Gravity and other forces conspire against conventional flight, but they are positively Machiavellian about inverted flight. Pity the first pilot who rolled inverted and sailed blithely along, only to hear the engine cough and die of fuel starvation when the gas settled in the top of the tank, or have the engine seize when the oil did likewise.
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Engineers and designers have conspired in return. Here are a few devices that enable inverted flight and transitional maneuvers, or at least make them less of a struggle.
Inverted fuel systems
Most aerobatic airplanes with inverted fuel and oil systems use fuel injection rather than a carburetor. When a carburetor is inverted, it can no longer meter fuel, and the float rises and cuts off the incoming supply. A fuel injector, which doesn't care what attitude it is in, measures airflow and meters the proper ratio of fuel to each cylinder so that each receives a constant flow of the same fuel-air mixture.
To ensure the flow from fuel tank to fuel injector, aerobatic aircraft with the fuel tank in the fuselage have a "flop tube," a flexible hose with a weight in the free end, plugged into the fuel tank. In normal flight, the weighted end of the hose flops to the bottom of the tank and draws fuel from there. When the airplane rolls inverted, the weighted end flops to the top of the tank (1 in diagram below), with the fuel. Regardless of the aircraft's attitude, fuel and flop tube end up in the same spot.
Aerobatic airplanes that have fuel tanks in the wing use a small "header tank," which is connected to the wing tanks. In normal flight, fuel gravity-feeds down to fill the header tank, which is connected to the suction side of the fuel pump. When the airplane rolls inverted, the header tank is now above the engine, and fuel gravity-feeds from the header tank to the fuel pump. A check valve in the line from the main tank to the header tank stops fuel from draining back into the main tank when the airplane is inverted.
Inverted oil systems
Engines that use an external oil tank-"dry sump" engines-have a device similar to a flop tube that can reach oil in almost any attitude. In wet sump engines, in which oil is stored internally in a sump at the bottom of the crankcase, an oil pickup line near the top of the engine (2 in diagram above) as well as in the oil sump ensures that oil is available in any attitude. A valve with two steel balls separated by a spring is connected to the top and bottom of the engine; like a flop tube, the balls (and the oil) go where gravity dictates, alternately covering and opening the appropriate oil pickup point.
Most airfoils are cambered, or curved, on top but flat on the bottom. As a result, they fly better upright than inverted. Symmetrical airfoils, which have the same curvature on both surfaces (3 in diagram below), perform exactly the same upright or inverted, and so are favored by aerobatic pilots. In order to fly at all, however, a symmetrical airfoil must be positioned at a slight positive angle-leading edge high-with respect to the flight path; otherwise the airflow around the upper and lower surfaces would be the same, and no lift would be created.