Constant Speed Prop vs Variable Engine Timing

(Dan Thomas) wrote:

Horsepower is a function of torque multiplied by RPM. A Lycoming
engine in an older Cessna 172, for example, produces 150 HP at 2700
RPM under standard conditions (sea level atmospheric pressure and
59°F). In the takeoff roll with the fixed-pitch prop, RPM will be
around 2300 RPM, which, according to the POH, would indicate a
horsepower output of about 61% of 150, or about 92 horses. Not very
good, is it?

snip

Dan,

Those numbers can not be correct. The power curves in the Lycoming
operator's manual show that in standard sea level conditions at 2,300
RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or
88% of full rated power. Interestingly, your 92 hp figure closely
matches the propeller load curve at 2,300 RPM. The propeller load
curve, however, is not a full throttle curve. Rather, it is a
variable throttle static run-up curve using a fixed pitch test prop
(or club) chosen to achieve max rated engine RPM at full throttle. If
your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300
RPM full throttle in standard sea level conditions, then it is wrong
by a wide margin.

While I'm here, I'd like commend you on your typically spot-on
explanations and your generosity in frequently answering questions
here. Unfortunately, business and other matters keep me from
participating here as much as I'd like. It is folks like you who make
the difference here, not the... (well, I'll let that go). I Hope you
stick around for a long time.

David O -- http://www.AirplaneZone.com