Cracked it!

In case anyone’s interested – I should have saved this for the book, but I am very chuffed. That mystery third number, the ‘design number’, in DH blades.. I know what it means!

Without going into the arcane background (I have been working from AERADE and NACA reports, translating blade solidities in chords, plotting curves and looking for patterns), the simple answer is that there were only 5 basic designs of DH controllable-pitch blade going into the war, and they were scaled up and down according to their shank size.

The designs were defined by the position along the blade length of the broadest chord. The ‘base model’ for any given shank size (thousand series) had the fattest part at 50%. This would be in a case where the design number matches the shank size.

If the design number was different from the shank size, it takes the position of the widest point from the corresponding shank size ‘base model’. If the design number was higher than the shank size, it was at a point further outboard than 50%, if design number was less then it would be inboard.

This is of course then modified by the last digit, the ‘cropping number’, which trims a defined number of inches off of total diameter.

Thus:

[ATTACH=CONFIG]242171[/ATTACH]

I have prepared a little table – this might help with identification of ‘found’ blades where the drawing number has gone.

[ATTACH=CONFIG]242172[/ATTACH]

In the example above, the Spit has a number 55409. So shank is size 5, design number is 4. It does in fact have a widest point at what WOULD be 50% for a 4,000 shank base design… from the table, that’s at 46.7% along the un-cropped 5,000 series blade (from the centre of the boss). The reason why it still appears outboard of 50% is because the blade has been cropped 4.5 inches.

The Roc (55262) has a widest point at what would be 50% for a 2,000 series base design blade.. 40.6% along this one (again, from centre) and has had only 1 inch cropped from each blade.

And to help get round the problem of ‘cropping’ obscuring the blade type, here’s a handy guide to maximum widths, which give you the shank size as a starter for 10:

1,000 series 7.35″
2,000 series 7.86″
3,000 series 8.41″
4,000 series 9.0″
5,000 series 9.6″

This is still a work in progress, and I would greatly appreciate any ‘real world’ data that anyone might have out there to confirm or confound this theory!

This does not apply to Hydromatic types, and I don’t think some of the shank sizes or blade design numbers appeared in real life?

David, you have a PM.

Matt