Texas, weather, props, etc.
Chuck,
Many years ago I drove cross-country through Texas and never forgot how different that state was from one end to the other. It's not like driving across New Jersey, more like driving from one part of the country to another. I know it can get dang cold down there, one of the worst ice storms I have ever seen was in Dallas.
Regarding those props, I learned that you can turn all the rpm you want but unless you have sufficient thrust for a particular airframe you aren't going anywhere fast. If rpm means everything how come pylon racers don't run 4 inch props turning 40,000 rpm? Not enough thrust. They learned from trial and error that the props they use are optimum for the airframe.
Clearly the 7.4 inch prop is too small for a Diamond Dust. The speeds I achieved did not even reach the 85% efficiency level of Mark Mallory's formula, in fact I fell well short of that.
I played around with Andy Lennon's thrust formula and it turned out a 9X10 at 16,400 rpm created 50% more thrust than a 7.4 X 8.25 turning 21,400. There has to be a way to tie those thrust and speed formulas together but I don't have the math skills to do it.
How does a P-51 Mustang Reno racer go 400 mph? An enormous variable pitch prop and the torque to turn it! I wonder how many inches of pitch that thing is set at for high speed, 30, 40, 50 inches?
The problem we have with model airplane engines is that they are rpm, not torque machines. We have to work at lower pitch ranges whether we like it or not. The trick is finding that "sweet spot" of diameter vs. pitch for the particular airframe we are using.