It's really not that hard. The engine has a torque/rpm curve. The prop has an rpm/torque curve. You overlay them, the gap between them is the excess torque available. The prop torque over the engine torque at any point is the percentage load. Where the curves cross is the max rpm achievable. You just need to decide what constitutes overload. The 80% assumption is just a guess, and there is no one right answer.
Determining the fuel economy is much more complicated, requires the fuel map, and can probably really only be determined with tests.
|