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How do I figure the percent of power at a certain RPM ?

The Kid

FOUNDER
Thompson Falls
My aerial photo flying requires me to not be screwing around "getting gas" when I'm out trying to do aerial photography. I want to stretch what gas I have as far as I can. Ben Visser (GA NEWS) the other day said 65% power will use less gas than 70% power. I know he is right. But just how do I figure out what EXACTLY is my percentage of power via the RPMs that I am turning? What math do I use? How do I do it? Does altitude affect it at all? Is there a point where your RPM setting is using less gas alright but at a cost of too much less speed? Maximum range comes down to MPG I would think? Again, what is 65% power in a Lycoming whose max RPM is 2700? And how is that RPM computed?
 
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Im sure some of the smarter people on here will have a formula, but aircraft with POH’s usually publish a chart...
 

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The Lycoming Operating Instructions booklet has charts and graphs. It's not nearly as simple as just RPM. Depends on temperature, altitude, manifold pressure, etc. I have the charts on my desk because I was wondering the same thing, and they are not real user friendly. In my opinion the instructions for their use are not well written.
 
For example: https://iflyei.com/product/fp-5l-fuel-computer/ Then add a GPS for ground speed input to determine MPG and other info.

Or at least resort to pilotage based upon previous power/rpm-fuel consumption-groundspeed-weather-mpg data. It helps to take notes of those when flying for later use.

Piper does publish an estimated rpm vs lean-rich fuel burn chart for a 74-56" prop in their PA-18 Owner's Manual.

Gary
 
For a fixed pitch propeller powered airplane, use the power chart which came with the plane. If the prop has been changed in pitch, diameter, profile, etc. from the original, the chart is no longer valid. In that case you will need both a manifold pressure gauge and a tachometer. There are other inputs involved but these two will be close.

This should help: https://www.kitplanes.com/determining-engine-power/
 
I burn about 7 gph at 2500 rpm down at 1100 feet with a 41" Borer. The Sensenich I put on to come up to the Crowe bar the first time burned about 8.5 gph.
 
I believe that's with a 74x56 prop. I use those fuel flows with the Borer and it seems to work out right.
Which seemed to work out right RPM or % of power?

I burn about 7 gph at 2500 rpm down at 1100 feet with a 41" Borer. The Sensenich I put on to come up to the Crowe bar the first time burned about 8.5 gph.
Were you also using 2500 rpm on your trip to Crowe bar? The increased fuel flow would indicate using more manifold pressure.
 
Which seemed to work out right RPM or % of power?


Were you also using 2500 rpm on your trip to Crowe bar? The increased fuel flow would indicate using more manifold pressure.

I use Fuel flow to arrive at % power by altitude from that chart
 
If getting the most out of each gallon of gas is the priority, it seems like a fuel flow gauge would be more useful than a chart.

Web
 
The instrument mentioned in #4 and 12 "Can be connected to a compatible GPS unit to further enhance the display capabilities to include Fuel to Destination, Fuel Reserve, Nautical Miles per Gallon and Statute Miles per Gallon."

Gary
 
The owners manual for my 53 C180 has a graph, not a table. Hard to decipher.

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IMHO the table version in the 1957 model owners manual is much more user friendly.

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Fuel flow for a given power can be all over the map. It's at best a "reasonable" guess at power.

I recently bought an EI fuel flow inst (thank you Steve P.). I can run 2400 RPM at anywhere from 6 to a little over 8 GPH at a "couple thousand" feet MSL.

Edit: PA-12, 82-42 prop.
 
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Instead of looking at a power chart, why not use the minimum throttle setting that will keep you in the air? Then monitor the fuel flow with the gauge to see how long before a fuel stop. I just don't get what a power setting on a paper chart is going to do for you when you're in flight and trying to minimize fuel burn.

Web
 
Lean a little bit leaner than best power (max RPM) at best range speed (I believe this is "Carson's Speed", but not absolutely positive)
 
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The EI FP5 shows real time fuel flow and can interface with a portable GPS with the correct cable. Pretty hard to beat that.

Mine is extremely accurate after tweaking it using the instructions that came with it. Essentially, you enter in how much it took to top the tanks, and then it compares what you added to what it thinks you have used and then zeros in on the correct flow compensation and gets more accurate the more often you enter the gallons actually used.

It will give you GPH, a derived HP percentage, fuel used/remaining, miles per gallon, fuel to next waypoint and/or destination, time to empty, (if you have the RS232 connected to a GPS source tracking a waypoint) and gives a reminder to switch tanks if you set it up that way.

They're about $600 at Spruce. A bargain.
 
I have my EI FP5L fuel flow gauge hooked up to my Garmin 496 and it will tell me how much full to destination. Kinda handy when going west of here to do the quick math. I just ordered a CGR30P for our new to us 182. Since it has the manifold pressure feature it will display horsepower as well. Be kinda interesting to see what it is at varying altitudes, throttle and prop settings. Oh yea, did I mention that I am an EI dealer? ;) I bet I can answer your questions better than Spruce too. ;)
 
A JPI FS450 provides fuel endurance. It's not a function I look at often but it's good to have. The G3X has it, too. Percent HP is unimportant to me. Fuel use and engine temps are what I look at. With those in range I let the airplane's vibration tell me its happy place.
 
I would have to agree with the others, you can do math all day long trying to plug in all the conditions for the days flight or just get an engine monitor with EGT/CHT/FUEL FLOW and have immediate information on you fuel burn and Cylinder temps.
DENNY
 
When one is Lean of Peak EGT (something FI does better than a carb) then Fuel Flow x 14.9 = horsepower. (not % hp.....just hp) So 10 gph is 149 hp. This multiplier applies to a Lycoming with 8.5:1 compression ratio. If your CR is different, then the multiplier will change. High CR are more efficient. This does not work when rich of peak.
 
I think it boils down to if you want the most mpg then you run the least RPMs that you can (speed and getting the oil temp up to 180 degrees will both suffer with low RPMs) and lean appropriately and maybe even put in a electronic mag to help with the fuel burn (advances the timing at lower RPM and or altitude) . The coefficient of drag on a super cub has to be high, compared to say a Mooney, and therefore more RPMs burns more gas, but doesn't correspond with the same increased percentage of speed in relation to the increased percentage of more fuel burn. The fuel burn gauges sound like a good idea too for sure.
 
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