More DH bracket propeller info/calculations

Further to my last, when I started to get an inkling of some pattern to the DH blade numbering system, below is my stab at what it all means.

The table relates only to DH bracket-type blades, not Hydromatic equivalents or otherwise. It gets a little hazy around the smaller end of things (1, 2 and 3 series) and also the 6 series. But the middle bit, I am now confident of. I plotted some known examples without cheating (ie not looking at photos for reference, just using the numbers) and when I did check on the results I was pleasantly surprised! I’m including them here too.

The key is where the widest part of the blade lies, as measured from the tip of the nominal design in each case. Of course the last digit alters where the real-life tip is. That’s why it’s been hard to work out backwards!

So to recap – 1st digit, prop type (5=alloy CP), 2nd digit = shank size, 3rd digit = position of broadest chord relative to tip, 4th digit = rotation PLUS MODIFIER TO INDICATE ROOT EXTENSION, and 5th digit = reduction in diameter by ‘shaving’ tip.

To explain the 4th digit – the basic system is 0 for RH and 5 for LH. But to indicate that the blade has been ‘stretched’ inboard of the widest point by multiples of 18 inches, another ‘1’ is added, so the stretched RH’s can be 1 or 2, and the stretched LH’s can be 6 or 7.

I would be very happy for someone with a real blade to measure theirs and tell me this is wrong – but not as happy as I would be if they told me I was right!

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