# Aboukir’s High-Altitude Spitfire (Original Modification)



## HF_Spit (Mar 13, 2017)

Hello
I would like to try & get the records straight about the event that took place in North Africa during WW2, where a modified Spitfire shoots down a JU86.

I am the Grand son of the mechanic who made this modification. My granddad's name is Bert Woods.
I hope you understand that this is important to me to get this straight & also important amongst aviation enthusiasts.

He told me about this event long before it was posted on the internet.
I am pretty sure he didn't realise the significance of what he was doing at the time but it certainly made a difference.
Although we were close he didn't say too much but he told me about this not long before he died so it must have been important to him.

This is what he told me.
There was a German reconnaissance aircraft flying every day over us & we couldn't do anything about it, it was flying much too high for any fighter. He said you could hear it but do nothing. Then every night there was a bombardment. He told me it was obviously spotting positions.

So they decided to do something about it. (I could tell Bert was annoyed while he was telling me this, he had his annoyed, angry face)

He made it very clear they only had one aircraft & that was a Spitfire that he maintained. He made this very clear because he told me how the conditions were pretty bad where he was.

The way he told me the story, it did not sound like a very planned modification at all. It was one done quickly & worked first time.

He told me about removing the Hispano Cannons and other details that match most of the Wikipedia information but
there is one big detail he told me that is not mentioned anywhere else yet about this modification. He went into detail about how long it took to remove many of the Spitfires main bolts. He told me that these took a lot weight up. I queried him about this but he insisted this is what he did. This detail is not mentioned anywhere else because no-one else knows apart from the real man who did it & who ever he decided to divulge the information to, I am assuming he told the Pilot too. Also the modifications that took place after this original most likely would not have had this done & so it would not have been recorded.

He said the pilot took off and shot down the reconnaissance plane.

He made it clear that after this the bombing stopped at night.

Bert Woods (Far Left)

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## Airframes (Mar 13, 2017)

Welcome to the forum, and thanks for the info.
When you mention "main bolts", do you know which bolts your Grandfather was referring too ?
The 'high flyers' in the UK itself, not only had the cannon and associated equipment removed, but also the armour plate, and any _unessential _equipment and fittings.


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## vikingBerserker (Mar 13, 2017)

That is awesome, thanks for posting this.


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## Robert Porter (Mar 13, 2017)

Very interesting, I too would love to know what he meant by main bolts.


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## stona (Mar 13, 2017)

Is this a No.145 Squadron Spitfire we are talking about? At least one of these was indeed lightened and shot down a Ju 86 P in 1942.

All high altitude adaptations required the removal of all non essential equipment, 145 Squadron reduced the armament to just two .50 calibre machine guns. The lack of radio (wireless in the 1940s) meant that the lightened aircraft operated in conjunction with a normally equipped aircraft which could maintain radio contact with controllers on the ground. Once visual contact was made the lightened aircraft would attempt the interception.

Later No.41 Squadron adopted a system of flying a pair of Spitfires, one armed with only .303 machine guns which would attempt to drive the target down to an altitude at which its cannon armed partner could finish it off.

There were many different attempts to lighten and tweak Spitfires for extremely high altitude interceptions.
'Bill' Kain (who may still have been with No. 127 Squadron) flew a modified Mk IX, MA504, to an altitude of 47,000ft on a test flight, 16th April 1944.

MA504 was later much photographed while serving with No. 41 Squadron.

Cheers

Steve

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## HF_Spit (Mar 13, 2017)

I have added a picture of Bert & his unit which my cousin has just sent to me.
Bert was clear when he told me only one Spitfire was used to bring down a JU86. 

Bert was also clear about this being the first modification like this. He was an excellent mechanic & worked on many interesting projects & British WW2 Aircraft throughout the war. 

Regarding the Bolts he talked of removing. This is something that stuck very clearly in my memory. What I do remember was that he told me there was a lot of them & this is what took time. I imagine only a real Spitfire mechanic would really understand.

It's amazing to think that this one Spitfire could make so much difference because it was not expected.


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## stona (Mar 14, 2017)

Great picture! When I was last in Egypt a few years ago you couldn't get near those monuments for the throngs of tourists!

The first specially modified Spitfire was the Mk V converted on No. 145 squadron as far as I know. I think this may be the one which your relative worked on. That aircraft was the first to make a successful interception of a Ju 86 and it would indeed have been a nasty surprise to the Luftwaffe, but it was not the only one, nor the last.
There were numerous other conversions, several Mk IXs later in the war. This is a matter of historical record.

There are plenty of people who know and understand how a Spitfire goes together and works. I don't think that they or a 'Spitfire mechanic' would know what 'main bolts' indicates. Most would interpret that as meaning something to do with either the wing or engine attachment, and those fixings were definitely not removed  
The weight was saved by the removal of non essential equipment, non essential in the sense that it could be removed without compromising the integrity of the airframe or its ability to fly. For example, all pilot armour was removed. It was considered non essential, but what the pilots thought about this might be different. Later all paint was removed, again deemed non essential, and the aircraft puttied and sanded to a smooth finish before being polished.

Cheers

Steve


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## HF_Spit (Apr 20, 2017)

Hi,
The reason Bert (My Grand Dad) mentioned main bolts to me because these were Bolts which could be considered by some to compromise the integrity of the Aircraft. This is why he made such a point about it to me & why i have made such a point about it here.
In his opinion the Spitfire would be ok without theses 'Bolts'. As he led me to believe they were not to do with non-essential equipment, that it was a calculated risk. He also made no mention of extended wing tips.

He did tell me some details to do with the shooting down.
The pilot only just managed & struggled to reach the altitude of the German aircraft, that the pilot & his Spitfire was barely able to get shots in & only damaged it just enough. He said we were really lucky.

I am sorry Steve but this is how he explained it to me. I can not go into more detail than this. I guess it is unfortunately open to interpretation.


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## HBPencil (Apr 20, 2017)

Cool, thanks for the info 



HF_Spit said:


> Hi,
> The reason Bert (My Grand Dad) mentioned main bolts to me because these were Bolts which could be considered by some to compromise the integrity of the Aircraft.



At the risk of exposing myself to ridicule as I'm no engineer but I can see how removing a number of bolts from, say, the empennage and wing roots may work given that the aircraft was not expected to be exposed to anything but moderate maneuvering given the extreme altitudes it was to operate at? I guess the pilot would've been obliged to descend gently and keep an eye on his airspeed.


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## Zipper730 (Feb 17, 2019)

HF_Spit said:


> The reason Bert (My Grand Dad) mentioned main bolts to me because these were Bolts which could be considered by some to compromise the integrity of the Aircraft. . . . In his opinion the Spitfire would be ok without theses 'Bolts'.


So, the idea was to remove some structural reinforcement because it would save weight, and up high, you wouldn't need to aggressively maneuver the plane since the airplane is barely even flying (just enough to maneuver on another airplane that's barely flying), right?


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## Colin (Jun 10, 2019)

HF_Spit said:


> Hello
> I would like to try & get the records straight about the event that took place in North Africa during WW2, where a modified Spitfire shoots down a JU86.
> 
> I am the Grand son of the mechanic who made this modification. My granddad's name is Bert Woods.
> ...


Well this is very interesting. My father Don Enticknap was also an aircraft mechanic and recounted a similar story. However he mentioned a modification to the engine fuel system. He was fed up of being marched out into the desert during the evening for safety. Every time they came back they had been bombed and he lost his kit. Whatever the modifications were they were all relieved by the result. He also mentioned fitting American engines to Spitfires but I don’t have any details. Another interesting modification he was involved with was fitting a cannon for tank busting but I don’t know which aircraft.


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## special ed (Jun 10, 2019)

My library is no longer in any order to find anything, However I remember reading about this and it is listed as the highest interception/combat of WW2 without a pressurized cockpit (by the Spitfire) using only oxygen mask. As I recall, the altitude was something like 42,500 feet at contact.


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## Zipper730 (Jun 10, 2019)

HF_Spit said:


> In his opinion the Spitfire would be ok without theses 'Bolts'.


Do you mean the stub-attachment points?


> He also made no mention of extended wing tips.


They were fitted with extended wing-tips...


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## fubar57 (Jun 10, 2019)

He hasn't been here since you last quoted him two years ago and what had extended wing-tips?

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## fubar57 (Jun 10, 2019)

5 Spitfire Mk.VIs in the form of improvised PR Mk VIs arrived in Egypt in 1943 after the threat of the Ju-86P had passed. They were also unarmed


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## fubar57 (Jun 10, 2019)

This may be the modified Mk.V, BR114





Da Web

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## Zipper730 (Jul 10, 2019)

fubar57 said:


> He hasn't been here since you last quoted him two years ago


Uh, I didn't notice that...


> and what had extended wing-tips?


The improvised Aboukir High Altitude Spitfire (modified Mk.V). From the picture you have, it would appear that you have the right aircraft. Pointed-tips and a four-bladed propeller, with an Aboukir filter.


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## fubar57 (Jul 10, 2019)

I posted a photo of it last month


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## Zipper730 (Jul 10, 2019)

fubar57 said:


> I posted a photo of it last month


It's a cool picture...

I'm curious how they boosted the engine's compression-ratio: Did they re-gear the supercharger, increase the RPM, something something else?


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## wuzak (Jul 11, 2019)

Zipper730 said:


> It's a cool picture...
> 
> I'm curious how they boosted the engine's compression-ratio: Did they re-gear the supercharger, increase the RPM, something something else?



I don't think they did anything to the engine.

Reducing the weight increased the aircraft's operational ceiling.


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## Zipper730 (Jul 11, 2019)

wuzak said:


> I don't think they did anything to the engine.


I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.

How would you boost compression ratio?


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## pbehn (Jul 11, 2019)

Zipper730 said:


> I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
> 
> How would you boost compression ratio?


why would you?


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## gumbyk (Jul 11, 2019)

Zipper730 said:


> I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
> 
> How would you boost compression ratio?



That's not a field mod.

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## pbehn (Jul 11, 2019)

gumbyk said:


> That's not a field mod.


A masterpiece of understatement gumbyk.


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## Zipper730 (Jul 12, 2019)

gumbyk said:


> That's not a field mod.


Well, it was technically a maintenance-depot mod: I remember reading about it, of all places, Wikipedia. Normally, this isn't a suitable source, but the source was "Spitfire: The History" by Eric B. Morgan and Edward Shacklady, which I own. The source listed pages 149, 155-156. 

While page 149 didn't seem to have anything of use, page 155-156 did: Under "MIDDLE EAST MODS", there's the following entry


> .......Another successful Middle East modification, again by No 103 MU, was the high altitude VC developed to meet the high flying Ju86P twin engined, photo-reconnaissance aircraft. Allied naval movements from Egyptian bases, so vital to the defense of the Middle East, were being monitored by a team of four Ju86s and there was little the RAF could do when the German aircraft flew over at heights of 37,000 ft plus. Normal operation ceiling of the VC was approximately 36,000 ft, but at that height it had to be an exceptional airplane and was, normally, an unstable gun platform. The staff of No 103 MU, practically built their own high altitude fighter by taking a new VC (BP985) that had arrived at the depot on 4 May 1942 and stripping it of all unnecessary weight.
> 
> ........It was not feasible to increase the supercharger gear ratio so it was decided to raise the compression ratio of the Merlin 46 by as much as possible. The cylinder block was modified by hand as were the liners. A four blade de Havilland 45/1 Hydromatic propeller was installed, plus an Aboukir filter which incorporated the 91/2 gallon oil tank. All armoured plate was removed together with the Hispano cannon installation leaving just the two inner .303 Browning machine guns. Finally, extended wing tips of local manufacture were added.


So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?


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## wuzak (Jul 12, 2019)

Zipper730 said:


> So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?



different pistons, shave the surface of the head and/or block. From the description you provide it looks like they shaved the block (and cylinder liners) to raise compression.


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## Zipper730 (Jul 12, 2019)

wuzak said:


> different pistons, shave the surface of the head and/or block.


So they basically reduced the volume inside the cylinders so the pressure would go up more?


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## tyrodtom (Jul 12, 2019)

No they didn't reduce the volume inside the cylinders, they reduced the volume in the combustion chambers.
The combustion chambers is on the top of the cylinders.
So that the fuel/air mixture in the cylinders is compressed into a smaller space when the pistons get to the top of their stroke.

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## Zipper730 (Jul 21, 2019)

I assume the higher compression ratio would produce a higher RPM? Or would the RPM be kept the same as before?


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## tyrodtom (Jul 21, 2019)

Higher compression has nothing to do with producing more RPM.
It means you produce more HP per power stroke of each piston.

Zipper you're asking questions that indicate you have little knowledge of the fundamentals of the internal combustion engine.
You need to address this shortfall yourself rather than rely on this forums members to constantly educate you a tidbit at a time.


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## pbehn (Jul 21, 2019)

Zipper730 said:


> Well, it was technically a maintenance-depot mod: I remember reading about it, of all places, Wikipedia. Normally, this isn't a suitable source, but the source was "Spitfire: The History" by Eric B. Morgan and Edward Shacklady, which I own. The source listed pages 149, 155-156.
> 
> While page 149 didn't seem to have anything of use, page 155-156 did: Under "MIDDLE EAST MODS", there's the following entry
> So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?


The compression ratio, is the ratio of the volume of the cylinder at BTC compared to when it is at TDC. Basically how much of the swept volume is compressed into the "combustion chamber" which is what is left when the piston is at the top of its stroke. With a supercharger or turbo charger the compression ratio of the engine is a bit academic because the gas is already compressed when the piston is at bottom dead centre. This is what all the discussions of pressure and fuel grades is all about. A Merlin or any other supercharged engine has a very low compression ratio compared to any normally aspirated high performance engine.


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## Zipper730 (Jul 27, 2019)

pbehn said:


> The compression ratio, is the ratio of the volume of the cylinder at BTC compared to when it is at TDC. Basically how much of the swept volume is compressed into the "combustion chamber" which is what is left when the piston is at the top of its stroke.


I understand that. What I'm curious about is how the increase in power would be expressed.

Spin the shaft faster?
Absorb more power by adjusting the propeller for the given engine/propeller RPM?


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## pbehn (Jul 27, 2019)

Zipper730 said:


> I understand that. What I'm curious about is how the increase in power would be expressed.
> 
> Spin the shaft faster?
> Absorb more power by adjusting the propeller for the given engine/propeller RPM?


No idea what you are asking, what does "expressed" mean? What is the increase in power?


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## wuzak (Jul 27, 2019)

Zipper730 said:


> I understand that. What I'm curious about is how the increase in power would be expressed.
> 
> Spin the shaft faster?
> Absorb more power by adjusting the propeller for the given engine/propeller RPM?



The extra power would allow the propeller to be run at a coarser pitch.

The engine speed would not necessarily change, and certainly wouldn't for a WW2 aero engine.

Usually for a higher compression ratio you can't run as much boost. For this particular application that wasn't as important, since the performance gain they were seeking was well above the engine's critical altitude.


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## pbehn (Jul 27, 2019)

wuzak said:


> The extra power would allow the propeller to be run at a coarser pitch.
> 
> The engine speed would not necessarily change, and certainly wouldn't for a WW2 aero engine.
> 
> Usually for a higher compression ratio you can't run as much boost. For this particular application that wasn't as important, since the performance gain they were seeking was well above the engine's critical altitude.


Is it possible that they increased the compression ratio to get a slight improvement at very high altitude trading lower performance at lower altitude because using maximum boost would cause detonation?


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## wuzak (Jul 27, 2019)

pbehn said:


> Is it possible that they increased the compression ratio to get a slight improvement at very high altitude trading lower performance at lower altitude because using maximum boost would cause detonation?



Yes.


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## Zipper730 (Jul 28, 2019)

Okay, so the modifications would serve to drive up the normal pressure within the cylinders, which would increase horsepower for a given throttle setting, and allow the propeller to be driven at a higher pitch setting for the same RPM. With the pressure within the cylinders being higher, the risk of detonation would be higher and, the engine would need a bit of throttling at lower altitudes, in order to prevent this, and a greater throttling loss would occur in this area?


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## pbehn (Jul 28, 2019)

Zipper730 said:


> Okay, so the modifications would serve to drive up the normal pressure within the cylinders, which would increase horsepower for a given throttle setting, and allow the propeller to be driven at a higher pitch setting for the same RPM. With the pressure within the cylinders being higher, the risk of detonation would be higher and, the engine would need a bit of throttling at lower altitudes, in order to prevent this, and a greater throttling loss would occur in this area?


It isn't my "major" but as I understand it the higher you go the less the twin supercharger works. Eventually you reach a stage where the total effect of the twin supercharger and intercooler delivers a charge that is at the level (or close to) sea level atmospheric pressure. Then you use the "tricks" that an atmospheric engine uses, like increasing the compression ratio. As I see it the engine was optimised for one use and would be of little use anywhere but extreme altitude.


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## Zipper730 (Jul 28, 2019)

pbehn said:


> It isn't my "major" but as I understand it the higher you go the less the twin supercharger works.


The Aboukir Spitfire used a single stage supercharger with two speeds.


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## pbehn (Jul 28, 2019)

Zipper730 said:


> The Aboukir Spitfire used a single stage supercharger with two speeds.


Its the same principle. The density of the exploding charge is what governs the power output. To achieve that density the supercharger becomes less effective the higher you go. The Merlin and Allison engines are not highly tuned for atmospheric engines the tuning is in the supercharger or turbo set up. A compression ratio of 6-1 is "cooking" in a normally aspirated engine.


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## wuzak (Jul 28, 2019)

Zipper730 said:


> The Aboukir Spitfire used a single stage supercharger with two speeds.



Single speed single stage.


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## Zipper730 (Jul 28, 2019)

wuzak said:


> Single speed single stage.


I thought the V's were twin-speeds...


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## wuzak (Jul 29, 2019)

Zipper730 said:


> I thought the V's were twin-speeds...



Nope.

The Mk.III had 2 speed (Merlin XX), some others were modified with the Merlin XX and maybe some Seafires. Otherwise all 2 speed engines in Spitfires were also 2 stage engines.

The Spitfire V was mostly fitted with the Merlin 45.


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