In aviation, a drop tank (external tank, wing tank or belly tank) is used to describe auxiliary fuel tanks externally carried by aircraft. A drop tank is expendable and often capable of being jettisoned. External tanks are commonplace on modern military aircraft and occasionally found in civilian ones, although the latter are less likely to be discarded except in an emergency. Overview The primary disadvantage with drop tanks is that they impose a drag penalty on the aircraft. External fuel tanks will also increase the moment of inertia, thereby reducing roll rates for air maneuvers. Some of the drop tank's fuel is used to overcome the added drag and weight of the tank. Drag in this sense varies with the square of the aircraft's speed. The use of drop tanks also reduces the number of external hardpoints available for weapons, reduces the weapon-carrying capacity and increases the aircraft's radar signature. Usually the fuel in the drop tanks is consumed first, and only when all the fuel in the drop tanks has been used, the fuel selector is switched to the airplane's internal tanks. Some modern combat aircraft use conformal fuel tanks (CFTs) instead of or in addition to conventional external fuel tanks. CFTs produce less drag and do not take up external hardpoints but some versions can only be removed on the ground. History The drop tank was used during the Spanish Civil War to allow fighter aircraft to carry additional fuel for long-range escort flights without requiring a dramatically larger, heavier, less maneuverable fuselage. Mitsubishi A5M, designed by Jiro Horikoshi, began carrying an external underside drop tank to provide fuel for extended range in 1937 and during World War II, the Luftwaffe began using external fuel tanks with the introduction of a light alloy model for the Ju 87R, a long-range version of the Stuka dive bomber, in early 1940. The Messerschmitt Bf 109 fighter also used this type of drop tank, starting with the Bf 109E-7 variant introduced in August 1940. Fitted also to the Focke-Wulf Fw 190, the tank, available in at least four construction formats — including at least one impregnated paper material, single-use version — and varying only slightly in appearance, became the standard volume for most drop tanks in Luftwaffe service, with a rarely used , fin-stabilized large capacity drop tank used with some marks of the Messerschmitt Bf 110 heavy fighter and other