Yes, it is a small world. I guess you probably know then about the ferrying operations from Great Falls during the war (hence my avatar of the 7th Ferrying Group's patch) and how military planes were being flown through this area from factories to bases and from bases to depots. Many of the planes headed for Alaska went through Great Falls, most notably several thousand Airacobras, King Cobras and other Lend-Lease planes for the Russians. We also had a B-17 training mission here, which is why the Air Force base was originally built in 1943.
I don't have the P-40 w/AccuSim package and I'm not at all an expert on the P-40 so I don't know how well I can address your specific question.
The vented tank collects unused fuel vapor that would otherwise be lost. If the vented tank is full, the recycled fuel running into it overflows and I think it is released out of the fuselage and into the airstream. Quite simply, the mechanism for salvaging unused fuel fails and the 'recollected' fuel is wasted. It is better to burn off some of that fuel in the vented tank at the beginning of your flight so that you create space for the salvaged fuel to go to throughout the rest of the flight. (On a similar note, I recall that P-47 pilots were supposed to switch back to the vented tank every hour--for ten minutes I think--to maintain that empty space.)
I'll argue that in the late 1930s the Army really didn't anticipate needing a fighter that would fly more than 100 miles to engage in combat--after all, the U.S. was concerned primarily about defending the continent against the incursion of a naval superpower and with defending our Pacific stations and territories. Japan's navy was eyed as our major threat. The Army wanted fighters that could scramble to engage enemy bombers approaching our factories, ports, coastal defenses, etc. I'll assume that this is why the P-40Bs did not have provisions for an auxiliary belly tank (or at least not drop tanks). Although the Army originally anticipated that operational units would only need a limited combat range, I suppose experience proved it was necessary to fly military pursuit aircraft long distances even if only to relocate them to new bases. Hence, plumbing for a belly tank was added with the 'C' model.
Let's assume that in 1941 (pre-war) if a P-40C pilot knew he was going to fly a long distance that would require an auxiliary tank, he'd already have his route planned and he'd know how much fuel he'd need to get there since total weight is a major consideration. I'm thinking that having a belly tank installed wouldn't be the normal configuration for a squadron on alert because the external tank automatically reduces the airplane's top speed of by 10 mph, and probably wreaks havoc on the airplane's flight stability as well. If I am correct, the belly tank was used primarily for ferrying operations and recon patrols, not for the direct interdiction role. Let's also assume the pilot isn't going to drop his tanks on some poor farmer's barn halfway between Base X and Base Y, if you know what I mean.
The pilot starts up and takes off with his Reserve Wing tank and burns approximately 15 gallons to give him space for unused fuel that will be recollected from the carburetor. He then switches to the Belly (Auxiliary) tank since he is carrying that extra fuel for a reason--he knows he needs that gas to get to his destination. He'll want to empty the Belly tank in case he has to make a belly landing; also, since it is not a gaged tank he'd probably want to use up the fuel and then switch to tank that gives him a needle to watch. The Fuselage tank is recommended next, probably because it is a large tank and is located behind the pilot and therefore affects the airplane's center of balance. When the Fuselage tank gets empty, the pilot can switch to the other gaged tank--the Main Wing tank which is located approximately under his seat. Finally, when the fuel in the Main Wing tank is consumed, he switches to the Reserve Wing tank, which is located fore of the Main Wing tank and approximately under his feet. By now the pilot should be very close to his destination and shouldn't be sweating the exact contents of the Reserve Wing tank. If the fuel selector that is illustrated in the vintage P-40D/E manual is the same as it was in the P-40C, the pilot will never have to move the selector position through an empty tank if he uses this sequence.
I'm sorry, what was the question?
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(Link to MySpace tribute page)120th Fighter Wing, Montana Air National Guard