RE: prop spinning too fast?
FF, the individual speeds have to be in the same direction for them to be purely additive [on an equal basis]. In the example of a turning prop on a moving plane, the speeds are added, but trigonometry is involved and you need to do vector math to figure out how much the forward speed of the plane contributes to the turning speed of the prop blades. I would never guess that it ends up being as much as the example given by Mike C. Math is such a useful and exact science, I wish I could make better use of it.