walkman

walkman

Well Known Member
I'm building a little spreadsheet so that I can track performance over time. I want to enter observations such as indicated altitude, airspeed, OAT, rpm and MAP, fuel burn etc and have it calculate density alt, true airspeed, % power etc.

This will help for some planning, but mostly to identify trends over time, potentially catch problems, and will help quantify any performance changes due to any changes I make.

I've got all the calcs except for % power. I have a power setting table but I'm a bit stuck on how I would use that in Excel. I'd really like to find a formula for calculating % power based on density altitude, rpm, and MAP. Any ideas?

This is for an O-360.
 
Thanks. The Kevins sheet for some reason throws all sorts of errors and doesn't work for me.

I think what I'll do is create a separate curve for each RPM in {2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700} for pressure alt vs MP. then use curve fitting to approximate the formula for each.
 
walkman said:
Thanks. The Kevins sheet for some reason throws all sorts of errors and doesn't work for me.

I think what I'll do is create a separate curve for each RPM in {2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700} for pressure alt vs MP. then use curve fitting to approximate the formula for each.
Click to expand...

Search Excel help for "look up tables"...
 
found an older version that works. go figure. I'll utilize that. thanks for the pointer.
 
MauiLvrs said:
(RPM/max RPM) * (MP/Atm P) * Rated HP = HP


(HP / Rated HP) * 100 = % power
Click to expand...

Well that's giving my wildly different results than either the lycoming power setting tables or Kevin's spreadsheet.

Example. 9,000' msl indicated, 30.18 in the window, gives me an absolute atmospheric pressure of 21.6". Running at 2,400 rpm and 21.9 map. Your calculation gives me 90% power whereas the others give me 75%.

Am I missing something?
 
walkman said:
Example. 9,000' msl indicated, 30.18 in the window, gives me an absolute atmospheric pressure of 21.6". Running at 2,400 rpm and 21.9 map. Your calculation gives me 90% power whereas the others give me 75%.

Am I missing something?
Click to expand...
atmospheric pressure not corrected for altitude.
Try 30.18
 
I'm intrigued by this discussion as I've had the same question bouncing around in my head for quite some time. To make sure I understand it, I applied Walkman's numbers with the following results:

(RPM/max RPM) * (MP/Atm P) * Rated HP = HP

(2400/2700) * (21.9/30.18) * 180 = HP (I'm assuming 180 rated HP)

0.88 * 0.726 * 180 = HP

115 = HP

(HP / Rated HP) * 100 = % power

(115 / 180) * 100 = % power

0.638 * 100 = % power

63.8% power (Final answer)

Does all this look correct? Does this result track with the power tables in the Lycoming manuals?

Thanks for the useful information! It'll help me with my O200 that has some pretty fuzzy graphs in the manual.
 
The lycoming power setting table gives about 75% for that setting/altitude. which is also what Kevin's spreadsheet returns. I have not looked at the sheet yet to see what it is actually calculating.
 
Kevin's spreadsheet computes the power according to the Lycoming published power sheets, interpolating between a series of data points taken from the plots.

I believe the variance you are seeing is primarily the effect of the reduced back pressure in the exhaust at lower air pressures, this effect becomes quite pronounced at higher altitude.

When you appreciate the complexity of this you also appreciate the amount of effort that Lycoming put in to characterize these engines in the early days.
 
You must log in or register to reply here.