A controversial Whirlwind Theory..

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Hello all

This might cause some discussion, or it might sink without a trace - but I am prepared for the arguments..

I have been doing some number-crunching, and rummaging around pre-war and wartime technical reports from the RAe and NACA. I have found out something that I am nervous of stating, for fear of being seen as an over-excitable amateur, but I will take a deep breath...

It might not have been the Whirlwind's Peregrine engines running out of puff with height that was the problem at altitude for the Whirlwind.

In fact the Peregrine had the same drop off in supercharger boost pressure with height as the Merlin II and III.

I believe it was the propellers that caused the major problem. The blades had unusually thick sections - approaching 0.1 t/c ratio at 0.7radius. I have the figures that show critical Machs for the specific RAF6 section of the WW's props at various thicknesses. Sure enough, at around 350 mph forward speed and 3,000 rpm (geared at 0.477:1), at around 16,000-17,000 feet, a significant and rapidly growing proportion of the Whirlwind blade hit that Mach figure, becoming 'useless', generating no lift, and very 'draggy'.

With height, the speed needed to reach this critical Mach decreases. So its not just the lack of supercharger boost, its the fact that the propellers loose all efficiency beyond a specific tip speed, which is variable with forward speed, at 16/17,000 feet and above.

This issue was identified with the Spitfire, and as early as 1940 it was realised that the MkII's Rotol blades (at around 0.89 t/c @ 0.7r) were too thick, and were hitting the compressibility wall - limiting the aircraft's performance to less than was expected with the new Merlin XII engine.

The Mark I De Havilland blade was around 0.76 t/c @ 0.7r, meaning that the onset of the effect was delayed until the aircraft was travelling at around 355 mph at 19,000ft. DH nevertheless went on to create an even thinner, improved blade for the Spitfire (0.6 t/c) to delay the effect even further.

Newer, faster aircraft had props that were of much thinner, NACA designed sections that allowed the tips to pass through Mach 1 while remaining aerodynamically useful.

The Whirlwind, and the Spitfire, were victims of being faster than almost anything else around at the time, and thus encountering these things anew. The difference is, Rolls-Royce, the RAF and De Havilland put their heads together to make the Spitfire even better - but nobody thought to put nice thin blades on the Whirlwind, instead finding it easier to blame the engines. I wonder whether RR kept quiet, wanting to be rid of the Peregrine so they could focus on the Merlin and Griffon.

I will show my workings, and some nice graphs, in the next WFP newsletter, for anyone interested.

Controversial, I know. Thoughts, please.

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I suspect it is a matter of placing effort where it would bring most value: there simply weren't enough Whirlwinds around to matter, and nor would there be enough to justify design and production effort of a new production line of propellers. There's always sufficient use for lower-altitude fighters anyway.

If you really want to investigate "conspiracy" where propellers are concerned, why was nothing satisfactory done to prevent the failures due to low temperatures of DH props/spinners after it had been identified? Why was it that the Spitfires sent to Australia had dedicated high-altitude Merlin 46s but were fitted with the DH props, despite their known problem in the role? Could it be that the Rotol props suffered from precisely these limiting Mach number effects you describe above, so were undesirable for the role, anyway? Nothing to do with Whirlwinds, I'm afraid.

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Interesting theory Beermat and probably part of the jigsaw that led to the lack of development of the Peregrine. I went to a lecture last week on the "100 years of Westland" and the chap giving the lecture, while praising the advanced nature of the Whirlwind, indicated that Rolls Royce were too occupied with churning out Merlins that they really wanted the Peregrine off the books.

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I am not claiming a conspiracy as such - it is more an interesting angle, and one that showed where the problem may have been in a purely technical sense.

But it is a fact that compressibility was understood by some at the point in time where the future of the WW was under discussion.. but what went on record was the Peregrines were choking at height. I agree with you, Graham.. the right 'resources' - human ones - were not applied, for simple reasons. The decision to move on had already been taken :)

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An interesting topic, Beermat and one that has seen me put on my thinking hat - but unfortunately with a similar lack of success!

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I don't know the date at which the Whirlwind was "written off". If you think that had already happened by April 1940 then the propeller tip story becomes irrelevant. If it hadn't then it becomes one more black mark, although it doesn't seem to have affected the Spitfire Mk.II too adversely. The timing and sequence of events is pretty important. We are approaching the time when priorities were given to a handful of types, at the expense of development of others. The Whirlwind certainly wasn't on that short list, so new props were not going to be considered during the crisis, but by the time priorities were relaxed it surely had been discounted.

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Decisions on munitions production - what to build, where, in what quantity - were not made by industry. RR had no say in dumping Peregrine (or Boreas, Exe, Vulture) in favour of Merlin: nor was it up to them to churn out Merlin, or anything. The Customer - us - paid the piper; organised supply, shipping, labour; built and equipped factories.

The twin-engined, 4 cannon fighter awarded 2/37 to little Petter's/Westland as more promising than others' schemes, was a challenge - new airframe, systems, engine, soon (1/41, when high altitude was added as Welkin) the novelty of pressurisation. There was no Prime Contractor in those days - Project Management was by the Customer, who, 7/38, caused the business insertion of John Brown and AEI (Westland was near-bankrupt), and planned to produce Whirlwind in Nuffield's Castle Bromwich (turf turned 8/38). AMDP Freeman added Spitfire II to CBAF, 4/39; then, as has been infamously misrepresented, its output was slower than hoped...but not because RR was overloaded trying to prove and build multiple products. See these contradictory reports:
- “this decision (i.e: to rely on N) cost (at) least 1,000 (Spits) which should have been in reserve when the (BoB) started” (Air Minister Swinton, 60 Years of Power, Hutchinson, 1966, P119, blaming N for output shortfall). cf:
- Spitfire was “in danger of being eliminated from the re-armament programme (due to delay, not at CBAF, but at V-S. Its) future was assured (due to the) stubbornness of 1 man (N, insisting) on producing (all of all) 1,000 (at CBAF, so Whirlwind was) squeezed out”’ A.M. memo, 11/7/39, in E.B.Morgan/ E.Shacklady, Spitfire, Key, 1987, P51.

The fall of France caused Minister of Aircraft Production Beaverbrook to "dump" awhile anything undeliverable NOW! Whirlwind thereafter did not recover any priority, and Westland was consumed in building and repairing Spitfires and acting as Design Authority on Seafire...not because Whirlwind, Peregrine or propellors were bad (or victims of conspiracies), but because lots of (cheapish) proven Spitfires were of higher priority than future better...maybe.

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It is actually surprisingly difficult to find a precise date on which the Whirlwind was 'written off'. Thanks to one of our researchers I am now looking at probably the entire record set held by the National Archives on the Whirlwind. The short version of the timeline is that on 8th May 1940, in response to the concern expressed by George Bullman (the Air Ministry's Engine 'Head') about the effect of continued Peregrine production on Rolls' output, WR Farrar, the AD to the DTD simply said in a memo 'No New Whirlwinds'. This is as close to an official decision as I can get.

[ATTACH=CONFIG]241143[/ATTACH]

However, before, during and after this decision was being made there was a discussion carrying on about the Mark II (AVIA15/317). This would have had Morris radiators, 100-octane-ready Peregrines, greater tankage, revised cannon feed arrangements and Hydromatic props, no mention of blade changes, just the greater pitch range imparted by the Hydromatic mechanism.

No decision was made on this until Dowding wrote to Beaverbrook in October 1940 saying (in response to a direct question) that although it was still to early to say for definite he'd not be unhappy if no further Whirlwinds were produced. I doubt very much that Whirlwinds were anywhere near the front of Dowding's mind at the time.

Beaverbrook replied with characteristic unilateralism (and in line with Alertken's post above) that he would therefore not produce any more Whirlwinds. It looks like the axe was ultimately wielded by the Minister for Aircraft Production acting independently and without official sanction (beyond a three-word memo five months earlier), but all official parties were happy to tacitly let it go, meaning they could stop thinking about the Whirlwind and focus on other things.

There was a last comment on the matter made at a meeting in February 1941, more a post-rationalisation of the decision made by Beaverbook for the Ministry three months earlier:

[ATTACH=CONFIG]241145[/ATTACH]

I would still argue that the propeller tip story is not irrelevant, however. It was a limiting factor on the aircraft before any decisions were made, it didn't just kick in after the event. Decisions were made upon the usefulness of the aircraft as against its impact on production capabilities. Performance at height, or rather the lack of it, figured very largely on the 'balance sheet'.

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..doesn't seem to have affected the Spitfire Mk.II too adversely.

It did a bit - it would have been 5mph faster 500ft higher with the DH blade. This table also handily includes the compressibility factor for each blade, you can see the issue with the Rotol. The one I highlighted was (I think) the production standard. It was the most efficient of the Rotols tested, and as is often the case it was also the one most affected by compressibility. This might be why the MkII was faster lower down, but slower higher up, than the MkI:

[ATTACH=CONFIG]241147[/ATTACH]

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Yes, fascinating material but I meant that it didn't affect the production of the Spitfire Mk.II. Similarly, had the Whirlwind been more successful, the tip problem may not have affected its future.

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Ah, I see.

The problem, if it's real, did affect its future. It made it sluggish at height, and this counted very greatly against continuing with production (I have that in writing), and development - and thus being thought a 'success'. The Spitfire was different - The Spitfire was a proven aircraft with a development 'roadmap' attached by the time the disappointing altitude performance of the MkII (a fast aeroplane, otherwise) came to light. By contrast the Whirlwind hadn't even been delivered to squadron strength at the time, and the programme was eminently 'haltable' - and it was.

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I have convinced myself. Critical Mach at 'tip' is in fact at 0.91 radius, after which the profile misbehaves as the thickness tapers towards the rounded tip.

You can see the maximum measured speed converging with the theoretical point at which the props will stop helping you go forward, starting at Full throttle height and agreeing -showing the limiting factor - above 20,000 feet.

This is why squadron pilots were on record as being confused as to how Martlesham got theirs to 31,000ft, when their aircraft stopped being of much use above 26,000.

All of the performance testing was done on prototype L6845, which had Rotols. The squadrons got De Havillands:

[ATTACH=CONFIG]241161[/ATTACH]

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I fail to understand the argument the OP is trying to make. He has launched into trying to establish a theory using peripheral RAE and NACA documents without actually stating what the problem with the Whirlwind was. A dropoff in performance as altitude increases is usual for all aircraft types. The reasons can be multiple.

Can the OP quote actual documentation and numbers to this perceived performance shortfall of the Whirlwind, or is the evidence simple anecdotal?

Did the type achieve its agreed specifications before service entry, and if not, what official reasons were given for failing to do so?

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My old mate F/L D.W. 'Doug' Sturgeon was an unusual chap in many ways not least because he flew Whirlwinds then Mossies, then Hornets so could compare the lot. He noted that whilst the single engine handling caught a lot of people out and is given as the principal reason for their failure (apart from engine fires, underpowered etc) what people never commented on were the HP slots - he told me that these were cable operated and could be locked but the cables had a nasty habit of snapping. It was a while ago now so don't quote me but I think he said the cable routed around the engine and the heat could cause one to snap - - one would then deploy with the other retracted - leading to a flick roll when one wasn't expecting it.

This is from memory now so I may be misremembering... he was ex- 263 sqn and a top bloke

TT

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I haven't done any research on the Whirlwind, never had the time, but my impression has always been that the aeroplane was made of some pretty exotic materials for the period and that these had to be imported. Given Beaverbrook's passion for limiting production of aircraft to a few important ones (the right decision at the time), the Whirlwind just had to go.
Having said that, the few aeroplanes produced lasted in squadron service until (I think off the top my head) 1944 so the extraordinary performance that these materials bestowed on the fighter so just what could have be achieved with a single seat twin engine fighter that could escort bombers to Berlin and back. A pity that Petter thought only in terms of metal and not wood.
Wasn't there a proposal to develop the Whirlwind with Merlins?

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Ex Brat, I have just seen this old thread promoted by a recent post, which I will come to next.

The performance drop off of the Whirlwind with height when compared to other types in production is there in the figures provided by the RAE and respective manufacturers. It is extant as a series of figures and graphs which are public record and which do not require regurgitation in order to then explore reasons for an unexplained difference.

Dowding did say in writing that he was unwilling to commit the aircraft in its primary role because of the altitude problems - which is pretty terminal for a new type. He then entered into correspondence with Beaverbrook, who, from the correspondence, was the one to order a halt in production somewhat arbitrarily as was his style - though for sound reasons that emerge from the correspondence, all of which is on record at the National Archives.

The largest operational problem with the Whirlwind was clear and is a matter of record.

If you do not understand the argument then please read it again, or perhaps take a look at my TAH article on the subject which spells it out very carefully, at length and with full references. It also explains that the aircraft did meet its agreed specifications at test, and this was because the aircraft tested had Rotol propeller blades and not the de Havilland ones that were a problem. It is alarming but absolutely the truth as documented in black and white that the two aircraft (tested prototype and production) were deemed 'identical' and the problems with performance at altitude that dropped below those at test were was called 'inexplicable'.

All I have attempted to do is explain them. Please don't be so derogatory, as you have been in this post and others. I have been carefully researching this for years and much as it might pain you to accept it it does actually stack up as a theory. It is not arm-waving or some kind of paranoid conspiracy - just something that was overlooked because trans-sonic theory wasn't really where most minds were 'at' at the time. Some were, and the Spitfre's 55409 was thinned as a result. It was generally known that the blades as they came from US Hamilton blanks were too fat and thus inefficient, though it was less well understood at the time why.

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TT, that's interesting, it's always good to get first hand info. In the wrong order - the slats were locked shut fairly early on in the aircraft's career at which point this stopped being a problem - whether this was 'known' by those encountering the Whirlwind would depend upon when they did so.

Single-engine performance wasn't so much lack of power as a lack of feathering capability on the bracket-type props - there were no 4,000 series Hydromatic equivalents available as the design just hadn't come from the US.

Engine fires were, it would seem, largely rumour and bar anecdote - I cannot find one example actually documented.

The performance drop off of the Whirlwind with height when compared to other types in production is there in the figures provided by the RAE and respective manufacturers. It is extant as a series of figures and graphs which are public record and which do not require regurgitation in order to then explore reasons for an unexplained difference.
Yes you do need to show your working and thought process.

How can you compare one aircraft type with others then in production without making a through investigation and comparison of ALL aspects of each aircraft type? You are not quantifying the problem, simply supplying the reader with supposition.

*******
You say that the Whirlwind performed less well at altitude than other aircraft types of the time. One very important factor that will limit an aircraft’s ceiling is wing loading. It’s not the only one, but for the purpose of this argument I think it is useful. Did you consider that?

Compare the Whirlwind, Spitfire Vb and Hurricane 2c using wing loading data from about the same time period:

Whirlwind 45.78 lbs/sq ft.
Spitfire Vb 27.68 lbs/sq ft
Hurricane 2c 33.89 lbs/sq ft.

The three types that different wing sections; Clark YH, NACA 2213/2209.4, and NACA 23015. All three aerofoils have very similar max Cl at comparable Re. No real surprise then that the Whirlwind had a ceiling some 6,000 ft lower than the other two is it?

You will note that I haven’t shown my working, but since “figures and graphs which are public record, and which do not require regurgitation…”.

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You think I'm making this up? Why would anyone do that? OK. Let me show you these, at the risk of boring anybody else not so determined to demonstrate my inability to read a graph.

[ATTACH=JSON]{"alt":"Click image for larger version Name:\tww.png Views:\t0 Size:\t296.6 KB ID:\t3870853","data-align":"none","data-attachmentid":"3870853","data-size":"full"}[/ATTACH]

The background image is from an RAE test of a standard Spitfire I R6770, added is Whirlwind from trials in 1942 with bombs. You can find the data through the National Archives, apologies for no reference here. Both have equivalent performance with the same curve up to FTH. Above this there is variation - the absolute maximum speed drops off faster with height with the WW than it does with the Spitfire. The difference is in the degree to which altitude impacts performance. but only above FTH. This last tells us it's related to thrust, not aircraft drag. So, what changes with altitude? The speed of sound. What was critical? Mach around the prop blades. That's why it's called critical Mach. What affects critical Mach on prop blades? Thickness. What was different between these two aircraft (and also the difference between this production WW and the 'performing as expected' prototype)? Blade thickness.

Blade - or wing - thickness doesn't make a difference to drag until it does. It does when transonic speeds are reached. You put excess drag on a CP prop blade and what does it do? It turns. It reduces pitch. That reduces thrust. It does take a lot of mental picturing to get the whole thing, but it really does work and the texts are out there.

Sorry to present it like this, but you wanted to follow my reasoning. Here you have the data and a simplified version of my thought process. As I say, I have written about this in greater detail elsewhere, and all of the references are available. If you struggle to understand after absorbing all of this that is not my fault, please make more effort. If you need more data - and I do have it - I suggest you do the rest yourself. Yes there is something odd about the difference beginning at around FTH, but if you think about it enough you will understand that with a CP prop this is how it is going to manifest itself - this and a slight variance in apparent FTH from that specified by the engine manufacturer (which also is in the data). No, I will not work this out here for you, it's tedious - as an expert able to dismiss others so readily I am sure you can work it out yourself.

Of course we could look at all aspects of the aircraft, but this is a theory about one of them. It would only be worthwhile to explore other things in relation to it if you were desperate to find some other reason why the theory isn't feasible. Maybe you will, but wing loading won't cut it. Personally I can't see why you have such a fly up your intake about this.

I do appreciate that you are a very experienced and knowledgeable person. Sometimes, however, this leads to a form of arrogance that means that anyone who comes up with something you hadn't thought of must a) be wrong and b) be slapped down. Please make sure you understand the principle already put forward here and in much greater detail elsewhere (rather than demanding I show you my working like some teacher who doesn't really get it himself), before dismissing further. I say this with all due respect.

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To add - propellers have always had what Douglas Adams called an 'SEP field' around them. Everyone from the A&AEE when they claimed their tested WW was no different to those in service to Mr. Brat today seem to believe it is the engine which provides the 'thrust' component of the famous four-arrow lift/drag/weight/thrust diagram for a propeller aircraft. It is not, it is the propeller. Change the propeller and you might as well have changed the engine.

In order to prove that the idea does not work, rather than staring at graphs one would have to demonstrate one of the following:

  1. Thick RAF-6 section aerofoils do not in fact take on wave drag at lower Mach than thin Clark Y ones.
  2. The speed of sound does not reduce with altitude
  3. A constant speed propeller's response to increased rotational drag is not to reduce pitch
Until one can show this, I would suggest that this has more bearing than wing loading - which did not change between prototype as tested at A&AEE and production.

The problem we have in quantifying the problem with absolute figures is that there are no plots of performance apart from those for the Rotol prototype or variations on production - wing thickening and bombs. However, these latter DO show an increased gradient in the max speed curve compared to prototype and other comparable aircraft, regardless of absolute figures (which of course should be how we look at his anyway).

It is the difference in the rate of change, not the absolute figures, which is the 'smoking gun' when we come to consider the physics. I hope I am explaining this point well enough - what someone who does not understand the principle would think was a weakness (no absolute figure comparison) is in fact just the correct approach as one cannot usefully compare absolute figures. One can look at "if and how" those figures change as external parameters change and then look to mechanisms that might cause this.

This is a basic principle of engineering and applied maths - calculus over algebra, If you want to apply it to the graphs that do exist, be my guest, but it won't prove anything that a visual check on the contemporary graphs and a grasp of what is going on with the aircraft won't.

The graph in your post last evening means nothing without labels. As I now understand it, your research is trying to establish;

Why was the maximum speed of the WW at altitude less than that of the Spitfire I - is that correct?

And that you put this lack of comparative performance down to your idea that this was because of the propellers fitted to the WW reduced the available thrust by their having higher drag than the one installed on the Spitfire, and the reason for the greater drag (you say) was due to the blades becoming Mcrit? Am I correct?

And you have not considered;

  1. Engine hp available against weight of the aircraft
  2. Drag of the aircraft, both profile and induced
  3. Wing planform
Did you consider the comments made by Harold Penrose regarding modifications made to the WW after it had undergone testing at Farnborough? [QUOTE]In June the first prototype was ready for testing with a new nacelle-ducted exhaust system, but decreased top speed was inevitable because of the greater drag and loss of injector effect.[/QUOTE]