The Airplane - Jay Spenser [34]
This was an enormous machine with a maximum gross weight of almost 19,000 pounds (8,500 kilograms) and a wingspan of about 102 feet (31 meters). Although it flew well in tests, the Zeppelin firm saw its hopes for production dashed by the Inter-Allied Commission of Control, which—noting the craft’s military pedigree and potential—ordered it destroyed under the terms of the Treaty of Versailles. Sadly, despite the company’s impassioned pleas to be allowed to sell or donate the E.4250, it was scrapped in November 1922. Had this singular airplane instead entered commercial service and spawned successors, all-metal semi-monocoque construction might have blossomed earlier.
The Zeppelin-Staaken E.4250 of 1920 might have hastened the adoption of all-metal, stressed-skin construction had it not been ordered destroyed under the terms of the Versailles Treaty.
As for Adolf Rohrbach, he relocated to Denmark to avoid Versailles Treaty limitations and opened an airplane plant that built land planes and flying-boat airliners for Lufthansa. He relocated to the United States at the end of the 1920s and returned to Germany the next decade, where he died on the eve of World War II at age fifty.
Who did first put it all together? The honor of designing the world’s first production all-metal, semi-monocoque airplane goes to John K. “Jack” Northrop in the United States, with his Alpha. Northrop, a self-taught aeronautical engineer, is one of the most influential airplane designers of all time. Working at various times for companies such as Lockheed and Douglas, Northrop always wanted his own company and had one at different stages of his career. Best known for the airplanes that bear his name, he also helped shape other people’s designs, ranging from Lindbergh’s Ryan NYP Spirit of St. Louis to the famous Douglas DC-3 airliner.
An avid follower of aviation trends, Northrop came up with the idea for an aerodynamically clean and very fast high-wing cabin monoplane in the mid-1920s. The result was the Lockheed Vega, which embodied Northrop’s ideas for semi-monocoque construction combining pressure-molded wooden halves with an internal framework. Sleek and capable, the Vega was an instant success that only improved as more powerful engines and aerodynamic cowlings became available. But Northrop was now thinking in terms of metal, not wood.
The Northrop Alpha of 1930 was the first production airplane to employ all-metal, semi-monocoque construction.
National Air and Space Museum, Smithsonian Institution
In 1928, Northrop left Lockheed to produce the first airplane to bear his name. The Northrop Alpha flew in early 1930. Sleek and shiny from its fully cowled engine to the tip of its tail, this breakthrough mail plane combined a fully cantilevered wing with an enclosed cabin for airmail, freight, or up to six passengers. Coming at the start of a new decade, the prototype Alpha was a transitional monoplane with holdover biplane design shortcomings. First, it had an open cockpit behind its enclosed cabin that placed the pilot far aft, where visibility was poor. Second, it lacked a retractable landing gear, although production Alphas were fitted with aerodynamic gear fairings to reduce drag.
Just what is stressed-skin construction? Imagine you’re holding a thin sheet of aluminum about a meter square. This piece of sheet metal is very light. Asked to try your strength on it, you confirm it’s too strong to be pulled apart. You also find you can’t distort its square shape into a parallelogram. In engineering terms, this material has just demonstrated excellent resistance to tension and shear loads.
Now, donning heavy gloves to protect your hands, you push inward on its sharp