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Low Power Cruise settings


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I guess this is a model specific question, mine is a J.

‘I know many are all about speed, but sometimes your not in a hurry and efficiency has its own attractions. So what do you J model guys use for a low power cruise manifold and rpm setting wise?

I know in theory that low RPM increases both propellor efficiency and reduces engine drag so the lower the RPM, the more efficient on paper. But I’ve done some testing with a different airframe and found that while I’m sure that’s a correct statement that the differences weren’t measurable with a known accurate to .1 GPH fuel flow gauge and indicated airspeed.

My motor seems “happiest” vibration wise not going lower than 2350, but I’ve not extensively studied this, there may be sweet spots at lower RPM’s that I’m unaware of, plus I don’t know how well my prop is balanced as it’s a new to me airplane, so what power settings do others use?

I’m not meaning this to get into a LOP discussion, although it may end up delving into that, just wondering what others use for a power setting and why those numbers is all.

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Do you really mean low power?  Or do you mean best economy?

For low power I'd think throttle just above the horn, RPM just above the yellow arc, and about 30-50 LOP would give you the maximum time aloft.  Shoot for about 90 KIAS (l/d max) clean and adjust power to get there.

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Primary efficiency gains from LOP is from slowing down, not so much from an increase in BSFC. However there are a few people who stand to gain financially from selling products that if they are to be believed are required for LOP operation so it’s not surprising there are converts.

Bottom line, if you keep power output low enough you simply can’t hurt your engine with the mixture knob, and rich mixtures are only required at very high power outputs which are of course inefficient.

Maybe I should come “clean” a little, I have a background in aircraft testing, not really piston engine aircraft, but testing is testing. I’ve done some testing in my Maule with an IO-540W1A5D Lycoming that had Gami’s and fine wire plugs, engine was well broken in with a couple hundred hours on new Millennium cylinders and I had a MVP-50 that was multi point calibrated within 90 days. Anyway what I came up with was that the majority of efficiency increase from LOP came from the rather serious reduction in power output and not so much from any increase in actual engine efficiency gained from LOP. Said another way, set an exact airspeed say at peak EGT, write down the  fuel burn, than set that same airspeed LOP and see what fuel burn is. You may find that the biggest variable is airspeed and if your after absolute minimum fuel consumption, that poking along at Vy is likely to give you the max range and endurance, but who can stand flying that slow?

However what I was asking is whats some good settings that the engine runs smoothly and keeps oil temp above 180F in a typical warm day and what airspeed do you get and the fuel burn.

In theory if your after absolute min consumption, full throttle to reduce pumping losses and very low RPM for prop efficiency and engine drag reduction and leaned as far as it will run to reduce power output as low as possible and airframe drag due to lower airspeed is the answer, and that’s ignoring altitude, which is of course not an insignificant variable.

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2 minutes ago, Bob - S50 said:

Do you really mean low power?  Or do you mean best economy?

For low power I'd think throttle just above the horn, RPM just above the yellow arc, and about 30-50 LOP would give you the maximum time aloft.  Shoot for about 90 KIAS (l/d max) clean and adjust power to get there.

Just looking for power settings for when we are just out for the fun of flying or maybe sightseeing, looking at the trees turning color or whatever, not trying to maximize anything, a nice comfortable cruise is all, one that generates enough heat to keep the oil above 180f etc.

I just got my Mooney and it will be a couple of months until the finances recover enough for me to install an engine monitor so all I have is the 40 yr old factory gauges so I only have approximations of what oil temp and cyl head temps are

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I just aim for whatever gives me 8-8.5gph and 120kias depending on altitude. The M20J just really likes 120kias. It's a little faster than Carson's speed, it provides a good level of quiet and comfort, and the pitch provides good visibility. Slower, and the nose gets pitched up too high.

I use 120kias in the climb, cruise, and descent. That requires full power and target egt in the climb, power back some in descent but I use a shallow one at 300fpm, and 8gph LOP in cruise. For xcountry, weather permitting I try to go high enough that 120kias provides maximum ktas.

In terms of RPM, it likes 2500RPM or more for best speed and efficiency. However, it seems to put less wear and tear on things keeping the RPM lower like 2300-2400RPM in cruise. The oil would come back cleaner and look less used cruising 50 hours at those lower settings than less hours at higher settings.

Probably the absolute best efficiency you can get is about 12,000ft, WOT, 2600-2700RPM, 7.5GPH, at or slightly lean of peak, making around 120kias, will yield around 150ktas. But, this isn't always practical.

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For flightseeing, just set for a speed that lets you see. For me, low altitude is generally 23" / 2300 but will be different for your J.

Loafing along on Plane Ride Day with 172s and Cherokees, i also use 2300 but pull the throttle far enough to maintain spacing and not overtake, call it 105-110 mph and whatever MP that is.

For real efficiency, fly at Vz, Carson's Speed. Should be several threads here about that, too.

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I had to look up Carson’s speed because I had never heard of it, but it’s apparently the right side of what in the Military was referred to as “bucket” airspeed or close anyway.

If you look at any L/D chart, even helicopters you’ll notice that it’s bucket shaped and Carson’s speed is apparently the front edge or right side of that “bucket”

A nice compromise in speed vs fuel consumption

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Just looking and correct me if I’m wrong, but Carson’s speed is 1.316 time VG? Understand I learned what Carson’s speed was about 1 min ago.

A J model’s VG is 90 KIAS? If so then 1.316 times 90 is 118 or so, which is so close to 120 that you can consider 120 as being the best speed for endurance ‘ fuel consumption and the desire to actually get somewhere?

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15 minutes ago, bluehighwayflyer said:

Plus one, except for the last paragraph.  Obviously the max range and therefore the "absolute best efficiency" is much slower at L/D max, but nobody in their right mind wants to fly around that slowly.

I stand corrected. I did not mean "best efficiency", I meant "the best bang for the buck efficiency" or the best "get there fast on as little fuel as possible efficiency."
 

2 minutes ago, A64Pilot said:

I had to look up Carson’s speed because I had never heard of it, but it’s apparently the right side of what in the Military was referred to as “bucket” airspeed or close anyway.

If you look at any L/D chart, even helicopters you’ll notice that it’s bucket shaped and Carson’s speed is apparently the front edge or right side of that “bucket”

A nice compromise in speed vs fuel consumption

Best glide in M20J varies from 85 to 91 knots from 2300 to 2740lbs. This puts Carson's Speed at 112-120kias. Theoretically you should cruise a little slower when lighter but who cares? It gets a little more efficient lighter anyway so enjoy the speed. 120kias in climb and descent have proven themselves very efficient as well. Basically the speed gain from best glide to carson's speed is substantial while the fuel consumption is minimal. On the other hand, once you start going faster than this, you are spending a lot more fuel to go a little faster.

Keep in mind that there are other costs besides fuel per hour and some of those costs add up if you fly too slow and cancel out the savings. This is why at or slightly above Carson's speed works out better than going slower. If you're going somewhere.

 

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In regards to climb, someone on here did analysis and came up with the "Vz" climb as most efficient. Basically it's max power and up to cruise altitude as quickly as possible while maintaining 120kias in the climb. Don't power back in the climb. More time climbing has an adverse affect on efficiency. However, too high an airspeed add parasitic drag as well. So essentially you still maintain Carson's speed in the climb and you use excess power to climb quickly.

I did some of my own experimentation to find the same to hold true for the descent stage of a trip as well. Most Mooney guys are in too much of a hurry to get to the shitter that they go full speed on the descent. What they aren't realizing is that the high speed in the descent does burn away as parasitic drag as well and cancel out some of the efficiency they sought in cruise.

 

So, 120kias all flight long, 8gph cruise, and LOP are the way to fly a Mooney fast and as efficient as you can get while going fast.

 

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8 minutes ago, A64Pilot said:

So without going out and flying it, what power setting gets you 120 KIas?

Depends on altitude and conditions. But like @bluehighwayflyer said, 8-8.5gph LOP is the engine sweet spot and you try to pick an altitude (usually 6,000-9,000ft) that gives you 120kias and the best ktas you can get for it.

I used to pay a lot of attention to degress LOP, and slowly leaning and all that. For most of my flying for the last 8 years I just do the big pull to the fuel flow and forget it. You do have to experiment a little in the beginning to know under which conditions this will work but at 8gph you can't hurt anything no matter how badly you lean it.

Edited by 201er
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Hello 

I usually use for climb 25 inch / 2500rpm / full reach till 6000 ft,

Then from 6000 ft to 9000ft full throttle and 2500 rpm for « fast » flying 150kts+ with 10gals 50F rich of peak

Or using 2400rpm and 9,5gals and much less noise in the cabin losing few knots 2-3 only

I have MTProp 3 blades installed
Philip

D867CFD2-AAFA-4C68-A416-24D5C2EE1D60.jpeg

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A few points to ponder:

1. Prop efficiency: Bob Kromer (former Mooney test pilot) has stated that the M20J prop has maximum efficiency at 2500 rpm. However, with a constant speed prop, the efficiency curve is relatively flat, so it probably doesn't make a big difference over the range of normal cruise rpm. Lower rpm does reduce the horsepower lost to friction. On the other hand it reduces power once the throttle is wide open.

2. LOP: Leaning reduces both BSFC and power. One way of thinking about it is to run as lean as you can and still get the power you need for whatever speed you want. WOT always provides the greatest efficiency for two reasons: less pumping loss (work done sucking air around the restriction of the partially closed throttle plate) and the engine doesn't exhaust burned gases as well at lower intake/exhaust pressure ratios which lowers combustion temperatures.

3. Carson speed: The best range for fuel burned is at L/Dmax speed. Bernard Carson's contribution was to recognize that the utility of airplanes has a lot to do with speed and he came up with a speed that minimizes fuel consumed per unit of velocity (kind of like SFC is fuel consumed per unit of power). Mathematically it works out to the speed for  (CL1/2/CD)max which is 1.32 * max L/D speed. I've attached Carson's original paper which is an interesting read.

Skip

 

AIAA.1980.1847.B.H.Carson.pdf

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It would be very unusual for a prop to be more efficient at high RPM, the reason is drag, at higher RPM, the drag increase is quite substantial.

But there is a lot of misunderstanding about props, due largely to marketing.

Engines are geared to slow down the prop, even the old R-1340 was both direct drive and geared, gearing adds weight and complexity so of course direct drive is preferred, but that means you need to make power at a low RPM due to prop limitations and that’s tough, so it’s a compromise as everything is.

‘However even if a prop was more efficient at high RPM, the engine isn’t, the reason it’s not is drag, it takes quite a lot of energy to spin a motor and that energy is of course “wasted” as in it’s not being used to drive the airplane.

‘So from a efficiency perspective the lowest RPM that will allow you to make the required power will almost always be the most efficient.

‘However as of course horsepower is torque times RPM, once you reach the torque limit an engine can produce the only way to get more power is by turning up the RPM.

The Toyota Prius is of course an example of an engine designed and operated as efficiently as possible, the Prius is computer controlled, and most often the Prius is run at wide open throttle with the computer turning down the engine RPM via the planetary gear system so that at full throttle it’s output matches what’s required. It’s not uncommon  to see a Prius at 1200 RPMor less even at highway speed if your going slightly down a hill etc.

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Well A64...

You have a reading assignment tonight...   :)

https://mooneyspace.com/search/?q=Carson speed&quick=1

Carson, is new to a lot of people...

But, as a Mooney pilot...

We get two choices.... Speed and efficiency....  How fast do you want to go? How efficiently do you want to go?

Norman @testwest has lead many a discussion on Vz and Carson...

He wrote an interesting paper that included it, I believe...

So.. if you want to fly all day at Mooney speeds... Go Vz, and LOP....   :) (our Vz is faster than their Vz...)

Best regards,

-a-

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Also a J here:

I am consistently using 2600 RPM. Keep in mind that this does NOT mean the propeller pitch is always the same. It varies with airspeed. But my blue lever remains at 2600 RPM. I don't believe in more wear because of (somewhat) higher RPM, but cranking down to 2500 reduces air flow and thus available air for combustion. I would even keep 2700 RPM from takeoff to landing if not for noise considerations.

Quick notes on the other levers: once above 4000ft or so, power is limited to about 75 percent anyway, so I do the "big pull" to 10.1 gph (that is 75 percent) or less, and from then on don't worry about red boxes and the like, since 10.1 gph limits engine power to 75 percent in any case. (I do that "big pull" even in climb, of course with open cowl flaps, and rarely see CHTs over 360-370 even at Vy)

The throttle remains wide open throughout the flight unless MP goes above 25.5 inches when descending (in that case I throttle back, since more MP will just be useless air not needed for cooling). And the mixture remains tuned to 10.1 unless MP goes below 24.4 when climbing. Below that, I use a table to find the FF optimal for complete combustion (LOP). Basically a proportional reduction, e.g. 22.7 MP corresponds to 9.4 gph, 21.0 MP corresponds to 8.7 gph, etc.

Thus, somewhat surprisingly, I no longer use EGT for leaning since the above procedure keeps me out of the "red box" anyway, and my concern is simply the most efficient mixture for cooling and combustion. (That is, unless I cruise below 4000ft, which I rarely do, and in that case, simple tune to 24.4 MP and 10.1 gph).

I see CHTs around 300-330 in cruise.

For RPM values other than 2600. most values above need to be adjusted.

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Where do you find this lean of peak table your adjusting fuel flow to? That would be helpful as X fuel flow at X RPM should equal X percentage of power output, within limits of course.

Previously I had to find peak on the richest cylinder, then lean to get desired LOP setting, anything below -20 made the airplane unacceptably slow on my 540, and at the altitudes that gave me the highest true airspeeds (9 to 11,000) I lost too much airspeed LOP so I abandoned it, LOP works, it does reduce fuel consumption, but for me in the last airplane that was instrumented and equipped for it, it did so I believe primarily by reducing power so much.

50 LOP is a huge power loss, and at altitude there is no “simply increase throttle to recover power lost” your already there, turbo may be different, I speak of normally aspirated  

 

Oh, and on edit, wear is directly tied to RPM, one theory is its because of the kinetic energy of the rotating components, the higher the kinetic energy that there is, the higher the wear. I believe it’s largely due to the forces required in a reciprocating engine in reversing directions thousands of times per min.

Plus of course for things like rings and bearing surfaces, the higher RPM results in more distance per unit of time they are drug across cylinder walls etc.

RPM is the biggest reason why slow turning stationary motors last so much longer than their higher turning cousins, generators for instance in your part of the world to get 50Hz a single pole generator turns 3000 RPM, a double pole, 1500. The 1500 RPM generator lasts longer, twice as long? maybe not, but it’s generally accepted that the life is longer.

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1 hour ago, A64Pilot said:

Where do you find this lean of peak table your adjusting fuel flow to? That would be helpful as X fuel flow at X RPM should equal X percentage of power output, within limits of course.

Once you hit peak egt and leaner, HP is mostly a function of FF.  With the 8.5:1 compression ratio in your J the magic formula is around 14.9hp per 1 gph.  Example: 8.5gph would be about 63% power.  8.5*14.9/200  

 

1 hour ago, A64Pilot said:

50 LOP is a huge power loss, and at altitude there is no “simply increase throttle to recover power lost” your already there, turbo may be different, I speak of normally aspirated  

Most LOP settings with a IO-360 would be 0-15 LOP.   50 LOP would not be practical in most cases for the reason you mention.  The Continental IO-520 usually runs well at 50 LOP, most Ovations are very happy there.

Cheers,

Dan 

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Yes the C-210 with an IO-520 I flew a lot was OK with LOP, OK meaning didn’t fall on its face.

My Parallel valve IO-540 wasn’t, it lost too much power to be usable, Unsure how my angle valve Lycoming in the Mooney will. However what’s interesting is that it will operate smoothly LOP until power just goes away, meaning fuel flow is sufficiently balanced with stock injectors, no Gami’s needed.

I think I will likely stick with Lycomings recommendation and operate at peak at 65% or lower power, I assume Lycoming recommends this due to possibly their combustion chamber shape simply losing power at LOP, it will run of course but loses power, and that may make customers unhappy, so don’t recommend it.

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2 hours ago, A64Pilot said:

Where do you find this lean of peak table your adjusting fuel flow to? That would be helpful as X fuel flow at X RPM should equal X percentage of power output, within limits of course.

 

This shows some relationships of power-mixture-BSFC (from Lycoming)

image.thumb.png.e6c4e7ffde0ac514ed844d693139fdc6.png

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Wear vs. rpm and the effect of rpm on engine output:

A Lycoming IO-360 has a stroke of 4.375". Assuming it runs for 2000 hours at 2600 rpm vs 2500 rpm, the piston rings will travel within each cylinder an additional:

4.375 in./stroke x 2 strokes/rev x 100 rev/min x 60 min./hour x 2000 hr. / 12 in./ft. / 5280 ft./mi. = 1,657 miles.

Much of the internal friction comes from the cylinder/ring interface and that friction dissipates as additional heat raising CHTs. The only two reasons I can think of for running a higher rpm is if you need the additional power or you find a rpm that generates smoother operation.

Engine torque is a function of BMEP which is in turn a function of volumetric efficiency (among other things) which is a measure of how efficient the engine is at filling each cylinder with air during the intake stroke. Brake torque is a measure of the work output of the engine. Power is the rate at which work is performed. So, BHP = Brake torque x rpm.

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