howardnmn wrote:seems to me, with fixed pitch % power is just a direct function of max rpm.
I wish it were that simple! In fact, for either carbureted or fuel injected ingines, % power is a function of air passing through the carb or injection airbox. You can see that RPM doesn't tell the whole story, if you consider that a given RPM at one altitude will produce a very different power level from the same RPM at a different altitude (considering that air density varies with altitude). Plus, you can point the plane downhill and throttle back, producing a very high RPM at almost no power. Thus, rather than RPM, the best direct indicator of power is intake manifold vacuum (usually, erroneously called manifold
pressure). But even that is not a simple linear relationship.
Aircraft power is actually a function of three factors: engine power (a function of airflow for a proper fuel/air mixture), engine torque (a function of engine RPM), and thrust (a function of propeller RPM). The engine RPM that produces optimum torque is generally quite high. The propellr RPM that optimizes thrust is
significantly lower, because of drag effects. This is why Rotax, for example, uses a prop gearbox - so both torque and thrust can be optimized simultaneously. As for engine power, since it's related to air flow (and the corresponding fuel flow to achieve efficient mixture), turbocharged engines obviously produce more constant power than the normally aspirated ones most of us fly.
Bottom line: "75% power" is actually a meaningless designation. But, to see a typical breakdown of performance at various power levels for a typical LSA, see the table at
http://avsport.org/acft/performance.pdf.
pjcampbell wrote:if max is 5800, i thought people cruise around 5200-5300?
Yes, and according to the above-referenced chart, that's midway between what I call "normal cruise" and "max cruise" for a Rotax 912ULS-powered LSA. Note that Rotax specs the engine at 5500 RPM max
continuous, and 5800 RPM for up to 5 minutes maximum.