P-51 fuselage fuel tank

drgondog · Jul 11, 2024

MIflyer said:
One thing I have wondered about is just how interchangable the different tanks were. For example, the P-47 had that "flat" 108 gal belly tank that I do not think that was used on any other aircraft. Of course its design was to accommodate that low hanging belly of the P-47.

Also no mention of the 165 gal steel drop tanks Lockheed was building. That Av Week article is dated June 1943, so presumably they were in full production before then. Perhaps they were reserved for the Pacific?

The early drop tanks used on Hurricanes had built-in pumps to get the fuel out. I wonder just when the approach of using the exhaust from the the vacuum pump was adopted? Since we had drop tanks long before the RAF I assume it was a US invention.

I have often thought that I'd like to write a book on The Other Stuff:

1. Drop Tanks
2. Radios
3. Oxygen Systems
4. G-Suit Systems
5. Environmental Control Systems
6. Automated Controls

The collective impact of all that Other Stuff was enormous.

The 108gal was cylindrical paper composite Bowater, the 110gal steel tank and 115gal flat top were all hung on C/L - later the 108gal Bowater was hung on pylons. Also, the 150gal flat top was self sealing and designed for c/L or pylon - both with std 14 1/2" center to center lugs. Also made were 150gal steel tanks which may or may not have been self sealing - but the 165s dominated.

The Lockheed 165 was often referred to as the 150gal steel (same as 330/300gal Ferry tank). I am not sure what the pressure test threshold was, nor do I recall whether the P-38 adopted the vacuum pump exhaust slave to pressurize tanks from P-38J-15. The earlier P-38models IIRC were Not pressurized.

The P-38 pylons were home grown (ditto Republic and North American) but all imbedded B-10 until S-1 was adopted in 1945.

I finally found a reference in Fifth Air Force Huon Campaign to booster pumps immersed in the 200gal 'so called Brisbane tank' but I am near certain that Ford did not produce beyond the 4000 originally ordered - consequently 5th AF began conversion to B-7 centerline and modifying wings in December 1943.

Source FAREP Rpt No. 6 dated 20 November, 1943 Memo to the Chief of the Air Staff --------.> MG Barney Giles

MiTasol · Jul 11, 2024

MIflyer said:
One thing I have wondered about is just how interchangable the different tanks were. For example, the P-47 had that "flat" 108 gal belly tank that I do not think that was used on any other aircraft. Of course its design was to accommodate that low hanging belly of the P-47.

Also no mention of the 165 gal steel drop tanks Lockheed was building. That Av Week article is dated June 1943, so presumably they were in full production before then. Perhaps they were reserved for the Pacific?

The early drop tanks used on Hurricanes had built-in pumps to get the fuel out. I wonder just when the approach of using the exhaust from the the vacuum pump was adopted? Since we had drop tanks long before the RAF I assume it was a US invention.

I have often thought that I'd like to write a book on The Other Stuff:

1. Drop Tanks
2. Radios
3. Oxygen Systems
4. G-Suit Systems
5. Environmental Control Systems
6. Automated Controls

The collective impact of all that Other Stuff was enormous.

The first attachment lists drop tank interchangeability in April 45 and the the P-39 and P-40 used the same 52 gallon tank but that is about my limit. The second attachment is a summary of the orders which may have more details.

Low Battery · Jul 11, 2024

So this sounds distinctly like what my self and others have suspected, which is that the "pump mod" was not on or in the P-47 but rather inside the drop tank itself. No wonder nobody used this, because putting a fuel pump in every single drop tank would be a massive logistical undertaking and maintenance issue.

MIflyer · Jul 11, 2024

Well that is interesting. According to a USMC pilot who flew F4F's from Guadalcanal the tanks they used were made of a "Bean pod" material (presumably nonmetallic) and required the pilot to manually pump the fuel out of the tank.

Looking at the F6F and F4U, it does not appear they used the same drop tanks.

Postwar, supposedly a nonmetallic F4U drop tank was used to build the fuselage of the Piper Skycycle, but they did not build many of them.

Shortround6 · Jul 11, 2024

MIflyer said:
Well that is interesting. According to a USMC pilot who flew F4F's from Guadalcanal the tanks they used were made of a "Bean pod" material (presumably nonmetallic) and required the pilot to manually pump the fuel out of the tank.

The F4F may have used two different aux fuel tanks. There was an early tank that was not droppable, supposed to be used for ferry only? but in the SP who knows? Carriers only had so much storage space so there were also conflicting orders/requirements for bring tanks back and/or landing with fuel in them and having them fall off during the arresting landing.

An interesting line of questioning.

MiTasol · Jul 11, 2024

Low Battery said:
View attachment 788306
So this sounds distinctly like what my self and others have suspected, which is that the "pump mod" was not on or in the P-47 but rather inside the drop tank itself. No wonder nobody used this, because putting a fuel pump in every single drop tank would be a massive logistical undertaking and maintenance issue.

As well as requiring wiring modifications, at least one extra switch and CB as significantly increasing the cost of the tank.

Engineman · Jul 12, 2024

PAT303 said:
But it was okay for P51 pilots to have 85G of fuel in the same spot, or Me109 and P47 Thunderbolts that had the pilots seat bolted to the fuel tank?. The rear fuel will be burnt off before engaging in combat, you know that.

Lets get it right about the Bf 109. The pilots seat was not "bolted to the fuel tank". The fuel tank was an L shape that fitted behind and under the cockpit tub floor and rear bulkhead and was isolated from the cockpit interior. The F and early G series fuel tank was often a rubber reinforced self sealing type, but later this reverted to an alloy rigid type. The laminated armour plate was fitted behind the fuel tank and blanked-off the rear fuselage. In the case of the Bf 109 under fuselage drop tank, it delivered its fuel to the single main tank directly, when selected to transfer by turning-on the air feed that transferred the fuel, purely by air pressure.

Eng

PAT303 · Jul 12, 2024

Engineman said:
Lets get it right about the Bf 109. The pilots seat was not "bolted to the fuel tank". The fuel tank was an L shape that fitted behind and under the cockpit tub floor and rear bulkhead and was isolated from the cockpit interior. The F and early G series fuel tank was often a rubber reinforced self sealing type, but later this reverted to an alloy rigid type. The laminated armour plate was fitted behind the fuel tank and blanked-off the rear fuselage. In the case of the Bf 109 under fuselage drop tank, it delivered its fuel to the single main tank directly, when selected to transfer by turning-on the air feed that transferred the fuel, purely by air pressure.

Eng

I get what your saying, the notion was the Spit pilot was somehow more venerable if fitted with the rear aux tank where's the 109 pilots sat on it's tank but as you stated in both cases the tanks were outside the cockpit tub so the risk in both situations is the same.

PAT303 · Jul 12, 2024

tomo pauk said:
No, not the Mk.III...

The Seafire MkIII was the first model to have the leading edge tank not the Mk XV, it also had the stubs removed and bulges streamlined to lower drag. It was an all over better aircraft than the previous models.

PAT303 · Jul 12, 2024

Shortround6 said:
The two videos show the progression and show problems with the rear tank on the Spitfire. Yes they yanked 40lbs of ballast out of the rear of the fuselage for the 29 gallon tank. Also be aware of prop and engine changes affected CG issues. Also the radio fit and change in oil tankage.

A lot of stuff to consider above "they had room for it! shove it in there!"

Yet they fitted the 33 gallon lower 42 gallon upper. Everyone keeps going on about COG changes etc but the P51 had the identical problems yet the Yanks just got on with it.

PAT303 · Jul 12, 2024

EwenS said:
Given that the designers were not idiots, there had to be a good reason for not using it.

They managed to fit almost every variant of the Merlin, various models of the Griffin, fit it with bombs, rockets, JATO packs, machine guns, cannons, cannon and machine guns, make it a premier PR aircraft, have it reach the highest speeds a piston engined plane could reach, fly the highest of any plane, it has the highest piston engine kills of the war, it flew off carriers and fought in every theater, it was even used as a glider tug, but getting another 13 gallons in an empty space in the wing leading edge was beyond them?. Sorry not buying it.

Reluctant Poster · Jul 12, 2024

MiTasol said:
The first attachment lists drop tank interchangeability in April 45 and the the P-39 and P-40 used the same 52 gallon tank but that is about my limit. The second attachment is a summary of the orders which may have more details.

Thanks for this. Very interesting.
To me there seems to be a lack of focus here. So many paths are being explored.
I work with a lot of recent grads and there are two rules I try to get them to follow
1. Before you start to solve a problem you should know the answer. This is a glib way of stating that you need to define the problem as much as possible before you start. If you have an idea of the answer should be you can quickly discard the path chosen and try again. There is tendency to try multiple paths in the hopes that one of them works. This wastes a lot of time and effort. The correct decision can actually be lost in the morass.
2. The second is a corollary to the first. Any decision is better than no decision. It is best to pick what you think will work. If it is the correct decision you are golden. If it's the wrong decision you can quickly come to that conclusion and you will learn from the mistakes and should be able to come to the correct decision quickly. This in my experience takes less time than trying all possible solutions.

To be fair I wasn't there at the time and can never fully understand the situation as they saw it. World War II was fought in an incredibly short time and decisions were made under tremendous time pressure. Its amazing that so many good decisions were made. Its certainly more fun to critic the "poorer" decisions than to praise the many brilliant decisions that were actually made.

Shortround6 · Jul 12, 2024

PAT303 said:
Yet they fitted the 33 gallon lower 42 gallon upper. Everyone keeps going on about COG changes etc but the P51 had the identical problems yet the Yanks just got on with it.

We don't know if the Yanks had an identical problem or a similar problem. It is not identical unless there was an identical shift in center of gravity and an identical change in aircraft behavior. IDENTICAL, not similar.

Looking at some of the tests on WWII Aircraft Performance

It appears that Spitfire MK Vs had a CG that ranged from 7.3in aft to 8.5 in aft. depending on exact model and equipment. And weights varied for the ones they had CG info on from 6525 to 6975lbs.

Now the weight and balance chart for the MK IX with five 17.5lbs weight stuffed behind the tail wheel, shows a CG of 4.4in aft to 4.8in aft. (they made the chart for both eight .303 guns and for standard two 20mm and four .303 armament. The eight .303 guns and ammo load was lighter but the CG was 0.4in further aft.
O.4in is probably not enough to cause problems.
However it shows that the MK IX was a lot less tail heavy than earlier versions, even with all the ballast shoved in the tail. Perhaps something to do with with that two stage Merlin and the 4 blade prop?

Saying they did something on the MK IX and that proves it could be done on the earlier versions may not hold water.

MIflyer · Jul 12, 2024

Shortround6 said:
The F4F may have used two different aux fuel tanks.

The lower tank you can see in this picture of a F4F aft fuselage is the reserve fuel tank. So it is possible he might have been pumping fuel from that source, although I think that unlikely. Of course, we do not know if he was in an F4F-3 or F4F-4, either.

Interesting that Lockheed inquired whether the drop tanks were for low altitude ferry flights or high altitude escort flights. You would need pressurized drop tanks for high altitude flights.

Zipper730 · Jul 12, 2024

Barrett said:
Seems self evident:

"With the internal fuselage tank filled with 85 gallons of fuel, the airplane is so unstable longitudinally that violet pullouts or tight turns must be executed with caution, as stick loads rapidly reverse..."

I wouldn't have thought that stick loads would reverse: I would have thought the plane would have merely pitched up violently and you'd either stall and dive into the ground or you'd snap your wings off.

MiTasol · Jul 12, 2024

Late war the RAAF removed the IFF from P-51D&K aircraft when the rear fuselage tank was used. By that time the allies had air superiority in the SWPA.

33k in the air · Jul 12, 2024

MiTasol said:
The first attachment lists drop tank interchangeability in April 45 and the the P-39 and P-40 used the same 52 gallon tank but that is about my limit. The second attachment is a summary of the orders which may have more details.

Regarding that first document, I have a couple of questions:

(1) Do you have the entire document, or know where it can be downloaded from? I'm interested in seeing the full publication.

(2) On page 7 of the PDF (document page 70) it shows the remainder of the data for the 165 gallon knockdown tank. Underneath that is an image of the 150 gallon drop tank used by the F6F Hellcat. But there is no descriptive text visible accompanying the photo. Does the source document have text that didn't translate to the PDF? Or is it missing from the source as well?

PAT303 · Jul 13, 2024

Shortround6 said:
We don't know if the Yanks had an identical problem or a similar problem. It is not identical unless there was an identical shift in center of gravity and an identical change in aircraft behavior. IDENTICAL, not similar.

If you read up on both aircraft they both had the same handling problems with a full rear fuel load, in both cases it was advised to plan for the rear fuel to be burnt off before enemy action was expected, the fact that people are now getting pedantic about exact wording leads me to believe that the argument against the fitment of rear tanks is being lost.

http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf

http://www.wwiiaircraftperformance.org/mustang/P-51B_Fuselage_Tank_4-43-23-1.pdf

PAT303 · Jul 13, 2024

Shortround6 said:
However it shows that the MK IX was a lot less tail heavy than earlier versions, even with all the ballast shoved in the tail. Perhaps something to do with with that two stage Merlin and the 4 blade prop?

Saying they did something on the MK IX and that proves it could be done on the earlier versions may not hold water.

I did make the argument about the rear tank being used to balance the heavier engine as the MkIX did have lead ballast in the tail, the MkV had the rear 29 gallon tank fitted so it was viable and George Buerling fitted his MkV he had in Malta with twin side by side drop tanks so if mods needed to be done they were. If you go back the the original question about the P51 it had all sorts of problems that presented themselves as the airframe changed, going to the bubble canopy caused fishtailing but did they say no good and stop using it, hell no, they fitted a fillet down the spine and problem solved.

Engineman · Jul 13, 2024

Zipper730 said:
I wouldn't have thought that stick loads would reverse: I would have thought the plane would have merely pitched up violently and you'd either stall and dive into the ground or you'd snap your wings off.

Then you are wrong.

Eng

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