Drop tank

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 jettisonable. 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 the event of emergency.

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. During World War II, the German Luftwaffe began using external fuel tanks with the introduction of a 300-liter (80 US gallon) 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 300 liter tank, available in at least four differing construction formats — including at least one impregnated paper material, single-use version — and varying only slightly in appearance, became the standard volume for most subsequent drop tanks in Luftwaffe service, with a rarely used 900 litre (238 U.S. gallon), fin-stabilized large capacity drop tank used with some marks of the Messerschmitt Bf 110 heavy fighter and other twin-engined Luftwaffe combat aircraft.

The first drop tanks were designed to be discarded when empty or in the event of combat or emergency in order to reduce drag, weight, and to increase maneuverability. Modern external tanks may be retained in combat, to be dropped in an emergency.

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