LOP & Percent Power for Dummies®

[A64Pilot]

I start at 22 squared for LOP myself, because that’s close to 65% power and will vary some due to DA, but it’s a safe place

I can get I think about 145 kts ROP at best power and I think that’s about 9.5 GPH, unsure because I don’t run there.

8 GPH is about peak and maybe 140 ish, kts 

7 GPH is LOP and 135 kts, this is often where I cruise when I’m not in a hurry.

6 GPH is real LOP and about as LOP as she will run smoothly and is 120 kts, it’s my just out to cruise setting, but that’s 20 NMG, and that’s not shabby at all.

‘These are at lower altitudes, speed ought to get better up higher in thinner air

[Bob - S50]

8 hours ago, A64Pilot said:

I understand that 75% is 75%.

However what chart do you use to determine 75% LOP? I’m not aware of one and anyone’s monitor that displays percent power is a rough estimation at best, and of course percent power is proportional to mixture, so an LOP chart would be tough to develop

You can determine percent power LOP by first setting 75% at best power mixture, recording the airspeed, then going LOP and adjusting manifold pressure, (if possible) to achieve the same airspeed. Way too much trouble for me, I just want to save fuel sometimes when I’m not in a hurry.

‘I have if possible in parenthesis because it’s often not possible for a NA motor to achieve due to altitudes manifold pressure restrictions if you stay lean enough to be safe

But why bother? Plus your pushing the limit, and that can be costly.

The procedure I spelled out is for those who want KISS, and a way to safely run LOP without expensive instrumentation and without any mods to the engine.

‘Even though many will tell you that you need an analyzer to run LOP you really don’t, you do if you want to push the limit, but if you stay well away from it you don’t.

 

As long as you don't go too deeply LOP, 75% is 10 GPH. 65% is 8.7 GPH. 55% is 7.4 GPH.

I set the RPM and MP I want then lean to LOP. I can tell by the fuel flow how much power I'm making.  If you have a favorite power setting (RPM/MP) after you've done it a few times you can just pull the mixture back to get the same fuel flow and it will be very close.  Pretty easy too.

[Ragsf15e]

12 hours ago, A64Pilot said:

Define optimal.

I agree with you that it will most often put one so lean the power just “goes away” and that’s regardless of timing advance too. Timing doesn’t make a whole lot of difference, some but it’s not 10 kts difference.

So to keep us safe, just don’t lean aggressively until your at a HP output that you can’t hurt anything with mixture, I believe Lycoming puts that point at 75%, but as I’m risk adverse I use 65%. Those numbers are derived from best power mixture which is what the charts show, I think. Not actual power when leaned, much of the fuel savings from LOP comes simply from reduced power, more than you might think, but who cares where it came from, so long as it shows up.

Now it’s not for people that are in a hurry, and some will say that slowing down just is not what they bought a Mooney for, and that’s fine, for those that want to cruise at high power, just go ROP, the cost isn’t as high as you may think, especially when you reduce the high fuel consumption by the reduced time aloft and reduced time on the engine etc.

‘You can get into trouble by wanting your cake and eating it too, and decide you want to go fast, and do it LOP, for me the risk isn’t worth it.

‘If you have an Ipad or Iphone, there is a neat app called Aircraft Power that seems to work very well for calculating percent power, just enter OAT, manifold pressure, RPM and indicated altitude and it gives you percent power, stay at or below 65% and you can’t hurt anything 

That app is cool, but it’s old enough that it doesn’t work on newer phones unfortunately.

[PT20J]

Our fascination with percent power is interesting. I guess that we have to have some guide for setting power and the typical POH lists power settings by percentage of rated power, so we've become used to it. But one could easily figure out what the combinations of MAP and rpm yield 75% power for Lycoming and 65% power for Continental (the Key number concept) and then use anything that suits you below those limits without ever figuring out that it is 67.375821%. I once asked Mike Busch what percent power he uses for cruise and he said, "I have no idea, probably 65% or less but I never bothered to try to figure it out." And what's magic about 65% or 75%? Mooney could just as easily have made tables showing 62.5%, 74.8%. 

The truth is that there is no way to get more than a close approximation of percent power with the instrumentation we have. The POH will be reasonably accurate ROP. LOP you would need the BSFC curve for your engine. But these are not readily available and even if you get one from Lycoming or Continental, it is for the engine on a test stand and not as installed in your airplane with it's imperfect induction and exhaust systems. 

The approximations that Advanced Pilot Seminars came up with for LOP are based on the well known fact that Otto cycle  thermal efficiency improves with increasing compression ratio (to a point) and from looking at old Continental manuals that had BSFC curves for engines of various compression rations and then generating some rules of thumb.

Some engine monitors use proprietary algorithms to display percent power and since it's digital, it must be correct, right? Well, in reality, it's only as good as the algorithm and the data input to the algorithm. It's going to be an approximation at best.

So, do what Mike does: Find a few power settings that get you the performance/economy tradeoff that you want and are below the manufacturer's limits and enjoy the ride. And if some formula says it's 67.321785% so be it.

Skip

 

[carusoam]

ROP formulas used in engine monitors can be funky... when calculating or looking up the %hp...

LOP formulas are a direct calculation using CR and FF...

All you have to know is how your engine monitor knows what it is telling you...

When it comes to POH power charts...

Mooney got pretty good with the %bhp numbers while LOP in the mid nineties...  the LOP charts are in the POH... for the M20R...

There is no reason this data can’t be developed and printed for easy use...

The MAPA safety team has been printing the ‘key numbers’ for quite some time...

Doesn’t get any easier than that...

Best regards,

-a-

[A64Pilot]

The only thing that’s magic about percent power is knowing at what percent power your safe and staying below that, beyond that it’s meaningless.

‘Just as knowing your 20 degrees or 25 degrees LOP of your richest cylinder, or what your “Gami spread” is, they are all meaningless. 

‘What is meaningful is are you below the point where you can hurt your motor or not, and is it running smoothly?

My IO-360 runs so much smoother with its stock system than either my IO-540 or IO-520 did with Gami injectors it’s shocking, the IO-360 seems to be the poster child for LOP.

‘But try something for kicks, run a medium airspeed, the one you usually run LOP, then run it 50 ROP and adjust manifold pressure to achieve the identical airspeed, leave RPM the same, that’s how much fuel your actually saving LOP.

Report back, it may not be as big a number as many seem to think.

 

[gsxrpilot]

24 minutes ago, A64Pilot said:

The only thing that’s magic about percent power is knowing at what percent power your safe and staying below that, beyond that it’s meaningless.

‘Just as knowing your 20 degrees or 25 degrees LOP of your richest cylinder, or what your “Gami spread” is, they are all meaningless. 

‘What is meaningful is are you below the point where you can hurt your motor or not, and is it running smoothly?

My IO-360 runs so much smoother with its stock system than either my IO-540 or IO-520 did with Gami injectors it’s shocking, the IO-360 seems to be the poster child for LOP.

‘But try something for kicks, run a medium airspeed, the one you usually run LOP, then run it 50 ROP and adjust manifold pressure to achieve the identical airspeed, leave RPM the same, that’s how much fuel your actually saving LOP.

Report back, it may not be as big a number as many seem to think.

 

I agree and follow your SOP running my engine at low enough power that I can't hurt it no matter what. When I flew a naturally aspirated engine, I just made sure I was at a high enough altitude that the engine would only make 60 or 65% power. With the turbo, I have to set the power at 65%.

If your engine runs smooth at the LOP setting you want, then obviously your GAMI spread is small. And often the IO360's do that well with factory injectors. But sometimes they don't. And its nice to know why and what to do about it. If the difference in fuel flow between the first cylinder to peak and the last cylinder to peak is half a gallon per hour or less, you're probably very happy with the way it runs LOP. If the difference in fuel flow is more than half a gallon per hour, you're probably not able to get LOP and still run smooth. If the situation is the latter, then GAMI injectors can help. But if they're not needed, they're not needed.

At the same low power setting, the difference between LOP and ROP full settings is not huge. But my practical application is to use ROP to be able to run high power, and therefore go as fast as possible without prematurely killing my engine. My go fast settings would be around 75% power and 100° ROP. The other setting for me is economy cruise which would be about 65% power and 25° to 30° LOP. I figure if I'm going slow, I might as well do it on the least amount of fuel possible. I can't say for sure this was the reason, but my turbo Continental did go past TBO without needing any cylinders or turbo work.

The difference between my "go fast" and my "economy cruise" is 10% to 12% speed and 30% to 35% fuel.

[A64Pilot]

I see all of the time,  “I run LOP and save 30% on fuel”

Well that’s not really the case, what is, is “I run LOP and slow down and save 30% on fuel”

Most of that fuel saving is in slowing down, to quantify the difference if people are interested they need to remove all the variables except the LOP part, which means of course keeping speed and RPM the same.

Years ago I did it in my Maule, and the difference was small enough that I just didn’t bother with LOP anymore, but that was a draggy airframe, and the 540 even with Gami’s and a small as in less than a half gallon spread just didn’t run well LOP, it lost power like you wouldn’t believe, but it was a parallel valve motor too, so maybe that was it?

LOP “worked” in the C-210 though, go figure, but it wasn’t 30% either or even close, but as the 210 wasn’t mine and I didn’t pay for fuel, I always cruised it at 25 squared or usually wide open 2500 RPM and ROP for the best speed I could get which was usually 155 kts on average, it was flown for business and time is money, plus if I went fast maybe I wasn’t so late getting home that day.

Continental’s are in my opinion more “hot rods” than Lycoming, just listen to an IO-520 idle, it sounds like a drag racer, that’s the radical cam, where a typical Lycoming idles smooth, it doesn’t lope.

So I believe you made TBO where most don’t not so much due to LOP, but because you ran low power so much, and most don’t.

Its been known for 100 years or more that an engine run at lower power will last longer, within limits of course, but there are so many that believe especially for Diesels that the way to long life is to run them hard, and you can show them all kinds of proof that isn’t true but they choose not to believe it.

Edited May 6, 2021 by A64Pilot

[Bob - S50]

32 minutes ago, A64Pilot said:

I see all of the time,  “I run LOP and save 30% on fuel”

Well that’s not really the case, what is, is “I run LOP and slow down and save 30% on fuel”

On the IO-360 I think it's more on the order of 1 to 1.5 GPH which would be 10 - 15% for a given power.  However, just saving gas (money) is not the only reason to run LOP...

1.  The obvious, save money.

2.  Safer.  The engine makes much less CO when LOP than it does ROP so we are much less likely to succumb to CO poisoning if our exhaust system has a leak.

3.  Better for the environment.  Each gallon of gas I save is that much less crap I'm dumping into the air and a lot less CO2 I'm adding to the air too.

4.  Better for the engine.  I'm much less likely to get deposits on my exhaust valves and spark plugs so I'm much less likely to develop morning sickness or a fouled plug.  That improves my dispatch reliability.  Not having to fix those problems also saves money.

5.  Better range and payload.  If I save 1.5 GPH, on a 4 hour flight that's 6 gallons or 36 pounds of payload I can carry that I might not be able to carry when ROP.  Or, if I'm weight restricted which limits the amount of fuel I can put on board, it extends my range by that 10 - 15% listed above.  And if you are the type that uses a really sharp pencil, if I want to land with an hour of fuel, that might be 9 gallons LOP or 10.5 ROP so that gives you another 1.5 gallons to burn enroute instead of leaving it in the tank at touchdown.  That extra range would be in addition to the range extension due to the 10-15% lower burn.  Or you could use it to give you another 9 pounds of payload capacity.

[A64Pilot]

I’m not saying don’t run LOP and I understand some of its advantages, although honestly I hadn’t considered some like CO output.

‘All I am suggesting is go out yourself and determine how much fuel flow difference there is, I haven’t done it yet on my Mooney, just don’t do what many do and say I burn 10 gls an hour ROP, but only 7 LOP so therefore I save 30% on fuel.

Go out yourself and as I said set yourself up at your favorite LOP cruise, then go to 50F ROP, and adjust your manifold pressure to achieve your LOP cruise speed, that’s the actual fuel savings.

‘Why 50F ROP, well you have to pick something and I believe that’s Lycoming’s recommend cruise mixture, so may as well pick that.

Edited May 6, 2021 by A64Pilot

[Tim Jodice]

I am in the "I did not buy a Mooney to go slow camp" that said I try to be LOP when possible. 

Unless the is a reason to be high I stay between 4-8K feet. that typically allows me to make 75-85% (10-11GPH) power LOP. In my experience 20 degrees LOP is the sweet spot regarding max power while keeping the cylinders temp around 380.

I run ROP when the density altitude won't let me make 75% then I take what I can get.

Playing with RPM I find 2600-2700 is best. It does seem logical that slower is better for both reduced frictional losses in the engine and increased propeller efficiency but in my experience it is negligible if at all measurable. I am not an engineer but my guess is in the big picture both of those reductions in efficiency are very small relative to better BSFC running LOP.

Pick any power setting from the book note the speed and then go LOP and raise the rpm as needed to return to the same speed. In my experience there has never been a combination that is more efficient running a lower RPM ROP than a higher RPM LOP. It is only 1 GPH  less at 75% but it it is a free 10% increase in MPG.

Now let's throw in some temperature related reality. Book numbers seem to be the notorious 50 ROP. If you are going to run high power ROP and want to have the same CHTs as LOP then you need even more fuel than book. I find it takes 100-120 ROP or about 13 GPH to keep them around 380. That eeks out about 3 maybe 5 knots.

However if the density altitude is high (9K+)and cylinders are easy to keep cool I set up 50 ROP. I will post some pictures later.

 

[PT20J]

5 hours ago, A64Pilot said:

I see all of the time,  “I run LOP and save 30% on fuel”

Well that’s not really the case, what is, is “I run LOP and slow down and save 30% on fuel”

Most of that fuel saving is in slowing down, to quantify the difference if people are interested they need to remove all the variables except the LOP part, which means of course keeping speed and RPM the same.

Good point. It's pretty easy to figure out what the actual LOP vs ROP fuel saving is, apples-to-apples.

Looking at the chart for a Continental IO-550G at 65% power:

Best power ROP = 78 pph = 13 gph

Minimum BSFC (50F LOP) = 67 pph = 11.2 gph

Fuel savings 14%. Any additional savings would come from running at a lower power LOP than ROP.

Skip

 

 

[gsxrpilot]

Just now, PT20J said:

Fuel savings 14%. Any additional savings would come from running at a lower power LOP than ROP.

Yep, 14% from LOP and 20% from reduced power = 34% fuel saving.

I'm also in the camp of "I didn't buy a Mooney to go slow" But if a 34% fuel saving allows me to skip the fuel stop, I'm a lot faster A to B than if I was running higher power/higher fuel burn.

A typical flight for me is Denver Austin.
At 65% power and 9.5 GPH its a 4 hour flight.
At 75% power and 14.5 gph it's a 5 hour flight. (fuel stop)

[PT20J]

10 minutes ago, gsxrpilot said:

Yep, 14% from LOP and 20% from reduced power = 34% fuel saving.

I'm also in the camp of "I didn't buy a Mooney to go slow" But if a 34% fuel saving allows me to skip the fuel stop, I'm a lot faster A to B than if I was running higher power/higher fuel burn.

A typical flight for me is Denver Austin.
At 65% power and 9.5 GPH its a 4 hour flight.
At 75% power and 14.5 gph it's a 5 hour flight. (fuel stop)

That's the beauty of a turbo -- you get a lot wider operating envelope that translates into more options. Of course, the trick is to understand the operating envelope and know what your options are and how best to use them which you have excelled at.  

[A64Pilot]

25 minutes ago, PT20J said:

Good point. It's pretty easy to figure out what the actual LOP vs ROP fuel saving is, apples-to-apples.

Looking at the chart for a Continental IO-550G at 65% power:

Best power ROP = 78 pph = 13 gph

Minimum BSFC (50F LOP) = 67 pph = 11.2 gph

Fuel savings 14%. Any additional savings would come from running at a lower power LOP than ROP.

Skip

For some reason I can't seem to upload files right now. I'll add the chart later.

 

That still doesn’t tell the story, not really because do many of us actually cruise at best power which is what about 150F ROP?

Conti’s are a different motor, they do behave slightly differently too.

That’s why I was saying pick your favorite LOP setting, record the speed, and then duplicate that speed at 50 ROP, in a 360 Lycoming to prevent mixing of apples to oranges.

‘I suspect from my previous experience with a 540 Lycoming and a 520 Continental, that for whatever reason the Continental does better LOP, I’m defining better as less power loss.

But the proof is in the pudding and it’s easy to do, beyond a fuel flow meter it doesn’t require any special instrumentation.

‘I’ve not done it, so I don’t know the answer, but I’m expecting about 10%, because a 10% increase in BSFC is huge, 1 or 2% is actually nothing sneeze at, the Airlines would kill for 1 or 2%.

[A64Pilot]

9 minutes ago, PT20J said:

That's the beauty of a turbo -- you get a lot wider operating envelope that translates into more options. Of course, the trick is to understand the operating envelope and know what your options are and how best to use them which you have excelled at.  

A turbo is slightly more efficient, even without any playing with mixtures, a turbo takes what is normally waste energy and extracts power from it, heat from the exhaust of course.

Of course they also add weight, there is never a free lunch.

[PT20J]

5 minutes ago, A64Pilot said:

That still doesn’t tell the story, not really because do many of us actually cruise at best power which is what about 150F ROP?

I was just able to post the chart. I chose best BSFC and Best Power because that's the biggest spread to illustrate your point. But you can see that the chart includes other operating points as well.

Skip

[Shadrach]

13 minutes ago, A64Pilot said:

That still doesn’t tell the story, not really because do many of us actually cruise at best power which is what about 150F ROP?

Conti’s are a different motor, they do behave slightly differently too.

That’s why I was saying pick your favorite LOP setting, record the speed, and then duplicate that speed at 50 ROP, in a 360 Lycoming to prevent mixing of apples to oranges.

‘I suspect from my previous experience with a 540 Lycoming and a 520 Continental, that for whatever reason the Continental does better LOP, I’m defining better as less power loss.

But the proof is in the pudding and it’s easy to do, beyond a fuel flow meter it doesn’t require any special instrumentation.

‘I’ve not done it, so I don’t know the answer, but I’m expecting about 10%, because a 10% increase in BSFC is huge, 1 or 2% is actually nothing sneeze at, the Airlines would kill for 1 or 2%.

I know of no box stock aeroengine that consistently runs LOP as well as the angle valve four-cylinder Lycoming. Most of them will run smoothly (depending on manifold pressure) to 50LOP.  I’m not sure if the extra 5° of timing on earlier birds has anything to do with it, but mine will run well past 100 at 25” and above. 
 

Your speed comparison is the best way to determine fuel savings at a given speed. Alternatively you buy a twin and do real time engine to engine comparisons like this...

 

 

[jaylw314]

41 minutes ago, A64Pilot said:

A turbo is slightly more efficient, even without any playing with mixtures, a turbo takes what is normally waste energy and extracts power from it, heat from the exhaust of course.

Of course they also add weight, there is never a free lunch.

I thought turbocharger engines have a lower compression ratio?  That means their BSFC is lower

 

3 hours ago, A64Pilot said:

‘Why 50F ROP, well you have to pick something and I believe that’s Lycoming’s recommend cruise mixture, so may as well pick that.

I've done that, I wrote it down in my records:

150 KIAS, 6500' MSL, Alt 30.02

• Best power (125 ROP and 60%) - MP 21.0, RPM 2500, 10.2 gph
• Mooney economy (25 ROP) - 9.2 gph
• 25 LOP - 8.8 gph

Of note, for 25 ROP, I had to bump up MP a little to keep the same airspeed.  For 20 LOP I had to bump it up more, but I didn't write those numbers down.

Grain of salt--my fuel pressure sensor was...challenging, maybe +/- 0.2 gph with fluctuations

So 25 LOP only saves about 5% gph at the same power for me over Mooney economy, and about 15% over best power.

 

[PT20J]

For some reason I had a lot of problems posting the Continental chart in my earlier post. It's up now.

Skip

[Tim Jodice]

Here is about 85%

[jaylw314]

1 hour ago, Tim Jodice said:

Here is about 85%

Oooh, that really is a slick display!

Edited May 6, 2021 by jaylw314

[PT20J]

Here's an article some of you might find interesting. Of special interest is the old internal Lycoming paper on how to calculate power from fuel flow and the author's spreadsheet to automate the calculations. Try comparing Lycoming's method to whatever method you or your engine monitor use and post the results.

https://www.kitplanes.com/determining-engine-power/

Skip

[A64Pilot]

5 hours ago, jaylw314 said:

I thought turbocharger engines have a lower compression ratio?  That means their BSFC is lower

 

 

Not all turbo motors do, but usually one designed form the beginning to be a turbo motor as opposed to a motor that a turbo was added to, almost always has a lower compression ratio, often also forged pistons and oil jets spraying on the bottom of the pistons and many times a beefier crankshaft and stronger block all to withstand higher power outputs and for cooling.

‘But that’s a static compression ratio, by lowering the static compression ratio, that allows more boost because boost adds air, which raises the compression because the air is already under pressure, before the piston begins its travel upward.

This in effect makes a turbo motor “bigger” meaning that a 360 with a turbo may have the same air charge of a 400 cu in engine, and as the air was under pressure to begin with it has the effect of even higher compression ratio. A turbo motor can be thought of as a variable compression ratio engine, and a variable displacement engine too.

 That explains the power increase, but the efficiency increase comes primarily from harvesting waste heat and using it.

It can get complex, if a turbo is used for HP increase, it can have a lower BSFC due to needing extra rich mixture to stave off detonation, but if used for efficiency and power is limited to a level where a lean mix is safe, then they are more efficient, meaning a better BSFC.

[carusoam]

Tim’s monitor re-org’d...

-a-