The infamous Vokes Air Filter

[Fatboy Coxy]

Hi all

I'd like to know more about the British infamous Vokes Air Filter, used in the North African Campaign from 1941. I say infamous, because I've read that it had a negative impact on the speed and maneuverability of aircraft it was fitted to, but operating in the sand and dust environment of that theatre of war, something like it must have been needed. I'll provide what little I know, or have read, and hopefully people can fill in the missing bits, correct me or confirm my 'facts'.

Vokes was (is I believe still) a developer and manufacture of engine filters, and was asked to provide a special 'tropicalised' filter to deal with large amounts of sand and dust in the air, and so save on excessive engine wear. What they produced was a large dust filter that fitted over the carburettor air intake under the nose, and was in service from mid 1941 in North Africa. All well and good so far, however the size of the 'air scoop' underneath caused considerable drag, affecting performance. They were fitted to both the Hawker Hurricane and Supermarine Spitfire, but I don't know if the American P40 series of aircraft were fitted with them or a different dust filter, as it had the ability to bypass the filter once having taken off.

Performance figures I have read suggest the Spitfire lost 16 mph due to its air filter, the Hurricane lost 20-25 mph, and I'm unaware what performance was lost on the P40s.

A RAF MU (Maintenance Unit) based at Aboukir, Egypt, worked on refining the Vokes Filter, reducing lost performance, but was only made in small number, this was called, not surprisingly the Aboukir Filter.

Hurricanes and Spitfires sent to the Far East (Singapore, Burma, Australia and India) also used the Vokes filters.

However at some stage I think the original Vokes filters disappeared, this may be with the change up of model of aircraft being operated and a better filter being introduced, or an acceptance of a shorter engine life time, in exchange for retaining that potential life saving speed.

And that's it, all very shaky info to be honest.

 

[Juha3]

Some info about Vokes Volkes filter - what's inside?

Spit Mk. IXs and VIIIs fitted with tropical filter had instead of the Vokes a Supermarine tropical air intake with integrated filter element working in the same princible than the filters on Merlin Mustangs and Bf 109s. Incoming air could be directed through the filter when needed, otherwise it bypassed the filter.

 

[MIflyer]

When RAF Typhoons were deployed to Normandy they found the dust on those rough airfields was a problem. So they had a representative from a UK filter company come over and in short order he devised a way to install filter cloths in the existing engine hardware and this proved to be effective.

Now, why were US aircraft never seen with filters? That is, except for the Spitfires flown by the USAAF in the Med? Well, for one thing the Allison V-1710 was a downdraft intake engine, where the air came in through the top of the cowl, requiring a long duct that was suitable to carry air filters. The RR Merlin was an updraft intake engine and so the air intake came right off the bottom of the fuselage. On the Merlin Mustang the air intake was located right under the prop, once again providing a long intake duct that could be used for a filter.

 

[GreenKnight121]

We just had a thread where this was discussed..... the "Pink FR/PR Spitfires" thread - many posts about both the Vokes and Aboukir filters: Pink FR/PR Spitfires

And an earlier one "How good a plane was the P-40, really?", which has scattered discussion of air filters on P-40s and other aircraft: How good a plane was the P-40, really?

 

[Fatboy Coxy]

Some info about Vokes Volkes filter - what's inside?

Spit Mk. IXs and VIIIs fitted with tropical filter had instead of the Vokes a Supermarine tropical air intake with integrated filter element working in the same princible than the filters on Merlin Mustangs and Bf 109s. Incoming air could be directed through the filter when needed, otherwise it bypassed the filter.

Hi Juha3, wow what a find, hats off to Magpie22, he knows his onions!

 

[Fatboy Coxy]

We just had a thread where this was discussed..... the "Pink FR/PR Spitfires" thread - many posts about both the Vokes and Aboukir filters: Pink FR/PR Spitfires

Hi Greenknight121, thank you for this, I can see that its very easy to determine which is the Vokes and which is the Aboukir filter now.

And an earlier one "How good a plane was the P-40, really?", which has scattered discussion of air filters on P-40s and other aircraft: How good a plane was the P-40, really?

And in this thread 'Wild Bill Kelso' mentions performance, which is where my figures may have come from

Please forgive me as I lost track of who posted the link for this one, but this is pretty enlightening vis a vis Tropical filters:

Looks like the Tropical filter on the Spit V cost 16 mph, and the Vokes filter on the Hurricanes cost 20-25 mph.

The Allison engined P-40s were set up to use the filter only on takeoff. Same for the Bf 109F, which lost 15 mph due to the filter. This is all very good data.

View attachment 767890

But regarding performance lost by drag Pat303 says

The MkV lost 20mph carrying a 90G drop tank, the Volkes didn't have that much drag

no source quoted though.

 

[Fatboy Coxy]

I also have Greyman disputing the 25 mph loss

A&AEE has the Hurricane II losing 7 mph and 2,800 ft in full-throttle height when tropicalized.

These were two different aircraft, so not a perfect comparison. Also, the tropicalized aircraft was a IIa vs. the normal IIb tested, so the tropical conversion is probably a 9-8 mph loss.

I'm not sure what to make of the Australian 20-25 mph figure.

reinforced by Pat303.

Running 9psi boost instead of 16?. I don't believe for a moment that the Volkes filter is as bad as it is made out to be, if you look at other aircraft like Beaufighter it only lost around 3-4mph when tropicalised, likewise UK MkV Trops only lost around 7-8mph, for the RAAF MkV's to lose 20-25mph there has to be more too it than just the filter.

 

[Fatboy Coxy]

We also have some detail on engine life

So did most other aircraft. Some had more problems than others. The Hurricane was marginal in speed compared to other allied fighters and that was before they hung the Vokes filter on it.
The Hurricane climbed better than P-40s, but thay may not show up as often. Some Japanese aircraft had trouble with cooling due to small openings in the cowl.
Hotter, thinner air is hotter, thinner air. It doesn't care who make the plane, It matters what the wing loading is and perhaps the boost being used by the engine. But the changes are are going to be percentages.

And we get to individual aircraft. P-40F & Ls never got Vokes filters. They also went through engines at a much higher rate than the P-40E/K/M/N did.
Part of the difference between the over cowl intake and carb intake being in radiator/oil cooler opening in the Merlin P-40s.
This is why the British gave the Americans 600 used Merlins as a source for rebuilds.
Maybe the P-40Fs should have gotten a better filter even at the cost of performance?
A slower plane that is flying is of more use than an plane waiting for a replacement engine (or crashed).

Shortround6 mentions the P40 which indicated that models without the Vokes filter went through engines quicker, even though they still had a filter fitted

And Wild Bill Kelso mentions that Spitfire engines were averaging 50 hours with the Vokes filter, while operating in Darwin, Australia.

So what is an average life of an engine. Does anyone have any figures for temperate climates like Britain, and then the Middle East, and what I am specifically looking for, Burma, Singapore, Dutch East Indies?

And please excuse my ignorance, but does a fighter aircraft put more strain on an engine leading to a shorter life than say a twin engined bomber or a Coastal Command patrol aircraft?

 

[Spits2]

79 Sqd RAAF changed to a locally manufactured smooth bottom cowling when it was escorting B24,s out of Townsville.

79 Sqn RAAF Spitfire (AWM P02874.033 Stock Photo - Alamy

Download this stock image: 79 Sqn RAAF Spitfire (AWM P02874.033) - KX9JCK from Alamy's library of millions of high resolution stock photos, illustrations and vectors.

www.alamy.com

 

[Fatboy Coxy]

79 Sqd RAAF changed to a locally manufactured smooth bottom cowling when it was escorting B24,s out of Townsville.

79 Sqn RAAF Spitfire (AWM P02874.033 Stock Photo - Alamy

Download this stock image: 79 Sqn RAAF Spitfire (AWM P02874.033) - KX9JCK from Alamy's library of millions of high resolution stock photos, illustrations and vectors.

www.alamy.com

Hi Spits2

Thank you for this, RAAF 79 Sqn was formed in May 1943, quite a bit latter than I was looking at, but if they were altering the cowling and still keeping the Vokes Air Filter, in my opinion it points to the fact that the air filter was worth keeping for improved engine life, and the reduced flight performance was acceptable. Changing the cowling is another one of those efforts that looks at incremental improvements in an effort to gain a margin of superiority over your opponent.

 

[Spits2]

Hi Spits2

Thank you for this, RAAF 79 Sqn was formed in May 1943, quite a bit latter than I was looking at, but if they were altering the cowling and still keeping the Vokes Air Filter, in my opinion it points to the fact that the air filter was worth keeping for improved engine life, and the reduced flight performance was acceptable. Changing the cowling is another one of those efforts that looks at incremental improvements in an effort to gain a margin of superiority over your opponent.

It wasnt just the filter but the Tropical Oil tank was longer and held more oil to keep the Merlin cooler in Tropical climates.It gives the cowl a Pigeon breast appearance.Same as the PR Spits.

 

[EwenS]

79 squadron RAAF wasn't operating in Northern Australia from the time of its formation which may explain a lot. It only went to Sattler (south of Darwin) in Jan 1945 to convert to the Spitfire VIII and left again for Morotai in Feb.

Aircraft history for JG740 / A58-173 (the only JG74* serial delivered to Australia) from ADF Serials site:-

"SS Austral Star 11/03/43. Rec 1AD ex UK 11/03/43. Rec 79Sqn RAAF ex 1AD 14/05/43. Rec 1AD ex 79Sqn RAF for Temperate Cowl(Tropical Volks intake removed) 30/05/43. Rec 79Sqn RAAF ex 1AD 06/06/43. As JG740, Coded UP-U and later named "Sweet FA" on Port side, forward of cockpit. Flown by 79 Sqn CO S/L A C Rawlinson. Rec 6AD ex 79Sqn RAAF 16/06/44. Rec CGS ex 6AD 11/09/44. All over metal finish except for black anti-glare panel, white empennage & leading wing edges and Coded "E", while with CGS. Accident 1000hrs 21/12/44 during camera gun exercise, when following engine failure due to coolant leak, aircraft made forced landing near South East Merredith Railway Station Victoria. Pilot; F/Lt J S Archer CGS not injured. Aircraft received limited damage. Vehicle Accident during recovery transit when RAAF Trailer Reg#AF11224 (With Spitfire A58-173 being carried on) was hit from behind by Civilian Vehicle Reg#AG000 collided, causing more damage to rear fuselage of A58-173 at Merredith 22/12/44. AMSE Approval to Write-off per File #9/16/1779 to convert to components 26/12/44 at 1CRD. Pics Held 79Sqn and CGS fin"
From the Australian War Memorial Site
79 Squadron was formed at Laverton, Victoria, on 26 April 1943. The squadron received its first Spitfire VC aircraft a month later, when it was ordered to deploy to Goodenough Island, off New Guinea's east coast. The advance party from the squadron left Australia in mid-May, while the pilots and aircraft followed in early June.
The airfield on Goodenough Island was later known as Vivigani.

Goodenough Island - Wikipedia

en.wikipedia.org

A quick look through the ADF listing shows JG796 / A58-175 underwent the same modifications on 2 June 1943. And JG807 / A58-176, JG891 / A-58-178, JG912 / A-58-80. All with 79 squadron. You can probably find more in a detailed search. So given where they were operating from there was, at least in theory, no need for dust filters.

ADF Serials - Spitfire

By Aug 1943 they had moved to Kiriwina Island and there are photos of 79 squadron with the Vokes filters again. The AWM descibes the operating conditions there as "adverse". And then on to Momote in the Admiralty Islands in March 1944. Both airfields appear to have been very dusty, which might explain the need for the filters again. These island strips were often had surfaces made from crushed coral, which didn't do a lot of good to aircraft engines.

 

[Spits2]

Yes the smooth cowl was only in use for a short amount of time...Coral dust was found to be more abrasive than normal dust off memory and caused a lot of drama.The Australian Archives does have the Australian Smooth cowl GA drawing and offset measurements to create one.Was a straight bolt up job to the existing structure.There was some modding though to move gun cooling ducting and other stuff.This I heard third hand from Langdon badger Jr who has talked to ex 79Sqd airframe fitters while they were alive.

 

[Fatboy Coxy]

Aircraft history for JG740 / A58-173 (the only JG74* serial delivered to Australia) from ADF Serials site:-

"SS Austral Star 11/03/43. Rec 1AD ex UK 11/03/43. Rec 79Sqn RAAF ex 1AD 14/05/43. Rec 1AD ex 79Sqn RAF for Temperate Cowl(Tropical Volks intake removed) 30/05/43. Rec 79Sqn RAAF ex 1AD 06/06/43. As JG740, Coded UP-U and later named "Sweet FA" on Port side, forward of cockpit. Flown by 79 Sqn CO S/L A C Rawlinson. Rec 6AD ex 79Sqn RAAF 16/06/44. Rec CGS ex 6AD 11/09/44. All over metal finish except for black anti-glare panel, white empennage & leading wing edges and Coded "E", while with CGS. Accident 1000hrs 21/12/44 during camera gun exercise, when following engine failure due to coolant leak, aircraft made forced landing near South East Merredith Railway Station Victoria. Pilot; F/Lt J S Archer CGS not injured. Aircraft received limited damage. Vehicle Accident during recovery transit when RAAF Trailer Reg#AF11224 (With Spitfire A58-173 being carried on) was hit from behind by Civilian Vehicle Reg#AG000 collided, causing more damage to rear fuselage of A58-173 at Merredith 22/12/44. AMSE Approval to Write-off per File #9/16/1779 to convert to components 26/12/44 at 1CRD. Pics Held 79Sqn and CGS fin"
From the Australian War Memorial Site
79 Squadron was formed at Laverton, Victoria, on 26 April 1943. The squadron received its first Spitfire VC aircraft a month later, when it was ordered to deploy to Goodenough Island, off New Guinea's east coast. The advance party from the squadron left Australia in mid-May, while the pilots and aircraft followed in early June.
The airfield on Goodenough Island was later known as Vivigani.

Goodenough Island - Wikipedia

en.wikipedia.org

Hi EwenS, thank you for this, I love the detail, especially the bit about the road accident!

 

[Fatboy Coxy]

It wasnt just the filter but the Tropical Oil tank was longer and held more oil to keep the Merlin cooler in Tropical climates.It gives the cowl a Pigeon breast appearance.Same as the PR Spits.

Hi Spits2, thank you for this, love the diagrams, but even more so the bit of detail you've given me. It been staring me in the face but I've been so focused on how important the air filter was because of the dusty conditions at many of the airfields the aircraft were operating from that I lost site of the fact that the Vokes Air Filter was a special 'tropicalised' filter, and not just a large air filter. As you say it housed a bigger oil tank, to help keep the engine cooler, something that would be needed throughout the Middle East, and Far East. So any aircraft going to the 'Tropics' would get one regardless of whether the airfield was dusty or not. And given the advantage of air power, that it can be shifted from one airfield to another, you might not know if you're going to need a bigger air filter for the dust or not, but the hotter climates would remain regardless.

A more sophisticated approach might have been to be able to exchange the large cowling and air filter with a smaller one, while still retaining the larger oil tank. And on the subject of the oil tank, was a larger cowling and greater air flow an advantage in cooling the oil within the oil tank?

 

[EwenS]

Hi Spits2, thank you for this, love the diagrams, but even more so the bit of detail you've given me. It been staring me in the face but I've been so focused on how important the air filter was because of the dusty conditions at many of the airfields the aircraft were operating from that I lost site of the fact that the Vokes Air Filter was a special 'tropicalised' filter, and not just a large air filter. As you say it housed a bigger oil tank, to help keep the engine cooler, something that would be needed throughout the Middle East, and Far East. So any aircraft going to the 'Tropics' would get one regardless of whether the airfield was dusty or not. And given the advantage of air power, that it can be shifted from one airfield to another, you might not know if you're going to need a bigger air filter for the dust or not, but the hotter climates would remain regardless.

A more sophisticated approach might have been to be able to exchange the large cowling and air filter with a smaller one, while still retaining the larger oil tank. And on the subject of the oil tank, was a larger cowling and greater air flow an advantage in cooling the oil within the oil tank?

Many of the PR Spitfire Marks got the enlarged oil tank without the Vokes filter, e.g. the PR.XI

Spitfire pilots and aircraft database - Spitfire EN427

Some earlier British based PR.IV/VII were similarly equipped.

 

[Fatboy Coxy]

Many of the PR Spitfire Marks got the enlarged oil tank without the Vokes filter, e.g. the PR.XI

Spitfire pilots and aircraft database - Spitfire EN427

Some earlier British based PR.IV/VII were similarly equipped.

Hi EwenS, thank you for this. The earlier Hurricane and Spitfire aircraft used for Photo Recon were, I believe, given the Aboukir Filter, instead of the Vokes, but that's around 1941, whether that practice continued up to 1943 and onwards, I can't say.

 

[Spits2]

Filtered air kept the piston rings alive..Cool oil kept the Crankshaft and cam bearings alive..Also in the Tropics there were mods to the fuel system as well as tank pressurization stopped vapor locks and starvation problems..

 

[Jabberwocky]

Performance figures I have read suggest the Spitfire lost 16 mph due to its air filter, the Hurricane lost 20-25 mph, and I'm unaware what performance was lost on the P40s.

On the performance hit, I think the unfortunate answer is "it depends on which figures you want to use".

The tests of tropicalised Mk Vb with Vokes filters I can find has them achieving the following speeds

• 354 mph at 17,400 ft (AB. 320, tested April 1942 by A&AEE)
• 357 mph at 11,200 ft (JK 940, no test date. Same aircraft given as 358.5 mph at 10,700 ft for 'New Tropical' filter, 383 mph (!) at 10,700 ft with 'Temperate' filer and 354 mph at 11,500 ft for 'Aboukir' filter)
• 351.5 mph at 10,200 ft (EF 541, no test date. Same aircraft given as 348.5 mph at 11,000 ft for 'New Tropical' filter and 354 mph at 11,200ft with 'Temperate' filter).

The second and third aircraft are from a chart on page 177 of Morgan & Shacklady's Spitfire: The History.

RAE/A&AEE testing of new Mk Vbs generally had them achieving top speed between 368 mph and 372 mph, at full throttle heights of 19,000 to 20,800 feet. Data sheet speed is listed as 371 mph.

So it would appear based on this data point that the Vokes filter cost this aircraft somewhere between 2.5 and 26 mph in top speed and as much as 10,000 feet in full throttle height.

Another datapoint is from the RAAF. Their testing of a tropicalised Mk Vc has it achieving 365 mph at 22,000 feet. Non-tropicalised versions achieved 368 to 372 mph in testing. So on this basis, the Vokes filter may have only cost the aircraft somewhere between 3 and 7 mph in top speed.

This is backed up by some comparative testing from the RAAF. According to Cooper in Darwin Spirfires , pilots were claiming a loss of 10-15 knots in speed from the Vokes filter. So the RAAF's Directorate of Technical Services conducted direct performance tests of Mk Vs with 'temparate' intakes and with Vokes intakes and found the tropicalised aircraft were "no more than 5 knots slower than the temperate intake aircraft".

Average speed of tropicalised aircraft at 21,000 ft was found to be 310 knots - which is just short of 357 mph and very much in line with what in service aircraft were achieving in Europe. The A&AEE performed tests on 10 different Mk Vs and Mk VIs in late 1942. Of the six Mk Vs tested, top speed varied between 352 mph and 368 mph and full throttle heights varied between 20,200 feet and 22,600 feet.

So, does the Vokes filter cost as much as 26 mph in speed, or only 2.5 mph?

Personally, I'd lean towards the RAAF finding of ~6 mph for an in service aircraft.

Remember that a BUNCH of other factors are in play here - engine performance, airscrew fitted, carburettor type, airframe details, overall fit and finish and other factors can have a huge influence on performance. The Vokes is just one factor that could have degraded speed compared to a brand new, fresh off the production line aircraft.

 

[ThomasP]

re

The tests of tropicalised Mk Vb with Vokes filters I can find has them achieving the following speeds

• 354 mph at 17,400 ft (AB. 320, tested April 1942 by A&AEE)
• 357 mph at 11,200 ft (JK 940, no test date. Same aircraft given as 358.5 mph at 10,700 ft for 'New Tropical' filter, 383 mph (!) at 10,700 ft with 'Temperate' filer and 354 mph at 11,500 ft for 'Aboukir' filter)
• 351.5 mph at 10,200 ft (EF 541, no test date. Same aircraft given as 348.5 mph at 11,000 ft for 'New Tropical' filter and 354 mph at 11,200ft with 'Temperate' filter).

I believe there is a misunderstanding re

. . . and as much as 10,000 feet in full throttle height.

Airframe AB320 results were for 3000 rpm at +9 lbs boost. Engine critical altitude w/o tropical filter was ~16,250 ft, so the Vmax at 17,400 ft was with tropical filter and RAM. Based on tests of other Spit Mk Vs the Vmax at 3000 rpm and +9 lbs boost would have been at around 19,000 - 20,000 ft w/o tropical filter but with RAM. So 2,000 - 3,000 ft loss of critical altitude due to the filter?.

Airframes JK940 and EF541 results were for 3000 rpm at +16 lbs boost. Engine critical altitude w/o tropical filter was ~9,000 ft, so the Vmax at 10,700 - 11,500 ft was with tropical filter and RAM. Based on tests of other Spit Mk Vs the Vmax at 3000 rpm and +16 lbs boost would have been at around 13,000 ft w/o tropical filter but with RAM. So a loss of 1,500 - 2,300 ft of critical altitude due to the air filter?

I think this is correct.