Spitfire Propeller Question

[Edgar]

The early Merlins did actually have three prop shafts:-

The "A" shaft (known as "Full Diameter Shaft,")which was restricted to the Merlin I (nos 63 - 401,) and Merlin II (nos 407 - 2905) could take the fixed-pitch, or de Havilland 2-pitch, or de Havilland V.P. airscrews, but with various "add-ons" and "take-offs."

The "B" shaft (known as the "First Necked Down Shaft") was fitted to Merlin III serial nos 2907 - 5605 (plus some Merlin IV & X) and could take the fixed-pitch, or de Havilland bracket type, or Rotol Internal Cylinder no 5, but not the deH Hydromatic, certain deH bracket types, and Rotol external cylinder airscrews.

The "C" shaft (known as the "Universal Necked down Shaft) was fitted to Merlin IIIs from serial no 5607 onwards, and could take the fixed-pitch, de Havilland two-pitch, variable pitch and Hydromatic, plus Rotol internal and external cylinder airscrews.

Edgar

[JackG]

Looking at the diagrams of Rotol's coloured discs, doesn't look like they made any metal based propellers during the war - would that be correct?

I've been wondering, during the course of the Spitfire's history, which type of prop was employed most, metal or wood - or isn't even possible to attempt such an answer?

Anyhow, I've found an interesting pdf file of some kind of printed document announcing Rotol's first constant speed propeller made of wood, dated May 23rd 1940:

http://www.wwiiaircraftperformance.org/hurricane/Flight_Rotol_23_May_1940.pdf

regards,

Jack

Edited September 25, 2013 by JackG

[Test Graham]

An interesting file, but it does imply, if not say in quite as many words, that the first Rotol propellers were of magnesium alloy.

I suspect that there were more Rotol props used on Spitfires than DH types, as the DH prop was only used on Mk.Is and some Mk.Vs (and Mk.VIs? but they were few). So wooden props would be the more common.

[Fernando]

Hi, gingerbob,

I am currently building a bunch of early Merlin PR Spits IN 1/48, and therefore researching a bit on them. You have said that "... PR Spits were early recipients of Rotols". However, I have not come across any photograph of a PR Spit so equipped, not even the PR.IVs, which were supposedly built alongside the Mk. Vs; even when the early ones received the Merlin 45 (as most survivors did) the prop looks the same. Even late PR.IV ones serving with the 681 Sqn in Burma. They all seem to have been fitted with the De Havilland and early round exhausts.

In modelling terms, I never use the wide-based prop in the Tamiya Vb kit, but the one in the MDC or Airwaves set (as a DH). I also guess that the DH prop in the Tamiya Mk.I has too thin blades, or perhaps it is an early model?. The DH prop in the Airfix Mk.I/II/IIa looks wide enough, as the one in the SH/CA/Ed/Italeri. How's the DH prop on the Hasegawa kit?.

Fernando

[Test Graham]

It was discovered that DH props tended to freeze at altitude leading to failure of the constant-speed mechanism. The early solution - and I don't know of any later one - was to ensure that the mechanism was "worked" at regular intervals. However, because PR work required the long periods of steady flying at constant engine setting and cold temperatures, Rotol props were to be preferred for PR work. Finding photos of them are indeed another matter.

When I say that no later solution seem to have been found, this is because DH props were on the Spitfires sent to Australia in late 1942. There were many failures in combat at the higher altitudes of these Japanese penetrations (up to 28-29000ft) leading to a number of losses and more lost opportunities. Together with jammed cannon and poor leadership, this lead to a disillusion with the type and ill-feeling to this day. I recommend the book Darwin Spitfires by Anthony Cooper for a thorough and even-handed account of the sorry saga.

[Seawinder]

Looking at the diagrams of Rotol's coloured discs, doesn't look like they made any metal based propellers during the war - would that be correct?

I've been wondering, during the course of the Spitfire's history, which type of prop was employed most, metal or wood - or isn't even possible to attempt such an answer?

Anyhow, I've found an interesting pdf file of some kind of printed document announcing Rotol's first constant speed propeller made of wood, dated May 23rd 1940:

http://www.wwiiaircraftperformance.org/hurricane/Flight_Rotol_23_May_1940.pdf

regards,

Jack

I'm pretty sure there were Rotol metal blades used at least on some early Mk. IX's. I had a long discussion at another forum last year when I was building mine, and the conclusion we reached was that Rotol used pale blue disks to label them. They had more rounded tips than the Jablo blades:

Cheers.

[Fernando]

Hi, Graham,

I know that photos may be elusive, but... ALL the photos showing De Havillands and NONE a Rotol? Not even the one of BR416 in its "High Altitude" guise, with extended wingtips and all?

Fernando

Edited September 26, 2013 by Fernando

[Magpie22]

It was discovered that DH props tended to freeze at altitude leading to failure of the constant-speed mechanism. The early solution - and I don't know of any later one - was to ensure that the mechanism was "worked" at regular intervals.

When I say that no later solution seem to have been found, this is because DH props were on the Spitfires sent to Australia in late 1942. There were many failures in combat at the higher altitudes of these Japanese penetrations (up to 28-29000ft) leading to a number of losses and more lost opportunities. Together with jammed cannon and poor leadership, this lead to a disillusion with the type and ill-feeling to this day. I recommend the book Darwin Spitfires by Anthony Cooper for a thorough and even-handed account of the sorry saga.

Hello Graham,

The Aussies did develop a partial solution to the runaway prop problem. I quote Bill Hardwick a pilot with 457 Sqn:

"A basic design fault was revealed in the airscrew constant speed unit. Simply, the constant speed unit was set to give the Merlin engine a maximum speed of 3,000 rpm. It was the job of the hydraulicaly-operated CSU to maintain the pre-selected engine revs, irrespective of the attitude of the aircraft. At the sub-zero temperatures experienced at high altitudes the hydraulic oil would congeal and, with the bleed-back ports between the front (cold) and back (warmer) chambers being too small, the unit could not operate properly. On nosing over into a dive, the airscrew would act like any fixed pitch airscrew and over-ride the pre-set maximum setting of 3,000 rpm. With the engine running up to 30% above design speed, the scene was set for certain disaster. The squadron fitters enlarged the ports and felt muffs were made in an effort to maintain circulation. Following these modifications the heavy losses of Sunday May 2 were never repeated."

Over the years, I have had the opportunity to interview many of the pilots, including commanders, of the Spitfire squadrons. There was certainly some disagreement among them as to the tactics that should have been employed but, I came across very few that thought that they were subject to poor leadership: in fact most were highly complementary of their leaders, particularly Walters and Calwell who are subject o much criticism in Cooper's book. It is very easy to sit back, with the benefit of 20/20 hindsight and be critical of those who fought in, and led, those Spits. I agree with you re the quality of Cooper's book. It is highly detailed in its description of the battles but, I feel, fails in its assessment of the the tactics employed and the qualities of the leadership and the pilots involved. Cooper has made good use of the records now available but has failed to talk to those involved in sufficient detail to enable him to present a better balanced picture.

Incidentally, the problem of jammed cannon was not peculiar to the RAAF, it was a problem that affected Spitfires in all theatres, particularly those involved in high altitude operations. I find the comment about "disillusion with the type and ill-feeling to this day" rather strange. I have never spoken to an RAAF pilot that had anything but a great respect, and in most cases, love, for the Spitfire. The only matter of contention seems to have been which was the nicest to fly, the VC or the VIII?

Magpie 22

[gingerbob]

Hi, gingerbob,

You have said that "... PR Spits were early recipients of Rotols". However, I have not come across any photograph of a PR Spit so equipped, not even the PR.IVs, which were supposedly built alongside the Mk. Vs; even when the early ones received the Merlin 45 (as most survivors did) the prop looks the same. Even late PR.IV ones serving with the 681 Sqn in Burma. They all seem to have been fitted with the De Havilland and early round exhausts.

Interesting. I'll have to go study some photos. I confess that my confidence stemmed from a number of comments in documents that this would be happening, and perhaps even was happening (early '41, with the appearance of Merlin 45). I've also got it in my mind that I've seen early PR Spits with Rotols (the blunt-spinner Mk.II variety). It is quite possible that when the DH Hydromatic became available (late '41 or early '42?) that became the preferred prop.

I haven't studied different kit props against each other. If I do, I'll report back!

bob

[Test Graham]

The heavy losses of May 2nd may not have re-occurred but the failures did not stop happening. I agree that Cooper is not perfect: although he speaks of the problem not being found in the Middle East I have since found a few examples of Spitfires on Malta suffering from failures of the CSU. I assume that more will have escaped mention in the popular histories. Certainly there were not as many or as concentrated in time, and as I said above the higher altitude of the Japanese raids appears to be the key difference that triggered the failings. I do feel that there appears to be a short-sightedness in the position of the appropriate authorities in the UK: that more stringent steps should have been taken earlier to remove the problem altogether. However, it may well be that the problem is inherent in the design, and as always in war the "good enough" will be accepted if the alternative is significant delays and a shortfall in production. The interim steps taken, and instructions issued, must have been presumed adequate, and/or the particular circumstances at Darwin not foreseen. Given the emphasis on higher-altitude combat in the RAF's planning, this is perhaps surprising.

The Hispano's failings may be (I suspect probably are) similarly related, but again do not seem to have occurred elsewhere with anything like the frequency. After all, with WW2 technology it was unavoidable that some guns did jam sometimes. This point could do with enlarging upon by an appropriate historian. Cooper does not enlarge upon the reason for the guns failing (IIRC). However, I do feel that even in Cooper the RAAF groundcrews seem to have escaped scrutiny to anything like the same extent as the planners and the pilots. Yes they worked hard in poor conditions at the dog-end of the supply chain, but were they supremely - and uniquely - beyond all question? Or were they as ill-prepared as many of the pilots?

I don't think that hindsight is necessariy 20/20, unless it takes account of all factors. And not necessarily correct even then. However, if well informed, distance can lend a perspective which is inevitably missing at the time, or lacking amongst participants even later. Closeness can add blinkers. Finding out what was right, and what wrong, is the entire purpose of looking back at all.

PS I see Bob has slipped in. The Hydromatic suffered the same problems as the earlier DH - it was the Hydromatic that was employed at Darwin. The value of the Hydromatic was the wider pitch range permitting shorter take-offs, which is why specific trials were carried out on HMS Furious and a rush programme followed to fit all aircraft on that delivery to Malta with Hydromatics. I don't think that this would be specifically helpful to the PR role. Though it no doubt would help take-off with the heavier fuel loads it suffered the same problems that led to the advice in the first place.

In particular I agree that Cooper was critical of Caldwell (and, to be fair, others). I think he produced enough evidence to justify that.

Internationally pilots have loved the Spitfire as a flying machine. So much so it is almost surprising to find pilots (except perhaps ex-Hurricane pilots in the BoB, with their own special position) who will say anything against it. In the Australian case, if you look beyond the pilots involved to the internal RAAF and external Australian politics, and the history books, other people are more than willing to condemn it. I feel Cooper's book provides a necessary counterbalance to the simplistic negativity of some commentators.

Edited September 26, 2013 by Graham Boak

[Fernando]

Hi, gingerbob!

Going back and forward my archives (all published photos, in well known books, no Holy Grial here...) I found K9791, a PR.IB shot down and exposed at the infamous Krakow "Victory in the West" exposition, which had a metal Rotol prop. The early, typical of Mk.II fighters one, not the broad-base typical of Mk.Vs; combined with the "blunt" boss. It is stated that it had a Merlin XII; a picture shows the Coffman starter bulge, so, probably, the reason for using that prop is that it was a Mk.I (from the first production batch, actually) upgraded to Mk.II (Merlin XII), then converted to PR. If so, another candidate could be P7505, an Armstrong built Mk.II converted to PR.VII (said to be the only such known in the Ventura book).

Fernando

[gingerbob]

Hi Fernando, I was just reading through a file about "PRU aircraft requirements"- for a different reason, naturally- and came upon this comment:

68B Notes on meeting RAF Benson re Spitfire D spares 13/8/41

taking into account number of spares which had been supplied to service 100 a/c during the last 18 months, following suggested:

28 Port, 22 Stbd wings

10 Port wing oil tanks

5 Dummy oil tanks (fuselage)

10 Windscreens

25 Blister hoods, with Martin Baker jettison device

60 KDG wing tank gauges

12 Camera control box

12 Glycol cockpit heater

From contractors:

30 Rotol prop

15 Spinner

~~~~~~~~~

I ain't saying this proves anything, but for obvious reasons I noticed!

bob

p.s. Those late IVs in Burma you mentioned might (pure speculation) have had DHs for the benefit of metal blades. I did spot one Rotol on a PR Spit- I think it was the one Type E. Is this the one you already cited?

[OttoVonSkidmark]

Apologies for the thread resurrection! I’m currently building the Revell 1/32 Spitfire mkii a. As you know the spinner is quite pointed! I can only find the quickboost mkivb as a replacement. So, is this acceptable as a replacement? I think it is the rotol and the spinner looks blunted enough.

 

Thanks all.

Edited May 25, 2020 by OttoVonSkidmark
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