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Optimum Fuel Consumption Tasks

Bever

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This may be trivial to most, but its something to do to occupy my time and justify flying. Can only eat so many $100.00 hamburgers.
I have taken on the tasks of establishing optimum fuel burn without the use of a fuel flow gauge. Don't have one.
1968 180 w/ 0-360 Both tanks filled to tabs (17gal ea.) Take off and climb to altitude, level off and lean on the R tank then switch to L tank for one hour at a steady state then back to R tank. Currently using 2450 RPM and as suspected getting better GPH at higher attitudes. Std day getting 8 GPH @ 4000' and 7.8 GPH @ 6000'. These numbers are pretty consistent. Local flying buddy and intense engineer has shared that on these engines the ideal situation is being at an altitude that requires full throttle to produce 65% power. My eyes are crossed looking at so many performance graphs and not being able to decipher that information. Is getting this information only possible by using a manifold pressure (vacuum) gauge. Anyone with input on this?? No matter what, I still use 10 GPH for flight planning.
 

av8ator1

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On my archer I get the follow via a fuel flow gauge
Climb 2650 rpm at 17.7 gallons
Cruise 2300 rpm at 7.9 gallons
Cruise 2400 rpm at 9.1 gallons
 

Orest @ CYFD

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Many folks fly way too low, and waste a lot of fuel and time. In still wind, on a standard day, you will be very close to optimum fuel burn at about 2/3rds of your service ceiling.

Easy to manage, climb at full throttle up to that altitude, then level out. Don't touch the throttle, but lean it out as you wish. On descent, leave it full throttle, just point the nose down and fly fast. Only pull back the throttle when you approach an overspeed for the conditions, as you descend.

KISS.

* Orest
 
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Bever

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av8ator 9.5 @ 2400. I guess many variables. But I'm filling exactly to the tabs then run one hr and then fill to the tabs again. It's been pretty consistent. Do my numbers sound to low. I am using EGT gauges.

Full throttle all the time. That would eliminate any need for calculations.... IIRC the service ceiling for the 28-180 is ~15K' so using your methodology the optimum fuel burn (std day) would be 10K' thereabouts? I'll have to test that.
Right now, if I level off at say 3500' the RPM will easily exceed red line. Is that typical? Static RPM is 2470
 

Orest @ CYFD

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...
Full throttle all the time. That would eliminate any need for calculations.... IIRC the service ceiling for the 28-180 is ~15K' so using your methodology the optimum fuel burn (std day) would be 10K' thereabouts? I'll have to test that.
Right now, if I level off at say 3500' the RPM will easily exceed red line. Is that typical? Static RPM is 2470
Very good, and yes.

* Orest
 

Morgan3820

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av8ator 9.5 @ 2400. I guess many variables. But I'm filling exactly to the tabs then run one hr and then fill to the tabs again. It's been pretty consistent. Do my numbers sound to low. I am using EGT gauges.

Full throttle all the time. That would eliminate any need for calculations.... IIRC the service ceiling for the 28-180 is ~15K' so using your methodology the optimum fuel burn (std day) would be 10K' thereabouts? I'll have to test that.
Right now, if I level off at say 3500' the RPM will easily exceed red line. Is that typical? Static RPM is 2470
On my Warrior, I recall static rpm as 2300-2350. Underproped?
 

Canuck

David Megginson (he/him)
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On my archer I get the follow via a fuel flow gauge
Climb 2650 rpm at 17.7 gallons
Cruise 2300 rpm at 7.9 gallons
Cruise 2400 rpm at 9.1 gallons
What altitude? That would be extremely low power (with surprisingly high fuel consumption) at 8,500 ft DA, but not too far off expectations at 2,500 ft DA (assuming rich of peak).
 

Canuck

David Megginson (he/him)
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This may be trivial to most, but its something to do to occupy my time and justify flying. Can only eat so many $100.00 hamburger.
Unless you're flying into a very strong headwind (40–50+ knots), best cruise fuel consumption will be at 55% power at any given density altitude (you'll probably run out of patience before you run out of gas, so most people choose 65% and sacrifice a tiny bit of fuel economy for an extra ~10 knots). Best efficiency will come from leaving your throttle wide open and pulling back the mixture to your target RPM (if your engine still runs smoothly that way).

Figuring out the right RPM involves knowing the density altitude first, then doing a simplified lookup. Here's a table I made for my O-320-D3G (yours will likely be slightly different):

http://megginson.com/Aviation/o320-d3g.html

A nice shortcut for initial power setting is that your indicated airspeed will be almost identical for any given power setting up to 10,000 ft, even as the true airspeed is increasing with altitude.

For example, I know that my Warrior indicates about 110–112 KIAS at 75% power, lightly loaded (about 5 KIAS less at max gross). So at first in cruise, I just yank back the red lever and tweak a bit until my indicated airspeed ends up here:

55% power — 90–92 KIAS
65% power — 100–102 KIAS
75% power — 110–112 KIAS

then I crosscheck the RPM to make sure it's about where the power tables require (which it usually is).

D
 

Bever

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Ah ha.. That chart is exactly what I'm working towards. Academia information is plentiful and often times conflicting. This "leaning" issue is a prime example. It's the empirical knowledge, to me anyway, that provides the most value.
So far, all the graphs from Lyc and Piper reference a Manifold Pressure in the equation.
"The red area represents a power range that is too high for cruise (possibly resulting in detonation and engine damage),"

The TCDS below contains [For all operations, 2700 r.p.m. (180 hp)]

I'll generate a Blank Spreadsheet and start filling the blanks to include BHP & GPH
Using this and OREST's method will help me sort this out.
Great input...thanks

TCDS for S# 28-5091
Engine Limits
S/N 28-671 through 28-1760, and 28-1760A (except S/N 28-1571 and S/N 28-1573)
(See NOTE 4):
Maximum permissible takeoff, 2475 r.p.m.
For all other operations, 2700 r.p.m. (180hp)
S/N 28-1571; 28-1573; 28-1761 through 28-5859; and 28-7105001 through 28-7205318:
For all operations, 2700 r.p.m. (180 hp)
Propeller and Propeller Limits
Sensenich M76EMM or 76EM8 on S/N 28-671 through 28-1760, and 28-1760A (except
S/N 28-1571 and S/N 28-1573).
Sensenich M76EMMS or 76EM8S5 on S/N 28-1571, 28-1573; 28-1761 through
28-5859; and 28-7105001 through 28-7205318.
Static r.p.m. at maximum permissible throttle setting not over 2450 r.p.m., not under 2275 r.p.m.
No additional tolerance permitted.
 

Canuck

David Megginson (he/him)
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I'm lucky with my Warrior II, because the POH authors must have waited until the nervous lawyers were away on summer vacation before slipping in this genuinely-useful information: :)

For Best Economy cruise, a simplified leaning procedure which consistently allows accurate achievement of best engine efficiency has been developed. Best Economy Cruise performance is obtained with the throttle fully open. To obtain a desired cruise power setting, set the throttle and mixture control full forward, taking care not to exceed the engine speed limitation, then begin leaning the mixture. The RPM will increase slightly but will then begin to decrease. Continue leaning until the desired cruise engine RPM is reached. This will provide best fuel economy and maximum miles per gallon for a given power setting. See following CAUTION when using this procedure.

CAUTION​
Prolonged operation at powers above 75% with a leaned mixture can result in engine damage. While establishing Best Economy Cruise Mixture, below 6,000 feet, care must be taken not to remain in the range above 75% power more than 15 seconds while leaning. Above 6,000 feet the engine is incapable of generating more than 75%
The same advice should apply to all PA-28s with four-cylinder carbureted engines and fixed-pitch props. These planes are incredibly simple to operate LOP compared to more-complex setups (either it works LOP-WOT, or your engine runs too rough and you need to stay ROP and burn the extra fuel; you'll know pretty quickly).
 
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