LOP & Percent Power for Dummies®

[cwaters]

On 4/29/2021 at 6:44 PM, Ibra said:

I think MP =< RPM and few simple sittings makes a generic transition easy & short but one is free to explore other setting latter on their own with specific POH at hand or ask in MS or those who know their types very well (same as running LOP on high power, it's very important when you travel with proper instrumentation but not the smartest move during a short busy training)

Thanks, I'm still learning and trying to decipher the vast amount of information out there on engine ops. I'm trying to currently find a side of the fence for LOP vs ROP for my 201. there is so much out there (and here on MS) about it on both sides of the fence (to operate LOP or not). For reference I have an EDM 900 for monitoring.

[Hank]

Just make copies of your Performance Tables, stick them on your kneeboard and go fly. You'll figure it out pretty soon.

Hey, it worked for me. Then you can tackle "Key Numbers". 

[Ibra]

35 minutes ago, cwaters said:

I'm trying to currently find a side of the fence for LOP vs ROP 

Seems you can split any pilot room 50/50 on the topic 

 

Nothing very scientific about the choice, the splits will be clustered into those who fly 2h vs 4h legs, 0h or 2000h left on engine, cheap vs expensive gas, and those who enjoy it vs those in a hurry 

The EDM900 is a good tool to go LOP with more data than just by feeling ! 

Edited May 3, 2021 by Ibra

[A64Pilot]

LOP isn’t anything new at all, My Father used to get his 210 up high and lean it as much as he could and it run smoothly.

‘This was in 1968, many, many people did. You just give up 10 kts cruise or so but gained phenomenal range,mot weren’t willing to give up the speed.

An engine analyzer is a good tool to have in the box, but you don’t have to have one to run LOP, just run less than 75% power and you can do anything you want too with the red knob and it won’t hurt the motor (NA motor only).

‘Now that 75% power number is a number that’s valid at best power mixture, so LOP it’s actually well less than 75%.

Personally as I’m risk adverse I use 65% not 75%

LOP or rich of peak, or even peak is fine, just run rich of peak if your in a hurry, trying to go fast and be LOP or peak can get you into trouble, not saying it will, but why risk it.

You can within reason control power solely by fuel burn, try it. Set 22 squared and 9 GPH, then 8 GPH, then 7 GPH, and even 6 GPH, once you get below 8 you’ll notice a power decrease, probably 

‘A Diesel engine controls power output solely with fuel, it does not restrict air intake, there is no throttle valve.

Now we aren’t Diesels of course, but within limits we can run over a pretty wide set of mixture ratios

 

[PT20J]

3 hours ago, cwaters said:

Thanks, I'm still learning and trying to decipher the vast amount of information out there on engine ops. I'm trying to currently find a side of the fence for LOP vs ROP for my 201. there is so much out there (and here on MS) about it on both sides of the fence (to operate LOP or not). For reference I have an EDM 900 for monitoring.

Pilots can make stuff so complicated.  Actually, it’s pretty simple. 

1. If you follow the power charts in the POH, you cannot go wrong.

2. Below 75% power in your IO-360, you can do anything you want with the mixture without hurting anything.

3. With constant manifold pressure and rpm, ROP mixtures will yield more power (speed) in return for less economy (lower mpg). Conversely, LOP mixtures give greater mpg and slower speeds. You can decide what’s more important to you and operate accordingly.

4. Most stock Lycoming I0-360s run pretty well LOP. You don’t even need any fancy instrumentation. Just lean carefully until it gets a little rough and richen just enough to smooth it out.

Skip

[Ragsf15e]

Here’s my technique (M20F)...

When my wife and I have our twin 5 year olds on board and we’re going more than 100nm, I’ll go ROP every time to save the 3 minutes and hopefully outlast their kindle battery. I will get approx 145kts on ~11.5gph fully loaded at 10k.  The CHTs are nice at 350ish, it costs a few extra gallons, but it saves a couple minutes.

When it’s just me, I’ll run LOP at 140kts on around 9gph at 10k.  It’s cooler CHTs (sometimes too cool), but it definitely saves some fuel.  You can do the math on the extra time required (145 vs 140kts), but it’s tiny.  
 

In my opinion, both techniques should be in your toolkit.

[carusoam]

Helps to read up on the GAMI spread test...

Helps to have a FF gauge to run a GAMI test....

If your GAMI spread is close to zero...  your cylinders are operating very close to being identical.

when your cylinders behave in an identical fashion... you can lean them deeply...

Deep leaning at 5K’ can be near 90°F LOP...

Deep leaning at 10k’ may only be about 50°F...

Deep leaning leaves a lot of power behind...

Deep leaning can compress a lot of air unnecessarily...

The coolest thing about a low GAMI spread... you can lean deeply without the engine getting rough...

So... if you use the lean til rough method...   your engine will turn off before it gets rough...  

To run a GAMI spread test... no GAMI injectors are required...

PP thoughts only, not a CFI...

Best regards,

-a-

 

[gsxrpilot]

I've only been seriously interested in running LOP since 2016. So that's only the last 5 years. Before that it either wasn't my airplane, was renting on a "wet" rate, or was flying a carbureted engine. But in the last 5 years, I've taken live classes, online classes, and read/studied everything I can get my hand's on regarding the operation of our piston engines.

As a relative newbie to this, I've heard a million times how it's so easy. Just lean to rough and then richen it up just a bit. That might work when you've got 50K hours behind these engines and can smell the difference between 10° LOP and 30° LOP. But for someone just trying to figure it out, and yet on the hook for the engine if you screw it up, that isn't enough for me.

• For one, a very well fuel balanced distribution, will never run rough but will run smooth all the way to idle cut off.
• With a poor fuel distribution, it might start to run rough with some cylinders LOP while others are still ROP. Richening that up "just a bit" might put you in the worst possible place to run the engine.
• What is "rough" and what is happening there?
• Avoiding any LOP operations above 65% power would seem to be good advise with a good margin for error.
• Advice that running any mixture at 75% power or below?? This is contradicted by some sources. Who's correct?
• Cirrus says anything below 80% power is ok. Maybe that requires a parachute?

Modern engine monitors are easy to read. And with a little education are very easy to understand and interpret. 

• Fuel flow spread between the first cylinder to peak and the last cylinder to peak. If the spread is too large (>0.5gph) "rough" will probably happen before all cylinders are LOP. 
• Number of degrees LOP based on last cylinder to peak or degrees ROP based on the first cylinder to peak.
• A read out of % of HP without having to triangulate the numbers on the very faded power chart in the 40 year old POH.
• Quick diagnosis of intake leaks, weak ignition systems, or the specific plug fouled.

When you don't have enough hours to have experienced a bad engine or know one from a good engine. And you can't tell the difference between "rough" from being a few degrees too lean and "rough" from being over open water or high mountains without any engine change at all, and you really need that engine to get within shouting distance of TBO or you might be walking... an engine monitor can be the most valuable and comforting instrument in your panel

[gsxrpilot]

I run ROP if I can do so and still make the destination without a fuel stop. 

I run LOP anytime that will allow me to skip a fuel stop.

LOP saves me 33% fuel for a 12% speed penalty.

When flying in the flight levels, not stopping for gas saves time measured in hours.

[PT20J]

Paul, 

The 75% number is for the Lycoming IO-360 which the OP has. I believe Continental says 65%. Most airlines ran the radials at around 45%. My point was that below a power recommended by the manufacturer, you can’t hurt things with the red knob.

Engines will all start to run somewhat rougher as the mixture gets very lean even if you had perfectly matched mixture to all the cylinders (which is never possible). The reason is that each cylinder has cycle-to-cycle variation in the combustion event and that causes power variations which become greater as the mixture gets leaner. In fact, that’s the reason that the BSFC curve starts to rise again after it hits a relatively flat minimum as you continue to lean. But it is subtle; it’s not like the engine is shaking itself to death. In the NA IO-360 you can notice it if you lean carefully. The procedure is in the Lycoming Operator’s Handbook. The late Walter Atkinson told me that’s the way he used to lean his twin Beech and the R-985 being a supercharged radial has about the best mixture distribution you can get. 

Skip

[PT20J]

1 hour ago, gsxrpilot said:

I run ROP if I can do so and still make the destination without a fuel stop. 

I run LOP anytime that will allow me to skip a fuel stop.

LOP saves me 33% fuel for a 12% speed penalty.

When flying in the flight levels, not stopping for gas saves time measured in hours.

That makes a lot of sense to me. You have a turbocharged engine which gives you a lot of options to choose an optimum altitude and fuel flow. And, you can fly high enough to run LOP at 65% or less and still have great true airspeeds.

My little M20J with a Lycoming normally aspirated IO-360 doesn’t have as many options. Down low just fooling around I run LOP and save gas. But in the mountainous west where I live, going anywhere generally requires me to fly high enough that LOP is too slow (for me) and I generally run at peak.  

Skip

[carusoam]

Speaking of the late Walter Atkinson...   

We credit APS for giving the guideline of 65%bhp as the do no harm, lean all you want guideline...

With a NA aspirated engine this is an easy guideline to follow...if unsure climbing another 1k’ to experiment isn’t too difficult...

when leaning really deep....  some seat of the pants sensors are probably more sensitive than others...

Best regards,

-a-

[jaylw314]

A point worth noting is that the ignition system is another factor as to how LOP you can get.  If your ignition system has any weakness, it may produce roughness LOP before you get any roughness from fuel imbalance and can produce wonky results on an engine monitor.

[Steve Yucht]

So to add a little avgas to this fire...  I run ROP pretty much all the time to keep my hottest CHT at 380 deg. In my O2 this comes out to about 23 squared at 8-9000 ft (my preferred altitudes), 170 kts and 14.8 gph with no wind factor. I never fly more than 2:30 legs.  Why bother, you are just increasing your risk for a blood clot and one of those makes a top overhaul look cheap!  That's about the limit for my bladder anyway.  I could see if you are flying solo and dont mind peeing in a bottle, otherwise I view LOP as a way to save fuel to avoid a stop I want to make anyway.  That said, I absolutely use LOP when I am trying to juggle fuel flow to land with 16 gal and a full plane load of passengers and luggage.  I then usually start out ROP and switch to LOP if I need to slow down the fuel flow.  There is no one correct answer, just a bunch of tools in our toolchest.

Edited May 4, 2021 by Steve Yucht

[A64Pilot]

9 hours ago, PT20J said:

But in the mountainous west where I live, going anywhere generally requires me to fly high enough that LOP is too slow (for me) and I generally run at peak.  

Skip

For us simple NA guys with little motors LOP works best for the East Coast.

There are very few rules that work best for all altitudes GWT and air speeds 

But it’s just like the advice of don’t lean below 5,000 ft. For those that don’t really inderstand what’s going on, following that rule will ensure they don’t hurt their motor.

‘But altitude has nothing to do with leaning, manifold pressure does, on 30 min trips etc I’ll lean the snot out of it and I’m at 2,500 or so, but I’m at 22” manifold pressure.

‘My factory stock ignition and fuel system on my motor will continue to run smoothly so far LOP that it just doesn’t develop enough power to keep flying.

‘For instance I can set 22 squared and lean to 6 GPH, she slows down to 120 kts but is still smooth and  I need cowl flaps to close fully to keep CHT in the green.

‘I fly like that when the mission is just to fly around for the enjoyment of flying, no destination. Makes me wish I could open the window and hang my arm out.

Edited May 4, 2021 by A64Pilot

[Shadrach]

23 hours ago, PT20J said:

Pilots can make stuff so complicated.  Actually, it’s pretty simple. 

1. If you follow the power charts in the POH, you cannot go wrong.

2. Below 75% power in your IO-360, you can do anything you want with the mixture without hurting anything.

3. With constant manifold pressure and rpm, ROP mixtures will yield more power (speed) in return for less economy (lower mpg). Conversely, LOP mixtures give greater mpg and slower speeds. You can decide what’s more important to you and operate accordingly.

4. Most stock Lycoming I0-360s run pretty well LOP. You don’t even need any fancy instrumentation. Just lean carefully until it gets a little rough and richen just enough to smooth it out.

Skip

Agree with the above but would add the caveat that one needs to know enough about how engines behave to identify that the engine is LOP or isn’t. Most properly maintained NA Lycs will run very far LOP (especially at higher MPs) but if your behind one that doesn’t, it’d be good to be able to recognize the situation. At low power it doesn’t matter. I will also say that leaning until rough and enrichening until smooth would put my engine way leaner than optimal at most any altitude. This may be more true for engines timed to 25°.

[A64Pilot]

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 

Edited May 5, 2021 by A64Pilot

[Marauder]

On 5/4/2021 at 8:13 AM, Steve Yucht said:

So to add a little avgas to this fire...  I run ROP pretty much all the time to keep my hottest CHT at 380 deg. In my O2 this comes out to about 23 squared at 8-9000 ft (my preferred altitudes), 170 kts and 14.8 gph with no wind factor. I never fly more than 2:30 legs.  Why bother, you are just increasing your risk for a blood clot and one of those makes a top overhaul look cheap!  That's about the limit for my bladder anyway.  I could see if you are flying solo and dont mind peeing in a bottle, otherwise I view LOP as a way to save fuel to avoid a stop I want to make anyway.  That said, I absolutely use LOP when I am trying to juggle fuel flow to land with 16 gal and a full plane load of passengers and luggage.  I then usually start out ROP and switch to LOP if I need to slow down the fuel flow.  There is no one correct answer, just a bunch of tools in our toolchest.

I am a ROP guy as well. The only guy One guy on this board is 201er. He who routinely uses LOP to make sure he can fully soil his cargo pants to live up to his "Stinky Pants" moniker. 

[Bob - S50]

1 hour 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 

First, 75% ROP and 75% LOP will yield the same speed.  However, to get 75% power, it will take more MP when running LOP so you can get 75% ROP at a higher altitude than you can LOP.  I actually seated the rings in our engine after a Top OH by running 75% LOP.

To define optimal.  To me it would mean running LOP at the point of most efficient mixture or BSFC and with optimal peak pressure timing.  From what I've read, that would be about 30 - 50 degrees LOP and PP at about 16 degrees after TDC.  LOP is pretty easy to find with a graphic engine monitor.  We have nothing to show us PP location.  Since we are using the mixture knob to find LOP, we can't then use it to move PP.  We can always use the prop control to move PP but the throttle can only be used to move PP if we are at lowish altitudes.  Higher RPM moves PP further past TDC, low RPM moves it toward TDC.  High MP moves it toward, and low MP moves it away.  Timing (20 vs 25) will have a definite affect on when we hit PP.  Those with engines timed at 20 should hit optimum at lower RPM than those timed at 25.  Too bad we don't have a PP indicator.

I found that at cruise altitudes up around 10,000' my #3 CHT (A3B6D 25BTDC engine) would flirt with 400F.  By increasing RPM from 2500 to 2600, I actually dropped the CHT by about 5 or 10 degrees while going faster.  The drop was probably due to a few factors; higher RPM moved PP further from TDC, higher RPM probably also dropped MP by a fraction of an inch due to friction which also moved PP further from TDC, and the higher speed resulted in more cooling air going over the cylinders.

[A64Pilot]

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.

 

[A64Pilot]

I’m saying that LOP doesn’t have to be complex, as I said my Father ran LOP in the 60’s, it wasn’t called LOP back then it was just called leaning it out.

But the “new” ways have people believing that you have buy expensive equipment and resolve complex problems inflight, and y9u don’t. There is a KISS way to run LOP for those that don’t want to make a science project out of it.

[Hank]

1 hour 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.

 

The injected guys have a formula based on fuel flow to calculate power when LOP. My carb doesn't run LOP very well, I don't have fuel flow, and the formula is engine-specific [at least O-360 is different than IO-360], something like 13.5 or 13.9 * fuel gph = hp, so the target for 75% is 150 hp and for 65% is 135.

Maybe @201er will come along with the real formula. It's not rocket science. But my O-360 bleeds airspeed like a shooting victim when I try LOP, so I don't use it much . . . .

[A64Pilot]

Any formula would be based upon the Brake Specific Fuel Consumption, which is a fancy way of saying how efficient an engine is, the BSFC will change some when LOP and will vary with how LOP you are.

‘If it didn’t then there would be no reason fuel consumption wise to run LOP. With mixture to some extent your swapping power production for efficiency, it’s not much, but it is real.

 

 

Edited May 5, 2021 by A64Pilot

[N231BN]

When at peak EGT or leaner:

8.5:1 Compression ratio = approx 14.9 hp/gph
7.5:1 Compression ratio = approx 13.7 hp/gph

[jaylw314]

3 hours ago, A64Pilot said:

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

Just write down your IAS at 75% ROP.  When you hit that same airspeed LOP, you're at 75% power!   Write down your fuel flow and you have your one and only LOP setting.  Since LOP, power is limited by fuel flow, MP and RPM won't matter.

Theoretically, RPM will slightly affect efficiency through friction.  Being super lean will also slightly decrease efficiency, but the effects are pretty small practically, since as you go super lean, you'll lose so much power anyway.  LOP already causes enough impatience!  Between peak and 50 degrees LOP, the BSFC curve is fairly flat.

Of course, good luck actually getting 75% power LOP in an IO-360.  I might be able to get that below 3000' MSL, but where I am it's a bumpy ride at top speed that low...

I know my J is a couple knots slower than POH at 70% power, and I can match that speed with 9.4 gph, which is within the range of error from the calculated FF that mentioned above

Edited May 5, 2021 by jaylw314