Prop Blades Too Thick, And Peregrine Blameless, On Whirlwind?

That’s great Matt, more than I was hoping for. Now I can dot the Ts and cross the Is 😉

This note totally overlooks the different propellers:

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Will do when I am in front of the right machine.

Matt,
Would it be possible for you to post Martlesham’s speed v alt trial data for L6845? Most grateful if you could share.
Thanks

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

Its a pity that we do not know the source document for the tabulation of Spitfire propeller data, I suspect that there may be other gems in it.

It’s only circumstantial evidence, but according to ‘Reinventing the Propeller’, Jeremy Kinney, the trials with 54 Squadron in 1939 showed that their Spitfires with metal Rotols had, among other advantages, better ceiling than those with the DH. Does anyone have those documents?

In the meantime, this shows an altitude effect (as well performance only on a par with the WW up to 16,000ft):
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(A&AEE trials, Boscombe, from aircraftperformance.com)

and here are some t/c ratios – only at 0.7 sadly – and their effect – note ‘compressibility loss’, though at what altitude / Mach number isn’t explained, making absolute values meaningless. I expect it represents loss under the conditions recorded in the row, ie max speed conditions – so only hints at the susceptibility of each blade design to compressibility, not the full effect towards the ceiling. http://www.spitfireperformance.com/spit2prop-b.jpg. The WW was 9.6%, for comparison .Also note how speed / altitude around FTH is affected.

Kinney also mentions that the reason the RAF had a lot of aircraft with inferior props when they wanted the Rotol units was simply availability. Makes the apparent instruction to Rotol to concentrate on wooden blades seem like a material supply thing, but that would only make sense if there really was no understanding of the importance of blade thickness at all, and they thought it was just something about the hub? Hard to credit, especially in the light of these results, but it’s looking increasingly possible.

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?

Ah, my bad. Must learn to read :eagerness:

Ah, no, the Whirly’s is 54409. The first 4 is shank size – ‘4,000 series’. The Spit was 5,000 series, hence 55409.

The first digit is type (5=metal controllable), and the third digit is drawing number within the series, fourth is rotation and fifth how many inches “removed’ from the ‘full length’ pattern blade of the drawing – or rather from the projected prop diameter described by it.

Still working on whether the third digit indicates anything more than a sequential number.

Do you have any ideas why the Whirly’s DH prop is drawing no. 55409 and the Spitfire’s is no. 55409B considering that they are quite different?

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.

No slouch. Depends what you are comparing it to? Better than the DH version, however you look at it. The A&AEE had the Rotol version pegged at 31,000ft ceiling, the RAF could only manage 27,000 before RPM and thus pressures began fluctuating. The same engine, same boost. Constant (not increasing) speed, 3,000 rpm in both cases until puff ran out, maintained by a CSU that would then hit the stops.

I completely disagree with the assertion that the DH props were of sufficiently thin section to cope. The work of John Stack and many experiments on that very aerofoil demonstrate that they were not thin enough along all of the outboard section (which is a known profile) to avoid compressibility drag rises really very early compared to, say, a Spitfire 55409B blade, just 2% thinner at 7.6%.

From the scant data available it appears that the Whirlwind was not exactly a stellar performer with the original Rotol props (of section, profile etc. unknown at this point).

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’

If ony it was that simple. Spitfire I trials were run at a weight and power that comes out as 5.8 – 6.1 lb/hp. For the Whirlwind it was 5.9.

Just standing back from this a bit and having skimmed the posts to date; I think it is entirely possible that the WW just ran out of power at altitude?

Each Peregrine must have been producing sub-1000hp so that doesn’t add up to much for such a potentially fast aeroplane. Bear in mind also the indisputable fact that prop size/blade area is directly related to engine power. Efficiency drops off with height and increased RPM. You can’t just keep on increasing (or just maintaining) power and airspeed by increasing the RPM and pitch of the prop, one or the other (or both) run(s) out eventually.

de-H prop blades were of much thinner section than the wooden Rotols due to obvious structural reasons. The de-H props should have been of sufficiently thin section to cope but, as I alluded, if you run out of power it doesn’t matter how good your prop is. A pair of Merlins would have been the answer 😉

Rotol originally touted their propellers as being able to have aluminium alloy, mag alloy or wood as a blade material. I’d suggest that their eventual direction to use entirely wood was probably driven by the Ministry and war material shortages. Why use mag or ally when wood (a non-strategic material) would do perfectly well for nearly all applications.

Anon.