 
Well I was always taught to go with the higher rpms and lower manifold pressure to reduce wear and tear. Other than that I'm not sure why you would choose one of the other if the fuel flow is the same.
 
Inverted25 said:
Well I was always taught to go with the higher rpms and lower manifold pressure to reduce wear and tear. Other than that I'm not sure why you would choose one of the other if the fuel flow is the same.
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See, this goes against everything I was ever tought as an auto mechanic. Lower the RPM's to reduce wear. Every rotation of the engine takes life away from it. Thats why high RPM engines in cars usually don't last as long as low ones. Think about how many more miles you get out of a diesel that turns a max of 3400RPM's (cummins Diesel) Vs a gas motor that constantly spins at 3400RPM's and turns a max of 55-6000RPM's.

It leads me to belive that there are many things in aviation that deal with operating engines that are purley myths/urban legand/wifes tales.
 
No myths or legends here. We all KNOW that every time we fly over squared a puppy dies.

Seriously though, I run at the lowest allowed RPM for the given power setting. I hate noise, and I'd like to limit wear.
 
mshunter said:
It leads me to belive that there are many things in aviation that deal with operating engines that are purley myths/urban legand/wifes tales.
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Yes. I've seen the argument go both ways, but in the end, they're only arguments. There is a severe lack of hard data on the best way to operate an aircraft engine.
 
Codyjp said:
Seriously though, I run at the lowest allowed RPM for the given power setting. I hate noise, and I'd like to limit wear.
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And you can find this Sea Level and Altitude Pressure chart, located in the aircraft engine manual. Here is one example:

http://www.lycoming.textron.com/supp...Operations.pdf

Page 45.

Find it for your engine and post it up. From there we can see the numbers that work best for your aircraft.



OP: I agree completely with mshunter, the idea to operate at a higher RPM is silly. A lower RPM results in:

  • Reduced engine wear
  • Better fuel flow
  • Cooler engine temps
  • Longer time between overhauls

The longer time between overhaul isn't an actual increase in TBO, it will still be, for instance, 2000 hours. However, overhauls are based off the engine tach time. A lower RPM results in less time on the engine each flight, thus more flying between overhauls.
 
I agree with everyone and have read many articles on the subject, and there seems to be many ole wives tails in aircraft engine operation. I like to run the props slower versus faster just for noise alone, then use a mp setting that is suitable for the flight and that is authorized in the POH.
Lower noise
lower fuel flow
no real loss of TAS
it just makes sense the ole never let the prop go lower than the mp is ridiculous look at turbo charged engines they are boosted well above square the whole time(i know i know thats a whole different animal)
 
Do the lowest RPM for the power. You're not going to hurt the engine and it's quieter.
 
I know a fella who has an older Bonanza. He cruises the thing at WOT no matter where he goes. After takeoff the throttle never leaves the firewall unless he needs to descend or it's out of the green in cruise. He just pulls the engine back with the prop and leans the mixture till egt's peak then enrichens about 75*. The engine has about 3000hrs on it, compressions are still good, and oil analysis always comes back clean. :dunno:

BTW, he has owned the airplane since it was new. IIRC it's a mid '70's.
 
mshunter said:
I know a fella who has an older Bonanza. He cruises the thing at WOT no matter where he goes. After takeoff the throttle never leaves the firewall unless he needs to descend or it's out of the green in cruise. He just pulls the engine back with the prop and leans the mixture till egt's peak then enrichens about 75*. The engine has about 3000hrs on it, compressions are still good, and oil analysis always comes back clean. :dunno:

BTW, he has owned the airplane since it was new. IIRC it's a mid '70's.
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It will certainly work and is frequently the suggested method for best economy cruise.
 
mshunter said:
I know a fella who has an older Bonanza. He cruises the thing at WOT no matter where he goes. After takeoff the throttle never leaves the firewall unless he needs to descend or it's out of the green in cruise. He just pulls the engine back with the prop and leans the mixture till egt's peak then enrichens about 75*. The engine has about 3000hrs on it, compressions are still good, and oil analysis always comes back clean. :dunno:

BTW, he has owned the airplane since it was new. IIRC it's a mid '70's.
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That is how I fly constant speed systems. Except I don't go to the limit, I use the engine chart and find the lowest difference on the chart. For an IO-360 this is about 4.5 difference, i.e. if the power was 24" you could have 1950 RPM. Then I added a small fudge factor, so in an IO-360 I will go to a maximum difference of 4 for any power setting. Even though some cruise settings allow for a difference of 6, I worry to much about forgetting when it is and isn't ok.

Just my 02 though.
 
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It will knock you on your butt.
 
High throttle/low RPM increases the pressure in the cylinders. Basically, the lower RPM means the valves will be open for a longer period of time, and more air will enter the cylinder on the intake stroke.

However, for this pressure to be high enough to do damage you would have to be running at too low of an RPM to get any real power out of the engine.
 
tgrayson said:
They don't have fleet data and that's the only thing that could ever establish any truth to this subject.
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They may not have fleet data but they do have the history of how companies learned to get longer life out of the big recips such as the 3350 and 4360 which was to run them lean of peak.

Also, for a long time the myth of not running oversquare has continued when it was disproved by Lindbergh as a technique that almost doubled the range of the P-38s was THE reason the -38s were able to fly the intercept/shootdown of Yamamoto. And that was in the 1943.

Conclusive? Maybe not but surely enough to challenge if not dismiss plenty of the myths and misconceptions. One thing seems to repeat on various sites and posts.. running lean of peak is the way to increase engine life.
 
Orange Anchor said:
They may not have fleet data but they do have the history of how companies learned to get longer life out of the big recips such as the 3350 and 4360 which was to run them lean of peak.
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Anecdotes aren't history and it appears the LOP motivation was/is primarily reduced fuel consumption. While it doesn't appear to hurt engines, I'm skeptical they have statistically significant data that indicates that it helps them. The GAMI people tend to be as susceptible as anyone to confirmation bias.
 
tgrayson said:
Anecdotes aren't history and it appears the LOP motivation was/is primarily reduced fuel consumption. While it doesn't appear to hurt engines, I'm skeptical they have statistically significant data that indicates that it helps them. The GAMI people tend to be as susceptible as anyone to confirmation bias.
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Not from what I read. It was engine life. Althought not a radial, my P-38 ace friend said if they got 50hrs out of a 1710 they were happy. Later they did lengthen the life of the engine with new procedures.

The B-29 suffered early losses more to engine problems than enemy fighters and the 3350 originally was not making it to much more than 30hrs. Later versions of 3350s went for more than 3500hrs.

http://www.gami.com/articles/frugalflyer.php

Believe it. Don't believe it. But supply some info as to why these guys are wrong.
 
Orange Anchor said:
Not from what I read. It was engine life.
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From that last link:
This discussion about lean-of-peak operations has been going on an awfully long time, said Braly, who began evangelizing on its benefits 10 years ago at the Advanced Pilot Seminars he taught at the companys Ada, Oklahoma, headquarters. Weve got some NACA research from 1943 that was classified as top secret at the time because they wanted to know if they could run lean of peak to extend the range of bombers and other U.S. military aircraft.
​
Believe it. Don't believe it. But supply some info as to why these guys are wrong.
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It's up those asserting some position to supply the evidence. All they have is anecdotes, which is what psychic healers and UFO trackers have. It may or may not be true, but you can't honestly assert something is true just based on stories. Personal testimony is very, very, very unreliable.

I'm puzzled why you'd think that anecdotal evidence published by people who have a financial stake in your being convinced are a reliable source of information.
 
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