Carson Speed/Altitude/Engine & Prop Settings M20K (231)

[Ethan]

 

Hello Folks,

Just read Mike Busch's article in AOPA on most efficient airspeed for both speed and fuel burn. According to Busch, in most cases it's the Carson speed or 1.32 x V l/d (best glide). Of course, in the Mooney 231 the V l/d varies depending on weight - at light weight it is 76 kias, at max gross it is 87 kias. That means the Carson speed varies between 101 and 115 kias, depending on weight.

Busch also says our airplanes run most efficiently at wide-open-throttle (WOT) and at a low propellor RPM. 

In a Mooney 231, WOT is impractical because WOT is 40" until the high-teens (I have a Merlyn waste-gate) and it is impractical to cruise in the high teens and low twenties for most of my flights < 300 NM. Running high manifold pressure settings also has the downside of high CHTs so I don't cruise with anymore than 31" of MP.

So, my question: Have any of you experimented with what combination of altitude, propellor RPM and MP is best for achieving the Carson speed for the given weight of your airplane? If so, will you share it and your thoughts about it?

Thanks,

 

Ethan

 

 

[skykrawler]

Maximum range airspeed is slower that what Mooney owners want to fly.

[midlifeflyer]

8 minutes ago, skykrawler said:

Maximum range airspeed is slower that what Mooney owners want to fly.

Maximum range airspeed is slower than what any owner wants to fly any airplane. Busch is talking about max efficiency. Of course even that may be slower than you want to fly. Most of us are willing to exchange some level of efficiency to get where we are going sooner.

[PT20J]

Maximum range speed is the speed where the gallons of fuel consumed per mile is minimum.

Carson speed is the speed where the gallons of fuel consumed per knot is minimum.

 

[1980Mooney]

11 hours ago, Ethan said:

Just read Mike Busch's article in AOPA on most efficient airspeed for both speed and fuel burn... in most cases it's the Carson speed or 1.32 x V l/d (best glide). .. in the Mooney 231... That means the Carson speed varies between 101 and 115 kias, depending on weight.

So, my question: Have any of you experimented with what combination of altitude, propellor RPM and MP is best for achieving the Carson speed...

If you are at 5,000 ft. then you are flying 120 - 131 ktas

If you are at 10,000 ft then you are flying 132-145 ktas

If you are at 20,000 ft then you are flying 162 -177 ktas

If you are at 25,000 ft then you are flying 189-198 ktas.

Down low you will be flying like a C-172

Up high you will be WOT.

 

Edited July 17, 2022 by 1980Mooney

[PT20J]

1 hour ago, 1980Mooney said:

If you are interested in how to fly between 101 and 115 kias, why don't you just buy and fly a Cessna 172?  This reminds me of the gas crisis in the 70's when it was decided that we should all drive 55 for best fuel economy.  I don't understand the interest in "achieving the Carson speed".

Airplanes have excess horsepower available in cruise flight. The question is how to best utilize it when flying a fixed distance from point A to point B

A. We can maximize speed, which minimizes travel time, but that burns the most fuel.

B. We can maximize miles per gallon, but it takes a long time to get to the destination.

C. We can chose something in between which is a tradeoff between the two extremes.

If you really need to get somewhere as fast as possible and don't care what gas costs, choose A.

B is hardly ever practical. 

Most of us fly some version of C. For instance, LOP is slower than ROP but burns less gas. And 65% power is slower than 75% but also burns less gas and may improve engine life. Carson speed is just this tradeoff carried to the optimum point. It's nice to have options.

Skip

[Ethan]

Carson speed is slow, I agree, but with gas prices so high, and air travel quick even at slower speeds, and because a Mooney is about efficiency, I want to know if other folks are doing it and how they're doing it.

I tried lean of peak for awhile and frankly, didn't really like it. If I set the power to 65%, the highest safe LOP setting, the TIT just seemed too high and twitchy. I kept having to adjust the mixture to keep it reasonable. By reasonable, I mean below 1650. I also don't like LOP because the engine runs just a scosh rough and just feels anemic. In contrast, while ROP - between 28" - 31" and 2400-2500 RPM, the engine sounds good and feels strong. I also go faster and the TIT is more stable. It just seems better that way.

Busch's Carson speed discussion is interesting because it can be done ROP. 

[Pinecone]

1 hour ago, 1980Mooney said:

If you are at 5,000 ft. then you are flying 120 - 131 ktas

If you are at 10,000 ft then you are flying 132-145 ktas

If you are at 20,000 ft then you are flying 162 -177 ktas

If you are at 25,000 ft then you are flying 189-198 ktas.

Down low you will be flying like a C-172

Up high you will be WOT.

 

Exactly.  Carson speed is INDICATED air speed.

Fly higher and TAS gets to be very nice.  Perfect altitude would be where WOTLOP = Carson speed.

Which may not be doable if turbo.  My 252 book says, 26,000 feet, standard day with 65% power is slightly faster than Carson speed.

Edited July 17, 2022 by Pinecone

[Will.iam]

2 hours ago, Ethan said:

Carson speed is slow, I agree, but with gas prices so high, and air travel quick even at slower speeds, and because a Mooney is about efficiency, I want to know if other folks are doing it and how they're doing it.

I tried lean of peak for awhile and frankly, didn't really like it. If I set the power to 65%, the highest safe LOP setting, the TIT just seemed too high and twitchy. I kept having to adjust the mixture to keep it reasonable. By reasonable, I mean below 1650. I also don't like LOP because the engine runs just a scosh rough and just feels anemic. In contrast, while ROP - between 28" - 31" and 2400-2500 RPM, the engine sounds good and feels strong. I also go faster and the TIT is more stable. It just seems better that way.

Busch's Carson speed discussion is interesting because it can be done ROP. 

Your turbo TIT is too high because you are not deep enough LOP. Your engine running rough LOP is because your injectors are not flowing fuel evenly enough. Known as the gami test anything over i think 1 gallon spread if i remember correctly  is  to rough to go LOP. You can get gami tuned injectors to even out your spread to below .5 gal per hour between all six injectors. My spread is .2 from my most rich injector to my most leanest injector. I fly LOP all the time with my TIT below 1550 or 100degrees below redline. I use 25” MP 2500 RPM and 9 gal per hour shows me approximately 22 degrees LOP and that gets me 150 at sea level 160 at 10,000ft. 
i can drop rpm to 2200 rpm and the turbo temp and fuel flow will drop alot more as there are less combustion events per minute but the speed also drops alot more too. 
you do have an engine monitor with temp probes on all six cylinders right? Most consider that as a minimum so that you are not accidentally cooking a cylinder that is not lean enough LOP to stay cool. A fuel flow recording monitor will make finding your gami spread easier and more  precise Than trying to do it by watching the hopskin fuel totalizer and trying to match up your recording with the data later after download i tried that and thought i had a .5 spread. Once i got my jpi to record fuel along with my egt temps as i went from full rich to full lean i found out my spread was really .8 so i got gami injectors and they got my spread down to .2 which is much better to go deeper LOP if i need to but I haven’t really needed to as going further LOP than about 20 to 22 degrees and the power starts to really drop off and hince my speed as well. 

[Z W]

@Will.iam how smooth is your engine at higher power settings LOP?  For example, 32/2500 and 11.5GPH?  Or 28/2500 and 10.5GPH?

Mine is smooth at the settings you describe (25/2500 and 9 GPH) but if I'm going somewhere I don't like the 15kt speed penalty compared to running peak TIT or ROP.

When I run higher power settings LOP like I described, the plane will do it and the temps settle in nice and low but like Ethan says, I can feel the slightest bit of engine roughness and I can't stand to fly around like that.

I have never tested the spread and should do that.

For a good mix of economy, speed, and engine temperatures, I run our TSIO-360-MB at 27" or 28" MP, 2500 RPM, leaned to peak TIT, which is around 12 GPH.  Yields 135-140 KIAS which is 160-185 KTAS from 9000-FL180.  When it's hot out, reducing to 27" MP keeps the peak TIT under 1600, or alternatively, you can leave it at 28" MP and increase fuel flow to 13.5 GPH to accomplish the same thing and go a little faster ROP. 

At all of these settings the CHT's will be under 380 degrees, though you have to crack the cowl flaps to keep them there.  My philosophy is CHT's under 380 and TIT under 1600 to take it easy on the engine and turbo.

[Fly Boomer]

9 hours ago, Ethan said:

If I set the power to 65%, the highest safe LOP setting, the TIT just seemed too high

While that theory is beloved by many around here, that's not the "highest safe LOP setting" -- it's just a setting where you probably can't damage your engine with the red knob. That said, some engines don't like LOP at any power setting, but that's a different set of problems.

[Ethan]

3 hours ago, Z W said:

@Will.iam how smooth is your engine at higher power settings LOP?  For example, 32/2500 and 11.5GPH?  Or 28/2500 and 10.5GPH?

Mine is smooth at the settings you describe (25/2500 and 9 GPH) but if I'm going somewhere I don't like the 15kt speed penalty compared to running peak TIT or ROP.

When I run higher power settings LOP like I described, the plane will do it and the temps settle in nice and low but like Ethan says, I can feel the slightest bit of engine roughness and I can't stand to fly around like that.

I have never tested the spread and should do that.

For a good mix of economy, speed, and engine temperatures, I run our TSIO-360-MB at 27" or 28" MP, 2500 RPM, leaned to peak TIT, which is around 12 GPH.  Yields 135-140 KIAS which is 160-185 KTAS from 9000-FL180.  When it's hot out, reducing to 27" MP keeps the peak TIT under 1600, or alternatively, you can leave it at 28" MP and increase fuel flow to 13.5 GPH to accomplish the same thing and go a little faster ROP. 

At all of these settings the CHT's will be under 380 degrees, though you have to crack the cowl flaps to keep them there.  My philosophy is CHT's under 380 and TIT under 1600 to take it easy on the engine and turbo.

I see things similar to ZW and fly similarly: ROP and lower MP settings if I want to save gas. I do have GAMIs and a good JPI so I can see hot cylinders and EGTs. Like ZW, at 27" - 28" MP settings, In the summer, I need to crack the cowl flaps to "in-trail" to keep a couple of cylinders under 400. When I want to go a bit faster I might drive the MP up to 31", counterintuitively, at that higher MP setting, I can get away with completely closing the soul flaps and still keep the head temps under 400. For the last couple of years, I have been cruising using 2500 rpm for best speed but would like to experiment with lower RPM settings - consistent with the Busch recommendation for best efficiency, Carson speed, etc.

[Will.iam]

3 hours ago, Z W said:

@Will.iam how smooth is your engine at higher power settings LOP?  For example, 32/2500 and 11.5GPH?  Or 28/2500 and 10.5GPH?

Mine is smooth at the settings you describe (25/2500 and 9 GPH) but if I'm going somewhere I don't like the 15kt speed penalty compared to running peak TIT or ROP.

When I run higher power settings LOP like I described, the plane will do it and the temps settle in nice and low but like Ethan says, I can feel the slightest bit of engine roughness and I can't stand to fly around like that.

I have never tested the spread and should do that.

For a good mix of economy, speed, and engine temperatures, I run our TSIO-360-MB at 27" or 28" MP, 2500 RPM, leaned to peak TIT, which is around 12 GPH.  Yields 135-140 KIAS which is 160-185 KTAS from 9000-FL180.  When it's hot out, reducing to 27" MP keeps the peak TIT under 1600, or alternatively, you can leave it at 28" MP and increase fuel flow to 13.5 GPH to accomplish the same thing and go a little faster ROP. 

At all of these settings the CHT's will be under 380 degrees, though you have to crack the cowl flaps to keep them there.  My philosophy is CHT's under 380 and TIT under 1600 to take it easy on the engine and turbo.

Mine is smooth at higher power settings but the turbo TIT goes up. I try to stay below 1550 but have gone up to 1600 before, knowing that is putting more wear on the turbo. I can lower the RPM and that reduces turbo temp but speed also suffers. Reducing RPM also increases my over square which has been debunked as ok to operate but my engine is at TBO with high blow by that seems to blacken the oil faster when i fly big oversquare ops. I. E. My oil stays yellow longer when i fly LOP and square than if i fly oversquare and/or ROP like at TO power. 

[Will.iam]

9 minutes ago, Ethan said:

I see things similar to ZW and fly similarly: ROP and lower MP settings if I want to save gas. I do have GAMIs and a good JPI so I can see hot cylinders and EGTs. Like ZW, at 27" - 28" MP settings, In the summer, I need to crack the cowl flaps to "in-trail" to keep a couple of cylinders under 400. When I want to go a bit faster I might drive the MP up to 31", counterintuitively, at that higher MP setting, I can get away with completely closing the soul flaps and still keep the head temps under 400. For the last couple of years, I have been cruising using 2500 rpm for best speed but would like to experiment with lower RPM settings - consistent with the Busch recommendation for best efficiency, Carson speed, etc.

Down at low altitude i would think at carson speeds running a 2200 or 2300 RPM would gain you more efficiency with recouping friction losses but up in the FL’s i think the RPM would have to go up to keep the prop efficiency loss from excessing your gain from friction loss. I have not flown enough up high to experiment to see which has the greater impact. Most  likely a comprise between the  two will give best results. 

[1980Mooney]

7 hours ago, Will.iam said:

Down at low altitude i would think at carson speeds running a 2200 or 2300 RPM would gain you more efficiency with recouping friction losses but up in the FL’s i think the RPM would have to go up to keep the prop efficiency loss from excessing your gain from friction loss. I have not flown enough up high to experiment to see which has the greater impact. Most  likely a comprise between the  two will give best results. 

But if you are down at low altitude (let's say 5,000) then the Carson speed is 120 - 131 ktas.  Who wants to fly at that speed?  Compromise is right!

Edited July 17, 2022 by 1980Mooney

[Brandt]

20 minutes ago, 1980Mooney said:

But if you are down at low altitude (let's say 5,000) then the Carson speed is 120 - 131 ktas.  Who wants to fly at that speed?  Compromise is right!

True.  At that speed you might as well own a Cirrus…

[A64Pilot]

Some very general rules applicable to ALL engines.

Lower RPM is more efficient due to less friction losses, wide open throttle is more efficient due to less pumping losses.

The Toyota Prius with its nearly infinitely variable planetary transmission will commonly pull the engine RPM down to where at WOT power is equal to power required,  these are small improvements, but they add up.

My recommendation is to run a lower RPM, but pick one that’s smooth, not necessarily the bottom of the green, go for smooth, then run LOP at a power setting that can give you a speed you can live with. Vibrations are hard on everything, so avoid them.

My J that’s 22 squared and 6.5 GPH down low and results in 50C LOP and about 125 kts indicated, which gives me roughly 20 NMPG, you can probably beat that, but not by a whole lot, for instance 50C LOP is less efficient than 15 to 25 LOP, but again, not by much.

I think that puts me at about 50% power

I don’t travel that way, but flying out to breakfast with Cubs and an Enstrom helicopter etc., who’s in a hurry?

The greatest increase of efficiency is from slowing down, just like cars, remember the 55 MPH speed limit?

Many people are certain that there are higher speeds that their car etc are more efficient than lower speeds and within reason unfortunately that’s not true, you gain fuel efficiency in a piston engine by slowing down, not necessarily true for a turbine, but that’s a different story.

Note, the WOT efficiency is really more theoretical than anything, basically it’s saying you burn less fuel making say 75% power at WOT and a lower RPM than you do at a higher RPM and lower manifold pressure, but if we are trying to save fuel at normal or average altitudes, that pretty much means slowing down, up high your WOT anyway

[Pinecone]

5 hours ago, 1980Mooney said:

But if you are down at low altitude (let's say 5,000) then the Carson speed is 120 - 131 ktas.  Who wants to fly at that speed?  Compromise is right!

Owners of non-Mooney aircraft?

 

[carusoam]

On 7/16/2022 at 11:32 AM, Ethan said:

 

So, my question: Have any of you experimented with what combination of altitude, propellor RPM and MP is best for achieving the Carson speed for the given weight of your airplane? If so, will you share it and your thoughts about it?

Thanks,

 

Ethan

 

 

Ethan,

I think you are mixing several things together….

 

When flying an M20K dressed up as well as yours is….

You have several options….

Before getting down to Carson’s speed…

 

1) Knowing your TIT limit…

2) Knowing how to fly it LOP… big pull etc.

3) Selecting your favorite power settings…

4) Selecting your favorite altitude…

5) By the time you have selected Carson’s speed as an answer… you are already flying 2knm, non-stop for the flight.

6) How often do you fly tremendous Mooney distances?

7) Flying your engine probably isn’t done on feelings…. 

8) Choose your weapons for best economy and use them…

9) The magic of flying slowly is lowered air resistance…

10) The M20K’s magic is its ability to fly in the teens where the air is thin, and its engine can easily run LOP…

Go turbo!

Lets invite a couple of people to the conversation… @testwest our most knowledgable Vz guy… @201er a solid long distance MSer….

Best regards,

-a-

[jaylw314]

On 7/16/2022 at 8:32 AM, Ethan said:

 

Hello Folks,

Just read Mike Busch's article in AOPA on most efficient airspeed for both speed and fuel burn. According to Busch, in most cases it's the Carson speed or 1.32 x V l/d (best glide). Of course, in the Mooney 231 the V l/d varies depending on weight - at light weight it is 76 kias, at max gross it is 87 kias. That means the Carson speed varies between 101 and 115 kias, depending on weight.

Busch also says our airplanes run most efficiently at wide-open-throttle (WOT) and at a low propellor RPM. 

In a Mooney 231, WOT is impractical because WOT is 40" until the high-teens (I have a Merlyn waste-gate) and it is impractical to cruise in the high teens and low twenties for most of my flights < 300 NM. Running high manifold pressure settings also has the downside of high CHTs so I don't cruise with anymore than 31" of MP.

So, my question: Have any of you experimented with what combination of altitude, propellor RPM and MP is best for achieving the Carson speed for the given weight of your airplane? If so, will you share it and your thoughts about it?

Thanks,

 

Ethan

 

 

Are those best glide numbers accurate?  My J lists best glide at gross as 91 KIAS, down to 85 KIAS when light?

In theory, for short flights, the most efficient profile would be a continuous climb halfway, then a continuous descent to the destination.  If it was possible to do a LOP climb at Carson speed, I assume that would minimize fuel burn per knot.  Whether you'd be able to keep within engine limits is another question, though...

I can't test that theory out with an NA motor  FWIW, in a J, 10k' MSL, WOT and 2400 usually gets you near Carson speed

[Ethan]

My best glide speed for (confusingly) my probably heavier Mooney is between 76 KIAS and 87 KIAS.

[A64Pilot]

The energy you burn in a climb, you don’t  get it all back in descent, or said another way the speed loss in climb isn’t made up in the descent.

Of course if the distance is far enough the decreased skin friction at altitude will give less fuel burned, moreso if your a turbine or turbo. if you really get complex with your planning, you step climb, increasing altitude as fuel is burned off, decreasing weight.

What your looking for is max range airspeed, which is different than max endurance airspeed sort of like Vx and Vy are different. Max range give best MPG of course.

Good rule of thumb, not precise, but good enough

https://www.aopa.org/news-and-media/all-news/2013/september/20/question-sept-20

https://studyflying.com/maximum-range-speed-v-s-maximum-endurance-speed/

[jaylw314]

4 hours ago, Ethan said:

My best glide speed for (confusingly) my probably heavier Mooney is between 76 KIAS and 87 KIAS.

Huh, that's interesting...I figured best glide would be similar at similar weights??  That's like a  7-8 knot difference?

Edited July 19, 2022 by jaylw314

[jaylw314]

2 hours ago, A64Pilot said:

The energy you burn in a climb, you don’t  get it all back in descent, or said another way the speed loss in climb isn’t made up in the descent.

Of course if the distance is far enough the decreased skin friction at altitude will give less fuel burned, moreso if your a turbine or turbo. if you really get complex with your planning, you step climb, increasing altitude as fuel is burned off, decreasing weight.

What your looking for is max range airspeed, which is different than max endurance airspeed sort of like Vx and Vy are different. Max range give best MPG of course.

Good rule of thumb, not precise, but good enough

https://www.aopa.org/news-and-media/all-news/2013/september/20/question-sept-20

https://studyflying.com/maximum-range-speed-v-s-maximum-endurance-speed/

I talked about climb because climb speeds are closer to Carson speeds than most people normally cruise.  Psychologically, it's easier to accept losing out on speed if you're in the business of climbing

[A64Pilot]

Got it.

We try to make things complex, it’s our nature, but often close is good enough as in the difference between good enough and precise is pretty insignificant, so if your really trying to burn the least fuel for a given distance, you operate the engine and propellor at their efficiency peak, and at a power setting that gives you max range airspeed.

So operate as low an RPM as is prudent, LOP and up high if the trip is long enough to justify the climb at 1.8 times stall speed (clean) and your going to be very close to the least fuel burned that’s possible.

Its not unusual to significantly increase range, and quite often if in doing so you can skip a fuel stop for the trip to take less time than a fast cruise. You don’t have to slow to max range to increase range, but know it’s max range, go any slower and of course range decreases.

It’s just another tool for the box is all.

So for me as I think my clean stall speed is roughly 60ish kts, 1.8 x 60 = 108 kts, figure 110 kts as it’s a nice round number to remember, so if for any reason I became concerned about if I had enough fuel to make the next possible stop, I know to reduce to 110 KIAS, 25 LOP and the lowest RPM that she will run smooth.