The Airplane - Jay Spenser [35]
You have just found aluminum sheeting’s Achilles’ heel: poor resistance to compression loads. Thick aluminum plate could resist these loads, but that would make for an airplane too heavy to fly. Here was a key structural challenge for those seeking to build out of metal.
The genius of semi-monocoque construction is that it calls on thin aluminum to carry the loads it can withstand (tension and shear) and spares it the one it can’t (compression). It does this by clever use of an internal framework far less substantial than a full load-bearing skeleton. By itself, what’s beneath the skin of one of today’s all-metal jet- liners could not begin to support the craft’s weight on the ground, let alone the loads imposed by flight.
As worked out by Rohrbach, Northrop, and others, this internal framework comprises circumferential members (ring formers and occasional bulkheads) with fore-and-aft or longitudinal members (longerons and stringers). Together these elements create sturdy compression-resistant bays over which aluminum skin is pulled taut and riveted in place. The result is a light yet strong metal structure in which the skin and what’s behind it share the job of carrying the loads and stresses of flight. Channel sections, flanges, built-up assemblies, and other bits of engineering magic further enhance strength while keeping overall weight to a minimum. Advancements in metallurgy, fabrication techniques, stress modeling, and structural design have continued to improve this breakthrough construction method over the decades.
About the same time that Jack Northrop’s company flew the Alpha, Boeing in Seattle flew its first all-metal, semi-monocoque airplane. The Boeing 200 Monomail was a mail plane of similar over-all configuration except that it featured a retractable landing gear. Although the Monomail wasn’t produced, it immediately gave rise to other Boeing stressed-skin airplanes that were.
Carrying ten passengers at 180 mph (290 km/h), the Boeing 247 of 1933 was the world’s first modern airliner.
Boeing
The culmination of this cutting-edge activity in Seattle was the Boeing 247, a ten-passenger airliner that took wing in 1933. In terms of configuration and construction, the Model 247 marks an enormously significant historical emergence: three decades after the Wright brothers invented the airplane, here was the world’s first modern passenger airliner. In contrast to other transports of the day, it alone possessed the sleekness of semi-monocoque construction.
The Model 247 also boasted retractable wheels, the latest radio gear, gyro instruments for blind flying, and other cutting-edge technology. Unfortunately, though, the airplane itself carried too few passengers to find solid commercial success. Moreover, passengers didn’t like the wing spars invading its passenger cabin, which created obstacles they had to climb over to reach their seats.
Douglas Aircraft saw a chance to do better. The next year, they began deliveries of the DC-2, a fourteen-passenger airliner with all-around better performance than the 247 as well as a larger cabin that sat above the wing spars so its aisle was uninterrupted. But it was this California company’s next product that changed the world.
The Douglas DC-3 forever changed the world when it entered ser vice in 1936.
Museum of Flight, Seattle
Placed into service in 1936, the Douglas DC-3 would be the most dominant airliner ever and one of the most significant airplanes in history. A bit bigger all around than the DC-2, it had a wider fuselage that allowed for ten railroad-style Pullman berths on night flights. In a day-plane configuration, this cabin accommodated twenty-one passengers seated three abreast, two on the left side of the aisle and one to the right.
Here at last was a magic combination