By Bob Alvis, special to Aerotech News

In the first installment of my look back at the historic Lockheed XC-35, we learned of the run-up to the first flights of the aircraft.

Lockheed test pilots had to make sure that they would be handing off an airframe to the Army Air Corps engineers and pilots that was safe to fly, along with any instructions that would be helpful to the pilots, who would be the first to see if a pressurized aircraft was feasible.

Here, in part two, we will now take a look at a very interesting flight test program that was like nothing ever experienced before in an airplane.

The first exciting phase of the test program would not take place in the skies, but in a hanger in Burbank, Calif.

The idea of blowing up an airplane like a balloon brought some very interesting questions from engineers charged with the task: “Sure, we can make a sea plane that can keep water out – but what will happen when the forces and air are pushed the other way?” In that Lockheed hangar, the day finally came when those Wasp engines were put to the test. They were equipped with turbochargers that had two centrifugal compressors on the same shaft. One supplied air to the engine, while the other supplied air to the cabin.

The cabin that was to be pressurized extended from the nose of the aircraft, to a pressure bulkhead just forward of where the tail surfaces were attached. The pressurized cabin was divided into two compartments. Either or both could be pressurized. The fuselage was of conventional all metal, semi-monocoque construction, except that thicker skin gauges were used and special attention had been paid to sealing all openings and riveted joints. Thus the idea was that an air tight seal had been achieved and the day came to put it to the test.

The test was to cycle the fuselage through several pressurizations on the ground to test cabin tightness and to check the operation of the discharge and emergency valves. As the hangar filled up with curious onlookers, the stage had been set for what was considered by many a comic opera. As the test began, it was found that the air seals around the doors and windows had not been completely adjusted. When the cabin was pressurized, many leaks began to emit whistling sounds of varying pitches. After some concerned glances, the gathered crowd became uneasy and started to decline in size. As the pressure increased, the whistles turned to loud screeches which were louder than the two running aircraft engines! At this point, the crowd of curious who were left were making hasty exits!

Adjusting the pressure down to 15psi helped to abate the screeching, but the fear now was, as the windows were seen bowed out by about a half inch, that the plane was going to explode like an overinflated balloon!

Much work was needed before air tests could commence. The only way to cure the leaks on the XC-35 was to go over every joint and seam with, of all things, soapsuds, to pin-point the leaks. Finally, the airplane was deemed flight-ready for the first airborne tests. On May 25, 1937, the first pressurized aircraft in the world took to the skies over Burbank.

The first flight had its shares of gremlins and even though they did not reach the altitude they desired, the crew did reach 20,000 feet in comfort and without the use of oxygen masks. Several more flights took place while instruments were adjusted, and they tested the general operation to the plane, finally reaching an altitude of 28,000 feet.

These flights became pretty routine, until one day when a Lockheed engineer wanted to show off to one of the lead pilots and demonstrate how effective the cabin pressure superchargers were. This engineer began operating the levers like a switchman in a busy freight yard and soon the cabin equivalent altitude was being taken up and down three- to four-thousand feet, several times a minute. The heads of the passengers felt as though they were expanding and contracting like a blacksmith’s bellows! The crew became enraged and started to voice their disapproval, but the renegade engineer continued on his mad-man demonstration. That came to a decisive end, when the crew grabbed the engineer from his seat and dragged him to the back of the plane, strapping him into a seat for the duration of the flight! The crew at this point was so shaken up that a quick end came to that day’s test flight. On the next day the test flights were canceled, as the entire crew had splitting headaches!

After a few major changes in equipment and systems, the official acceptance was completed. The XC-35 was flown back to Wright Field, Ohio, in August 1937. On the trip from Burbank, the plane cruised at 25,000 feet at a speed of 210mph. The crew reported it was a pleasure to be up there in comfort, instead of sitting bundled against the cold sucking on an oxygen tube!
A new era of transportation had arrived in the skies of America.

In 1938 the Collier Trophy, which has been awarded each year since 1911 for the greatest achievement in American aviation demonstrated by actual use during the preceding year, was given to the XC-35. The citation read: “To the U.S. Army Air Corps for having designed, constructed and completely equipped the XC-35 substratosphere plane, the first pressure–cabin airplane to be flown successfully in the world.”

The Air Corps continued to use the Lockheed XC-35 as a flying, high-altitude laboratory at Wright Field for several years. In 1941, the XC-35 was turned over to NACA at Langley Field in Virginia for further high-altitude research.

There is so much more to the story of this incredible plane and what it did for modern transportation. We can only hope that someday the Smithsonian sees fit to find a proper venue, restore the aircraft and display it for the world to see. Every plane that lands in Washington full of tourists who are bound for the museums that tell of this nation’s history should be able to see the very first plane that made their trip in comfort possible!

Until next time, Bob out …