With a fixed pitch prop, is it necessary to know anything other than RPM to ascertain what percentage of the engine?s rated horsepower is being produced? That is, would manifold pressure, altitude, mixture, etc., all be ?accounted for? by the RPM actually being produced?
More specifically, could a person flying behind an O-360, with a fixed pitch prop, test fly the aircraft in the same cruise configuration, at different RPMs (which would mean different manifold pressures), but at a single altitude and mixture setting, record the data necessary to calculate horsepower using the Lycoming power charts, correlate the RPMs from the raw data to the percentage of power calculated from the charts, label the steam gauge tachometer with those percentages, and have a quick and accurate reference for percentage power even when flying at altitudes and mixtures different from those used during the test flight?
My gut tells me that: 1) ROP vs LOP would matter with a constant speed prop because, as power varied with mixture, the governor would vary the blades? pitch accordingly to maintain the same RPMs; but, 2) blade pitch obviously won?t vary like that with a fixed pitch prop. If a fixed pitch prop has more power delivered to it, then it spins faster and vice versa with less power. Right?
My gut also tells me that altitude would matter with a constant speed prop because, as power varied because of the varying air charge density and amount of exhaust back pressure, the governor would again vary the blades? pitch accordingly to maintain the same RPMs. But, again, the blade pitch won?t vary like that with a fixed pitch prop. Again, if a fixed pitch prop has more power delivered to it, then it spins faster and vice versa with less power. Right?
Stick and Rudder suggests that the reason a normally aspirated engine, driving a fixed pitch prop, loses RPM as altitude increases is that the friction resulting from a given RPM is constant despite the altitude increase. In other words, there is a certain friction loss at every RPM regardless of the altitude at which those RPMs are generated. It?s just that, at progressively higher altitudes, the HP required to overcome the internal friction of the engine is a progressively higher percentage of the total power available. So, does that mean a fixed pitch prop being operated at high altitude would slightly underreport, via RPMs, the percentage of rated power the engine was actually generating as compared to what that engine would report, via RPMs, at a lower altitude, because a greater proportion of the power would be needed just to overcome friction? Or, am I over-thinking this aspect?
So, in the scenario that I?ve described, does % power = RPM? If not, what am I leaving out?
More specifically, could a person flying behind an O-360, with a fixed pitch prop, test fly the aircraft in the same cruise configuration, at different RPMs (which would mean different manifold pressures), but at a single altitude and mixture setting, record the data necessary to calculate horsepower using the Lycoming power charts, correlate the RPMs from the raw data to the percentage of power calculated from the charts, label the steam gauge tachometer with those percentages, and have a quick and accurate reference for percentage power even when flying at altitudes and mixtures different from those used during the test flight?
My gut tells me that: 1) ROP vs LOP would matter with a constant speed prop because, as power varied with mixture, the governor would vary the blades? pitch accordingly to maintain the same RPMs; but, 2) blade pitch obviously won?t vary like that with a fixed pitch prop. If a fixed pitch prop has more power delivered to it, then it spins faster and vice versa with less power. Right?
My gut also tells me that altitude would matter with a constant speed prop because, as power varied because of the varying air charge density and amount of exhaust back pressure, the governor would again vary the blades? pitch accordingly to maintain the same RPMs. But, again, the blade pitch won?t vary like that with a fixed pitch prop. Again, if a fixed pitch prop has more power delivered to it, then it spins faster and vice versa with less power. Right?
Stick and Rudder suggests that the reason a normally aspirated engine, driving a fixed pitch prop, loses RPM as altitude increases is that the friction resulting from a given RPM is constant despite the altitude increase. In other words, there is a certain friction loss at every RPM regardless of the altitude at which those RPMs are generated. It?s just that, at progressively higher altitudes, the HP required to overcome the internal friction of the engine is a progressively higher percentage of the total power available. So, does that mean a fixed pitch prop being operated at high altitude would slightly underreport, via RPMs, the percentage of rated power the engine was actually generating as compared to what that engine would report, via RPMs, at a lower altitude, because a greater proportion of the power would be needed just to overcome friction? Or, am I over-thinking this aspect?
So, in the scenario that I?ve described, does % power = RPM? If not, what am I leaving out?
