P-51 fuselage fuel tank

[Barrett]

The subject arose again in a couple of emails. Thought I'd share especially some members such as Bill Marshall can speak authoritatively.

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

The 500+ lbs of 85 gals. fuselage fuel was accepted for an aft CG.

Notice 3.b comments

Confirms what one of my Iwo Jima sources (21st FG) said, "The P-51 with a full fuselage tank did not fly like anything resembling an airplane."
An ETO guy (55th FG) who loved the 38 called the 51 "The funny plane" for the same reason.

 

[pbehn]

The subject arose again in a couple of emails. Thought I'd share especially some members such as Bill Marshall can speak authoritatively.

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

The 500+ lbs of 85 gals. fuselage fuel was accepted for an aft CG.

Notice 3.b comments

Confirms what one of my Iwo Jima sources (21st FG) said, "The P-51 with a full fuselage tank did not fly like anything resembling an airplane."
An ETO guy (55th FG) who loved the 38 called the 51 "The funny plane" for the same reason.

What are the comments in 3.B that should be noted? Why not take notice of comment 4. C. 5 ? Quote "Missions should be so planned that the fuselage tank should not contain more than 40 gallons where the pilot has a reasonable expectation of engaging in combat". Did the P-38s propensity to turn into a lawn dart and its need for dive brakes not earn it any epithet like "funny plane". How does the range of the funny plane with 40 gallons in the fuselage tank and maximum external fuel load compare to a P-38?

 

[Barrett]

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..."

 

[pbehn]

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..."

Yes, so dont do violent pull outs, in fact dont "do" combat. 45 gallons of the 85 gallons should be considered as external fuel, but 45 gallons at economical cruise is approximately 1 hour of extra range. Now, about my question, how did the P-51B with all tanks full but only 40 gallons in the fuselage tank compare to the P-38 (and P47)? How did a P-38 do when performing violent pull outs and tight turns with 500 gallons in external tanks?

 

[drgondog]

The subject arose again in a couple of emails. Thought I'd share especially some members such as Bill Marshall can speak authoritatively.

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

The 500+ lbs of 85 gals. fuselage fuel was accepted for an aft CG.

Notice 3.b comments

Confirms what one of my Iwo Jima sources (21st FG) said, "The P-51 with a full fuselage tank did not fly like anything resembling an airplane."
An ETO guy (55th FG) who loved the 38 called the 51 "The funny plane" for the same reason.

Unquestionably a full 85 gal fuel tank introduced aft CG issue - resulting in very low stick force per G. The common result was stick reversal in a turn or a snap roll. Consider that AAF decided to keep the 85gal tank, rather than change to 50 or 55gal tank. That decision ultimately killed the P-51D and D-1 in October 1943. Despite the lingering issue, all subsequent Mustangs save the Lightweights and early F6 types were delivered with 85gal tanks.

SOP for post war USAF was to limit fuel to 65gal. Ditto 8th AF, but at discretion of mission commander could order entire tank fill prior to mission, if particularly long with uncertain weather/winds aloft. That said, it was also typical to burn 20+ after takeoff and form up to use in climb. IIRC the flight test report stated that normal maneuver cg limits were achieved with 40 gal burn off.

The D was slightly worse than the B due to extra .50's and 600 rounds of ammo placement aft of Cg.

Also typical to introduce replacements to flight with full tank as orientation in Clobber College training - before 1st combat mission.

As an aside, it was safer in normal flight than say a compressibility dive in either the P-47/P-38 pre-dive flaps.

 

[drgondog]

Yes, so dont do violent pull outs, in fact dont "do" combat. 45 gallons of the 85 gallons should be considered as external fuel, but 45 gallons at economical cruise is approximately 1 hour of extra range. Now, about my question, how did the P-51B with all tanks full but only 40 gallons in the fuselage tank compare to the P-38 (and P47)? How did a P-38 do when performing violent pull outs and tight turns with 500 gallons in external tanks?

First, the P-51B with 184gal internal but w/2x75gal tanks had same combat radius as P-47D with single 150gal tank and 305 internal gal. I speculate without proof that a P-51B with only 40gal in fuse tank - all else being equal, still had more range with 2x108gal tanks than P-38 with full internal 410gal and 2x150/165 - but it would be close either way.

A positive feature about the P-38 design was that the LE 55gal tanks in P-38J-10 were located in front of the CG and close enough that they had no effect whatever to CG travel. IIRC the P-38 in ETO never carried more than 2x150/165gal tanks, but if caught by surprise without ejecting the tanks before combat, was in deep trouble. A P-38 with external drop tanks forced to maneuver was definitely an issue. McGuire discovered that issue too late.

The sway braces for the big tanks were inadequate for maneuver sideloads on all the fighters.

 

[MIflyer]

P-51 pilots first had to select a wing tank as the fuel source so that the excess fuel coming from the carb during cruise would have a place to go and not get dumped overboard. Then, once that tank was burned down a bit select the fuselage tank so you could burn it down a bit and THEN select the drop tanks.

A friend of mien was assigned to help ferry pilots in WW2. If they had a problem with their assigned airplane they parked it somewhere and left it for him. One day they got a call that a ferry pilot had a problem with a P-51. My friend flew over to Nashville to respond to the call for help. The ferry pilot said he had been ordered not to fly with ANY fuel in the fuselage tank but the one on the Mustang he was flying kept filling up with fuel with no effort on his part. Turned out that on that particular P-51 the Merlin carb overflow had been routed to the fuselage tank rather than the wing tank.

 

[drgondog]

P-51 pilots first had to select a wing tank as the fuel source so that the excess fuel coming from the carb during cruise would have a place to go and not get dumped overboard. Then, once that tank was burned down a bit select the fuselage tank so you could burn it down a bit and THEN select the drop tanks.

Strictly speaking, until the XP-51F/G/J and H the left main was the carb overflow.

A friend of mien was assigned to help ferry pilots in WW2. If they had a problem with their assigned airplane they parked it somewhere and left it for him. One day they got a call that a ferry pilot had a problem with a P-51. My friend flew over to Nashville to respond to the call for help. The ferry pilot said he had been ordered not to fly with ANY fuel in the fuselage tank but the one on the Mustang he was flying kept filling up with fuel with no effort on his part. Turned out that on that particular P-51 the Merlin carb overflow had been routed to the fuselage tank rather than the wing tank.

I've heard of that issue but never any source documents - save an obscure 8th AF P-51 pilot manual. It would be a field conversion as the drawing sare clear that carb return line go to the LH Main,

 

[MIflyer]

The sway braces for the big tanks were inadequate for maneuver sideloads on all the fighters.

On the Yamamoto Mission I think it was Rice that could not get his tanks to drop so he just pulled some G's and ripped them off.

Interesting item is that in the F-82 pilot's manual they state that if you have the anti-G suit system it will not work fully with drop tanks in use. Of course both the anti-G suit valve and the drop tanks used the vacuum pump exhaust as the pressurization source. Difficult to imagine the situation where you would need both operating.

 

[drgondog]

On the Yamamoto Mission I think it was Rice that could not get his tanks to drop so he just pulled some G's and ripped them off.

Interesting item is that in the F-82 pilot's manual they state that if you have the anti-G suit system it will not work fully with drop tanks in use. Of course both the anti-G suit valve and the drop tanks used the vacuum pump exhaust as the pressurization source. Difficult to imagine the situation where you would need both operating.

The G-suit was in continuous operation, and the output of the Vacuum pump nominally delivered 5-6psi through 30,000 feet. So, I might be wrong (gasp) but the G-suit was always in competition so long as the external tank was connected.

 

[Barrett]

Yes, so dont do violent pull outs, in fact dont "do" combat. 45 gallons of the 85 gallons should be considered as external fuel, but 45 gallons at economical cruise is approximately 1 hour of extra range. Now, about my question, how did the P-51B with all tanks full but only 40 gallons in the fuselage tank compare to the P-38 (and P47)? How did a P-38 do when performing violent pull outs and tight turns with 500 gallons in external tanks?

Good questions. The P-47 pilots I knew are deceased, but then so are nearly every other variety. Of the six remaining WW II aces, three flew Hellcats and the AAFers are two 51 pilots and a 38 guy whom I don't know. Presumably the 38 and 47 manuals or test reports would contain info similar to the 51...

 

[pbehn]

Good questions. The P-47 pilots I knew are deceased, but then so are nearly every other variety. Of the six remaining WW II aces, three flew Hellcats and the AAFers are two 51 pilots and a 38 guy whom I don't know. Presumably the 38 and 47 manuals or test reports would contain info similar to the 51...

The link you posted was an investigation type, test report. There is and was no doubt that the fuselage tank caused CoG issues when full. But the tank didnt have to be filled. The fuel in the tank could be burned off until the CoG was not affected, there are many comments in the text of your link about limits and usage and pilot training/ familiarisation. The whole discussion reminds me of an old joke. A man goes to his doctor and says "doctor, every time I put my arm behind my back and try to touch my head, it hurts" The doctor says "well stop doing it you fool". With the rear fuselage tank full the P-51 was problematic on CoG, but there was guidance about what to do. However that extra fuel was an additional contingency, for when things don't go to plan. For example an external drop tank may not drop, but hang up, and the extra drag eats up your fuel going home. That extra fuel could be the difference between making an emergency landing at Manston in Kent, ready for work he next day or drowning in the Channel or being taken prisoner after a forced landing. As far as I know all these planes were loaded to a level that isnt allowed today by civilian regulations, the situation was worse with bombers. You may see a B-29 or Lancaster flying today, you wont see one with a full crew, fuel load and Tall Boy or Grand Slam on board.

 

[MIflyer]

This is the G-Suit section from the 1949 F-82E pilot's manual. The 1948 version is nearly identical.

Currently we use about 12 PSI for pressurizing drop tanks, but they had a lot less to work with, lacking jet engines.

Interestingly enough, the challenge of supplying pressure for G-suits in piston powered aircraft led to a needless requirements that bedeviled us for decades thereafter. One approach used in the P-51 was a rubber bag with the aircraft battery atop it. Obviously, there was not much air there, although the pressure did go up if you were pulling G's. As a result, to prevent from losing too much air they had a very tight leakage requirement for the G-suit valve, a max of 600 CC of air per minute when it was putting out max pressure at 8G. This was leakage out of the valve into the cockpit. They wanted as much air as possible to go back into the rubber bag after the G-load was removed. G-suits in prop driven piston engined aircraft disappeared quickly when jets came along, and jets had plenty of pressurized air coming out of the engine. So the G-Suit valve leakage requirement was still maintained and while engineers struggled with meeting that requirement. none of them gave any thought to why it existed. Finally in 1977, with the F-15 and F-16 coming into service, AFSC decided to revisit the G-suit specification and hired a company to study the subject. They found the no-long-valid nature of the leakage requirement and that was a big help to me when I was working on how to overhaul F-111 G-suit valves, which were quite different from other standard G-suit valves and were also POS.

 

[PAT303]

The COG issue is a non issue because all the excess fuel causing it was burnt off before combat. No Pilot flew into enemy airspace with full aux tanks, it's the one thing that pisses me off about the Spitfire, it could have done so much more with 42+33G rear fuel tanks and a 90G dropper but the effect on the COG seems to be the reason they were never fitted in wide spread use.

 

[drgondog]

The COG issue is a non issue because all the excess fuel causing it was burnt off before combat. No Pilot flew into enemy airspace with full aux tanks, it's the one thing that pisses me off about the Spitfire, it could have done so much more with 42+33G rear fuel tanks and a 90G dropper but the effect on the COG seems to be the reason they were never fitted in wide spread use.

Somewhat subjective - specifically the linkage of 'excess fuel' qty to stick force per G and how skilled each pilot was in maneuvering in the lower rang of stability. Other factors contributing to the aft movement of the CG were the extra guns and ammo in the P51D vs P-51B - both with individual center of mass more than 12" past rear CG limit.

I haven't seen a weight and balance chart for Spitfire. That might not even exist, but that is quite a bit of fuel and comparable to the Mustang fuselage tank, maybe with center of mass further aft of nominal CG than Mustang?

Republic was forced to redesign the middle of their P-47 to add more fuel to the Main tank, rather than increase the existing auxiliary 100gal tank aft of cockpit.

 

[PAT303]

I haven't seen a weight and balance chart for Spitfire. That might not even exist, but that is quite a bit of fuel and comparable to the Mustang fuselage tank, maybe with center of mass further aft of nominal CG than Mustang?

Didn't seem to bother the MkIX or MkXVI, every MkXVI had the 42-33G rear tanks from the factory.

 

[PAT303]

Somewhat subjective - specifically the linkage of 'excess fuel' qty to stick force per G and how skilled each pilot was in maneuvering in the lower rang of stability. Other factors contributing to the aft movement of the CG were the extra guns and ammo in the P51D vs P-51B - both with individual center of mass more than 12" past rear CG limit.

All I'm saying is the rear aux tank(s) fuel will be burnt off and drop tanks jettisoned before engaging in combat, it was forbidden to engage in any maneuvers before that is achieved, from what I've read both the Mustang and Spitfire behaved normally without flight restrictions with 35G and 33G respectively of fuel left behind the pilot. The Spitfire had two separate tanks, the P51 got internal baffling after it was found the fuel sloshing about it's one large tank was detrimental to it's handling.

 

[drgondog]

All I'm saying is the rear aux tank(s) fuel will be burnt off and drop tanks jettisoned before engaging in combat, it was forbidden to engage in any maneuvers before that is achieved, from what I've read both the Mustang and Spitfire behaved normally without flight restrictions with 35G and 33G respectively of fuel left behind the pilot. The Spitfire had two separate tanks, the P51 got internal baffling after it was found the fuel sloshing about it's one large tank was detrimental to it's handling.

Again - there is no 'sharp line' between normal and 'abnormal' - only that stick force per G declined as CG moved aft - to near zero with full aft cg of fuel and ammo.

Forbidden to fight is also subjective - counseled against fight is perhaps better. My father entered combat with one tank 'stuck' on June 20 1944 and somewhere between victory number 1 and two in turning fight with JG 300, lost the extra baggage. I suggest 'do what ya gotta do' is better advice when there is no time for a measured process.

 

[MIflyer]

one thing that pisses me off about the Spitfire, it could have done so much more

Shucks, the Spit never even had a decent capability for drop tanks. The Spit IX could carry two 500 lb bombs over an absurdly short distance, but they never hung more than that little "slipper" tank under the belly operationally.

By the way, the P-51's out of the UK used different drop tanks depending on the mission. They did not always hang the big paper ones. That famous shot of LOUIV leading a flight was a photo op, not a real mission.

 

[PAT303]

Again - there is no 'sharp line' between normal and 'abnormal' - only that stick force per G declined as CG moved aft - to near zero with full aft cg of fuel and ammo.

From what I've read the trim couldn't be set and the plane couldn't be flown hands off with a full rear tank and all maneuvers done with extreme caution, once down to 35G it was business as usual. There seems to be a line between acceptable and difficult handling around the 40G mark.