Calculating Horsepower, % power

[flyboy1963]

thanks Dave,
I tried plugging my numbers into your equation; after about 10 tries of getting 2% horsepower, I pasted it into an online calculator....
..presto! I gained 63% power!!

...and this, after only 2 years of college, math, calculus, beer, physics, beer, statics, beer....hmmmm, seeing a trend here eh?

[BobTurner]

Dave,

Can you point me toward a derivation of the ROP formula? I'm curious how it came about, and the approximations involved (e.g., we know that power falls a bit (maybe 4%) leaning from best power mixture to peak EGT, yet the ROP formula doesn't reproduce that.

Bob

[Kevin Horton]

Originally Posted by brad walton

I have an old Lycoming paper I got several years ago, can't recall where from. It permits determining HP from fuel flow. If I recall correctly, for fixed pitch prop fuel flow at peak exhaust temperature is 85% of max HP at the same RPM and manifold pressure. You then determine fuel flow at max HP and reference a compression ratio table to determine pounds of fuel per horsepower per hour. From these numbers, HP can be calculated. For example, 8 GPH X 6 pounds per gallon divided by 0.4 pounds per HP per hour equals 120 HP. 120 divided by 180 equals 66.66% HP. There is also a method in the paper for constant speed props. I would be happy to fax or mail a copy. The copy quality is poor.

I have what must be the same paper. I posted that Lycoming power from fuel flow document as a PDF on my site.

I also created an OpenOffice (or NeoOffice if you use OS X) spreadsheet that does the calculation described in the document. No, I don’t have an Excel version, as I would have to convert the calculations from OpenOffice Basic to whatever Excel uses today. I don’t own a copy of Excel, so I’ve never been motivated to do that.

If you prefer working with python, the lycoming_power module in my AeroCalc module also does this calculation.

[rzbill]

Originally Posted by Kevin Horton

I have what must be the same paper. I posted that Lycoming power from fuel flow document as a PDF on my site.

Kevin,
I got the pdf to download but it took some magic. There are boogers pasted on the end of the URL you entered in the link above. Better edit.

Thanks for the document.

[Kevin Horton]

Originally Posted by rzbill

Kevin,
I got the pdf to download but it took some magic. There are boogers pasted on the end of the URL you entered in the link above. Better edit.

Fixed it, I think. Thanks for pointing it out.

[David Paule]

Originally Posted by Kevin Horton

This formula suggests that the O-320 B and D series will produce 160 hp at 2600 rpm. Is this correct? I would have expected 160 hp at 2700 rpm, and a bit less at 2600 rpm.

I don't have the power chart for these engines at hand, so I can't check myself.

I looked at a recent edition of the Operator's Manual and in Fig. 3-5 for the O-320-B and D, it shows 160 hp at about 2,710 rpm. The specs page says 160 hp at 2,700 rpm.

Dave

[brad walton]

Yes i think i may have pulled the document from your website. I found it very interesting and used it as my primary method of determining where my various percent power settings are. It seems to correlate well with expected results.

[flyinga]

This is all interesting, but please tell me what the "real world' usefulness of percent power is.

[tommylewis]

I use the percent power to repeat past performance. I know what percent power we usually run at, so I go to that setting. I know it is not exactly accurate, but I do not really care, just that it is the same power level I used last time I ran at that percent power.

[BobTurner]

Originally Posted by flyinga

This is all interesting, but please tell me what the "real world' usefulness of percent power is.

I'd say the most important use is if you follow Lycoming's advice to not lean agressively above 75%.