C Model Fuel Burn Rate

[MBDiagMan]

What fuel burn rate do you flight plan for.  I usually run at about 22 inches and 2,500 and seem to burn about 9GPH.

What do you plan for?  I don’t cut it close at all when it comes to fuel, but I really would like to be practical rather than stopping too much.

[Hank]

Above ~7000 msl, I pull the throttle just enough to make the MP needle move, and set 2500. This disengages the auto-enrichment circuit in tbe carb. 9gph is about it on block time, I don't have fuel flow.

[MBDiagMan]

Thanks Hank! 

Are you saying that you have everything forward to 7,000’ then pull back throttle until MP needle moves then turn down RPM to 2500?

[TTaylor]

Generally, it is:

1. Set rpm

2. Move throttle until mp drops

3. Lean until rough and then rich until smooth (usually 1/4 to 1/2 turn)

 

On the C, I block planned at 9.5 gph. 

[carusoam]

With no way to measure the FF in flight... and old crusty float fuel gauges...

Leaned ROP Using the sole numberless EGT...

I used 10gph for fuel planning with my C.  I Also stopped half way on any long flights.

FF, accurate gauges, and sharing fuel burn knowledge via MS sure would have made a difference!

Best regards,

-a-

[Hank]

31 minutes ago, MBDiagMan said:

Thanks Hank! 

Are you saying that you have everything forward to 7,000’ then pull back throttle until MP needle moves then turn down RPM to 2500?

God, no! Everything forward for takeoff. Lean by Target EGT when I remember, or WOT/Full Rich all the way up. Level off, accelerate then reduce throttle, set RPM and lean.

FYI Anthony:  my EGT has numbers every 100°F from 1200 to 1600, marks every 25° and redline at 1650°.

[Hank]

@MBDiagMan, I'm at the point now that I use mostly three power settings:

• Low altitude (< 3000 or 3500 or so):  23"/2300
• Medium altitude (~4000-6500 or 7000):  22"/2400
• High altitude (where I can't exceed 75% at 2500):  WOT - a tad/2500 as described above

Works for traveling (high), burger runs (low), even ATC step climbs & descents, instrument approaches before the IAF, etc.

But I do have most of the Performance Tables on my kneeboard for easy reference without having to pull the ancient, fragile, browning Owners Manual from the seatback pocket.

[Andy95W]

I plan 10 gallons per hour because then I'm sure I won't screw up the math.

[Raptor05121]

47 minutes ago, Hank said:

@MBDiagMan, I'm at the point now that I use mostly three power settings:

• Low altitude (< 3000 or 3500 or so):  23"/2300
• Medium altitude (~4000-6500 or 7000):  22"/2400
• High altitude (where I can't exceed 75% at 2500):  WOT - a tad/2500 as described above

Works for traveling (high), burger runs (low), even ATC step climbs & descents, instrument approaches before the IAF, etc.

But I do have most of the Performance Tables on my kneeboard for easy reference without having to pull the ancient, fragile, browning Owners Manual from the seatback pocket.

Hank is my soul animal. I do the exact same thing. I don't dip my tanks when flying local, but long distance I calculate my fuel/oil burn and speeds religiously. I flight plan for 10gph:

I takeoff, WOT, 2700RPM, full mixture and climb to cruise (8-10k). Leave throttle WOT, pull prop to 2500 and lean until smooth and CHTs stabilize. Block-to-block over an hour flight time, I've never burned more than 9gph that way and she seems to hit 8.7 quite religiously. 145-147KTAS. On descent I'll leave power in and bring prop back to 2300rpm. As I come down, I'll slowly keep lowering the MP an inch or two. That usually keeps me about 150-160GS in the descent (close to Vne on ASI) and makes up for time-to-climb.

[Hank]

Raptor, you really shojld stick the tanks before every flight.

For descents, all I domis push the yoke for 500 fpm and trim it hands off. As I come down, I eass the throttle back and mixture forward to maintain my cruise MP and EGT. I try to enter the pattern at (or near) 90 mph with Takeoff flaps.

Easing the throttle back at cruise has another benefit:  the cocked throttle plate in the carb may induce some turbulence, leading to better atomization of the fuel and more thorough mixing with air. I can run smoothly up to 25° LOP, but don't cruise there because I don't yet have an engine monitor. 

[MBDiagMan]

I faithfully stick the tanks.  A habit held over from years of Cessna 140 flying.  On the Mooney, however, it rarely means actually using the stick, because when I pull the cap it is to the top.

My FBO has my American Express on file and a key to the hangar.  I ask them to top off the tanks after every flight.  I always pull the caps and sample both tanks though.

I changed FF to block for 10GPH and will use Hanks throttle method.

[DXB]

The impact of crappy cowl/baffle design on the C model's fuel burn shouldn't be underestimated.  I agree 9-9.5 gph is the expected economy at 2500rpm, WOT,  7-10k feet, but I can only lean this far in the winter.  I have to burn 10-11 gph to limit my #2 cylinder CHT to the 380s in the summer, which is when I do the most flying. The other three run 20-40 degrees cooler in cruise. I have addressed all possible cooling factors that might cause this problem repeatedly over a period of years with no benefit, and no need to rehash here again.  Note I would be oblivious to this issue and choose to burn ~9gph at all times if I only had the single #3 CHT (which is indeed hottest in climb), and I'd probably need a new #2 cylinder every couple of years. I suspect my experience is not uncommon among C owners, and I doubt anything but improving the basic baffle design will fix it.  

[ShuRugal]

@DXB, I find it odd that your #2 (front left) cylinder would be the hot one.  Usually I would expect #3 or #4 (rear row) to be the culprit of excess heat.

 

One more reason for me to pick up an EDM, as soon as i can budget for it...

[ShuRugal]

that is just 'plane' weird.  Anyone got access to some CFD software?  we could draft up a model of the M20C doghouse and see how air actually wants to flow through it.... probably come up with an acceptable user-supplied alternative part to improve it....

[ShuRugal]

1 minute ago, David Herman said:

We’ve sealed every place we could find around that doghouse baffling ad nauseum ... around alternator .. top ... bottom ... back ... front ... you name it ... an exercise in futility me thinks

entirely.  When i am sloppy with my oil topups, i end up with oil streaks on the front lip of the inlet, running from inside to out....

These planes don't inhale through the doghouse, they exhale....

[DXB]

1 hour ago, ShuRugal said:

@DXB, I find it odd that your #2 (front left) cylinder would be the hot one.  Usually I would expect #3 or #4 (rear row) to be the culprit of excess heat.

57 minutes ago, ShuRugal said:

that is just 'plane' weird.  Anyone got access to some CFD software?  we could draft up a model of the M20C doghouse and see how air actually wants to flow through it.... probably come up with an acceptable user-supplied alternative part to improve it....

 

I'm told airflow in this cowl design can actually reverse out part of the front cowl under some conditions.  I suspect a point of stagnation/reversal is near my cylinder 2 in cruise. I just have the LASAR cowl closure mod, which I think makes zero difference other than aesthetics. I would love for someone with the relevant skills to model the airflow in a way that replicates these effects and predicts the needed design change.  Perhaps it could be a joint project with @Sabremech, who is spending considerable effort on bringing an economical cowl/ baffle mod for the vintage birds to market.  

[ShuRugal]

3 minutes ago, DXB said:

I'm told airflow in this cowl design can actually reverse out part of the front cowl under some conditions.  I suspect a point of stagnation/reversal is near my cylinder 2 in cruise. I just have the LASAR cowl closure mod, which I think makes zero difference other than aesthetics. I would love for someone with the relevant skills to model the airflow in a way that replicates these effects and predicts the needed design change.  Perhaps it could be a joint project with @Sabremech, who is spending considerable effort on bringing an economical cowl/ baffle mod for the vintage birds to market.  

To avoid further derailment of this thread, i have spawned a new one on that subject.  I have (or can acquire) the relevant skills, just help me gather data to model with.

[MIm20c]

1 hour ago, David Herman said:

10 gph in cruise, more or less, 7000’-11,000’  ...Throttle reduced just enough to close carb enrichment circuit/ 2400 RPM/ mixture 100° ROP ... set with EDM “lean find” function. ... our burn is stlightly higher due to Challenger Intake and PowerFlow Exhaust. 

Unlike Hank we climb at 25/25 ... and keep increasing throttle in the climb until wide open ... I find this method keeps our CHTs cooler ... I tried WOT /Max RPM ... and the CHTs were HOT. 

... and climb 120 mph or faster ... to keep the CHTs cooler.

Like Hank ... (we set 2400 RPM first) setting cruise power and mixture ... throttle (is now wide open) reduce until MP needle moves ...and closes the enrichment circuit on the carb. I can see the FF on the JPI go from 14ish to 12.5ish when the enrichment circuit closes ... then to 9.5 to 10ish with the “lean find” 100° ROP. 

This is the way I’ve been flying. I closely monitor all temps and if you try for full forward to level in the summer you will be in the 425 cht range. 25/25 on climb immediately and dramatically reduces cht’s.  The good news is obviously these engines can handle hot temps and leaned to almost peak at high power settings. They will also keep up with all the trainers when pulled back to 6-6.5 gph. 

[Hank]

I've never had trouble keeping back with the trainers. Set 2300 and reduce throttle far enough to not run over them . . . 15-16"? Faster than slow flight at MCA, but that ain't saying much.

[kortopates]

With regard to temps, I'll offer a little of what our Savvy data has shown me since I see quite a bit Mooney data. Their are lots of exceptions for what I'll offer but generally speaking the carbureted  Mooney's tend to run with higher CHTs on the left side; especially in climb. This is not due entirely to airflow but a combination of airflow and mixture distribution. Mixture distribution is pretty abysmal in these engines at full power in that the rear cylinders are not getting their fair share of fuel compared to the front. if you're also unfortunate enough to have a lower than average max FF (~16.5 GPH) then you'll likely have some hot CHTs in climb at no fault of your own. Those with higher FF have it much easier in climb. But I have yet to see one where mixture distribution doesn't evens out very nicely once power is reduced to cruise power levels and then temps run very nicely as long as baffling seals are in good shape. Some of you will discover some of tricks to improving mixture with ever so slightly a change in throttle to alter the turbulence but without significantly reducing MAP (unless you are intending to reduce power).  This technique helps in cruise as well to improve mixture distribution, as does partial carb heat, but the latter is best done with a carb temp probe so that you can target a specific temperature.

Those of you that are Savvy clients can also run our Report card on your plane and see how many of your performance parameters compare to the rest of your Mooney cohort. This includes parameters like highest inflight and cruise CHT, max FF, percent cruise power and many more.  Its doesn't give actual cruise FF that stated this thread, but it does give cruise % power as well as other related parameters. The only drawback to these reports is that we didn't have enough A/B/C/D/E/G's to separate the carbureted O-360's from your E's IO-360's and they are all in the same cohort (F's are grouped with the J's). But as more of you folks add engine analyzers and upload data this will improve over time.   Here is a link to more info on Savvy report cards if interested: https://www.savvyaviation.com/ufaqs/report-card-and-trend-analysis/ 

Edited March 15, 2018 by kortopates

[DXB]

I also consistently get fuel flows in the 17-18 range on takeoff.  Mixture distribution in climb does seem to be a major player in determining hottest cylinder.

The order of cylinder temps in cruise however is an entirely different animal in my bird.  The fact that #2 always settles out 20-30 degrees hotter than the rest irrespective of the cruise power or mixture setting indicates that the problem is elsewhere. Before anyone asks, it is not an induction  leak.  That cylinder has also been off and back on the plane for IRAN, but the issue did not change at all.  It was also basically the same before and after extensive work to improve the tightness of the doghouse.  I am convinced the issue is intrinsic to the airflow design.

BTW I tried to make a poll on this topic a while back among C owners, and #2 cylinder won out as hottest in cruise, although numbers are too small to be definitive.  

 

[kortopates]

21 minutes ago, David Herman said:

We have 17+ ff at brake release for takeoff. Carb temp gauge ... the carb heat may help slightly ... but ... it’s lipstick on a pig

You're doing much better than many with your FF  

The carb temp suggestion is meant in cruise to help if you were trying to run near peak or even LOP.  It works for some folks, but the I think cracking the throttle works more commonly. 

[Skates97]

My FF is just over 15 at take off/climb. In cruise I see around 9-9.4 if up around 8,500-10,500. Cylinders 2-4 run within about 5-10 degrees of each other. At high power 3 is the hottest (by 5-10 degrees) and in cruise 2 is the hottest (again by 5-10 degrees) Cylinder 1 consistently reads about 65-75 degrees cooler which I attribute to the sensor as it mirrors the other three in temperature increase/decrease.

Before I had the EDM 830 I planned for 10gph and while I always came out under that I liked to err on the side of caution.

[cliffy]

I've got nothing but what the factory gave me in 64' except a FF gauge. I've done a hundred trips just by going FT/2700 and leaning some over 5000' to cruise at 9000- 11000 ft. In all that time I leaned to rough at altitude and enrich to smooth. Always showed 10 for the first hour, 9 for the second and 8 every hour after that. All measured by filling the tanks at the end of the flights. 10, 9, 8, 8, is what I flight plan but I usually never go beyond 3 1/2 hrs now without stopping. 

I can usually lean by going to smooth to with in 1/2 gallon per hr of what I can lean to using the FF gauge. Its worked for me for 1700 hrs in this D/C. 

My cruise is usually 2550, WOT and lean to smooth. 

If you lean below the wide open throttle enrichment point, that point has no meaning (or that throttle position). All the leaning is done downstream of that FF position. I.E., if you see it go from 14 to 12.5 as you move the throttle plate and then you lean with the mixture to 10 GPH the throttle position at 12.5 has no meaning (except maybe to even out distribution).

[MikeOH]

I have a '70 F  (Injected) and am fighting the same #2 CHT issue: 20-30 hotter than the rest.  I see just over 16 gph on takeoff.  I've pretty much learned to pull back power when the #2 CHT hits 400, and cruise/climb at 120.