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Spitfire Mk.XII Rotol propeller blades


steh2o
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Hello,

I'm currently building a Spitfire Mk.XII model (link below)

and from time to time I discover that this Mark  had peculiar elements that unfortunately are not easily traceable today (not for me at least).

I'm trying now to figure out the shape of the Rotol propeller blades of this Mark.  As remarked before, they are not the same design as in Seafire Mk.XV (Rotol design number RA.10167) because the blade root is visibly different. Though, looking at it more and more  I noticed a similarity with something else:

f0aef9df-3a43-4b40-be6d-521dd91ae7c4.jpg

The two blades above can be perfectly superposed (except for the  tip), and the blade root shape is almost identical.

At left, Rotol design number RA 10046, Spitfire Mk. VII/VIII/IX, photograph mirrored horizontally

At right, Rotol design number ???? Spitfire Mk.XII

I'm not trying to say that Mk.XII used a simply mirrored 10046 design though; Mk.XII's blades were 3 to 4 inches shorter than Mk.IX's ones, accounting for the fact that the spinner had a 3" bigger diameter and the propeller axis sat about 4" lower than in Mk.IX (hence the need of a 10'5" propeller diameter instead of 10'9" for not hitting the ground in tail-high position).  The maximum chord of the blade instead was possibly the same. In the photograph above, the Mk.XII blade is in coarser pitch than the Mk.IX one and, if scaled to the correct lenght, would look narrower due to the coarser pitch.

To make it short, I'm convicted that the Mk.XII used the same but specular profile of the Mk. IX blade condensed in a shorter length.

If anybody has more information about this aspect, I would be happy to know!

Stefano

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22 minutes ago, gingerbob said:

You can't just "turn around" a prop to make it opposite rotation.

 

Agreed, To anyone versed in CAD, they need to be completely mirrored.

If the prop blade is just turned by 90 degrees to spin in the opposite direction, the trailing edge would 'cut' the air instead of the leading edge

(resulting in inefficiency or maybe even stalling, because the blade profile is inverted).

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16 minutes ago, warhawk said:

If the prop blade is just turned by 90 degrees to spin in the opposite direction, the trailing edge would 'cut' the air instead of the leading edge

(resulting in inefficiency or maybe even stalling, because the blade profile is inverted).

If the prop blade is turned 90°, it would probably result only in a huge cloud of dust, as nothing "cuts" the air, but it would get a full broadside 😉 I know you meant 180°.

But even then the leading edge would be leading edge, I think, but if the blade has an airfoil section (which I think most props will need to some extent), the more bulbuous side would be at the rear, generating a faster airflow and hence "lift" on the wrong side - I think that's what you meant.

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A quick look on Spitfire XII propellers.

 

Ministry of Aircraft Production. Constant Speed Unit GRF/2B, shared with Firefly and Spitfire XIV.  Propeller Rotol Hydraulic 4 Blade, R4/4, B5/1.  The The Firefly used Rotol Hydraulic 3 Blade RS5/19.  No model numbers for Spinners or Blades.

 

https://www.key.aero/forum/historic-aviation/130516-propeller-types-for-spitfire-xii-and-24

 

Not XII, but

Morgan and Shacklady, Spitfire the History

Griffon III
Griffon IV Rotol R13/4F5/5 Dural blades (first 6 aircraft), then Jablo blades
Griffon IIB Rotol XH/54D-RM-55
Griffon VI Rotol R13/4F5/6

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50 minutes ago, tempestfan said:

If the prop blade is turned 90°, it would probably result only in a huge cloud of dust, as nothing "cuts" the air, but it would get a full broadside 😉 I know you meant 180°.

But even then the leading edge would be leading edge, I think, but if the blade has an airfoil section (which I think most props will need to some extent), the more bulbuous side would be at the rear, generating a faster airflow and hence "lift" on the wrong side - I think that's what you meant.

 

I agree, I was wrong about 90 degrees specifically, because that case only applies to 45 degrees angle of incidence.

What I meant was that there's no way or angle You can rotate the prop blade to make it turn in opposite direction (black arrow below).

As You said, either the leading edge and/or the bulbous side of the prop would be in the wrong place to generate thrust forward.

You need a completely new prop blade, shaped to the opposite direction of turning.

spacer.png

 

In conclusion, even if the Mk.XII shared the exact same diameter and shape of its long-nose Merlin counterparts,

You couldn't take, let's say Mk.IX prop blades and fit them to a Mk.XII simply by changing the angle.

 

But hey, if Your kit is a short-run - the plastic is probably thick and wide enough to reshape it the other way 😏

(I did that with my Azur IK-3, for example)

Edited by warhawk
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23 minutes ago, warhawk said:

You need a completely new prop blade, shaped to the opposite direction of turning.

You do, but you could manufacture right hand and left hand versions of the same design simply by mirroring it

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I've built three 1/48 Spitfire XIIs and one in 1/32 and I haven't been happy with any of them. The props seem to be the biggest problem for the kit makers for some reason. I am going to have one last go at building a decent XII in 1/48 using the Eduard Mk V & VIII wing and fuselage and an Airfix XIV nose. I think I can solve the prop problem by cutting down the spare blades from an Eduard Tempest V kit so maybe you could look at cutting down a 1/72 Tempest/Typhoon prop. As a matter of interest the single stage Griffon III or IV as fitted to the Mk XII developed 1750hp which is very close to the 1720hp developed by the Merlin 63 or 70 hence why the prop shapes are very similar, just turning in the opposite direction to the Merlin. hth

TRF

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Hello Fastterry,

we share the same plan (sadly I didn't have a decent Mk.Vc wing).

I think that you'd better try and reshape the Airfix Mk. XIV blades. Mk. XIV had the same blade length and maximum chord of the Mk.XII, so all is needed is reshaping of the blade root in a fashion specular to the Mk.IX blade. Great help comes from the Squadrons! book which has good photographs of the blade from different angles. I believe the Typhoon/Tempest blades are much longer than the Mk. XII blade.

Your remark about engine power ratings is quite interesting- the Mk.VII/VIII/IX and derivatives had a propeller which blades have a similar maximum chord as the Mk.XII's, but are possibly 3 to 4 inches longer. Moreover the propeller diameter is 4" bigger in the Mk.IX. Overall I would say that the Mk.IX's propeller "pushes" more that a Mk.XII's did, perhaps that small difference was compensated with a coarser pitch.

It is interesting to note that the Mk.IV Griffon prototype used a bigger diameter propeller (according to Shacklady, both 10'9" and 11' IIRC) but the operational variant Mk.XII and following used 10'5". While the two-stage compressor high altitude variants enjoyed more blade surface in to thin air with the simple expedient of a fifth blade, used the single stage, low altitude variants a possibly less effective propeller design? Another interesting point is the fact that the Mk.XV-fitted propeller adopted a modified blade design with the aerofoil profile further extended to the blade root (this for sure increased a bit thrust).

Spitfire was a fantastic testbed for so many advances in aircraft design...

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The Mk.IX propeller was however expected to be at its best, or at least as good as possible, at higher altitudes than that of the Mk.XII.  I suspect that this is one reason for longer blades.

 

However bear in  mind that timescale could have been an important factor.  The development of a 5-blade hub could well have lagged behind the introduction of the Mk.XII.

 

I'm (nowadays) sorry that my university course was the first year in which propeller theory was not taught.  At times like this it could have been useful.  It was reinstated for the following year.

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24 minutes ago, Graham Boak said:

The Mk.IX propeller was however expected to be at its best, or at least as good as possible, at higher altitudes than that of the Mk.XII.  I suspect that this is one reason for longer blades.

 

However bear in  mind that timescale could have been an important factor.  The development of a 5-blade hub could well have lagged behind the introduction of the Mk.XII.

 

I'm (nowadays) sorry that my university course was the first year in which propeller theory was not taught.  At times like this it could have been useful.  It was reinstated for the following year.

You can always go back and finish the course GB.🤪

TRF

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5 minutes ago, fastterry said:

You can always go back and finish the course GB.🤪

TRF

And all the rest...  But no, there are too many other things still to learn.  Like all the kits in the stash (or on Hannant's shelves) that would be great to make but for all the others in the way.

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That's why I keep my stash in constant rotation, like compost.  Or perhaps it is because I can't remember where I set it down last time I was looking (oh, there it is, right next to my desk in that stack...) so instead I spot a kit I haven't thought about for a while and get all interested in THAT one before seeing an RFI of a third type, which makes me start thinking about THAT, and...

 

Now, back to props (airscrews).  The XII/XIV was limited by using the standard main gear geometry.  Prototypes can get away with exceptions that wouldn't work in service, and when they were working up the concept, a bit longer legs was part of the plan (as eventually came out in the 20-series, which of course is what the Griffon Spit was meant to be, except that it was a moving target).  One way of compensating for a different diameter is to change the reduction gear ratio.  I don't have time right now to look up the various ratios, though.

 

bob

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Production

Spitfire XII October 1942 to September 1943
Spitfire F.VIII November 1942 to September 1943, then 20 in November.
Spitfire LF.VIII from May 1943
Spitfire F.XIV from October 1943
Spitfire PR.XIX 22 from April to June 1944, then from October 1944.
Spitfire F.21 1 in April 1944 then from July 1944

 

June 1943 Rotol produces its first 5 blade propellers and 5 blade spinners, then a pause in production until September.

 

From the 1946 Janes,

Merlin gearing 0.42 or 0.477 to 1.

Griffon gearing
II, III, XII, 61 0.451 to 1
IV, 65, 66 0.510 to 1 (and VI?)

 

Propeller diameter, from Morgan and Shacklady

VIII, IX, XIV 10 feet 9 inches
XII, XIX 10 feet 5 inches
XIV 10 feet 4 or 5 inches
F.24 11 feet 10 inches

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About time someone wrote the book on British propellers. all the firms, all the models (with scale drawings of course) and all the usages. Does the Manchester prop match the Typhoon? Even if just the Vulture Tornado. Enquiring minds need to know!  

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