Power setting M20C

[brndiar]

Hi,

For 3 Months a bought a M20C. Usually Fly 2300 RPM, 20 MAP which gives me cca 105-110 knots GS at 5000 Ft /less than in POH/.

Folks, mit what RPM/MAP do you fly your Mooney?. What is up to you most economical settings? Any suggestions for Cruise setting?  Thanks,

milos

[Jim Peace]

1 hour ago, brndiar said:

Hi,

For 3 Months a bought a M20C. Usually Fly 2300 RPM, 20 MAP which gives me cca 105-110 knots GS at 5000 Ft /less than in POH/.

Folks, mit what RPM/MAP do you fly your Mooney?. What is up to you most economical settings? Any suggestions for Cruise setting?  Thanks,

milos

lately I have been cruising at 22" and 2390 rpm......I am real time at 2400 RPM

[Hank]

I generally run ~70% power in my C. After a while, I've pretty much settled on these: 

--3500 and below:   23"/2300

--3500-6500/7000:  22"/2400

7000/7500 & up:  WOT- /2500

For WOT-, I reduce throttle just enough to make the MP needle wiggle, set RPM and lean. Airspeed is generally 140-145 mph. Up high that good for ~147 KTAS.

[brndiar]

Let me ask- why with rising altitude rising RPM? Thanks in forward.

milos

[Hank]

17 minutes ago, brndiar said:

Let me ask- why with rising altitude rising RPM? Thanks in forward.

milos

As altitude increases, manifold pressure decreases. Raising RPM brings power back up. Look at your Performance Chart. 

[steingar]

I'm pretty much with Hank.  If I'm flying locally for currency I use 21"/2300 RPM.  If I'm doing a short trip at low altitude it's 23 squared.  If I'm doing a longer trip at higher altitude (anything above 5K) it's often WOT and 2400 or 2500, depending on how high I am and how much MP I have left.  the POH has lots of performance charts, one can use any of the settings there.  I might try 17"/1800 RPM this weekend to fly some Young Eagles.  Don't want to run over any Skyhawks.

[brndiar]

OK,  But generally speaking, for any given power, the lowest possible RPM will reduce friction within the engine, and this may be the most important parameter. A high RPM may produce the maximum power, but at a cost in drag (and fuel). Props are working wie airfoil. We can go real fast, and run the parasite drag up, or we can slow down to minimize it.

Does anybody have experience with flying "oversquare" ?

Arguments against flying that way?

Thanks

[Rwsavory]

38 minutes ago, brndiar said:

OK,  But generally speaking, for any given power, the lowest possible RPM will reduce friction within the engine, and this may be the most important parameter. A high RPM may produce the maximum power, but at a cost in drag (and fuel). Props are working wie airfoil. We can go real fast, and run the parasite drag up, or we can slow down to minimize it.

Does anybody have experience with flying "oversquare" ?

Arguments against flying that way?

Thanks

This is an excellent discussion of the topic:

 

[S.C.]

I’m not an expert by any means and am still learning how to finesse for optimal cruise and efficiency.  My longest cross country to date was from Ohio to Texas (KDAY to KHYI) generally cruising  at 5000 with 21/23 (manifold/RPM). I was fortunate and got about 8.4 gal/hr.  I did give up a little speed for the efficiency, I was running about 125-130 knots.  Other shorter trips I’ve been playing with the altitude and settings from my Cruise Profile chart. I use the Qref Mooney M20C (Mark 21) Quick Reference Checklist. I also fly 25degrees rich of peak. 

As with any chart, results may vary, but for the most part I’ve been pretty close to the expectations set by my checklist cruise profile.  

Next big test will be this summer when I go up to Massachusetts. Planning on going higher than 5000 for ‘this trip probably 8000+ when winds are favorable.  

[mike20papa]

An engine guy explained to me how low power settings can reduce the ability of the rings to expand and seal sufficiently resulting in increased oil consumption and cyl. blow by.  (it's during break in you need to run an engine "hard").   I use to lope around below 2400 and 20 in. but now I try to avoid it - in my M20A.  

[jaylw314]

3 hours ago, brndiar said:

OK,  But generally speaking, for any given power, the lowest possible RPM will reduce friction within the engine, and this may be the most important parameter. A high RPM may produce the maximum power, but at a cost in drag (and fuel). Props are working wie airfoil. We can go real fast, and run the parasite drag up, or we can slow down to minimize it.

Does anybody have experience with flying "oversquare" ?

Arguments against flying that way?

Thanks

I don't know about the C model, but in the J model, the POH specifies 27 inHg MAP and 2400 RPM for 75% power on the cruise power schedule which is significantly oversquare.  Obviously, you'll only get that near sea level, but clearly the IO-360 is capable of operating there.  I think the O-360 has a lower compression ratio IIRC, so it should have an even larger detonation margin, right?

Higher RPM's do cause loss of efficiency due to higher friction and lower volumetric efficiency, but on my POH that amounts to the difference at 60% power between 8.4 gph at 2200 rpm to 9.1 gph at 2700 rpm.  That's about an 8% decrease in efficiency for a huge change in RPM, which may or may not be worth it.

I recall one of Don Maxwell's articles suggesting the Mooney props are most efficient at 2500 rpm, but I don't recall seeing any data on that.

Also, if you get your prop dynamically balanced, it will run smoothest at the speed you balance it at, so that may influence your decision.

[midlifeflyer]

5 hours ago, brndiar said:

OK,  But generally speaking, for any given power, the lowest possible RPM will reduce friction within the engine, and this may be the most important parameter. A high RPM may produce the maximum power, but at a cost in drag (and fuel).

Not fuel. As @Hank explained, the maximum MP available decreases as altitude goes up. As a rule of thumb, the 30" available at sea level will  decrease about 1" per 1,000 feet through most of the altitudes we fly. That means, at 8,000' you only have 22" max manifold pressure. 

Let's look at the M20C performance tables - I'm looking at the ones for a 1976 Mark 21. And let's suppose you want about 70% power. 

At 2,500' you have a lot of options to get that 70%. You choose 2300 RPM. To get your 70%, the tables tell you to use 23" MP - there's that nice 23-squared a number have mentioned. That gives you 71% at 10 GPH.

Now go to the table for 7,500'. Note the table only goes up to 22.5 MP. There's that inch/1,000 ft loss. You are not getting any more MP than that. Since you still have that 23" available (more or less), you can still match it with 2300 RPM to get about  70% power (72% in this case) at 10.2 GPH.

Let's go up to 10,000 feet. Now your max available MP is only 20". To get that same 70% power, you are going to have to come up to at least 2400 RPM with a wide-open throttle. 

Head up to the 15,000' table and notice you are only getting about 16" MP and even at the max 2600 RPM, you are only getting 59% power at a lower 8.9 GPH.

Fuel cost? If you've been following along, you've no doubt noticed that fuel flow is closely tied to percent power, not to RPM. The number for 70% power at all the altitudes is about 10 GPH and, despite the fact that you are using a much higher 2600 RPM you are getting more fuel economy, but that's only because you can't develop more than about 60% power.

Yes, I'm with you - I prefer the combinations which involve lower RPM. Not only nice for the engine, but nice for me and my passengers - less vibration. But increasing RPM to account for the reduction in available MP is pretty basic and doesn't produce the hit you think it does.

[jaylw314]

16 minutes ago, midlifeflyer said:

Fuel cost? If you've been following along, you've no doubt noticed that fuel flow is closely tied to percent power, not to RPM. The number for 70% power at all the altitudes is about 10 GPH and, despite the fact that you are using a much higher 2600 RPM you are getting more fuel economy, but that's only because you can't develop more than about 60% power.

Does the POH you have show the difference in one power setting (like 60%) across a range of RPM settings?  Is it also about an 8% increase in fuel flow going from 2200 to 2700 RPM?

[0TreeLemur]

"Oversquare" is an arbitrary definition, inches and rpms have nothing to do with each other.  If you use m.p. in mm Hg, and rpm in radians per second, whoa boy!

[jaylw314]

7 minutes ago, Fred₂O said:

"Oversquare" is an arbitrary definition, inches and rpms have nothing to do with each other.  If you use m.p. in mm Hg, and rpm in radians per second, whoa boy!

Hey, how come nobody uses gradians anymore, and why do they still have it on calculators if nobody does?  

[carusoam]

According to MAPA guidelines...

You can generally swap 1” of MP for 100rpm.  To maintain the same hp...

If you have a FF instrument you can generally show this to yourself, on your machine.

If you have fuel injection, this is best shown while LOP, so all the fuel going to the engine is producing power... when using a carb, go as lean as possible.

For running power experiments in your new to 2U plane, it may be best to have an engine monitor.  Know what the red box is, and how to avoid it...

For most efficiency... read up on Vz... Karsen’s/carson’s (?) speed... get used to flying slower... efficiency is very much about reducing drag.

Normally aspirated NA engines lose MP with altitude... it is hard to fly over square when you can only generate 20” of MP...

The latest POH for the M20C has power settings that are pretty useable... a few quirks but not terrible.

Lastly, know what the limitations are for rpm. Engine/prop/airframe.... avoid extended time in the zones marked on the tach... at prescribed MP settings...

PP thoughts only, not a mechanic or CFI.... my M20C didn’t have an engine monitor or FF indicator...

Pick an altitude first, then select your favorite power setting... WOT, above my favorite altitude of 10k’... the only thing left is select an RPM... also know what that secondary fuel nozzle is and how to turn it off...

Lots of glide distance on the East Coast When above 10k’...

Best regards,

-a-

[midlifeflyer]

2 hours ago, jaylw314 said:

Does the POH you have show the difference in one power setting (like 60%) across a range of RPM settings?  Is it also about an 8% increase in fuel flow going from 2200 to 2700 RPM?

Sure. All cruise performance charts for airplanes with constant speed props do that.

And yes, same MP but different RPM means different percent power and therefore different fuel flow.

But no, if the higher RPM setting is combined with a lower MP, resulting in the same percent power, the resulting fuel flow is substantially the same.

[midlifeflyer]

1 hour ago, Fred₂O said:

"Oversquare" is an arbitrary definition, inches and rpms have nothing to do with each other.  If you use m.p. in mm Hg, and rpm in radians per second, whoa boy!

I've never seen "oversquare" used to mean anything in aviation other than MP being greater than prop rpm/1000.

[jaylw314]

[jaylw314]

14 minutes ago, midlifeflyer said:

Sure. All cruise performance charts for airplanes with constant speed props do that.

And yes, same MP but different RPM means different percent power and therefore different fuel flow.

But no, if the higher RPM setting is combined with a lower MP, resulting in the same percent power, the resulting fuel flow is substantially the same.

Argh, I wasn't asking it well.  I was asking if the C POH lists the various MP/RPM combinations at a specific power setting like mine, and the corresponding fuel flows.  You can see above that with higher RPM at the same power setting, fuel flow does slightly but measurably increase

[1967 427]

I think most pilots will go to altitude if they are on a flight that will last more than 1.5 hours.  I normally go to  9.5 to 11.5k and at those altitudes I go to WOT, and back off (as others do) until I see the MP just start to drop.  Below you can see I am @ 10.5k ft., 2500rpm, 18.7MP, which calculates out to 60% power.  The engine loves it as indicated by the T&P, the last think I want to do is to hurt my engine, and I get to take advantage of the winds aloft.  If you look closely you can see that I'm showing 170kts across the ground.  Fun to see 170 in a 1966 C.

[midlifeflyer]

12 hours ago, jaylw314 said:

Argh, I wasn't asking it well.  I was asking if the C POH lists the various MP/RPM combinations at a specific power setting like mine, and the corresponding fuel flows.  You can see above that with higher RPM at the same power setting, fuel flow does slightly but measurably increase

And I wasn't answering it well. The C tables  do show the fuel flow at various combinations, with some minor variations, but not as wide a discrepancy as you see in your table. Part of the reason may simply be the J has a significantly wider range of "acceptable" values. I'm no engine guru, but I would expect a "weak" power plant producing only 19" MP driving a prop at 2700 RPM to be working much harder - and using more fuel - than a "strong" 24" power plant only pushing 2200 RPM.  

So I guess whether the numbers are "substantially the same" depends on how wide a range we are talking about. Even in your table, the GPH delta from 2200 RPM  to 2600 RPM is only 0.4 GPH. That last combo is a 0.3 GPH jump all by itself. I can't even imagine wanting to use that 19"/2700 RPM  combo unless my altitude required it.

Edited June 7, 2019 by midlifeflyer

[Hank]

15 hours ago, jaylw314 said:

Argh, I wasn't asking it well.  I was asking if the C POH lists the various MP/RPM combinations at a specific power setting like mine, and the corresponding fuel flows.  You can see above that with higher RPM at the same power setting, fuel flow does slightly but measurably increase

No. The Performance Tables for Cs (my 1970, and a 1975 book I found) are by altitude. You can usually find a closenpower setting at different altitudes, but you've gotta look hard. At each alititude, power is broken down by RPM, with a listing of MP every inch at that RPM. It's a full page per altitude, in 2500' increments. 

 

Edited June 7, 2019 by Hank

[Hank]

17 hours ago, jaylw314 said:

Does the POH you have show the difference in one power setting (like 60%) across a range of RPM settings?  Is it also about an 8% increase in fuel flow going from 2200 to 2700 RPM?

 

15 hours ago, midlifeflyer said:

Sure. All cruise performance charts for airplanes with constant speed props do that.

Welcome to the world of the truly "Vintage" Mooneys! As shown above, our Performance Tables do not do that, despite the constant speed prop having been in use by Mooney for over 15 years when my plane was built. It's a challenge looking up a particular power setting, say 65%, and following it's permutations across several altitudes--lots of page turning, and there are usually several combinations per page.

Edited June 7, 2019 by Hank

[midlifeflyer]

30 minutes ago, Hank said:

 

Welcome to the world of the truly "Vintage" Mooneys! As shown above, our Performance Tables do not do that, despite the constant speed prop having been in use by Mooney for over 15 years when my plane was built. It's a challenge looking up a particular power setting, say 65%, and following it's permutations across several altitudes--lots of page turning, and there are usually several combinations per page.

Well, it does. The data is there although the presentation is less, let's say, "user friendly." And, as the J tables indicate, they too have several combinations to produce the same power setting. We are still left to make choices. 

I'm not sure how much it has to do with being "vintage," except that I'm not at all surprised the format has changed through the years. Even in "modern" aircraft there are differences in the way performance data is presented by different manufacturers. Compare the Mooney (even new) takeoff distance graphs, which are prone to user error, with the simpler and arguably more user friendly Cessna tables where it takes all of two seconds to figure out the max gross takeoff distance over a 50 ' obstacle from  a 4,000 msl  airport when the temperature is 30C.