Zeppelin-Staaken E-4/20  The Zeppelin-Staaken E-4/20 was a revolutionary four-engine all-metal passenger monoplane designed in 1917 by Adolf Rohrbach and completed in 1919 at the Zeppelin-Staaken works outside Berlin, Germany. The E-4/20 was the first four-engine, all-metal stressed skin heavier-than-air airliner built. At a time when most aircraft were small, single-engine biplanes made of wood and canvas, the E-4 was a large (102-foot wingspan), all-metal, four-engine, stress-skinned, semi-monocoque, cantilevered-wing monoplane, with an enclosed cockpit, and accommodation for 18 passengers plus a crew of five, including two pilots, a radio operator, an engineer and a steward, as well as radio-telegraph communications, a toilet, a galley and separate baggage and mail storage. With a maximum speed of 143 mph, cruising speed of , a range of about , and a fully loaded weight of , it outperformed most other airliners of its day. The E-4 included numerous innovations, including its all-metal monocoque construction, onboard facilities such as lavatory, kitchen and radio communications, and its notable and sturdy monoplane load-bearing box-girder wing constructed of dural metal which formed both the wing's main girder and the structure of the wing itself. Skinned with thin sheets of dural metal to give the aerofoil shape necessary for a wing, the girder section wing had fabric covered leading and trailing edges attached to it. This superb and innovative wing was robust and self-supporting. The E-4 was completed in 1919 and test flown between 30 September 1920 and 1922 when it was broken up on the orders of the Inter-Allied Commission. The Zeppelin-Staaken E-4/20 was a product of the innovative Zeppelin Airship company. Count Ferdinand von Zeppelin, founder of the Luftschiffbau Zeppelin GmbH (Zeppelin Airship Construction Co.) was himself a major aeronautical innovator, creator of the groundbreaking giant aluminium alloy framed Zeppelin lighter than air dirigible airships and later developer of a series of R-Planes. Zeppelin was one of the first aeronautical pioneers to apply stringent scientific principals to the design of aircraft, focusing on issues like power-to-weight ratios of engines and using the then new metal alloy aluminium for structural components.