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Volkes filter - what's inside?


Scott Hemsley

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I'm tinkering with the KP Spitfire Vc (tropical) and as I was assembling the Volkes filter, it dawned on me that all the various kits I've seen or assembled that had a Volkes filter, represented the 'chin' as a vast, empty cavern with a hole in front.  Surely there's a screen or grill inside that empty hole that normally can be seen, isn't there?  Anyone know?

 

Scott

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The Vokes filter in the Sptfire Mk. VC (Trop) consisted of five layers of material - two of wire gauze between which there are two layers of fabric with a centre layer of special filtering material, (often cottonwool). These were stitched together and then corrugated into deep folds to increase the actual filtering area in the space occupied by the filter.

 

O.K., I'm being a smart alec. I realise that you are referring to the Supermarine designed fairing that housed the Vokes Aerovee filter and enlarged oil tank on the Spitfire Mk.VC (Trop).

 

Seriously though, many call the filter housing on the Spitfire VC, a "Vokes filter", and that on the Spitfire VIII, (and late Spitfire IX), an "Aerovee filter". In fact, both the MK.V and the Mk.VIII used a Vokes Aerovee filter. So describing one as a 'Vokes filter' and the other as an 'Aerovee filter' is incorrect - both aircraft were fitted with Vokes Aerovee filters. Vokes produced two basic types of air filter, the Aerovee and the Three-ply. The former was a dry type filter designed for use with liquid cooled engines. The latter was an oil-viscous filter designed for use with air-cooled radial engines. I realise that I am being pedantic but, as an engineer, I believe that it is important to separate the descriprion of the filter and its housing.

 

Back to your query, this diagram and rough sketch may help. 

 

a68f70a7-a917-420f-922f-fffdc66c1136.jpg

 

 

bd42ef86-9192-4729-9e2a-4beeb3add122.jpg

As you can see the ram air inlet feeds air directly to the Vokes Aerovee filter. The filter is mounted at an angle to keep the frontal area of the filter housing as low as possible, but also to present the maximum possible area of filter to limit airflow losses as the air passes through the filter. Vokes claimed a 95% efficiency, but there were doubts raised about this claim. The main drawback of this installation was that there was no direct ram air feed to the carby - air always passed through the filter so the power of the engine was slightly reduced when the filter was fitted. There was also a shutter that could close off the flow of air through the ram air intake and, instead, draw warm air from the engine compartment for running the engine under conditions that may produce icing in the carburettor.

 

The photo below shows the nose of a VC Spitfire with the filter housing removed. The larger oil tank on this variant is readily visible.

27a0b571-e592-4c8c-838f-4ca48c659be8.jpg

 

 

 

 

The main drawback of this installation was that there was no direct ram air feed to the carby, with a consequent reduction in engine performance. On the Aboukir designed filter housing, the filter was mounted above the ram air intake, and air was only diverted through it when required, direct ram air going to the carby under normal flight circumstances.

10ab0e45-00d9-49ba-b138-68202734daf5.jpg

 

 

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@Magpie22 That photo of the Mk Vc's with the cowling panels and props removed is very interesting. Looks like a postwar aircraft dump, as there appear to be a whole line of Spits in the same condition and the grass has grown up around the aircraft. Any details as to the location and date of the photo? Just curious!

Mike

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6 hours ago, Magpie22 said:

O.K., I'm being a smart alec.

 

Heck, when I first saw the topic I was going to answer, "Ooh, ooh, I know... a filter!"  Your post is an outstanding answer- and I, too, love that shot of the engine installation.

 

bob

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Fascinating stuff, thank you!

The 'Vokes' filter on the Mk.V always looked rather draggy to my eye. However, reading the above, I'm assuming the loss of performance usually attributed to filter equipped aircraft was more a result of the loss of ram air then?

Also, would anyone, perchance, have any diagrams of the insides of the later rather sleeker Mk.VIII/IX filter?

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18 hours ago, 72modeler said:

Any details as to the location and date of the photo? Just curious!

At a guess, looks like Aussie Mk.Vs . 

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1 hour ago, Gazontipede said:

Fascinating stuff, thank you!

The 'Vokes' filter on the Mk.V always looked rather draggy to my eye. However, reading the above, I'm assuming the loss of performance usually attributed to filter equipped aircraft was more a result of the loss of ram air then?

Also, would anyone, perchance, have any diagrams of the insides of the later rather sleeker Mk.VIII/IX filter?

On the Spitfire VC, reduced performance was due to both duct loss, caused by the filter and, also, to a much lesser extent, the increased aerodynamic drag of the fairing housing the filter. To the untrained eye, it appeared that the shape of the fairing was the major casue in the drop in performance when, in fact, most of the performance loss was caused by the filter reducing the 'head' of the airflow to the carburettor. The Australians finally realised this after they replaced the filter housing with a cleaner 'temperate' cowling and gained little increase in performance. Their major error was in retaining the original stub intake that collected the air from behind the filter. This was some 15% larger in cross sectional area than the 'ideal' ram air intake on aircraft not fitted with the filter, as the air flow behind the filter was of reduced pressure and, therefore, a larger duct was needed to collect enough air to feed the carburettor. Using this stub as a 'ram air' intake delivered too much air to the carburettor, upsetting the fuel/air ratio, affecting boost pressuer and causing rough running, particularly at lower altitudes. If a new lesser area intake, matched to carburettor and supercharger requirements, had been installed, performance gains wouldmay have been realised. The only performance gain was an increase in rated altitude of about 1,000 ft.

There are excellent drawings of the filter and duct arrangement of the 'Universal Air Intake' on page 282 of Spitfire the History by Morgan and Shacklady. The advantage of this filter was htat air was only drawn through the filter when required, not all the time as on the the VC arrangement. The filter element is annotated as Vokes C-9045. I don't want to infringe copyright by posting the diagrams here. PM me if you want a copy 'for personal use'. 

 

Peter M

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On 4/15/2021 at 9:02 PM, 72modeler said:

@Magpie22 That photo of the Mk Vc's with the cowling panels and props removed is very interesting. Looks like a postwar aircraft dump, as there appear to be a whole line of Spits in the same condition and the grass has grown up around the aircraft. Any details as to the location and date of the photo? Just curious!

Mike

Hi Mike,

Yes it is postwar. Most of the RAAF's flyable Spitfire aircraft, (and Kittyhawks), were flown back to the mainland, most ending up at No. 6 AD's salvage centre, at Oaket, about 120 kilometres west of Brisbane. Here, what was reusable was salvaged and the rest of the arframe broken up and sold as scrap. A few more photos attached - got the tissues handy? Photos taken circa 1946to1948.

Cheers,

Peter

 

fd5dd165-5b83-4843-900b-54f8636ebe39.jpg

 

53b614a1-db41-4122-9779-0cdce49d9f70.jpg

 

dfc366a4-76df-44e4-a113-39d716b996bc.jpg

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11 hours ago, Magpie22 said:

There are excellent drawings of the filter and duct arrangement of the 'Universal Air Intake' on page 282 of Spitfire the History by Morgan and Shacklady. The advantage of this filter was htat air was only drawn through the filter when required, not all the time as on the the VC arrangement. The filter element is annotated as Vokes C-9045. I don't want to infringe copyright by posting the diagrams here. PM me if you want a copy 'for personal use'. 

 

Peter M

Thanks for the concise explanation Peter, my eye certainly counts among the untrained. :)

I do have a rather ancient and tired (the binding is starting to disintegrate) copy of Morgan & Shacklady. I must admit I don't use it as much as I used to because it is starting to come apart. In this instance, with a precise page number though I shall make an exception! Thanks again!

Cheers,

 

Richard

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10 hours ago, 72modeler said:

@Magpie22

 

Yes, sad to be sure, but no different from the same scene in the UK Canada, and the USA. Those Spits are all probably Holdens, now! :giggle:

Mike

 

PS- thanks for sharing the photos.

 

Alas Holden is now another deadun. GM axed the Holden brand last year when they decided to pull out of designing and manufacturing right hand drive vehicles. The laboratory where I used to work was just round the corner from the Holden design offices. Over the years, we tested a few Holden designs and concept cars in our low speed wind tunnel - and, yes we also did Ford as well. Interesing days working with Allen Moffat on the racing mods for his Foulcan. Ah, memories!!

Peter

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27 minutes ago, Magpie22 said:

working with Allen Moffat on the racing mods for his Foulcan

A Holden man were you Peter? :D It must have introduced an interesting & different dimension to your work.

Steve.

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3 minutes ago, stevehnz said:

A Holden man were you Peter? :D It must have introduced an interesting & different dimension to your work.

Steve.

 

Actually, Steve, I have never owned a Holden or a Ford, but that never stopped me being a fan of V8 racing!!

Peter

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  • 1 year later...

Funny this thread should pop up again now, because I was thinking only the other day: what happened to all the sand and dust that these filters kept out of the engine?  Was it dumped overboard, or did they wait til they'd landed and tip it out like an ashtray?  Was there a limit on flight time after which the filter would be too bunged up to work any more?

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13 hours ago, BS_w said:

spacer.png

Wow, again. 
 

Guess it’s time to learn how to turn that sheet into a 3D design. Some incredibly frustrating blurring of the right most columns!

 

Also, some gaps in locating stations A, B, C, & I. Looks like Station 0 is .22” aft of the datum, with 10” between that & Station I. 
 

What I can’t determine (help!) is the spacing from 0-A, A-B, B-C, & C-I. 

 

spacer.png

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1 minute ago, Peter Roberts said:

Very timely for the upcoming Airfix 1/24 Mk IXc - a Mk Vc Trop anyone?

 

Thank you to all the contributors. 🙏


Drawing up the Vokes in 1:1 as I type. Will make the digital model available for anyone interested. Just need to get all the information gaps filled. 

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10 hours ago, BS_w said:

is it better now?

vokes_11.jpg


Very helpful. Last pieces of the puzzle are in the table below in red. Anyone with better eyes able to have a look?

 

spacer.png
 

Eta: one easy way to draw something up is to trace it. That method is reliant upon a good quality, undistorted drawing. Sadly, @BS_w drawing is a bit distorted. 
 

The more accurate method is use the table of offsets to position each point in three dimensional space. That’s what I’m trying to do. 
 

spacer.png

 

I’m using the drawing as a canvas to ground truth my read of the table of offsets, not to trace. 
 

Due to some gaps in the table, I’m not there, yet. 

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On 4/17/2021 at 12:30 AM, Magpie22 said:

Hi Mike,

Yes it is postwar. Most of the RAAF's flyable Spitfire aircraft, (and Kittyhawks), were flown back to the mainland, most ending up at No. 6 AD's salvage centre, at Oaket, about 120 kilometres west of Brisbane. Here, what was reusable was salvaged and the rest of the arframe broken up and sold as scrap. A few more photos attached - got the tissues handy? Photos taken circa 1946to1948.

Cheers,

Peter

 

fd5dd165-5b83-4843-900b-54f8636ebe39.jpg

 

53b614a1-db41-4122-9779-0cdce49d9f70.jpg

 

dfc366a4-76df-44e4-a113-39d716b996bc.jpg

When I looked at the initial photo, I thought Oakey. And now, seeing these 3...Where these a/c are parked up in the photos is now the site of the Army Aviation Museum & the old hangars. For many years after the war, there were rumours that many Spit & P-40 airframes were buried in the ground on the northern & eastern fringes.

Incidentally as you drive along the road in front of the base, you can still see the hard stands remaining from the war years

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You guys are amazing at finding this stuff! Yet another university-level course in Spitfire bits. Interesting notes on the Aboukir filter too; one wonders if the ability to bypass the filter was inspired by those on the Bf 109 and Macchi C.202?

 

To respond to the original question...you basically DO see just empty space when looking in the small front opening of the Vokes filter housing, no? The filter element is way back there, mounted high at a pretty flat angle.

Edited by MDriskill
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