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?