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..of course, the littlest one and the biggest one were home-grown.

That the twist outboard of the widest point was what mattered at the time – ie. the bit that could be changed simply by telescoping – is shown by these experiments – https://digital.library.unt.edu/ark:/67531/metadc66316/m2/1/high_res_d/19930091733.pdf – into precisely the effects of twist and pitch that Ralph highlighted.

This is how DH cleverly adjusted the basic 6127 design via telescoping outboard of the widest point – standard procedure for length changes – to adjust pitch distribution – at least for the 4,000 series.

They just took a 6105 and twisted it for the 5,000 series. The also telescoped the profile to get the 55409, but didn’t call it ‘T’ because they didn’t increase the twist in the way telescoping normally would.

Other changes to thickness, aerofoil or twist meant that the range went beyond 400 by the time of the Hydromatics. As such the details of the Lancaster’s 455800 matched those of the 6353, a later addition to the basic 6105 family.

I will look into that twist, provided in a handy table in HS 130B, and see if I can’t work out the full extended sequence of twists across the Hydromatics along with the Brackets.. but not tomorrow. I need to do some work, and say hello to my family!

That’s a great diagram!

I guess any blade that you intend to rotate has to be a compromise. I was wondering how they ‘got away’ with twisting the outer portion of a Hamlton / DH blade by telescoping it, de-focussing it in the process. I guess if you no longer have the geometric convergence assured anyway it matters less.

Having said that, I would presume an aircraft intended for speed would have the twist optimised for coarse pitch? Or, thinking about it again, for that part of the coarser end of the range at which the engine is at max RPM around max V?

Ha! My uneducated mind is running on the same lines. The twist is key, and it’s an unknown – I am not at all sure what the variations were yet. I have the twist of the Spitfire’s 55409, and a Typhoon four-blade (less), and can see the twist of the US Hamiltons varying with design speed.

The ‘book’ says that telescoping a blade will change the twist, as well – but if one is basing a new basic design on an imported telescoped blade, as apparently happened with the Spitfire, I imagine one would iron that out? I just don’t know yet. Truncating it shouldn’t alter anything – the important thing is twist-per-inch, ie. rate of twist, of course.

Base angles were defined by the prop manufacturer responding to the specifications given by the aircraft designer – rpm, optimum speed (ie forward speed of maximum efficiency), speed range, priority for take-off and low-speed acceleration against maximum speed. The propeller supplier would offer up the model they thought most appropriate (and design one if it didn’t exist, if they thought there was a chance of mass production). That would include the base angle.

It is hard to get the ‘wrong’ angle on a constant speed blade. The idea was that the blade would find the right (optimum efficiency) angle for 2,800 rpm as it would for 3,000 rpm. The 2,800 rpm would be a higher-drag configuration, ie coarser pitch. This would normally be a less efficient setting, not because of the higher drag necessarily (the lift(thrust)/drag ratio generally stays proportionate) but because the engine was rated at 3,000 for maximum output (higher revs, more horsepower up to this point).

As the pilot you set the rpm – by moving the pitch control (which is not gradated beyond <- coarse — fine ->) and watching the rpm counter. If you find that 2,800 gives more thrust than 3,000 – in other words, setting the ‘wrong’ rpm and thus pitch for maximum speed, then something else has gone wrong. You have induced a form of drag with the blade at 3,000 rpm that also removes lift (thrust), an effect which is reduced at a lower prop speed. This is compressibility wave drag.

In all this I am not giving propeller rpm, of course – there is a reduction gear involved – but it’s a constant.

Hopefully this shows how an ‘inaccurate’ blade-setting mechanism isn’t necessarily going to be a problem – the mechanism does what it has to do to reach a particular rpm. The only problem might occur when it can’t reach one extreme or another – it either ‘hits the stops’ or there’s a leak preventing full actuation. Then you’d get rpm fluctuations, of course. Its possible to hit the stops within the ‘envelope’ of the WW in the climb, caused by there being more drag – and thus necessary blade-fining – than was designed for. There were indeed rpm problems above 27,000ft. This could also be caused by the blade still not giving low enough drag to maintain climbing revs even in the minimum-drag angle of attack (about -1 degree). The ‘dumb’ mechanism would carry on ‘fining’ the blade, passing through the minimum drag angle and beyond..

I digress. Basically, you encounter prop compressibility and its a ‘world of pain’, as they say.

The whole thing was a very curious episode.

For example, designing an aeroplane with intakes in the wing roots squarely behind the inner half of the propeller arc on each side – and then making the propeller un-handed so on one side the duct is completely the wrong shape.

There were cooling trials that showed this in the overheat on one side. And yet no-one seemed to notice the cause (Thanks to Dodge Bailey for pointing this one out).

Sending an aeroplane for test with one set of propellers and then putting it into service with another. And when questions were asked about performance differences, claiming it was an identical aeroplane.

Running all kinds of trials on other aircraft that showed large variations in performance with prop changes – and still not trying it on the Whirlwind.

Sending one aircraft to Rolls Royce to have the supercharger intakes moved and adding 30 mph at a stroke – and then forgetting about it (after converting the aircraft back!)

..and then finally letting Rolls Royce continue to shoulder the blame. Yes, there was a war on – but one would have thought people would pay more attention, not less, to troubleshooting a new aeroplane.

No, it was the other way around, the DH propped WW flew first, first prototype, L6844, October 1938, always had DH props, Second prototype, L6845 1939, always had Rotols. This was my point all along, the Rotols only went on one aircraft and that was the one that was tested with everyone saying it was a production standard machine when it most definitely was not, for that very reason!

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So the thicker DH prop pre-dated the thinner one.

Re the 55409 section – I guess we don’t know for definite in the tested cases, but I have inspected a 55409 and it definitely wasn’t RAF section – and the 5,000 series drawing that P&P has shows a Clark-Y, albeit for a Fairey Battle.

But I was getting muddled before when I said about the WW Rotol having RAF sections – apologies – we don’t know that about the Rotols, and in fact they probably, on balance, weren’t.

Yes, and of course that performance differential of the two metal prop types properly documented on another type does provide the circumstantial evidence for my Whirlwind theory, coming full-circle on this.

With the Spitfire we do have evidence of higher efficiency ‘low down’ with the thicker (9%) Rotols than the 7.6% de-H’s. But we also have that thing about limiting revs to improve speed, showing earlier hitting of ‘the wall’.

With the WW we have the reverse, in that the de-H’s were thicker even than the (probably around 9%) Rotols. I expect the DH WW was better low down, took off better etc (than the Rotol one, not necessarily the DH Spitfire) – but as you say it’s very hard to tell, it was never really performance tested.

I have solved the thickness/chord problem – or rather spotted my own error in making an assumption.

So, the Hamilton Standard tables for the 6353 do describe the de Havilland 5,000-series bracket AND 55,000 series Hydromatic blades. They are telescoped in the edition I have, but they can be re-plotted to de-telescope them.

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Useful? I don’t know. But satisfying. Now for the other series..