Local Canard Groups > San Diego EZ Squadron

Third Thursday at Casa Machado 5/17 at 6:30 PM

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swinn:
The monthly third thursday EZ get together at Casa Machado (KMYF) at
6:30 PM will be show and tell night.  Bring photos or parts of your
projects and show everyone what you've been up to.  I'll bring the PDA
based engine monitor software I've written and am currently flying
with.  Bill Ingram has been running the show down here, but he asked
me to send out some notices about the meeting this month since he will
be out of town.  Hope to see you there.  With the long days of summer
and nice cool evenings, MYF would be nice evening flying destination
in your EZ!

gilbertexp:
Could you tell us a little more about the engine monitor you're using?  I currently fly with Co Pilot and Flight Master for my Palm.  They're both excellent programs.  Co Pilot does the flight planning and weight and balance and Flight Master does the color moving map with an attached GPS antenna.  Is this something you wrote yourself or a commercial product?

Thanks,

John

swinn:
The engine monitor runs on my PDA.  I wrote the software myself.  It uses an RS232 data feed from the Grand Rapids EIS 4000 which I have mounted in the back seat, and all the probes run to the EIS.  My software does graphic display, some analysis functions (like leaning, % power, etc...) and data logging to SD cards.  I can display the logged data in Excel or other spreadsheet/charting app.

Drew:
swinn,
Can you post your percent power equation?

Anonymous:
This was a difficult one for me.  I looked at the existing engine monitors that show percent power and realized they are only accurate when you are leaned for best power.  They don't take mixture setting into account at all.  I wanted something a bit more intelligent that this, but couldn't find a way to perform a full calculation with the information I had. (I would need the Air/Fuel ratio) which would mean I'd need a mass flow sensor.

I was looking through the Lycoming Operators Manual for my engine and found that they have two charts.  One is a 'Power and Fuel Consumption' that is a power/RPM chart for full rich operations.  The curve is really flat, so I modeled it as a linear function.

The equation is simply that of a line

I took two points off of the chart and found the slope:
(y-y0)/(x-x0) = m

Then I subbed in one of my points and solved for y.  The software subs in x and calculates y.  For O-235 L2C standard pistons, that comes out to:

HP=((0.055 * RPM) - 36)

This gives me the HP the engine is making based on RPM and assuming a full rich mixture.  It is accurate at 2200 RPM, and 2800 RPM, but off by 3-5% in the middle since I simplified the chart to a linear function.

I wanted to also see the power when fully leaned.  There is no chart for this in the Lycoming manuals, so I use a fuel flow based formula that I got from one of John Deakins engine articles:

HP = FuelFlow * 14.9

I don't know how accurate it is, but also don't know of a better way.

The Lycoming charts also show a 'best power mixture' chart.  There are several linear functions to integrate and pressure altitude is required for the calculation.  I don't have pressure altitude data available yet so I haven't done the math to integrate the chart into a usable function.

So to sum it up, I've got two calculations.  The % Power Full Rich shows up on my engine startup/climb page and the % Power Full lean shows up on the cruise page of my engine analyzer software.

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