IO-360 low rpm LOP cruise power?

[Immelman]

Hi everybody,

 

Let me preface this thread with my overt goal: to run my airplane as efficiently as possible, and yes, sacrificing speed, while being kind to my engine and its long term health. I know that many Mooniacs want to fly fast, and at times I do too and know how to do that. However since I began to fly professionally a couple of years ago, speed means less to me than lower fuel consumption; what I fly at work trues out at 300kts, and really back in GA it makes little difference to me whether I'm flying at 150ktas or 120.

 

So what I am after are sources of data to figure out what the maximum amount of power I can *safely* run at low RPM during cruise (i.e. at 2000 rpm). My M20E has an IO-360 and standard Hartzell prop with a restiction to avoid continuous operation between 2100-2350 RPM, so those prop speeds in the 'sweet spot' of an M20J, for example, are off limits. I've got to come way back to 2000 or so.

 

So far the only thing I have found data-wise is the Lycoming operators manual for the -360 series engine which does include a cruise power chart including some low RPM settings. However, this chart lists maximum power based on manifold pressure, and implies rich of peak mixture. For low-power operations I run lean of peak and my stance is that manifold pressure is meaningless LOP; I can determine % power using fuel flow.

 

So how does one figure this out? What percent power can I run at 2000rpm without hurting something? By the way... what spurred this on was some experimentation. After years of cruise (LOP or ROP) at 2400-2500 RPM I tried the equivalent fuel flow (LOP) at 2000 RPM for 50% power and found a nice increase in speed and efficiency; its obvious that the friction losses are substantial. With $6 avgas I want to do what I can...while being kind to my engine.

[EDNR-Cruiser]

The only thing you should keep in mind is the maximum manifold pressure - which applies at low altitudes only - at higher (density) altitudes this won't be a problem any more...

I use this simple overview for my M20F with the same engine and prop as your "E":

2400 RPM at 19" = 100 HP = 50%
2400 RPM at 18" = 92 HP = 46%
2350 RPM at 19" = 97 HP = 48%
2350 RPM at 18" = 90 HP = 45%

2100 RPM at 19,8" = 90 HP = 45%
2000 RPM at 20,8" = 90 HP = 45%

2400 RPM at 19,0" = 100 HP = 50%
2350 RPM at 19,6" = 100 HP = 50%
2100 RPM at 22,5" = 100 HP = 50%
2000 RPM at 21,2" = 100 HP = 50%

2400 RPM at 21,5" = 120 HP = 60%
2350 RPM at 21,9" = 120 HP = 60%
2100 RPM at 24,2" = 120 HP = 60%
2000 RPM at 25,8" = 120 HP = 60%

 

Max Manifold pressures:
2000 RPM: 26"
2100 RPM: 26,7"

(HP and % for operation at sea-level).

That is all I need to know and in cruise (without time pressure) I can save some pounds per hour at a slighty higher TAS when I reduce RPMs and increase manifold pressure to compensate... - and at 2000 RPM it is much quieter, too...   :-)
This works only up to about FL90, above this it is difficult even at high QHN to maintain sufficient HP levels below 2100 RPM... - but it is great to run below at full open throttle, slightly lean of peak at 2000 RPM with a fuel burn of a Cessna 172 and 150% the speed...

[Immelman]

Right... and that matches what I see in the lycoming manual. But manifold pressure is, to me, relevant only for rich of peak ops. Once the mixture is lean of peak, its just a meaningless number, at least to me.

 

After looking again, I am seeing that they do not want you running more than 60% at 2000rpm which matches the data that you just gave me.  I will keep up on this but that is what I am looking for -- 60%.

 

I suppose I should also get an optical tach to see if I can run closer to 2100 without violating the restriction on my prop. I know my tach is a bit off on the high side so I am erring on the side of caution there to avoid going into the red band of RPM.

[kortopates]

If you want to run at 60% power for better fuel saving (miles/gallon) then why not run it at Peak. Once you're below 65% there is concern for running in the proverbial red box since it no longer exist at such low power settings and leaning further or even enriching further is just reducing the BFSC efficiency of your engine.  You should be fine using the LOP formula for fuel flow which should yield 8.0 gph for 60% power.

 

Similarly, I wouldn't assume 2000 RPM is more efficient than 2400, I would expect the higher RPM would yield better results but I'd collect some data and see to be sure if you haven't yet. 

[Immelman]

Paul, good point. I run LOP at lower power settings in general (<70%). I've got a 30-50dF injector "gami" spread (though I have stock injectors), so typically I am running the richest cylinder right at peak or just a few lean so as to maintain a balance of power coming out of each jug.

 

I don't agree though on higher RPM is more efficient, but admittedly my personally derived data is anecdotal therel, it would be cool to collect some. The lycoming operator's manual backs that up (http://www.monticellofc.org/aircraft/Lycoming%20operators%20manual%202005%20ed.pdf page 33)... but of course I know this also depends on how efficiently the prop works at one RPM and airspeed combination vs another.

[kmyfm20s]

With an IO-550 the power charts put an increase fuel demand of .1 GPH per 100 RPM. So there is less HP loss do to friction. As far as LOP at low altitude if you are running LOP with an IO-360 I believe it is 14.9HP per 1 GPH. 60% of 200HP would be 8.05 GPH. Set your RPM where you want it and play with MP and FF to get 8.05 GPH, just make sure you are LOP. As long as your are 65% or less it shouldn't matter how far LOP you are, go to the sweet spot where it runs smooth. I'm no guru so proceed at your own risk:)

[carusoam]

Does anyone have a concern for over square anymore?

Is OS officially dead?

At my preferred altitudes 10-12k' OS is not really an issue.

Immelmann, nice to see you pop in once in a while!

What do you fly at work?

Best regards,

-a-

[Bob - S50]

Does anyone have a concern for over square anymore?

Is OS officially dead?

At my preferred altitudes 10-12k' OS is not really an issue.

Immelmann, nice to see you pop in once in a while!

What do you fly at work?

Best regards,

-a-

The manual for our '78J lists ...lots... of over square power settings.  That makes it legal, however, I don't like to use them.

 

I'm not concerned about the red box at low RPM/low power settings, but I'm not sure about the stress on the bottom end.  Running low RPM moves the peak power point closer to TDC because the crank will not have rotated as far after the ignition event.  This will put more axial force rather than tangential force on the rod which means a higher percentage of load on the bearings.

 

Running LOP (or more than about 75F ROP) has the opposite effect.  It moves peak pressure further from TDC so it offsets the lower RPM somewhat.

 

How much impact those two have, I have no idea so I play it save and avoid OS.  If I want more power, it may not be as efficient, but I'll push the RPM up to avoid OS rather than add more load at the low RPM.

 

Of course, this is only a problem at low altitudes where MP is available to go over square.

 

Bob

[kortopates]

I don't agree though on higher RPM is more efficient, but admittedly my personally derived data is anecdotal therel, it would be cool to collect some. The lycoming operator's manual backs that up (http://www.monticellofc.org/aircraft/Lycoming%20operators%20manual%202005%20ed.pdf page 33)... but of course I know this also depends on how efficiently the prop works at one RPM and airspeed combination vs another.

 

I see what you mean WRT to the Lycoming data. Interesting how the POH contradicts that, their tables show the same fuel flow for the same % power level, independent of RPM, and the same speed for the same altitude. Although its best power, versus peak or best economy in the lycoming book, Its probably interpolated data rather than real. I'd trust the Lycoming data more with respect to % power you refer too on page 33. The APS guys are always talking about faster RPM being more efficient due to delaying the spark a little so I am surprised and I also find at high power settings I can't get as smooth LOP ops at low RPM. But I like to go fast! 

[Immelman]

Interesting stuff guys! I admit I am partial to the writings of Mike Busch who planted this seed in my mind both in his old avweb articles and then more recently in his webinars where lower RPM was discussed among other things. I am all about efficiency, but again don't want to harm the engine. Interestingly in one of Mike's older webinars he made the case for LOP operations being easier to achieve at low RPM!

 

On the work side, I flew a Beech 1900 (what an oustanding airplane) for about a year and then on to a different company where I fly a Brazilian turboprop. Before too much longer here, depending on the seniority game and what it will to do my schedule, I'll move up to a jet... though I have a soft spot in my heart for the turboprop flying and their capabilities.

[NotarPilot]

I routinely fly over square. However I have only been doing it less that a year. Most of my flights are short (less than 20 minutes) and at low altitude i.e. below 2,500 feet. I usually fly at 2200 RPM at about 27" MP while LOP. I find that the lower RPM does facilitate going a bit deeper LOP for me. The key number of 49 puts me at at about 72% power which could theoretically put me I'm the red box depending on where I put the mixture. But being the Mike Busch believer that I am, I monitor CHTs to determine if I'm abusing the engine. My engine runs fairly cool with the number 1 cylinder routinely running at just under 300F. The others are also fairly cool so even with the relatively high MP to RPM ratio I don't sweat it much. I believe CHTs are the best proxy for determining healthy engine operation. I'm happy to listen to differing opinions though.

[aaronk25]

I fly about 250 hours a year and I've noticed that peak efficiency is at 11,000 and up and at a fuel flow of about 7.2gph with mixture set between 20lop- peak. The one thing that is a given is that if a rpm setting is chosen that leaves gph below 7.2gph, there really isn't much more MPG to pick up.

So what that tells us is the io-360 peak mpg/bsfc is around 50-55% power.

I have a 77j so I only have a less than 1950 rpm limitation but if I were the OP I'd set the rpm at 2351 as the mpg gain of going to 2099rpm will be so small it won't be measurable and not worth the extra wear and tear to the engine and airframe vibration wear. If this was a 6-cylinder there would be less vibration at low rpm do to more power strokes but with our high compression 4 bangers there gets to be a lot of vibration down that low.....

What's conditions yield the best mpg? Fl170 2700rpm peak-15rop 6.5gph 148kts is 22.76mpg or next best and more practical 11.000ft 2400rpm peak-20lop 7.2gph 148kts 20.55mpg.

These numbers assume perfect near perfect rigging, fine wire tempest as perfect mags at 25degrees btdc.

[DaV8or]

Didn't the CAFE people work all this stuff out years ago? I thought either the M20J or M20E was their test bed. Then there is the Carlson number, isn't that supposed to be the most efficient speed/setting?

 

A lot of this stuff makes my brain hurt. Wanna save gas? Pull the throttle back. Pretty easy. Set up for say WOT, LOP at maybe 9 gph. Then from there just pull the throttle back and save even more fuel if you can stand the Skyhawk speeds. Ultimately, the only thing your wallet cares about is gph, so the flow meter is your friend.

 

I once wondered what the Mooney would be like at 152 speeds, so I tried cruising at 90 kts. You know what? It wasn't easy to do! Without adding power and flaps, the plane just doesn't want to cruise that slow. You will fiddle with the trim wheel a lot and the levers will be way back. I think it was about 5.5 gph. 110kt Skyhawk speed is easier at about 6 gph. It sucks though. You won't do it long.

[TTaylor]

I think the main theory for low RPM is there is less friction so you should be able to put more of the energy (i.e. fuel) into useful work.  Essentially it is a simple energy balance equation.  I have started flying my 20F at 2400 rather than 2500 rpm for that reason.  Same fuel flow should in theory produce slightly more power to the prop, 2000 rpm should be even better.

 

Tim (don't blame me, I only teach thermodynamics ;-)) 

[rbridges]

I think the main theory for low RPM is there is less friction so you should be able to put more of the energy (i.e. fuel) into useful work.  Essentially it is a simple energy balance equation.  I have started flying my 20F at 2400 rather than 2500 rpm for that reason.  Same fuel flow should in theory produce slightly more power to the prop, 2000 rpm should be even better.

 

Tim (don't blame me, I only teach thermodynamics ;-)) 

 

Not that it makes much difference, but that is why I cruise at 2400rpm vs 2500.  I figure a miniscule friction savings.  I would go lower but I have the RPM restriction, too.  I never thought about going down to 2100 or less.  

[Immelman]

Didn't the CAFE people work all this stuff out years ago? I thought either the M20J or M20E was their test bed. Then there is the Carlson number, isn't that supposed to be the most efficient speed/setting?

 

A lot of this stuff makes my brain hurt. Wanna save gas? Pull the throttle back. Pretty easy. Set up for say WOT, LOP at maybe 9 gph. Then from there just pull the throttle back and save even more fuel if you can stand the Skyhawk speeds. Ultimately, the only thing your wallet cares about is gph, so the flow meter is your friend.

 

I once wondered what the Mooney would be like at 152 speeds, so I tried cruising at 90 kts. You know what? It wasn't easy to do! Without adding power and flaps, the plane just doesn't want to cruise that slow. You will fiddle with the trim wheel a lot and the levers will be way back. I think it was about 5.5 gph. 110kt Skyhawk speed is easier at about 6 gph. It sucks though. You won't do it long.

 

Dave, that was part of my motivation...what can I get this thing to do on C152 fuel burn. What I found was that I can run 6gph, 2000rpm, LOP, low altitude, and get "carson's speed" (about 130+ mph indicated) in my plane. Its not behind the power curve or mushing along at all, which I know exactly to which you refer. But its not just throttle - you can run your bird with a higher RPM, ROP, and "burn" (more like "consume") 10gph while still achieving the same speed. Lets see, $36/hour fuel vs $60/hour fuel for the same result... which do I prefer?

 

I also agree about the best effciency at high altitude, and in my mind there is no question - for a long distance trip I want to be up high where I can get about 140kts true on 8gph, leaned right to peak, with the prop turning 2400 or 2500. Our wing is MADE for the "teens" altitude-wise, or higher with forced induction.

 

The thing is, with my new career my airplane "mission" changed. Its quite rare that I will fly long enough of a trip to justify a climb to high altitude. I can jumpseat pretty much anywhere for free. My mission has changed from long trips which the Mooney is suited well for, to recreation and enjoyable flying at a reasonable cost per hour, or shorter 1-1.5 hour trips where it just doesn't make sense to go that high. On the rare occasion that I take a longer trip of course I will fly higher, and spend a bit more in gas to get there... but I am not going to let my airplane sit idle between trips. I enjoy flying for the love of flying, and am not a guy to let my plane sit unused for weeks at a time as I believe its bad for it.

[EDNR-Cruiser]

Immelmann, I couldn't agree more! Same here, same motivation, same curiosity... - and whatever it is, less friction in the engine for sure but some also claim a higher efficiency of the original 2-blade-prop at lower RPMs, I do not care very much as long as I can save some bucks on fuel, travel at a much lower noise level and lower overall temperatures compared to high RPM operation - at the same speed!

My Mooney POH says nothing about oversquare settings or fuel efficiency... - back in 1966 they only cared about speed and range - and more importantly, the POH doesn't warn about running at low RPM. Mine actually shows 1950 RPM as "best endurance setting"...

Another point to make here is that the overall environment for people flying in Europe looks much different compared to flying in the US. A two hour flight is already considered as "long distance", especially in a Mooney. Flying above FL100 in Germany requires higher qualifications than some VFR pilots have, fuel is much more expensive than in the US and so on...
 

[jetdriven]

I did some checks at 9500' the other day. Funny thing was, at the same 8.5 GPH fuel flow, 2700 rpm was faster by about 3 knots over 2500 RPM, and it was faster by 5-6 knots at 2100 RPM on the same fuel flow. The engine may be more efficient but the McCauley C214 loses more than the engine saves.

[kmyfm20s]

I did some checks at 9500' the other day. Funny thing was, at the same 8.5 GPH fuel flow, 2700 rpm was faster by about 3 knots over 2500 RPM, and it was faster by 5-6 knots at 2100 RPM on the same fuel flow. The engine may be more efficient but the McCauley C214 loses more than the engine saves.

Interesting! Did you increase the MP for the lower RPM, I'm assuming all at equal LOP? I realize you are running an IO-360 but with the charts for the IO-550 there is approximately 1" increase in MP per 100 RPM decrease for the equivalent HP and for every 100RPM there is .1 GPH in FF, all at 50 LOP. Is there published data on the optimum RPM for a particular prop?

[jetdriven]

It was all full throttle. I leaned to 25F LOP at 2500 RPM as I usually do. Then I dialed RPM down to 2100 and richened to just ROP t the same FF. It was a lot slower. Then back to 2500 RPM, then 2700 rPM 50 LOP. Same ff. but there's 3-4 knots more at 2700 RPM compared to 2500 RPM on the same FF.

The engine may be more efficient but maybe the prop is not. This is a 1977 J with a roller cam A3B6 timed at 23 degrees with a McCauley C214 prop. Id be interested in seeing some other props repeat this. Go to 7-9K set 25 LOP 2500 RPM. note IAS and FF. then set RPM to 2100 and richer to same FF. wait 5 mins and note average IAS/TAS. Then set 2700 RPM, lean to the original FF, wait 5 mins and note average IAS/TAS.

[EDNR-Cruiser]

Not sure it your test is a good example... - why would I enrichen the mixture to keep the fuel flow at the same level as before? I might end up losing power because the mixture is then already too rich and fuel leaves the exhaust unused?
The difference between 2500 RPM and 2700 RPM is around 15 HP, leaning from 25 LOP to 50 LOP will cost you some horse powers but not as much as the higher RPM bring. What I wonder is why the fuel flow is the same, maybe because at your density altitude the air was already so thin...

I did a flight at 4000 ft AMSL (pretty much at ISA conditions) at my usual 2400 RPM with 20" MP, leaned to peak temperature. I then reduced RPM to 2000 and increased manifold pressure until the indicated airspeed was the same again as before which was at around 24" MP - basically at full throttle. I then fine tuned to peak again (not much adjustment required) and fuel flow was about 3 litres less than with 2400 RPM. Same IAS, same altitude but slightly less fuel burn and lots of less noise....
 

[Bob - S50]

There are so many variables...

 

All power settings should be at the same number of degrees LOP.

 

All should be adjusted to get the same IAS.  However, except on a calm day, changes in the wind, updrafts, and downdrafts as you fly along will vary your indicated as you maintain altitude.

 

In order to maintain a given IAS you would have to add MP as you decrease RPM.  That requires less than WOT for all but at most one RPM.

 

You then have changes in efficiencies caused by the throttle plate, engine friction (RPM), and propeller.

 

You would have to set a starting RPM and MP and then lean.  Read the IAS and FF.  Change the RPM and MP then lean again and wait for the speed to stabilize.  If it wasn't the same then adjust the MP and lean again.  Repeat until you get the same IAS and then read the FF.

 

When I look in the performance section of my POH (78J) and look at 55% power at 6000' for example, for the different RPM settings, they all show the same airspeed, but the lower the RPM, the lower the fuel flow.  That tells me that lower RPM is more efficient than higher RPM.

 

Bob

[Bob - S50]

Which is exactly why, after several years of recreationally studying both aerodynamics and combustion theory, 99% of the time I now just set fuel flow to 8 to 8.5 GPH and enjoy the view. 

What?!!! Enjoy yourself?  Good plan.

 

I don't sweat the small stuff either.  I like a compromise of speed and efficiency.  That's why I have a Mooney.  I just level off somewhere between 6500 and 9500 on long flights, set the RPM to 2400, and lean to about 10 to 30F LOP.  Then I sit back and enjoy the view.

 

Bob

[WardHolbrook]

You might be able to glean something from this thread from a few years back... 

 

http://mooneyspace.com/topic/3270-the-cafe-measure-of-efficiency-mooney-style/

 

Personally, I've been known to take that path on occasion, but when all is said and done the old 80/20 rule still applies - you get 80% of the benefit with the first 20% of the effort. The last 20% requires the most of the effort. Any way you slice it, the results are seldom very dramatic and from a practical standpoint any gains can be nullified along the route by a simple ATC vector or request to slow down or speed up. All that being said, it is a fun mental exercise. 

[1964-M20E]

Which is exactly why, after several years of recreationally studying both aerodynamics and combustion theory, 99% of the time I now just set fuel flow to 8 to 8.5 GPH and enjoy the view. 

ditto