I believe he is correlating the engine rpms to the actual prop rpms, as in a direct drive. Relatively speaking, the engine rpms are irrelevant, he is comparing the prop efficiency at various rpms.Dan:
What you're saying is equivalent to saying that somewhat lower rpm engines won't work well in aircraft. I don't believe it.
You have propeller mechanics working against you. A fixed pitch props, fans, even centrifugal pumps all behave in the following way:P.S. I'm only talking about relatively small differences in gearing. Like maybe 2:1 for cruise and 2.5:1 for climb. Assuming an automotive engine with the CVT being used as a redrive.
I was simply trying to get through the basics of why even modest gear ratio changes do not work well, and why it is just not done in airplanes nor in boats. Get into the reductions to the inflow velocity with decreased speed when the expressed power is reduced and yeah, things get even worse.Good point, wsimpso1, when you say "So let's slow down (....) 88% power is available". However I would make a slight correction: when RPM is reduced, your calculation is absolutely correct, since power= CP x ro x N^3 x D^5, therefore power is in fact directly proportional to cube of rotation speed. However when you reduce RPM, speed will drop a little, and so does the advance ratio V/ND, which causes a slight increase in CP, so a little more power is required to maintain the new RPM. I say "a little" because it is in fact is a very small change. Ref: NACA Report 640
lr27 is proposing that if we progress through the same cases starting from the opposite direction, the results will be better. Think about that... Can that work?I don't think it's advantageous to select a climb prop for use with a CVT unless the rpm anticipated is low enough to keep the tip speed down. Better to use a cruise prop, and then use the CVT to low the full power of the,engine to be used during the climb. I imagine that it doesn't make sense to use a CVT unless you are using an automotive conversion or other high rpm engine. Then the question is how much more the CVT weighs than an alternative PSRU. It's probably a bad idea to take out the control electronics from the car instead of controlling directly.
Anyway, in climb the advantage of using a CVT is the same as having a somewhat more powerful engine spinning the same prop.