Ah, Ralph’s the real Tarzan. Mine’s an inflatable fancy dress Tarzan suit. I am more like a duck – looks good on the surface but underneath little legs going like the clappers to make any progress at all.

I like the butter idea. Different blade twists cut the block different ways.. put the butter on some kind of low-friction bed, anchor the blade on a pivot beside the butter representing a hub, move the blade down and the more twisted one moves the butter more slowly as it cuts.

The engine cooling thing is interesting, but if there was some thrust intended for the cylinders then there would be little reason to not use it for propulsion if possible on an in-line engine. It looks to have been the other way around, in that there are quite narrow shanks to the Hamilton blades until they get outside the radius of the cowling of an air-cooled radial, while Rotol, who were designing for the UK market from the outset, had plenty of ‘proper’ aerofoil right up to the spinner.

When DH went on to design their own blades, and stated doing ‘paddles’ etc, then they also added some extra chord at the root end. Funnily enough this first happened when moving the de Havilland Mosquito on from prototype to production. There weren’t really the conversations one imagines happened between the aircraft designer and prop designer if not in-house – it seems on the evidence that the main conversation was between the procurement office and the prop salesman.

I have to hold up my hand and admit that these later DH-designed blades don’t (or shouldn’t) follow on in my blade-hacking-about series – I am really only looking at the early ‘needle’ blades.

On the other thing, by the way, I have asked around and all we have is the Peregrine AP. I can get that to you – it has some nice schematics – or just read it for what it is you needed. Was it con rod material?

Want to see something that turns it all upside down?

From NACA full-size tests on our very own 5868, report 658, published in April 1938:

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So much for theory. Looks like you need that twist to be ‘wrong’ for best results..

..and I could hazard an educated guess as to why.

What sort of speed would these blades be better for? By which I mean at the ‘point-of-focus’ as it were, minus 5 degrees, at 3,000 rpm x 0.477, what does the maths produce?

I wonder whether in fact I have it wrong, and making fine pitch more efficient was all about power delivery in take off and climb, prioritised over maximum speed. One could argue ‘who needs acceleration when you are already fast enough.. and who doesn’t need it at 150 mph in combat or when taking off’?

Having said that, I have just done a rough calculation and it looks like max efficiency of the 5868 would be at 50 mph and off the end of the fine pitch range, so I will need to revisit the DH data more carefully. Spitfire checks out at 150 mph and near enough right on the fine pitch stop.

A potential problem, but not so critical in the climb at around 200 mph TAS, so not the altitude issue. Unless the problem occurred at the ‘bottom end’ of the range.

Ah, that twist was done in the way some US charts did it, measuring twist from an arbitrary ‘zero’ at r/R=0.75, rather than one at 0.7. You can just shift the axis to put zero at 0.7, the curve is right. Sorry, I should have standardised on 0.7.

Word of warning, that twist is not to be taken as gospel along its whole length in that it comes from the 5868, the apparent pattern blade for the 6105, so two ‘generations’ removed from the 54409 (though sharing a t/c curve with that measured on one), backed up by a tiny fraction of the curve showing it’s pitch around 0.7 visible in a DH document. You will understand that we didn’t try measuring twist – extremely difficult to do on a blade in the first place – on a blade that had suffered an impact.

That’s interesting about the crossing out. Something going on there, definitely. Its like DH decided ‘on the fly’ not to twist with telescoping after all.

Working a few examples I have realised that I was thinking in high speed terms when I assumed faster meant less twist. I may have been over-complicating things. There is no doubt that each 4,000 series is telescoped in profile to a different degree, while the 5,000 series generally isn’t but the Spitfire’s 55409 is. But the twist? Maybe the hundred series wasn’t twist related at all – but then why the significant correlation to design speed in both the 4,000 and 5,000 series? The only other thing I can think of is ‘Activity Factor’. Still in the dark, really.

Variations on a theme:

Left, WW 54409 (un-telescoped but cropped 4.5 inches per blade), Middle 54350 (I read the number on the shanks of this aeroplane’s blades before they went on), Right – the only semi-visible image I have found in four years of looking of a Hamilton Standard 6127, the pattern for the other two (Ju52/3m SE-AER, Swedish Airlines, Malmo 1937).

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