LOP & Percent Power for Dummies®

[211º]

My Mooney has a JPI 700 which I quite like.  It isn't like the 900 or 930 (or 990 - the 2021 update) so it doesn't have percent power.

I really like flying LOP and at less than 65% power (to completely eliminate the concern of the red box)

In a recent post, I "screen-shotted" the formula for percent power calculation:  (14.85 * GPH)/200 = Percent Power = (area of cylinder * GPH)/ HP Engine.

If I want to make sure that I'm always below 65% power, is it as simple as:

14.85 * GPH / 200 = 0.65

GPH = 0.65 * 200 / 14.85 = 8.75 GPH

So for any setting with manifold pressure and prop where then engine is still running at 8.75 gallons/hr will be less than 65% power.

Is that correct?

[201er]

I don’t remember any of these formulas any more but I’ll just point out that at 8 or 8.5 gph, you’re quite safe LOP and very efficient. Between 8.5-10gph, LOP is fine too, you just gotta make sure you are 15-40 degrees LOP on the richest cylinder and make up to 75% power. Gets difficult to do above 4000ft without a turbo charger cause you can’t find enough air to make this much power so lean. 
 

I use the fuel flow technique when I gotta set the power quickly and not for too long. Like leveling off for airspace or ATC. When leveling off for a prolonged cruise, I’ll hit the lean find button and slowly lean out till about 15LOP in the 8-9GPH range. Keeps things cool and smooth.

[Ibra]

I think GPH as proxy of power is misleading if you are running at 2700rpm and 29MP as on takeoff on breaks producing 100% power then lean mixture very aggressive (one should not do that!) you will see 8.75GPH at some point no, is that ok for 65% power? now how about if you want 35% power will you lean to 4.5gph on 29MP&27RPM?

However, the GPH along an EGT curve (peak, 100ROP, 50LOP) indicates a proxy for % of power for the given MP & RPM setting: bellow 24/24, 25/23, 23/25 you can use GPH as proxy for LOP/ROP and even setting power % as per the formula, it just works well, above 25/25, 26/24, 24/26 just go full rich no LOP...

Otherwise why one can’t just fly wide open on throttle & propeller and use mixture to set power %?

Edited May 25, 2020 by Ibra

[jlunseth]

Your math is correct and the model is correct. I think the multiplier is actually 15 for normally aspirated engines but I fly a turbo so I haven’t really paid attention to the NA multiplier.

I don’t know why you would stick to 65% though. I run my turbo (TSIO360LB) routinely at 11.1-.3, which is 71%, and have a lot of hours on it that way. As long as you stay lean of peak enough there should not be a problem.

[Bob - S50]

29 minutes ago, Ibra said:

I think GPH as proxy of power is misleading if you are running at 2700rpm and 29MP as on takeoff on breaks producing 100% power then lean mixture very aggressive (one should not do that!) you will see 8.75GPH at some point no, is that ok for 65% power? now how about if you want 35% power will you lean to 4.5gph on 29MP&27RPM?

However, the GPH along an EGT curve (peak, 100ROP, 50LOP) indicates a proxy for % of power for the given MP & RPM setting: bellow 24/24, 25/23, 23/25 you can use GPH as proxy for LOP/ROP and even setting power % as per the formula, it just works well, above 25/25, 26/24, 24/26 just go full rich no LOP...

Otherwise why one can’t just fly wide open on throttle & propeller and use mixture to set power %?

I think what you are saying is you can't just use fuel flow to determine power, that you must have a combination of RPM and MP that will result in LOP operations in order for the OP's 8.75 gph to be 65% power.  I agree with that.

However, I think your second paragraph kind of muddies the waters.  I THINK what you were saying is that if the OP wants to run LOP, they should use a MP+RPM combination that would result in 65% or less and then run LOP.  However, if the OP does that, then 8.75 gph would likely result in ROP operation.  So I think you actually have it backwards.  At power settings that would yield more than 65% power if running ROP, and the OP leans to 8.75 gph, then they will get 65% power.  That assumes that the mixture is not so lean that it burns inefficiently, in which case it would make less than 65%.

[Ibra]

8 minutes ago, Bob - S50 said:

I THINK what you were saying is that if the OP wants to run LOP, they should use a MP+RPM combination that would result in 65% or less and then run LOP. 

Thanks for making that clear, it has to be 65% power combination to use that 8.7GPH, if it is 50% power combination you use 6.7GPH for LOP (an 8.7GPH is almost 100ROP) or keep that 8.7gph setting as low value for FF on higher combinations if you want to avoid LOP at higher power settings than 65% (or 75% & 10GPH as per the book)

Edited May 25, 2020 by Ibra

[Bob - S50]

57 minutes ago, 211º said:

My Mooney has a JPI 700 which I quite like.  It isn't like the 900 or 930 (or 990 - the 2021 update) so it doesn't have percent power.

I really like flying LOP and at less than 65% power (to completely eliminate the concern of the red box)

In a recent post, I "screen-shotted" the formula for percent power calculation:  (14.85 * GPH)/200 = Percent Power = (area of cylinder * GPH)/ HP Engine.

If I want to make sure that I'm always below 65% power, is it as simple as:

14.85 * GPH / 200 = 0.65

GPH = 0.65 * 200 / 14.85 = 8.75 GPH

So for any setting with manifold pressure and prop where then engine is still running at 8.75 gallons/hr will be less than 65% power.

Is that correct?

This is correct assuming you are using enough RPM+MP to generate at least 65% power.  If you were at 15" and 2200 RPM, then 8.75 gph would be ROP and you would not be making 65% power.  For my J, the combination is MP+RPM > 47.  I don't remember what it is for your E.  When we were seating rings in our engine, I routinely ran 75% power LOP (10 gph).

You can take your idea a bit further.  Go out and do some testing.  Think of a few power settings you like to use.  For example, I like to use 22" and 2600 RPM when cruising at low altitudes.  If it gets a bit bumpy I'll pull the power back to 20".  Maybe you like to use 17" and 2200 RPM for site seeing.  Whatever combinations you like, go out, set a combination, lean until you are LOP and note the fuel flow.  Write it down.  In the future, any time you set that combination of MP and RPM, you can simply lean to the appropriate fuel flow and you will be LOP.

And yes, if that fuel flow is 8.75 gph, then you are below 65% power.

[ArtVandelay]

Nobody uses the performance tables in the POH?

[aviatoreb]

13 minutes ago, ArtVandelay said:

Nobody uses the performance tables in the POH?

That would be in the ROP for dummies thread.

[jaylw314]

6 hours ago, 211º said:

My Mooney has a JPI 700 which I quite like.  It isn't like the 900 or 930 (or 990 - the 2021 update) so it doesn't have percent power.

I really like flying LOP and at less than 65% power (to completely eliminate the concern of the red box)

In a recent post, I "screen-shotted" the formula for percent power calculation:  (14.85 * GPH)/200 = Percent Power = (area of cylinder * GPH)/ HP Engine.

If I want to make sure that I'm always below 65% power, is it as simple as:

14.85 * GPH / 200 = 0.65

GPH = 0.65 * 200 / 14.85 = 8.75 GPH

So for any setting with manifold pressure and prop where then engine is still running at 8.75 gallons/hr will be less than 65% power.

Is that correct?

Realize that the IO-360 has a ton of detonation margin.  IIRC, there is a test stand graph where they measured detonation at full power leaned for best power, and only at full power at CHTs>400degF was there the start of light detonation.  Anything less than full power or those temps there was none detected.  For the life of me, I can't find a copy of those graphs, though.

If your CHT's run abnormally hot, those limits might be important, but if they're average, those are pretty conservative.  I have a fairly cool motor and I've run 9.5 gph while LOP without high CHT's or evidence of detonation on annual borescoping.

IIRC, the 14.85 number is for 8.5:1 compression ratios, so in theory the IO-360A3B6D would be something like 15.2 for the 8.7:1 compression ratio.  That means your theoretical 65% power is slightly lower,  around 8.6 gph while LOP.  The conversion factor also does change with how far LOP you are, so it's not perfect anyway.

Incidentally, upgrading the JPI 700 to a 730 is relatively inexpensive and takes 5 minutes for your A&P to install.  The screen is larger and the visual depection is more effective than the 700.

[jlunseth]

The method of operation is WOTLOPSOP, which stands for Wide Open Throttle, Lean of Peak, Standard Operating Procedure. For an NA engine you would leave the throttle wide open, you would not manage MP with the throttle at all.  You can bring the RPMs back if you want. You might try 2500. I use 2450 in my engine it just seems to like that speed.

Then percent horsepower is managed entirely with the fuel knob.  The simplest method is the Big Pull, you pull the mixture back until you feel a drop in power, that means you are on the lean side of peak. In the OPs case, he could just go straight to 8.7 GPH, that would do it.  

It is important to understand, though, that lean of peak or rich of peak operation are a fuel/air ratio, not a fuel flow number. We use handy things like fuel flow to get to a LOP setting quickly. But it would be possible to be at a setting that is not lean of peak in an NA even though fuel flow is 8.7 GPH. The way that would happen is that as the aircraft ascends the MP changes, it goes down. So if you start with an 8.7 GPH fuel flow at sea level, making 29”, and you ascend to, say, 18,000 where the MP is 14.5, and you leave the GPH at 8.7, you are probably now flying with a rich of peak or near peak fuel setting. The fuel/air ratio has changed. I know you probably can’t get to 18k, I used it for this example because the math is simple, the ambient air pressure is 1/2 ATM.  The point is, if fuel flow stays the same (8.7 GPH) as the aircraft ascends, the fuel/air ratio moves towards peak and then rich of peak simply because there is less air. I would not worry too much about it though, the engine probably has not exceeded 65% of its rated HP and you are not hurting it, you just are not LOP anymore. This is why we like to use an engine monitor to make a LOP cruise setting, or a ROP cruise setting for that matter. I would be a little concerned if you were to go to, say, 15k, whether you are running the engine too rich at 8.7 GPH and messing up your plugs.  I don’t know, I don’t fly a J.

8.7 GPH is a handy way in this instance of making a setting for use in the lower altitudes.

Edited May 25, 2020 by jlunseth

[carusoam]

Important details that 211° gets out of this conversation....

1) Using a proper formula for your situation is important...

2) The LOP formulas are sensitive to CR...

3) CRs are in the engine manufacturer’s data...

4) Often the CR data makes it to the POH...

5) Some POHs are loaded with valuable data...

6) Some POHs don’t even mention LOP.

7) TC’d engines have much lower CRs...

8) Some MSers have gone with higher CRs... 10:1...

9) Techniques of going LOP are helpful...

10) Techniques of staying ROP above SL can be helpful as well... it isn’t always full rich.

11) Avoiding the red box is easier with an NA engine... as MP is altitude dependent...

12) use caution with over simplified formulas... it takes extra effort to make sure they fit your situation.

PP thoughts only, not a mechanic or CFI...

Best regards,

-a-

[gsxrpilot]

Sorry to be late to the party here but that's my post that you took the screen shot from. I got the multiplier from @kortopates specifically based on the compression ratio of an IO360 engine. By the way, the multiplier for the TSIO360 is 13.7. 

I didn't read every post closely through this thread, but let me clear up a couple of things...

If you are running on the Rich side of Peak EGT, then by definition, you have excess fuel already and so % HP will be determined by the amount of Air which in our case is measured as Manifold pressure and it controlled by the Throttle. The Mixture has nothing to do with % HP as long as you are ROP. So the formula doesn't work for any situation where you are ROP.

If you are running on the Lean side of Peak EGT, then by definition, you have excess air already and so % HP will be determined by the amount of Fuel which in our case is measured as GPH and is controlled by the Mixture knob. The Throttle has nothing to do with % HP as long as you are LOP. And therefore the formula, as you worked out, will give you % HP for any MAP/RPM combination... as long as you stay on the LOP side.

But surely the MAP and RPM have something to do with something??? Yes, MAP and RPM can effect how far or how many degrees you are on either side of Peak. Here are some examples. These are made up numbers but hopefully they explain the effect of MAP and RPM on LOP...

Assume 21" MAP, 2500 RPM, and 8.75 GPH = 65% HP and 30° LOP

Then 23" MAP, 2500 RPM, and 8.75 GPH = 65% HP and 40° LOP (Here we've increased the MAP which adds air, thus leaning the mixture further, and therefore going further, more degrees from Peak EGT)

How about 19" MAP, 2500 RPM, and 8.75 GPH = 65% HP and 10° LOP (Here we've reduced the MAP which removes air, thus richening the mixture and therefore coming closer to Peak EGT.)

So what about @Ibra scenario of 29" MAP, 2700 RPM  and 8.75 GPH? This still equals 65% HP but so far LOP, probably close to 100° on the Lean side of Peak EGT, that the engine might not run. 

The formula works as long as you're on the Lean side of Peak EGT. I hope this helps.

[211º]

All good info - especially about the calculation being valid only when LOP due to fuel/air mixture.  That makes sense.

There is a part of my flying personality that would rather be a little slower and have more time in the air rather than save 15 or 30 minutes and use more fuel.  I frequently fly LOP between 5,000 to 9,000 and have a standard setting of 21" and 2350.  As I get higher, the %HP increases, but stays below 65%.  At this setting I can cruise at about 135mph IAS with a TAS of 149mph and 160mph -- all at 7GPH.  With 52 gallon capacity, that is a little over 7 hours in the air - which as noted, fits my flying profile/mission well.

After having written that I am going to double check how LOP my leanest cylinder is - there is no roughness, but there is that LOP slight power loss. 

Maybe I'll end up changing to a standard of 7.5gph or 8.0 gph

I appreciate all the input.

[gsxrpilot]

@201er is the hypermiler champion around here. Ask him to explain Carson's speed to you. No one goes further on less gas than he does.

[MikeOH]

Carson's speed explanation for dummies: 1.3 x Best Glide Speed

Edited May 26, 2020 by MikeOH

[PT20J]

https://cafe.foundation/v2/pdf_tech/MPG.engines/AIAA.1980.1847.B.H.Carson.pdf

[201er]

10 hours ago, gsxrpilot said:

@201er is the hypermiler champion around here. Ask him to explain Carson's speed to you. No one goes further on less gas than he does.

Simply put for dummies, fly the M20J at 120KIAS from climb to descent for the best bang for the buck. Climb full power at 120KIAS for good speed and cooling. Cruise high enough that you get max TAS while WOT at 120KIAS at 2500RPM at around 8-9gph. Power back to descend at 120KIAS till it’s time to put the wheels down. I don’t do any math while flying, I just fly 120KIAS the entire trip and then AOA on final.

[Ibra]

My guess is MP&RPM gets into picture as MP*RPM is your “AirFlow”: the air volume that gets pumped by cylinders when the propeller is windmilling & throttle is open (also depends on cylinder surface/displacement and compression ratios), from that you match “FuelFlow” and generate some heat to get some power, when LOP power is decide by FF but you need to keep some EGT levels for combustion to continue when LOP...

On efficient use of wings & gas: Drag/Lift vs speed (polars) are always modelled as quadratic functions A*(V/Vs)^2+B/(V/Vs-1)^2, without power on that curve the best glide optimum do sit around 33% above Vs which gives max range without gas, if you want to maximise the use of remaining gas on top of that you go for cruise at another 33% above that, so about 1.9-2.0*Vs which gives max range & optimal speed with power on remaining gas, for 65kts stall speeds you are around 110IAS-120IAS and maybe add few 10kts for headwinds and +/-5kts for weight if you care about flying over the ground with power or without 

Away from 1.3*VS-2.0*VS you are in “high power/drag regimes”, either backside of the drag curve or high power cruise, if I have any doubt on aircraft speeds or performance I fly near the low power range the aircraft seems to like it and settles there nicely 

Those +33% that comes anytime on top when optimising quadratic X^2 functions has something to do with the surface under, the average value or some linear tangent...

Edited May 26, 2020 by Ibra

[jlunseth]

13 hours ago, 211º said:

After having written that I am going to double check how LOP my leanest cylinder is - there is no roughness, but there is that LOP slight power loss. 

Maybe I'll end up changing to a standard of 7.5gph or 8.0 gph

I appreciate all the input.

If you are trying to figure out whether your engine is healthy running LOP you need to check how LOP the richest cylinder is. That is the cylinder that, when LOP, is closest to the “red box.” The only concern with the leanest cylinder is that it might be too lean to fire consistently, and if that happens you will know soon enough, the engine will run rough, but it is not hurting anything.

Has it occurred to you, though, that if you run that far lean, at 65% power or less, you are essentially flying an Archer/Arrow class aircraft? I will grant you, your fuel flow will be better than an Archer. But the aircraft itself would be cheaper, less complicated to fly. 60% is approach speed for me. My engine flies around for hours at 70-71%, faster and it now has several hundred hours doing that with no ill effects, in fact, knock on wood, I am about 100 hours over TBO with the TSIO360LB.

 

[211º]

Agreed on check LOP-ness on richest cylinder - IIRC, it is about 20°F LOP and the leanest is about 80°F LOP.  I can check my Savvy analysis for that.

[Piper cooment] Ha! & Ouch, man! But (for me) there is just something in my brain about flying a four-hour leg that nears closer to five hours and having seven hours in the tank that keeps my pulse and concern at moderate levels.

Although I didn't know when I bought it, it seems that I bought the E for its 52 gallons and not its speediness - my mission is most likely in the minority.

(It has even occurred to me that my old 150 with long-range 50-gallon tanks and an autopilot might not be too bad a platform - ok, and more wing loading too - I can do without the ragdollness).

[pwnel]

5 hours ago, jlunseth said:

Has it occurred to you, though, that if you run that far lean, at 65% power or less, you are essentially flying an Archer/Arrow class aircraft? I will grant you, your fuel flow will be better than an Archer. But the aircraft itself would be cheaper, less complicated to fly. 60% is approach speed for me. My engine flies around for hours at 70-71%, faster and it now has several hundred hours doing that with no ill effects, in fact, knock on wood, I am about 100 hours over TBO with the TSIO360LB.

Man that's an excellent statement.  Yes, I fly deep lean and very conservative..... but I could do that cheaply in an Arrow.  Real good point.

[Bob - S50]

I just don't understand why people want to run 'deeply lean'. I want my engine as efficient as possible. That means I want my engine about 50 LOP more or less and use a MP and RPM combination that gives me the power I want.

[Ibra]

I think there are millions ways to run an engine with 3 levers (unlike just one to set % of power and let ECU do it) and only few constraints to watch for, after having enough play with these now I got my simple way: 25rpm, 23mp and between 50LOP-pEGT or rich to lower CHTs, that delivers me 140-145ias all the way to 9000ft above that I can move the prop 

When I used to rent C182, I paid dry rent for tach hour and the fuel on my own, I used to run it at 22rpm, 25mp, deep LOP for some reason that made it lot cheaper per hour, less so per 100 nautical miles but I built load of hours that way  

Some may run their engine for best MPG? some for more TBO hours on 2700rpm tach? some for max cruise speed? max NM range? some for min noise? CHT less than 300? that should gives one where they should start...

Edited May 26, 2020 by Ibra

[pwnel]

38 minutes ago, Bob - S50 said:

I just don't understand why people want to run 'deeply lean'. I want my engine as efficient as possible. That means I want my engine about 50 LOP more or less and use a MP and RPM combination that gives me the power I want.

that's where I run - maybe I'm exagerating then when I say "deeply"