The problem can be further divided into two distinct issues: the power settings themselves and how to get into one. If we think of power settings alone, there should be little reasons to afraid of going into high MP, low RPM regime just as an example. But, if I shot a question to Average Joe, asking how low RPM is too low against a given manifold pressure, and on what basis does he feel okay down to that specific combo, it is not nearly every time that I get a coherent answer at all. If we simply told everyone to...
...does that mean the lever full-out? Or some specific charted RPM? If charted, in what source? What if the governor does not have a defined low RPM stop, or if it is excessively low?SaxTeacher wrote:run with the lowest possible RPM in cruise
Yes, one can usually build up a decent picture of what is reasonable by looking into engine manufacturer's data and combining it into other sources, but there are lots of flyers who never go beyond POHs. Further, even if such flyers start digging into engine charts, do they know to take into account the other sources of information, such as Continental's CSB09-11A, and evaluate them accordingly?
Another part of the problem are the dynamics. I wouldn't consider it an issue at all to make slight power changes with RPM first. But can we guarantee that these will always be done with due caution and with appropriate smoothness? Certain installations do have rather abrupt governor plus propeller behavior when making larger adjustments. When slowing down an engine by decreasing the prop RPM, we intentionally exceed the torque provided by the engine. Done smoothly, that's absolutely fine, but done abruptly...maybe not. Moment of inertia of an aircraft engine is fairly large, and I am not all convinced about the dynamic behavior of the classic GA pistons.
Old wives' tales on operation of GA piston engines are probably partly there to live by the most ignorant, so to say. Almost none seem to have any absolute wisdom behind them at all (think about 'squared' power settings), but sometimes provide means to ham-fisted operators to not need to know the limiting conditions and stay relatively, well, harmless, if not nearly optimal.
-Esa