by loucrane » Fri Oct 25, 2013 12:53 am
Hello again, Bob!
I expect you still have the mfr instructions for those HP 20's? They should include prop size recommendations for 2-bladers. For starters, try a 3-blader which is "down one" in either pitch or diameter. See how the engines like that one. E.g., if HP recommends a 9-5 2-blader, try a 9-4 3-blader, or possibly an 8-5.
Extraneous note: Nominal pitch - that marked on the prop by mfr - may or may not be what you actually get. There are, or were, pitch guages available.
Some prop brands were notorious (or famous, your choice) for measured pitch being lower than their rated pitch. The weakness in measuring prop pitch: We usually measure to the most often flat rear face of the prop, because we can work with reasonable confidence of accuracy.
Propellor blades are airfoils. Flat-bottom airfoils still "lift" when moving through the air parallel to the flat face. Measuring prop pitch there doesn't measure the working pitch, which is a bit higher than the flat-face measurement. How much? Varies with the prop, the brand, the model, the load and the style of prop (e.g., APC sells different series of props at the same nominal pitch and diameter specs, and the different series may not measure the same for the same numbers.)
So, all we'd get by measuring on a pitch guage is a reference number, AND info about whether both (or all 3) blades have equal measured pitch. That IS a degree of confidence, and worth it.