Deciding to use a fixed-pitch propeller or step up to the higher weight, cost, and complexity of a constant-speed prop in order to improve performance.
Amy Laboda has watched her first homebuilt (Kitfox) aircraft and her second daughter come of age in a way that illustrates some parallels between the two.
There's a large body of knowledge of good engineering practice for aircraft. Most of this knowledge was gained the hard way, and it should not be ignored lightly.