Hi Joe

I think all the numbers you are getting are about right. Your engine seems to be giving a fairly healthy power output.

The discrepancies in the numbers are due to difference in prop design which means that a 20x12 of one design will often spin at a different speed to a 20x12 of another design.

In particular APC props are thinner and tend to spin faster than other props for a given power. The 20x12 APC in particular has very thin blades and spins a lot faster than a normal profile prop. This probably accounts for the main discrepancy. If you put on a standard 20x12 2 blade you would expect to be down in the mid 4000s.

The standard prop power calc we use indicates that on the SP for a given design of prop (i.e. same blade shape, thickness etc) if you went from a 2 to a 3 blade prop at the same diameter you would expect to loose about 1100RPM.

Cutting the prop diameter by 1/2inch would gain you about 200RPM. So if you had managed to find an identical form 17.5x12 3 blade Master airscrew you would expect to see around 4600. The fact that you are 200RPM lower than this can only be due to differences in the detail design of the prop, and is well within the sort of variations we typically see.

The weight (i.e. mass) of the prop will not affect the max RPM, merely the spool up speed which should generally be pretty quick anyhow.

Its actually a hot topic this one! We are starting to think that using larger props on the SP range and loading them down to the mid 4000s might be a good way to run them, as long as you have enough pitch in hand for your forward speed. The following thread covers a similar topice:-


http://www.rcuniverse.com/forum/fb.asp?m=8635635


Anyhow hope this helps

Regards

Keith