Above It All
It took a maze of valves and venturis—and a trio of tycoons—to whisk passengers into the stratosphere.
- By Nick D'Alto
- Air & Space magazine, September 2009
(Page 2 of 3)
“The solution was a highly dynamic pressure system,” says Mike Lavelle, a director at Seattle’s Museum of Flight and an expert on the Boeing 307. As Price proposed it, the airplane would rise unpressurized to 8,000 feet, where medical experts believed there was enough oxygen to keep people breathing and functioning normally. Then the superchargers would introduce air to hold cabin pressure to 8,000-foot conditions as the airplane climbed. Higher than 16,000 feet and cabin pressure would fall, since the superchargers couldn’t pull in such thin air (at 16,000 feet, it would feel like 8,000 feet inside the cabin; at 20,000, like 12,000). Yet flying at 20,000 feet, the 307 would still top 90 percent of the weather and passengers would not get airsick.
“It was very well thought out,” Lavelle says. “We still pressurize most commercial airliners to that 8,000-foot standard today.” Yet Price’s invention also demanded exquisite balance. In the rarified air of high flight, the 307 would need to sacrifice some pressure in its cabin to ease the “bulging” effect high-altitude flight induced in the fuselage. But in descent, the airplane would need to repressurize to bear the inward forces its cabin experienced as the aircraft reentered thicker air. You’ve seen this effect in miniature if you’ve noticed a plastic water bottle bulge or dent as cabin pressure changes.
Key to the entire system was Price’s cabin pressure regulator. “It consisted of an inlet valve, which regulated the flow of ventilating air to the cabin, and an outflow value to control the discharge of air,” says Lavelle. In between, a maze of valves, venturis (short tubes with constricted throats), and sylphons (pleated metal bellows) interacted with one another, maintaining conditions inside the airplane as outside pressure varied. Incoming air was drawn via rows of slits on the wings’ leading edges, heated, and then pumped into the cabin; the 307 “exhaled” near its tail. It all happened largely automatically; reporters were so impressed they called the regulator a kind of “brain.” Boeing patent attorney Charles Reynolds saw a bright future for the device. “In a few years,” he told Price, “this ought to be in every plane in the country.”
But getting it to work was another matter. “Once engineers completed the working prototype, they felt the best approach would be to use an altitude chamber to replicate high-altitude pressures, Lavelle says. “But their request was denied, so they used an empty 50-gallon oil drum to fabricate a test rig.” By “supercharging” it with compressed air from a blower, Price and chief engineer Jim Cooper could make the pressure in the drum correspond to different “altitudes,” imitating what the 307 would experience in flight. To their delight, the new cabin regulator held these pressures perfectly—strong proof their concept was sound.
Still, nothing could replace the next step: actual flight tests. “Engineers would have to climb on board the aircraft with their parachutes, climb into the lower fuselage accessory compartment, and make adjustments on the regulators at altitudes of up to 20,000 feet,” Lavelle says. “Fleece-lined jackets—it was cold!” recalls Cooper. With so many parts, the regulator was somewhat delicate. “One failure was traced to human fingerprints left inside the bellows,” says Lavelle. “The oily residue was skewing altitude measurements by 2,000 feet.” Once perfected, production of the regulator was licensed to Garrett AiResearch of Los Angeles.
Aircraft of that day sometimes leaked when it rained, so making the 307 reasonably airtight (even after double rows of rivets) required some unorthodox quality control. All the skin joints were covered by “paw tape,” a Dupont neoprene product developed for the XC-35, recalls Bob Dickson, then newly hired on Boeing’s assembly line. “Then you drove the rivets through. To test for leaks, you covered the joints with Ivory soap and [after pumping in air] watched for bubbles,” he says. It worked. For a final test, a dozen workers soaped down each pressurized cabin, carwash-style, then watched for bubbles and listened for whistling air. (Engineers still leak-test airliners today, but they use much neater ultrasound instruments.) A test rig stressed each of the 307’s curved, plexiglass window panes to nearly triple their expected flight pressure. Cockpit windows also were coated with an alloy to block ultraviolet rays, as flying above the clouds risks sunburn. “After all,” pronounced legendary test pilot Eddie Allen in Boeing’s official account of the 307, “the great thing about pressurizing, is that you [shouldn’t] know you’re up there.” By late 1938, he’d taken the 307 to 22,000 feet.
Despite those successes, TWA major stockholder John Hertz, the rent-a-car king, canceled TWA’s order for five 307s, saying that at $315,000 each, the new airplanes were too expensive. Desperate, Jack Frye contacted Howard Hughes. The young billionaire, who was planning to fly around the world, liked the idea of an over-weather craft. In classic fashion, Hughes bought controlling interest in TWA.
“Hughes spent a lot of time poking around the plant,” Dickson recalls, “frankly, often looking like he’d slept in his car.” Perhaps seeking more comfortable digs, Hughes purchased one extra airliner, for himself.