Local Canard Groups > San Diego EZ Squadron

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

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Drew:
For the equations that don't have fuel flow, I am not sure how you get around not using MP (along with RPM)----unless this equation takes your specific fixed pitch propeller into account.  But hey---I don't know how to do it!

Anonymous:
Drew

You are correct.  The formula is for a fixed pitch propellor.  However, it is not for MY fixed pitch propellor.  I don't know what the error is.  I'm hoping to get a power curve for my prop from the manufacturer and re-scale the formula.  Manifold pressure doesn't give the whole story of airflow, you need to know the air density as well as the volumetric efficiency and the temperature of the air.  I would like to find someone that knows engine calculations really well to help me find a better formula.

Drew:
See this site:
http://go.phpwebhosting.com/~khorton/rv8/

On the left side of the page you can download an excel power chart for an O360.  This may or not be some value.  I used it to make a paper chart that was easy to use in the plane.

swinn:
Drew,

I found a new formula to use that seems to be a lot more accurate.  I found it in Kitplanes, Dec 06, article called 'Power Principle'

The formula is this:

HP = SLHP * (MAP / RATEDMAP) * (RPM / RATEDRPM)

So for my engine which is 118 HP @29", 2800 RPM the formula is:

HP = 118 * (MAP / 29) * (RPM / 2800)

I plug in MAP and RPM and get HP.  This works only for Rich of Peak.  

The other formula for Lean of Peak operations, HP = FuelFlow * 14.9 is also discussed in this article.  Evidently that formula is good for engines with 8.5:1 compression, and it varies with compression.  For a 7.5:1 engine, it would be HP = FuelFlow * 13.7

The article discusses the principles behind these formulas.  Of course they are approximations, but are pretty close.

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