"Blanche" wrote in message ...
Doug wrote:
Rule of thumb. So if you are burning 10 gallons an hour in a 180 hp
engine, you are developing (10x12) = 120 hp. 120/180 = .67, so that is
67% power. It's not exact but gets you close.

How does altitude get involved? According to the chart in the Lycoming
Operator's Manual, full throttle HP at Altitude there's roughly a
3% loss of HP for every 1000 ft. above sea level.


Assuming proper leaning and unchanging RPM,
falling manifold pressure decreases fuel flow.
Then, Doug's relationship remains independent of altitude.
It's derived from a simple statement of avgas' energy content.
It just assumes the engine converts chemical energy into go-power.
As Doug mentioned it's approximate, but useful.