David O wrote in message . ..
(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.

Right you are. The 92 HP figure is taken from cruising charts, less
than full throttle. My mistake in assuming that the 2300 RPM would
have a consistent HP.
We once did some physics calcs regarding the acceleration to
takeoff speed for the 172. We found that the energy to accelerate that
mass to that speed came to 24 HP, demonstrating the enormous losses to
prop and airframe drag and wheel rolling friction. Wouldn't it be
great if we could reduce those to a fraction of what they are and make
truly efficient flying machines?

Dan