MIflyer
Captain
A friend of mine had an unusual job in WWII. Ferry pilots, including some women, were used to move airplanes around the country and overseas. Many such pilots were not fully trained on the aircraft they were to fly and were limited in what they were allowed to do. My friend was dispatched to pick up up aircraft that the ferry pilots found they could not handle for some reason.
One day he was sent to Nashville to pick up a P-51. He got there and the ferry pilot explained the problem with the Mustang; he had been instructed to never fly it with any fuel in the aft tank, behind the cockpit. Fuel in the tank aft of the cockpit could result in Center of Gravity too far aft, making the airplane hard to handle, prone to tighten up in turns.
The problem with that Mustang was that while the aft tank was empty at takeoff, it started filling up with fuel in flight. Mindful of his orders, the ferry pilot landed and called for help.
As it turned out, the Mustang used a pressure carburetor, a design feature that eliminated the British problem with the engine dying with negative G's. Rather than a float type carburetor where the vacuum produced by the engine sucked the fuel out of the carburetor bowl, the pressure carburetor injected metered amounts of fuel into the supercharger inlet. But the pressure carb delivered more fuel than engine needed under cruise conditions. The excess fuel was directed back into the Left main fuel tank, and it could be up to 10 gallons an hour. Mustang pilots were told to first use fuel from the Left main tank, so that there would be room for the returned fuel, which otherwise would be vented overboard and be lost. After the Left tank had been drawn down a ways the pilot then would switch to the aft tank and draw it down to 25 gallons or less before switching to the drop tanks. The drop tanks used the air exhaust from the instruments' vaccum pump to pressurize them and force the fuel out; the pilot would alternate from selecting one to the other every 10 min or so to prevent an imbalance.
For some strange reason on that particular P-51 the return fuel from the carb had been plumbed into the aft tank instead of the Left one. My friend took the Mustang and flew it to the required destination; presumably they later fixed the problem.
One day he was sent to Nashville to pick up a P-51. He got there and the ferry pilot explained the problem with the Mustang; he had been instructed to never fly it with any fuel in the aft tank, behind the cockpit. Fuel in the tank aft of the cockpit could result in Center of Gravity too far aft, making the airplane hard to handle, prone to tighten up in turns.
The problem with that Mustang was that while the aft tank was empty at takeoff, it started filling up with fuel in flight. Mindful of his orders, the ferry pilot landed and called for help.
As it turned out, the Mustang used a pressure carburetor, a design feature that eliminated the British problem with the engine dying with negative G's. Rather than a float type carburetor where the vacuum produced by the engine sucked the fuel out of the carburetor bowl, the pressure carburetor injected metered amounts of fuel into the supercharger inlet. But the pressure carb delivered more fuel than engine needed under cruise conditions. The excess fuel was directed back into the Left main fuel tank, and it could be up to 10 gallons an hour. Mustang pilots were told to first use fuel from the Left main tank, so that there would be room for the returned fuel, which otherwise would be vented overboard and be lost. After the Left tank had been drawn down a ways the pilot then would switch to the aft tank and draw it down to 25 gallons or less before switching to the drop tanks. The drop tanks used the air exhaust from the instruments' vaccum pump to pressurize them and force the fuel out; the pilot would alternate from selecting one to the other every 10 min or so to prevent an imbalance.
For some strange reason on that particular P-51 the return fuel from the carb had been plumbed into the aft tank instead of the Left one. My friend took the Mustang and flew it to the required destination; presumably they later fixed the problem.
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