LOP & Percent Power for Dummies®

[Steve Yucht]

 

Thanks all for the advice.   I'm definitely trying to reconcile a bunch of different information from presumably very reliable sources.  Reading John Deakin and Mike Busch then comparing that to what is said here on Mooneyspace and the POH (and the final confounder is my 10,000 hr CFII A&P/AI who has 2000 hrs in his 1998J) has proved challenging.  I suspect if I was in quadruple digits for my flying hours I might have a different perspective but I'm still in the phase where I want to learn ravenously but keep it simple in the cockpit. Thanks so much for your guidance, it is incredibly appreciated! 

[Ibra]

There are some high level guidelines for healthy engine parameters that are not engine and aircraft specific (e.g. no over-square, reduce WOT above 1000ft, WOT above 6000ft, ROP above 75%), outside these you can go but then you need to pay attention to some extra details which now we have with accurate engine instrumentation, then the last factor is the weather and type of flying you operate under 

If someone says the prop has to be on 2500rpm for cruise, they may wish to talk about fuel efficiency, prop efficiency, engine noise, more power, CHT range, max mpg, max endurance? just remember you may have to disregard all of that when looking for an emergency climb on take off or with a mountain ahead (there is one unique engine config and one unique speed to tune) 

[cwaters]

On 5/29/2020 at 10:54 AM, Ibra said:

There are some high level guidelines for healthy engine parameters that are not engine and aircraft specific (e.g. no over-square, reduce WOT above 1000ft, WOT above 6000ft, ROP above 75%), outside these you can go but then you need to pay attention to some extra details which now we have with accurate engine instrumentation, then the last factor is the weather and type of flying you operate under 

If someone says the prop has to be on 2500rpm for cruise, they may wish to talk about fuel efficiency, prop efficiency, engine noise, more power, CHT range, max mpg, max endurance? just remember you may have to disregard all of that when looking for an emergency climb on take off or with a mountain ahead (there is one unique engine config and one unique speed to tune) 

can you define the "Over-Square" part of what not to do?

[gsxrpilot]

30 minutes ago, cwaters said:

can you define the "Over-Square" part of what not to do?

That refers to the RPM number being higher than the manifold inches number, i.e. 28" MP and 2500 RPM. 

You can ignore this old wives tale. "Over-square" is a non-issue with our engines.

[PT20J]

21 minutes ago, cwaters said:

can you define the "Over-Square" part of what not to do?

Back in the radial engine days there were issues with running high rpm and low manifold pressure because it reverse loads the master rod bearing and can cause it to fail. The rule of thumb was to avoid operation under square which meant to avoid combinations where the manifold pressure in inches was less than the rpm in hundreds (equal rpm and manifold pressure being considered “square”).

It appears that the concept got transferred (but in reverse) to the small normally aspirated engines we fly. The idea is that a power setting is safe if not over square meaning manifold pressure in inches should always be less than or equal to the rpm in hundreds. This is not a bad rule if you don’t have a power chart. But, if you do have a chart, any approved combination of manifold pressure and rpm is fine. Engines are more efficient at lower rpm and higher manifold pressure primarily because frictional and pumping losses are reduced.

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[cwaters]

15 minutes ago, gsxrpilot said:

That refers to the RPM number being higher than the manifold inches number, i.e. 28" MP and 2500 RPM. 

You can ignore this old wives tale. "Over-square" is a non-issue with our engines.

 

12 minutes ago, PT20J said:

Back in the radial engine days there were issues with running high rpm and low manifold pressure because it reverse loads the master rod bearing and can cause it to fail. The rule of thumb was to avoid operation under square which meant to avoid combinations where the manifold pressure in inches was less than the rpm in hundreds (equal rpm and manifold pressure being considered “square”).

It appears that the concept got transferred (but in reverse) to the small normally aspirated engines we fly. The idea is that a power setting is safe if not over square meaning manifold pressure in inches should always be less than or equal to the rpm in hundreds. This is not a bad rule if you don’t have a power chart. But, if you do have a chart, any approved combination of manifold pressure and rpm is fine. Engines are more efficient at lower rpm and higher manifold pressure primarily because frictional and pumping losses are reduced.

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Thanks to both of you. I was taught in my transition training to try and stay around square, but as you climb with a NA engine the MP drops so you end up with MP in the lower 20s and something around 2400-2500 rpm. Should the RPM be reduced to keep square or is there no real negative effect to operating in cruise with say 20" and 2400rpm ?

[gsxrpilot]

Just now, cwaters said:

 

Thanks to both of you. I was taught in my transition training to try and stay around square, but as you climb with a NA engine the MP drops so you end up with MP in the lower 20s and something around 2400-2500 rpm. Should the RPM be reduced to keep square or is there no real negative effect to operating in cruise with say 20" and 2400rpm ?

Our engines don't care. But even if you subscribe to the myth, the supposed problem is with MP over RPM, not the other way around. 

When I flew a naturally aspirated Mooney, I'd never touch the throttle after takeoff. I'd fly WOT (wide open throttle) all the time, once in cruise, I'd pull the RPM back to 2500 just for the efficiency, or 2400 for better economy. And lean as appropriate.

Now I fly a turbo Mooney and it's almost always over square. I climb at 39" MP and 2600 RPM. I cruise at 26" MP and 2500 RPM. Needless to say, I don't spend two brain cells thinking about "over square". It's just not a thing. 

[Hank]

1 hour ago, cwaters said:

 

Thanks to both of you. I was taught in my transition training to try and stay around square, but as you climb with a NA engine the MP drops so you end up with MP in the lower 20s and something around 2400-2500 rpm. Should the RPM be reduced to keep square or is there no real negative effect to operating in cruise with say 20" and 2400rpm ?

I cruise high at 19-20" when traveling, but I'm not going to pull the prop back that far! 2500 is smooth, fast and efficient for my C above 7500 msl.

At 10,000.msl, I'll be 19.5-ish and 2500 for best efficiency and higher speeds.

[Ibra]

2 hours ago, cwaters said:

Thanks to both of you. I was taught in my transition training to try and stay around square,

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)

Edited April 29, 2021 by Ibra

[jlunseth]

18 hours ago, 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)

Yes, I have shorthand power settings I use for training and maneuvers and I have also had to set power without the benefit of an engine monitor, when the monitor declined to report for duty (because the alternator was absent without leave). Best way to do it is with a really good engine monitor though, really helps you to stay on top of what the engine is doing.

[gsxrpilot]

19 hours ago, 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)

I'll agree with regards to LOP, ROP, cruise power settings, climb power settings, etc. There is lots to learn.

But MP=<RPM is just not relevant to our engines. It's a myth, an old wives tale, and shouldn't be taught to anyone.

[A64Pilot]

You actually can cause issues with over square, but it has to be pretty excessive to get there,

‘The issue is with too low an RPM with too high an MP, but we are talking about 29” manifold and 2000 RPM or similar and it takes time to get the heads too hot too. It’s essentially lugging the motor

Probably every single fixed pitch airplane there is, is over square during take off and climb

A lot of things are really not so much do this or you’ll trash your motor, they are more of a do this and your motor will last a good long time.

‘It’s really just like not running a cylinder head temp of over 400, because the redline is 500, so I’m good at 490 right?

 

[ArtVandelay]

FWIW, the certification process includes running the engine at redline at full power for 50 hours, then another 50 hours at 75% power at redline.

[A64Pilot]

By redline, RPM. cyl head temp or?

I know you can run one at 500F, but it’s going to last longer if you don’t.

Edited April 30, 2021 by A64Pilot

[Ibra]

1 hour ago, gsxrpilot said:

But MP=<RPM is just not relevant to our engines. It's a myth, an old wives tale, and shouldn't be taught to anyone

Most of the times it does not matter, especially with the right reflexes on increasing power with propeller & decreasing power with throttle but it's not open door for any ASI*MP*RPM combination, someone with M20J who wants 55% power can use 19/27, 20/24, 22/22, 24/20, 27/19

- On climb, 19RPM & 27MP just does not feel right at 70kts, one can try but the engine just cries loud !

- On cruise, 19RPM & 27MP for 130kts level cruise, it's not listed in POH but I guess one can get way with it, and maybe saving 1GPH vs 27RPM & 19MP 

[A64Pilot]

The theory of oversquare being more efficient it two fold, one reason is at lower RPM, there is less internal engine friction, the other reason is at lower prop speeds the prop is more efficient, both are proven actually it’s not a theory.

‘However my last airplane I installed an MVP-50 in it, and it had been multi point calibrated so it was very accurate. I had 14  temp readings, 6 cyl head, 6 exh, one oil and the OAT, and after sitting in a hanger for days, all 14 would read the same temp within 1 degree.

‘Anyway I tried several combinations of RPM and manifold pressures, I used speed to determine percent power, I didn’t care if I was 66% or 64%, I was after it being the same, and identical speed at identical atmospheric conditions means the power is identical.

‘Long story short, while I’m pretty sure low RPM and high manifold pressure is more efficient, the difference wasn’t measurable and the MVP-50’s fuel system could be calibrated to be very precise. It would keep track of what it measured the burn total to be, then you would enter actual burn volume and it would correct the K factor automatically, do that once to get real close, then burn a few tanks keeping up with the total of course and then enter it again, using 100 gls or so got it real, real accurate.

After that I would pick an RPM that felt smoothest, and a manifold pressure that woud give me the speed I wanted.

‘Actually in truth most of the time it was mute as I liked to fly high, which meant that the throttle was wide open, which brings up another point,  wide open is more efficient because there is no vacuum loss AKA pumping loss.

Guess what I couldn’t quantify that difference either.

Edited April 30, 2021 by A64Pilot

[A64Pilot]

Do any of you remember those wonderful cars of the 1970’s? You know when lead was taken out of the fuel and knock sensors weren’t installed yet, well remember every one of them, especially in hot weather would “rattle” when pulling a hill if you lugged them even just a little.

So what was the fix for the rattle? downshift, let the RPM build and the rattle would go away, you know what that rattle was don’t you?

‘Any of you own Diesel trucks and have a pyrometer? Well when EGT gets higher than your comfortable, the way to cool it down is to add throttle and force the transmission to downshift, and EGT drops immediately.

In those examples neither will “kill” a motor but over time if repeated over and over the belief is that it shortens the life of a motor.

Some engines are built to be run oversquare, you really don’t have any choice, and those of course it doesn’t harm.

‘The R-1340 for example a comfortable cruise where you weren’t running it too hard was 30/20 30” manifold and 2000 RPM, as it was supercharged it was designed for oversquare.

‘Guess what it’s minimum octane fuel allowed was? Being supercharged it had to have high Octane right?

The Pratt R-1340 was Certified to run on 73 Octane fuel, I don’t know where your going to find that low of octane, but the 1340 was designed to run on it.

[jetdriven]

The BMEP decreases with a higher RPM given a fixed throttle setting.  

[jaylw314]

On 4/29/2021 at 12:59 PM, cwaters said:

 

Thanks to both of you. I was taught in my transition training to try and stay around square, but as you climb with a NA engine the MP drops so you end up with MP in the lower 20s and something around 2400-2500 rpm. Should the RPM be reduced to keep square or is there no real negative effect to operating in cruise with say 20" and 2400rpm ?

Keeping RPM lower than MP was a thing in radials, it's not a practical issue with other motors

Keeping MP lower than RPM is a thing to avoid the risk of detonation and preignition, which is pretty hard to do with your motor.  IO-360's are pretty detonation resistant (there's a chart I can never find that shows a test stand IO-360 that could only produce light detonation at 100% power at the hottest CHT's), and without turbocharging, you'll never really be able to get your MP up enough to be an issue.

Practically speaking, it'd be pretty tough for you to do something with the black and blue knobs that will hurt your motor    

Edited May 1, 2021 by jaylw314

[PT20J]

The Wasp R-1340 was a great engine -- the first Pratt and Whitney built and the last radial P&W had in production. It powers my buddy's T-6G which I have greatly enjoyed when he let me fly it. But I believe that a review of the specs and TCDS will show it was certified for 80/87 octane fuel. It only has a 6:1 compression ratio.

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[A64Pilot]

Current TCDS for the R-1340 is dated I believe 1974, but back when the engine was first produced I don’t believe 80/87 existed.

R-1340 dates back to mid 20’s I think, and lead in aviation fuel didn’t happen until about 1930 or so I think, but I can’t find the first date 80/87 was produced. 80ish octane fuel was the standard until WWiI and I believe only the US produced 100. I feel pretty sure that just before the Battle of Britain that US supplies of 100 octane arrived and that along with allowing the Brits to produce a variable pitch prop under license really made the Spitfire a capable aircraft, without the fuel and prop it’s unlikely that the battle would have gone the way it did, RADAR or not. I think the fuel alone added 3o kts to the Spit and the prop greatly increased climb rate, before these modifications, the Spit was really sort of a dog compared to the ME-109

The Germans flew with 87 Octane.

Anyway I believe the R-1340 was Certified prior to lead in fuel and 80/87 and was Certified to run on 73 Octane. I believe that was in the Maintenance Manual, also where you found the procedure to toss into the carburetor Bon-Ami scouring powder if the engine was consuming excess oil. 

I’m assuming with no evidence that 73 octane fuel was some kind of 1920’s standard fuel.

On edit, my C-85’s TCDS is dated 1973 and it’s min octane fuel is 73, so maybe I just confused the two.

The 985 or Wasp jr was a “better” engine, only 400 HP but more reliable and would run apparently with a blown jug, while it’s 600 HP bigger brother would not.

About 10 years ago we did a production restart and resurrected a 1340 crop duster and built about two dozen for a contract, that was all my radial engine time.

‘I just realized that I have never been “checked out” in any airplane turbine or Radial, only flew with an instructor in airplanes with little flat motors.

Edited May 1, 2021 by A64Pilot

[N201MKTurbo]

http://www.oocities.org/fchemical/2.html#:~:text=The history of aviation gasoline%2C is as old,aeroplane engine that would enable it to operate.

[A64Pilot]

That helped, but not much. For some reason aircraft fuel history doesn’t seem to be well documented, prior to Jet that is.

‘I’ve read for instance that the Wright Bros fuel’s octane was in the 30’s and it’s believed that the engine began detonating on take off, and of course continued to do so. But then just because someone published it on the internet doesn’t make it true of course.

But I’ve also read that straight gasoline with no additives is the 73 number, but that is even in doubt because it seems some was less than that. Maybe it’s because what gasoline is, isn’t very defined, or wasn’t anyway.

‘I know that is THE problem with Certifying Bio-Diesel, because Bio-Diesel isn’t defined, has no standards you can certify to, it can actually be several different fuels, grouped under the name “Bio”

I know because I went or was actually drug down that route, and we wasted a lot of money too.

Edited May 1, 2021 by A64Pilot

[PT20J]

Never thought much about gasoline history before...

There's a pretty good article on Wikipedia: https://en.wikipedia.org/wiki/Gasoline

Figure 7.1 in this document ( aviation-tech-review.pdf  )lists a 73 octane fuel grade. It also states that  "In 1930, the U.S. Army Air Corps specified a Fighting Grade gasoline with a minimum octane number requirement of 87. This is believed to be the first instance in which the antiknock properties of an aviation gasoline (avgas) were defined in terms of octane number." (p43).

Here's an interesting chart from Charles Taylor's The Internal Combustion Engine in Theory and Practice.

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[A64Pilot]

I believe the R-1340 basically predated 87 octane fuel. I don’t know when it became available for civilian use.

‘But my C-85 came into existence much later, 1944 I think and it was Certified for a min of 73 Octane fuel, making me think that 73 Octane was available as late as 1944.

‘But as a kid in mid to late 60’s I guess,  I remember getting 80 Octane fuel with my Father in his C-182, and I think 100 in the C-210 in probably 69, it was green and the 87 red like most car gas.