XP-51 Lightweight/P-51H asymmetrical fuel tanks

[BarnOwlLover]

This is something I've been wondering about as far as a reason for it for a while. But why did the XP-51F/G and P-51H typically fly with asymmetrical wing fuel tanks? One was typically larger than the other, and sometimes the only fuel tank carried was in one wing. Was there any reasoning for such an arrangement?

 

[mjfur]

There's only a 5-gallon difference in the P-51H between Left & Right. Could be small bulge/dimple in the tank where the booster pump is located to keep it at the lowest point in the system.

Page 15 of the pdf.

https://www.wwiiaircraftperformance.org/mustang/P-51H_Pilots_Flight_Operating_Instructions.pdf

Kind of odd/interesting in the Use of Fuel box you use the fuselage tank last.

 

[BarnOwlLover]

On the lightweights, I've seen mention of 2x105 gallon tanks (one in each wing), 1 75 gallon and 1 105 gallon tank, only one 105 gallon tank (in one wing), and only one 75 gallon tank (in one wing). Been trying to figure out why this was done for a while as far as the asymmetrical tankage.

 

[cvairwerks]

The POH shows running asymmetrical loads when carrying 1000 pound bombs only. From the charts only 110 or 165 gallon tanks were approved for single carry with the 1000 pounders. Charts 39 and 40 at mjfur's link.

 

[BarnOwlLover]

The POH shows running asymmetrical loads when carrying 1000 pound bombs only. From the charts only 110 or 165 gallon tanks were approved for single carry with the 1000 pounders. Charts 39 and 40 at mjfur's link.

Those aren't drop tanks, but the internal fuel tanks on the XP-51F/G/J, and none flew with a fuselage tank. I'm trying to figure out the reasoning for the asymmetrical internal fuel arrangements that the planes were tested with.

 

[ThomasP]

Do you know if they were carrying anything else at the same time? Or is there any information on what they were testing at the time the flights took place (aside from the possibly of investigating asymmetric internal fuel load effects on stability and controllability)?

 

[drgondog]

This is something I've been wondering about as far as a reason for it for a while. But why did the XP-51F/G and P-51H typically fly with asymmetrical wing fuel tanks? One was typically larger than the other, and sometimes the only fuel tank carried was in one wing. Was there any reasoning for such an arrangement?

Yes. The left wing tank was removable for the Interceptor Mission for XP-51F/G/J/H. It was 75-gal at first to nearly equal the 2x92-gal main tanks for P-51B/D, when combined with the increased (105-gal) right main. At that time 100 gallons internal fuel was standard 'Fighter condition'.

The return line from carb overflow was re-directed to right wing - a scheme changed from ALL previous Mustangs, so the right wing 105Gal main was 'permanent' and had the booster pump submerged which was also capable of pulling fuel from left main tank, when installed.

The reason for the increase of the removable left main from 75-gal to 105-gal was an attempt to meet AAF demands for more internal fuel at the same time the 85-gal fuselage fuel tank was being added to P-51B and the 55-gal fuel tank was added to P-51D-1.

It was not sufficient so the only recourse, to avoid aft cg for any fuselage tank added to P-51F/G was to increase fuel from 2x92 to 105/100 plus add a 50-gaql fuse tank AND lengthen/move wing back for the P-51H.

 

[MIflyer]

As dragondog infers, on the earlier P-51's the normal procedure was to first burn fuel from the Left integral wing tank so to enable the return line from the engine to have a place to put the excess fuel going to the carb. Once you had created space in the Left integral wing tank, the next step in the procedure was to burn fuel from the aft tank to get the CG problem under control and then finally switch to the drop tanks. The alternative was the fuel going overboard. You could help the pilots out by making enough space in one wing tank so that the switching of tanks was unnecessary, or at least less critical. And of course if you could redesign the airplane to permanently fix the aft CG problem that would help a great deal.

A friend of mine was assigned the job of "rescuing" aircraft that were being ferried by dedicated "ferry only" pilots and was sent to Nashville because a ferry pilot followed his orders and refused to fly with any fuel in the aft tank. The aft tank kept filling up and that was because for some reason the return line had been routed to the aft tank.

One interesting aspect of the Mustang that people don't seem to comment on was that it was probably more "automatic" than any other aircraft of its time. In addition to an aneroid controlled high/low supercharger it also had automatic control of the radiator exhausts and a mike button built into the throttle. In contrast the P-47 pilot had to adjust his turbosupercharger settings and worry about the anti-compressibility "dive brakes" although in later models of the P-47N they did add a throttle mike button. The P-38 pilot had to make sure his turbos were both turning before he took off, had to turn the hydraulics on and off, had to turn the aileron boost on and off, had a "horn button" mike switch in the middle of the control wheel, had a complex series of fuel tanks to handle, and in later P-38L had to consider the cowl flaps that kept the intercooler from getting too cold, and had to find the drop tank jettison button in an awkward spot - and of course had two engines to manage. The F4F, F6F, F4U and P-61 had switches for supercharger speed and stage control, although the later F4U-4 and -5 went to more automatic controls. The Wildcat also had to crank his gear up and down by hand. No wonder they referred to the P-51 as the "Cadillac"; you just got in a drove.

 

[BarnOwlLover]

I've wondered if this would unbalance the aircraft in terms of left/right/port/starboard weight, or was that accounted for?

 

[drgondog]

I've wondered if this would unbalance the aircraft in terms of left/right/port/starboard weight, or was that accounted for?

aileron trim tab

 

[BarnOwlLover]

aileron trim tab

Is that why the lightweights and the P-51H have only the one on the port aileron? That's another feature that I've found to usually be symmetrical on aircraft but is again asymmetrical on the lightweights and the H