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It’s the basis of the data in M&S, which I don’t have in front of me. That might be referenced?

Just to get it ‘out there’, my workings so far on that drawing number.

For each ‘thousand series’ there are a number of ‘hundred series’ designs each six inches longer than the last. So a 54300 is 10ft 3in, a 54400 is 10ft 9in. A 55400 is 11ft 6 in, a 55700 is 13ft.

Then when you ‘trim’ the design by an amount of inches shown by the last digit you get the real world prop. Thus Whirlwind 10ft with a 54409, Spitfire 10ft 9in with a 55409, Battle 12ft 6in with a 55706. Blenheim left-hander 54350, 10ft 3in.

But what of twist? Well, if you give the same total twist to the blade you can then get whatever pitch distribution you want for your design by changing the basic diameter and ‘trimmed’ amount.

The faster you want your aircraft’s optimum efficiency relative to rotational speed, the more you go down the ‘longer pattern with more trimmed off’ route, to get less twist. Hence WW 10’9 basic but with 9 inches off, Blenheim 10’3 basic but with nothing off. The Blenheim will have more twist, as appropriate to a slower aircraft with roughly the same rotational speed.

Thickness/chord can also be standard for each drawing, and vary at any given station with choice of drawing and trimmed length. Note that the ‘oversize then trim’ technique for faster aircraft will tend towards thicker sections.

This is a very tentative suggestion, based on what examples with known drawing numbers and diameters fit into mathematically, nothing else.

But Rotol metal blade numbering remains a complete mystery. There must be someone out there who knows?

The thing about blades is that if you want to turn them at a constant rpm in the face of compressibility your major variables affecting horsepower taken up by doing this become altitude (and thus speed of sound) and blade thickness. More of one inevitably means less of the other. How much less depends on the blade profile, and the RAF profile was demonstrably poor in this regard.

While I agree that all engines will run out of power at some height, the point we have hit upon us that the engine has to drive the props, and if for any reason one set of props drags more than another and increasingly so with altitude the engine will run out of power lower down.

With the Whirlwind the production aircraft struggled to reach the altitude the tested prototype did. The only real difference was the props.

It turns out the Spitfire had altitude related performance variations when the same aircraft was tested with different props as well.

To put it another way, stick 9.6% thick RAF section blades on a Spitfire and it would suffer too.

Or to put it yet another way, how good your prop is, how much extra power is absorbed by just turning it, determines when you ‘run out of power’

Yes. It appears lot of inconsistent performance data of the period might be explained in this way.