Some pilots have demonstrated a lack of understanding with regard to the operation of the direct drive, fixed pitch, normally aspirated powerplants. When a power chart is provided, it will indicate that as the airplane is flown at different altitudes above sea level, it is necessary to use a higher RPM for adequate cruise performance with an increase in altitude. A typical example might be the O-360, 180 HP Lycoming powerplant. The power chart by the airframe manufacturer for this fixed landing gear aircraft lists 75% power at 7500 feet at 2675 RPM (no manifold pressure gage in their airplane). The pilot who does not understand the principles of operation in the thin air at altitude may observe that red line takeoff RPM is 2700 RPM, and is then reluctant to lean either for cruise or climb despite the altitude because he is pulling almost the same RPM as at takeoff.
However, he can and should lean the engine at these altitudes despite the high RPM, for the horsepower is down to 75% because of the thinner air. On the other hand, with any direct drive normally aspirated Lycoming engine, he can and should lean the mixture at any altitude as long as he is in cruise configuration at 75% power or less.
Let’s take a look at the airframe manufacturer’s power chart for the O-360, 180 HP engine and observe the gradual increase in RPM required with the increase in altitude, but maintaining 75% for cruise at each altitude. What the chart will not show here is that for flight above 7500 feet, it is not possible to achieve 75% power with a normally aspirated engine (meaning not turbocharged or supercharged).