Just 75 years ago, the Lockheed XC-35 made its first research flight, taking off on August 5, 1937, from the Army Air Corps’ Wright Field in Dayton, Ohio, thereby launching a series of tests of the airplane’s major innovation: a pressurized cabin. Although it wasn’t the first airplane to feature pressurization, it was the only one until that time with room in the pressurized capsule for a couple of passengers, in addition to the crew of three. (Germany’s Junkers Ju 49 had a pressurized compartment, but only for the crew; France’s Farman F.1000 had one too but with a seat that lifted the pilot up so he could see to take off and land; at high altitude the pilot would be sealed inside the compartment.)
The XC-35 test program was so successful that the Army felt confident in specifying a pressurized cabin for the planned Boeing B-29, and later, Boeing’s 307 airliner flew passengers in pressurized comfort (see “Above It All,” Sept. 2009).
Early on, aviators learned that the thin air at high altitudes could not sustain them, and if they lingered too long above about 15,000 feet without extra oxygen to breathe, they’d lose consciousness. Face masks to deliver pure oxygen were helpful to a point, but in September 1934, aviation pioneer Wiley Post demonstrated that a full-pressure suit similar to what astronauts wear today would enable pilots to fly to 40,000 feet, well beyond the altitude at which airliners cruised.
Airline passengers could hardly be expected to don bulky pressure suits, and military flight crew would be hampered by their bulk. The only answer was to provide the equivalent of a low-altitude environment in the cabin so passengers and crew could work in their shirtsleeves.
The XC-35 was an Electra 10-A similar to Amelia Earhart’s but reconfigured by Lockheed engineers working with a team from the Army Air Corps. The 1936 contract for the lone airplane promised $112,197 to Lockheed for delivering an airplane that met the requirements. One major change altered the shape of the fuselage. Where the Electra’s fuselage had flat sides, the XC-35 fuselage had a circular cross-section. When an airplane’s cabin is pressurized, it’s not that different from a balloon, and by eliminating any corners that might be overstressed under pressure, the circular shape ensured that the stresses were distributed over the entire structure. The Electra’s large, rectangular windows also had to go, in favor of much smaller portholes that could stand up to the force of the pressurized air.
By the 1950s, most airliners were pressurized, and once jet engines revolutionized aircraft performance, the ensuing ultra-high altitudes at which jets routinely operated absolutely demanded adequate pressure in the cabin. Where to get the air? The engines’ compressor stages, where incoming air was squeezed prior to being mixed with fuel. In modern jets, warm “bleed air” drawn from the compressor is cooled, then ducted into the cabin.
Boeing’s newest jet, the 787 Dreamliner, is the first current jet-powered airliner to use something other than bleed air as a pressure source. Instead, the 787 employs independent, dedicated compressors powered by electric motors that are energized by engine-driven generators. Boeing says the change saves a lot of weight, reduces parts count and maintenance, and, by retaining the compressor’s air for engine operation, increases fuel efficiency.
The XC-35 currently resides in long-term storage as part of the National Air and Space Museum’s collection.