BeermatOK. I had picked up on that possibility – And the ‘draggier’ the blade, the sooner this would happen? The response to blade drag increase would be dropped RPM when the engine starts running out of puff, which would cause the CSU to attempt to make the pitch finer. So surely the load on the engine against available power is the critical variable, and this is greatly increased as compressibility drag on the blades kicks in. And this happens earlier with the DH than the Rotol. Kind of my original contention.
On the DH blade, stops are stops. You would have to start shearing a lot of metal to go beyond them, and you’d have a loose blade if you did. I think any mismatch in torque problem (or in fact matching torque – the prop wouldn’t drive the engine in a climb) would have to lie before the stops are hit – perhaps as above? Also a bit of maths seems to show this, but its all very ‘back of fag packet’. You have to factor in forward air speed, but there’s nothing that says this can only happen beyond the blade’s minimum pitch setting, which is arbitrary.
The blade settings (min fine pitch, for example) though arbitrary, were not guessed at, they would have been defined knowing power, wing loading, rpm, etc etc. so as to never reach the stops and loose efficiency in this way.
The only thing possibly overlooked, and understandably so given the time, was blade response to compressible flow effects ( which would have been that reduction in pitch beyond design parameters).
BeermatAh, I touched on that in my initial ‘essay’.
EDIT – the version published had that bit removed!! I left this scenario out of the final version, apart from the line ‘against the stops at 28 degrees’. There were to many variables to be certain.
I suspect the Rotols had a greater range. They generally did.
That aside, the CSU was a ‘dumb’ mechanism in that it simply responded to a drop in revs by making the pitch finer. A higher than expected drag on the prop blades would mean a finer than expected – or designed for – pitch.
Is the situation you describe the same as the one I raised where the blades have gone beyond minimum drag to an angle of attack negative enough to essentially be resisting further acceleration rather than delivering thrust?
They would never be a net drag in the climb, when one thinks about it.. they won’t actually ‘windmill”. (But they can run out of usefulness and ‘block’ an intake on one side).
In this scenario there’s sill nothing stopping the blade continuing to rotate to the stops.
Whatever the limits of the design, prop characteristics would have been chosen to match – essentially ‘gearing’ the blades low enough to cope with the known (calculated/expected) speed/climb regime. It is the unconsidered effect of compressibility on the blades that would mean a finer than expected – or designed for – pitch. So a blade that hits additional drag from compressibility effects first will force the blade to the stops first. That would be the DH.
Would you say that’s a fair statement?
BeermatThanks!
Re. The increase in speed above full throttle height, granted it’s no mystery (though you do see a lot of graphs and tables that mark the altitude of max TAS ‘Full Throttle Height’, even when the engine data is available and contradicts this). It is the variation in how the curve looks above actual FFH, that is interesting, and how it correlates to prop efficiency all else being equal more so. I agree its no great revelation, and proves nothing about what is happening with the WW. So perhaps a herring of a slightly pinkish hue.
Can you explain how the CSU is driving the engine.. what is the sequence of events here?
Also, how to explain the clear and marked difference between L6845 which was supposedly a production WW with Rotols and all other production WW’s?
BeermatThis must be how Watson felt when Holmes went all distant and thoughtful.. you’re on to something, aren’t you?
BeermatNot at all off-beam, for me at least. What is happening around the intakes is pretty crucial – not forgetting they were right behind the props. I suspect any effect would be asymmetric as the blades are moving up on one side and down on the other.
BeermatHere is a note about the A&AEE trials, on Rotol-equipped L6845:
[ATTACH=CONFIG]244434[/ATTACH]
And for contrast, an almost contemporaneous letter from Dowding about performance in service (with DH’s)
[ATTACH=CONFIG]244435[/ATTACH]
It seems there was some ‘back and forth’ about the altitude performance in service not matching the quoted figures of ‘the first production aeroplane’ (shame there’s no transcript of that phone call), but no-one mentioned the blades being different.
BeermatMax speed level flight right, time to height left. These are the manufacturers figures, not tabulated anywhere but instead presented as part of a ‘descriptive handbook’, dated Feb 1940, and which also includes two photographs of L6845 with the Rotols. Nothing to confirm, but probable these are Rotol figures.
The later Martlesham Heath tests of L6845 with Rotols agree up to 15,000 feet, but show drop-off in max level flight speed attainable from 15,800ft upwards, 349.5mph TAS at 18,000ft, 342.5mph TAS at 20,000. They still have the aircraft climbing over 30,000 feet, though – unattainable by production aircraft.
Then again, who knows? As the blades were considered immaterial, maybe the Westland data is a ‘mix and match’ anyway?
BeermatFighting….the….urge… No! No off-topic politics!
I wonder how pivotal the Comet incidents were. Possibly crucial.. if you are a manufacturing company with orders, a skint country isn’t such a handicap.
BeermatThe giant flying *rse! How could I have forgotten that!
Beermat..and part of the explanation would be the same as the answer(s) to the bigger question ‘What happened to British manufacturing industry’?
BeermatOK, thanks. By the way, the graph I posted last shows very clearly how efficiency in the climb dropped off around 20 -25,000 feet. The fact is pressures decrease with altitude at a constant rate, whether ‘charged’ or not. So comparison with, in this case, the Spit shows something else to be going on. I am very interested to see what you come up with.
BeermatI don’t necessarily want to get this too bogged down in full throttle heights. I used that to show that variations in altitude performance are not all about superchargers and boost, and the discrepancies I used to indicate that (as they are what got me thinking about other altitude related variables).
It’s all connected, of course, but my main point was that the blades are encountering compressibility issues that reduce efficiency, and in the case of the Whirlwind that was not recognised and unexpected inefficiencies at altitude were blamed solely on an unspecified ‘problem’ with supercharging.
BeermatTAS – I should have said in the article that all speeds quoted are ‘True’.
The ‘book’ says 170mph indicated up to 10,000 feet, then subtract 2mph per 1,000ft
Actual climbing trials at optimum rate (full service load) showed 170mph IAS up to 10,000ft, but then a higher IAS after that (10 mph higher at 20,000ft), then settling back to more or less match the notes at 146 mph IAS at 26,000ft , which I make 220 mph TAS
Here’s the source of the 1,800ft statement – you can ignore the fastest Whirlwind curve, that’s a hypothetical case (at the time of the test) with 100 Octane:
[ATTACH=CONFIG]244396[/ATTACH]
The 15,000ft Full Throttle Height comes from Rolls Royce’s own figures.
Cheers,
Matt
BeermatThanks – I too am grateful for the ‘brains’ that come out of the woodwork when a serious question is asked!
Do we know if the carburetor had automatic altitude compensation/adjustment, or was there a mixture control for the pilot too and egt gauges fitted? If the later, any advice in the pilots notes for climb adjustment?
The answer as it turns out is yes and no, really. There was a two-position control, and carb-controlled adjustment too. The pilots’ notes say this (you may need to click to enlarge):
[ATTACH=CONFIG]244376[/ATTACH]
BeermatI don’t understand how ‘RP’ is ‘speaking properly’. It is an accent, and a lot of the mangled vowels you hear in pre-war broadcasting are a legacy of an attempt to ape the curious speech patterns of the English upper aristocracy among the aspirant middle classes. The accent thus ‘received’ by generations of public school kids (I don’t think it still is taught)? is in fact heavily German-influenced, overlaid on Norman French, if you care to go back far enough. But then so many of our regional accents are, as John pointed out with the example of Geordie.
I think the BBC edict, both on accents and the airing of alternative political viewpoints, was an attempt to do the ‘All in it together’ ‘call me Dave’ thing, to head off rumblings of being led into total war by our upper classes – which were gathering momentum. I am sure there were elements who feared some kind of uprising. Now, wasn’t this the department George Orwell was working in at the time? Interesting stuff.
Edit – no, I checked – Orwell joined the Eastern Service, but with a similar remit in that the aim was to prevent the colonies taking the opportunity to rise against Empire.
He was also writing “The Lion and the Unicorn: Socialism and the English Genius” at the time – I haven’t read it, but I suspect you’d love it, JG.
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