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Power setting M20C


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3 hours 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.

 

6 hours ago, midlifeflyer said:

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.

That's what I was missing since I don't have a C POH to compare, thanks guys!  I'm guessing somebody has gotten around to interpolating the numbers and reformatting them like they are listed in the J POH?  I imagine that would make life easier...

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31 minutes ago, jaylw314 said:

I'm guessing somebody has gotten around to interpolating the numbers and reformatting them like they are listed in the J POH? 

Not that I'm aware of. Feel free to have a go at it, there are several C model Owners Manuals in the Download section here.

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1 hour ago, jaylw314 said:

 

That's what I was missing since I don't have a C POH to compare, thanks guys!  I'm guessing somebody has gotten around to interpolating the numbers and reformatting them like they are listed in the J POH?  I imagine that would make life easier...

Not unless someone did it manually.

This is from the 1977 edition of the M20C manual. Notice this page  it only covers 5,000 feet at 41F (standard temperature). There are individual pages at 2,500' increments from sea level up to 12,500 msl, all at standard temperature for those altitudes. The data is there, but it's not as user-friendly as later charts. So, for, say 75% power at 5,000' at standard temperature, you are going to end up looking through the chart finding combinations that are close to what you want.

Lest you think that's too loose, bear in mind that like all performance charts, they are based on a brand new airplane. Plus, it is innacurate unless you make adjustment for density altitude So, when we see numbers with a decimal point, we are essentially talking about measuring  a football field with a micrometer.

And it could be worse. If you look at similar vintage Pipers, you'll find not tables at all, but a series of graphs, and you might have to run more than one of those to get numbers for an altitude.

image.png.5602ec7d5c0af87a23121ad34d9a0844.png

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5 hours ago, midlifeflyer said:

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.

Fly a Cessna then.  You easily figure out your takeoff distance and fly nice and slow so you can figure out lots of other things.

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2 minutes ago, steingar said:

Fly a Cessna then.  You easily figure out your takeoff distance and fly nice and slow so you can figure out lots of other things.

Of all the makes and models I've flown, I've liked flying Cessnas the least (unless I actually need to carry stuff). But I'm not sure what "liking" to fly them has to do with the choices made by manufacturers for their performance chart formats :rolleyes:

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... On The Page below, there Flight Operation and there Some Suggestions For More Efficient Engine Cruise Power Settings.

Found that article very interesting, just not sure with Full Thottle,  2500 RPM and 50 ROP in  low Altitudes (for Example below 3000 msl) ... So keep the throttle full open. The only time to back off the throttle in cruise in a normally aspirated engine would be 1) if you are flying below 3000 feet, where a full throttle would give some pretty high manifold pressures or 2) you're just out flying around to sightsee, where reduced power and slower speeds are called for. But for most cruise conditions where, you want to cross the ground quickly and in the most efficient manner, keep the throttle full open....

POWER.jpg

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@brndiar, if you can find a copy, the MAPA training manual also talks about power settings (among a number of other things).  It talks about optimum cruise settings in terms of a "key number" - a combination of MP and RPM/100. The key number for the C is 46, which translates to combinations such a 23/2300, 22/2400, etc,  You'll notice that reflects what a number of folks here have describe as what they use.

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While I use the Key Number method, I don't use WOT. My low altitude RPM is 2300, which limits me to 23" (my low level MP). To go slow for flightseeing or running with Cessnas, i set 2300 then whatever throttle is required for slowness (not running over Skyhawks is about 16-17").

Even when high, I don't use WOT, but back it off until the MP needle wiggles. At 10,000 msl, this can be a surprisingly large movement. This does two things:  shuts off the economizer circuit which pushes extra fuel through the carb for cooling at WOT; and the cocked throttle plate (hopefully) creates turbulence inside the carb for better fuel atomization and mixing, for more uniform distribution to all four cylinders. I call this "WOT-".

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19 minutes ago, Hank said:

While I use the Key Number method, I don't use WOT. My low altitude RPM is 2300, which limits me to 23" (my low level MP). To go slow for flightseeing or running with Cessnas, i set 2300 then whatever throttle is required for slowness (not running over Skyhawks is about 16-17").

Even when high, I don't use WOT, but back it off until the MP needle wiggles. At 10,000 msl, this can be a surprisingly large movement. This does two things:  shuts off the economizer circuit which pushes extra fuel through the carb for cooling at WOT; and the cocked throttle plate (hopefully) creates turbulence inside the carb for better fuel atomization and mixing, for more uniform distribution to all four cylinders. I call this "WOT-".

This is great information. 

The other thing interesting about the article is the 50 rich of peak. A standard for decades, it has recently come into some disrepute due to the lean-of-peak studies indicating 50 degrees rich is the worst place to be - solidly in the "red box" of high compression pressures ad high CHTs. (but that's another issue :))

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1 hour ago, midlifeflyer said:

This is great information. 

The other thing interesting about the article is the 50 rich of peak. A standard for decades, it has recently come into some disrepute due to the lean-of-peak studies indicating 50 degrees rich is the worst place to be - solidly in the "red box" of high compression pressures ad high CHTs. (but that's another issue :))

Above ~6500-7000 msl, it's not a problem. I've made dozens of lumch runs at 3000msl running 50ROP, with now 850+ SMOH in 2003?, and my new IA was impressed by how strong my engine was at annual this year.

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For longer higher flights above 6000 I try to get as close to 47 MP + RPM/1000 as possible. 2400 to 2450 seems to provide good oil and cylinder temps. The problem with my airplane is Since I can't get past peak mixture before I start to feel the engine start to miss I really have no idea what my ROP is. If down lower and in hotter weather will typically run 2400/20. Most flights I keep it pretty easy on the engine. It's impressive how much difference you get in speed depending on your settings.

no fuel flow meter but block number is 9 gallons per hour on higher longer flights. Climb cruise descend.  WOT with throttle just back to see the MP move and 2450 gives us around 145 knots true.

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Once you begin leaning with the mixture control the "economizer" circuit (and throttle position) has no effect on the fuel flow to the engine. It only works with a full rich mixture.  By moving the throttle plate toward closed  you open an air bleed (with full rich mixture) into the same channel that supplies fuel to the main discharge nozzle thereby "leaning the mixture a slight amount. Obviously you don't need all that extra FF to cool the engine if you are pulling the throttle back some. Once you lean with the mixture control (throttle full open or not) the slight leaning by the air bleed is surpassed by the more aggressive leaning with the mixture control.  You can leave the throttle wide open and if you lean with the mixture control you don't have any need to close the air bleed by bringing back the throttle.

The throttle plate being slightly angled "may" help with evening out fuel distribution after the carb downstream. 

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1 minute ago, cliffy said:

The throttle plate being slightly angled "may" help with evening out fuel distribution after the carb downstream. 

That's why I pull the throttle back some. By pulling until the MP needle wiggles, the throttle body cocks more at higher altitudes, where more even distribution is more desirable. Down low, WOT is just too much power . . . Which is why I don't go very far at 3000 msl. Even ny 40-minute runs to Spruce at KFFC are generally 6500 one way and 7500 the other. Plus it's cooler.  :)  No AC, etc., etc.

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Bob Minnis mentioned that it would be interesting if someone did some tests with something like those "vortex " devices that are advertised for cars that sit between a carb and the intake manifold spinning the intake charge for distribution. 

Can a more even fuel distribution be seen with an angled throttle plate on an analyzer with four EGT probes compared to a WOT position ? Has anyone tried this? Inquiring minds want to know. 

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37 minutes ago, cliffy said:

 

Can a more even fuel distribution be seen with an angled throttle plate on an analyzer with four EGT probes compared to a WOT position ? Has anyone tried this? Inquiring minds want to know. 

Yes, I can say without any hesitation or quibbling that angling the plate by pullling out on the throttle does even out the EGT graph on my JPI engine analyzer.  If I add just a touch of carburetor heat, they even out so much you'd swear it was fuel injected.

I'm finishing my annual inspection this week.  When I go flying I'll take some pictures.

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2 hours ago, Andy95W said:

Yes, I can say without any hesitation or quibbling that angling the plate by pullling out on the throttle does even out the EGT graph on my JPI engine analyzer.  If I add just a touch of carburetor heat, they even out so much you'd swear it was fuel injected.

I'm finishing my annual inspection this week.  When I go flying I'll take some pictures.

How much do you reduce throttle? I pull mine back until the MP needle wiggles, just a bit at 3000 msl and almost half travel at 10,000 msl. A comparison at different altitudes would be great! If I only had a monitor myself, but my upgrade fund is getting spent on other things this year [like fixing improper repairs from 17 years ago, undoing some screw-ups made at annual this year, etc.].

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Hank- I do the same as you, pull it back until there's a tiny movement of MP and then I leave it.  

I've never really stopped to think about the altitude differences, but my gut tells me it's more leveled at higher altitudes.  Not sure if that's due to more throttle-plate cockiness or the ability to lean more at altitude.

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If disruption of air flow is desired for even mixture distribution the angle of the plate will be the same at 3000' as it would be at 10,000'

If the distribution is even at 3000', the flow speed through the throat of the carb (if the RPM is the same) will be the same at a higher altitude. The same angle will accomplish the same mixture distribution.  

Again, once you pull the red knob for leaning, the economizer is out of the picture. 

Carb heat was tested in an R-985 radial many years ago and found on THAT engine to help to even out the cylinder flow of the fuel/air mixture. 

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  • 2 weeks later...

Well, as promised, I went flying today and took some pictures of my engine monitor to show how effective it is to retard the throttle a touch and add a bit if carb heat.

What isn't so effective is taking a digital picture of a digital engine monitor.

But trust me, it really works!

IMG_1996.JPG

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  • 3 months later...
On 6/23/2019 at 11:38 PM, cliffy said:

Would have been cool to see if it had worked  :-)   

To see it even out visually as you do it must be interesting

@cliffy - sorry it took so long.  I had to upgrade my JPI to get an answer to your question.  The pictures below were taken at 10,000 feet MSL, each after a few minutes to allow temperatures to stabilize. They definitively show the effectiveness of throttle reduction and carb heat addition.  I've added my comments directly to the pictures.

While the EGT difference might not be huge, the end result was that I was able to lean more and save more fuel while still running a smooth engine with cool CHTs.

IMG_2150.JPG

IMG_2145.JPG

IMG_2147.JPG

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There was an old thread about these cruise settings that I recall reading through. I took a long flight from Chino, CA to Sun Valley, ID last July and used Hank’s method, sans the carb heat. I flew back into a 25 knot headwind at 11,500’ and was in the air for 5Hrs 20mins - I had over ten gallons left in the tank when I landed. Settings were 2500 rpm WOT backed off a hair to see tiny move on MP gauge. Leaned until rough, then back in just a touch for smooth running. TAS was around 165mph - 140mph over the ground via gps. I’ve got basic speed mods, but nothing fancy; I was thrilled with the performance. These birds really like altitude.  I wanna say the MP was around 20.5” - but not positive on that. Same settings got me there in 4hrs 15min at 10,500’ with a nice tailwind 175-180mph over the ground via gps. 

Are you guys saying I could do even better with a touch of carb heat? Does this let you lean a bit more, before it runs rough?

Edited by Lukon
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