The fuel for the engine is usually held in a metal or plastic fuel tank, shaped so that fuel is drawn from the outside edge of the tank, as the fuel tends to be thrown to the side by centrifugal force as the airplane travels in a circle. A "clunk tank" as used in R/C is satisfactory, but dedicated tanks with wedge-shaped cross-sections are frequently used and tend to have better characteristics as the fuel runs out. A tank with a vent on the inner edge, or multiple vents, is usually called a 'suction' tank. The pressure of fuel delivery with a suction tank changes as the fuel runs out, causing the engine mixture ratio to become leaner as the flight proceeds. Tanks vented to only permit air to enter at the outside edge ("uniflow" tanks) provide constant fuel pressure over the duration of the flight and a constant mixture ratio.