Is It Better to Fly at 55% Power or 75% Power, in Terms of Engine Health?

Penguinforce

Penguinforce

Pre-takeoff checklist
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Penguinforce
Sometimes the time saved flying at 75% power outweighs the potential gas saved by flying at 55% power. So why not fly at 75% power all the time? My question is, in terms of overall engine health, is it better to fly at 55% power? Or is it better to fly half a cross country flight at 55% and the other half at 75%, or can you always fly at 75% power?
 
Someone explained it to me this way, in the context of a club where the rental is paid on tach time: unless you’re heading into a stiff wind, you’ll get more hours for your money at economy cruise. I haven’t run the numbers myself but the logic checks out.

As to engine wear, personally I bet running lower power settings (after break-in) will allow it to run longer, but will it be a substantial difference? Hard telling.

Surely someone else knows better than the likes of me though.


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I fly at "economy cruise" almost all the time. Because "time saved" is not important to me, whereas "gas saved" is. Gas costs money. Time, on the other hand, costs nothing, and rather just accumulates in my logbook, and translates into pleasure!

I'm not an expert on engine health, but my husband says that it's healthy for an engine to get the heat up with high power for a little while at least once on a flight, to "burn stuff out of there" or some such. His advice after a long flight in economy cruise is to power up and make some heat for a few minutes, say, before entering the pattern.
 
Penguinforce said:
Sometimes the time saved flying at 75% power outweighs the potential gas saved by flying at 55% power. So why not fly at 75% power all the time? My question is, in terms of overall engine health, is it better to fly at 55% power? Or is it better to fly half a cross country flight at 55% and the other half at 75%, or can you always fly at 75% power?
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Funny, I do the opposite. When I’m trying to get somewhere cross country I fly faster than when puttin around getting $100 burgers. I don’t worry too much about % power either. I just try to keep the cylinder temps below 370 in cruise.
 
kath said:
I fly at "economy cruise" almost all the time. Because "time saved" is not important to me, whereas "gas saved" is. Gas costs money. Time, on the other hand, costs nothing, and rather just accumulates in my logbook, and translates into pleasure!

I'm not an expert on engine health, but my husband says that it's healthy for an engine to get the heat up with high power for a little while at least once on a flight, to "burn stuff out of there" or some such. His advice after a long flight in economy cruise is to power up and make some heat for a few minutes, say, before entering the pattern.
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Time is the fire in which we burn (Dr. Soran)
 
Salty said:
Funny, I do the opposite. When I’m trying to get somewhere cross country I fly faster than when puttin around getting $100 burgers. I don’t worry too much about % power either. I just try to keep the cylinder temps below 370 in cruise.
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That’s a good way to look at things.
 
kath said:
I fly at "economy cruise" almost all the time. Because "time saved" is not important to me, whereas "gas saved" is. Gas costs money. Time, on the other hand, costs nothing, and rather just accumulates in my logbook, and translates into pleasure!

I'm not an expert on engine health, but my husband says that it's healthy for an engine to get the heat up with high power for a little while at least once on a flight, to "burn stuff out of there" or some such. His advice after a long flight in economy cruise is to power up and make some heat for a few minutes, say, before entering the pattern.
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I never thought about “revving up” the engine after using it at a lower power setting before entering the pattern. Thanks for the advice!
 
Some small engines don’t like being chugged… so model dependent
 
O-200s like to run harder than the bigger engines. They’re happiest and healthiest @ 2500rpm, or so it’s agreed upon by many who fly behind those little Continentals. I still like my fuel burn/ground speed @ 2250-2300rpm, though. Seems like it would be less wear and tear, it’s definitely a bit quieter, a smooth power setting, and still achieves a respectable cruise climb for a C150. (It will even climb a bit with 40 degrees of flaps, but that’s a different thread.)
 
Just run them often, if I’m not going anywhere, I run lower, if I’m doing cross country I run high enough that full power gets me 65% max anyway.
 
AndyMac said:
How does the 140 like to be run?


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the c85 and the 0-200 are cousins and I do run mine harder, she feels smoothest and the plane on step the easiest around 2450. 2550 or 2575 is redline so just a touch off max. Then it still runs cool with that kidney bean oil tank…
 
A while back, I came across an interesting thread on one of the forums discussing Carson speed. It was an interesting mathematical derivation I fond very useful. Here's one of many articles you can find on Google but gives a good introduction. Take it where you want. Hope this helps some.
https://www.flyingmag.com/very-best-speed-fly/

Dave
 
for a given flight, at 75% power, what is the time used as measured by the tach?

for the same flight at 55% power, what is the time used as measured by the tach?

if the engine lasts for more hours at 55% than at 75%, is that savings eaten up by the longer flight time required?

And, given the way most of the fleet is used, isn't calendar time the limiting factor?
 
FULL RENTAL POWER! :eek: ;)

I back a little off of full power on our, but not a lot. I'm usually cruising at 65%-70%, where almost all of the decrease is from altitude, not the throttle. If I'm flying I'm going somewhere, so no point in lollygagging around. There are some exceptions. I did my first two Young Eagle flights a few weeks ago. I powered up on the flight to the sightseeing locations, but then throttled back down so we spent more time around the lake or going around Stone Mountain.
 
The only way you're allowed to operate an engine:

squared.jpg
 
I’ve always been told to run them hard for longevity. I run my O-300 at 2450 RPM, but have only owned it a year. This engine has had several owners and there are a ton of variables involved.
 
Penguinforce said:
Sometimes the time saved flying at 75% power outweighs the potential gas saved by flying at 55% power. So why not fly at 75% power all the time? My question is, in terms of overall engine health, is it better to fly at 55% power? Or is it better to fly half a cross country flight at 55% and the other half at 75%, or can you always fly at 75% power?
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It’s a matter of what you are demanding from each cylinder. Two Lycoming 541 cu. in. 2000 hr TBO engines - the 235 hp O-540J3C5D used in Piper Dakota and the 350hp TIO-540AE2A used in the Malibu. The 235 engine the Dakota operated at 75% power will routinely make TBO without a top overhaul. The 350HP Malibu engine operated at 60% power will require a top overhaul in 700 hours.

At 75% power the cylinder in the Dakota is producing 30 hp. At 60% power the cylinder in the Malibu is producing 35 hp. One cylinder is loafing along. The other is being asked to 15% more HP at a power setting 15% lower. In a climb, 39 hp per cylinder v 58 hp per cylinder or 33% more HP per cylinder.
 
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It's not the HP....it's the heat.
Clip4 said:
It’s a matter of what you are demanding from each cylinder. Two Lycoming 541 cu. in. 2000 hr TBO engines - the 235 hp O-540J3C5D used in Piper Dakota and the 350hp TIO-540AE2A used in the Malibu. The 235 engine the Dakota operated at 75% power will routinely make TBO without a top overhaul. The 350HP Malibu engine operated at 60% power will require a top overhaul in 700 hours.

At 75% power the cylinder in the Dakota is producing 30 hp. At 60% power the cylinder in the Malibu is producing 44 hp. One cylinder is loafing along. The other is being worked to the max.
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Whichever one lets you fly the plane more often.
 
Lachlan said:
O-200s like to run harder than the bigger engines. They’re happiest and healthiest @ 2500rpm, or so it’s agreed upon by many who fly behind those little Continentals. I still like my fuel burn/ground speed @ 2250-2300rpm, though. Seems like it would be less wear and tear, it’s definitely a bit quieter, a smooth power setting, and still achieves a respectable cruise climb for a C150. (It will even climb a bit with 40 degrees of flaps, but that’s a different thread.)
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Don't tell Dan!
 
Bob Noel said:
for a given flight, at 75% power, what is the time used as measured by the tach?

for the same flight at 55% power, what is the time used as measured by the tach?
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Depends on the tach. Some electronic tachometers measure tach time differently than mechanical tachometers.
 
At Oshkosh I attended a technical forum hosted by Continental engines. Most of it was way over my head, but it was quite interesting. One of the things the speaker mentioned is each engine has a percent power that is optimal and is usually specified in the engine manual or technical ops. For example, I fly a Continental I/O550D and they recommend running 65-75%. According to the speaker, as long as you keep it within the manufacturer's recommended range other things like LOP/ROP, MP/RPM, etc matter very little.
 
Bob Noel said:
for a given flight, at 75% power, what is the time used as measured by the tach?

for the same flight at 55% power, what is the time used as measured by the tach?

if the engine lasts for more hours at 55% than at 75%, is that savings eaten up by the longer flight time required?

And, given the way most of the fleet is used, isn't calendar time the limiting factor?
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Law of diminishing returns, increased speed require an exponential increase of fuel flow and HP.
If your cylinders run below 300° that’s not good either, more lead build up and stuck valves.
 
RPMs are a simple forumula, the fewer times your piston rubs against the cylinder, the longer it will last. Unless you go too low and let it rust.

But that isn't the whole formula. I don't have the reference, but I recall a rather technical article from either KitPlanes or Sport Aviation about valve wear. That was more related to temperature than RPMs. There is a best temperature to run the engine at and that is probably a much better indicator of durability than straight RPMs, especially since the RPMs which produce a certain temperature will vary by the OAT.
 
I've typically cruised at 65-75% power with proper leaning, and gotten excellent engine life that way. In the AA-1/AA-5 series this is typically 100-150 rpm below redline, a good compromise between speed and economy, with a good overspeed margin in turbulence.
 
Checkout_my_Six said:
you can get the heat from any HP setting....all depends on how the engine is dialed in and fuel flow.
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You missed your calling. I am sure there are a plenty of Malibu owners willing to pay you to teach them how to make their cylinders go 2000 hours when no one else has been able too,
 
bflynn said:
RPMs are a simple forumula, the fewer times your piston rubs against the cylinder, the longer it will last.
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I’m not an engineer, so take this with a grain of salt…

My understanding is that if your piston is actually rubbing against the cylinder, you maybe have a few minutes before your engine seizes. In normal operation the piston rings are floating on a thin layer of oil, and there’s virtually no metal-to-metal contact. That’s why you can still see honing marks on cylinders with lots of hours - they’d be gone in a minute if there was actual contact between the piston rings and the cylinder.

My further understanding is that most engine wear occurs at startup, where there is momentarily metal-to-metal contact until oil pressure builds up. Which is why on most opposed engines “pulling the prop through to limber up the oil” is generally not a good idea.
 
Dunno about engine life, but I find 65% to be a sweet spot in my plane for lower vibration and noise with no detonation.

XC power settings are a tradeoff between cruise speed and time to climb. Once I expend the time and fuel to get up high, I want to stay up there, especially if winds are favorable. Again, 65% seems like a sweet spot for good speed and endurance.

Fuel capacity is a limiting factor in my plane though, with only 40 gallons. At 9 GPH, I'm looking to land at 3 hours. In a C182 with 7 hour tanks I would probably fly at full rental power.
 
I'm not an engineer either, so ignore me when I am not precise and say "rubbing against the cylinder".
 
I'm no engineer, but I am an IA. Most engines I've seen go to TBO are simply run by the book. Generally, the manufacturer specifies 65-75 percent power.

When I fly the 310, I'm at 8-12k feet and WOT leaned to 25-50 degrees ROP. Engine is happy, performance is optimum. When it's time to come down, I reduce throttle until the MP needles budge and start down at 700 or so fpm unless ATC needs me to show off. Then approaches are 17 inches, 700fpm, 120 indicated and configured. Works every time.
 
kath said:
I'm not an expert on engine health, but my husband says that it's healthy for an engine to get the heat up with high power for a little while at least once on a flight, to "burn stuff out of there" or some such. His advice after a long flight in economy cruise is to power up and make some heat for a few minutes, say, before entering the pattern.
Click to expand...

Odd and unnecessary. "Burn stuff out of there" is a reference to plug fouling issues. Plugs foul at idle/ground operating RPM with the mixture left too rich. Plugs don't generally foul at any sort of cruise power setting, so it's silly to go to high power in an attempt to burn off non-existent plug deposits just before entering the landing/taxi phase... which is where plug fouling is ACTUALLY likely to happen if you don't manage the mixture properly.
 
Penguinforce said:
Sometimes the time saved flying at 75% power outweighs the potential gas saved by flying at 55% power. So why not fly at 75% power all the time? My question is, in terms of overall engine health, is it better to fly at 55% power? Or is it better to fly half a cross country flight at 55% and the other half at 75%, or can you always fly at 75% power?
Click to expand...
If you're just talking about gas savings, you need a very strong headwind before you save gas by flying at 75% power (I calculated 60 kt for a Cessna 172p, and even more, of course, for faster planes). You'd have to have a good bladder, though, to fly for any amount of time into a strong headwind at 55% power in a 172. :)

https://lahso.megginson.com/2004/12/20/wind/

In terms of engine health, they're all fine (from everything I've read), but you need a bit more care to manage your engine above 60% power, because it's possible to drive CHTs (and internal pressures) above safe levels depending on where you lean. The main thing is to avoid anywhere near 50°F rich of peak EGT — either go 50°F lean of peak, or 150°F rich of peak, and you'll be well clear of the red zone even at 75% power.
 
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