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Just past the standing figure, a chamber with movable sidewalls controls the Mach number of air entering the diffuser. (NASA Glenn)

The Perfect Wind Storm

In the 1950s, engineers at Cleveland's brand-new supersonic wind tunnel battled shock waves, unstarts, and the local power company.

In May 1956, Cleveland was hit with record wind storms, and residents were still digging out of the debris when the Lewis Flight Propulsion Laboratory in nearby Berea, Ohio, debuted what was then the world’s most powerful wind tunnel (and one of the most powerful today). Edward Sharp, director of the propulsion arm of the National Advisory Committee for Aeronautics’ propulsion lab, introduced the 10 x 10, named for the dimensions of the tunnel’s test section.

The 10- by 10-foot supersonic wind tunnel was the preeminent achievement in a post-war response to the V-2 rockets and high-speed fighters Germany had fielded in World War II. The 1949 Unitary Plan Act had established supersonic wind tunnels for the NACA in Virginia, California, and now Ohio, as well as Air Force facilities in Tennessee. Congress wanted to ensure that the United States would never again lag in aeronautical technology.

Much of the press corps must have listened with only one ear while thinking up such headlines as “Big Wind Blows Good”—anything to play up the irony of a wind tunnel’s unveiling being preceded by a wind storm. Sharp acknowledged the storm’s power by securing his tie with two clips shaped like Century Series fighters.

At the presentation, tunnel designer Harold Zager noted the stainless steel walls’ subtle curvature, designed to transition the flow from subsonic to supersonic speeds in an hourglass-shaped nozzle upstream of the test section. “I worked for the guy who designed the supersonic nozzle contours. We laid those out with the method of characteristics,” he says today, referring to a design procedure that enabled him to solve the complex equations governing supersonic flow using little more than a slide rule and a drafting board. “That makes the flow in the test section shock-free, if you do your homework right.”

The goal for any tunnel designer is to create airflow in the test section that matches the conditions—airspeed, pressure, temperature—that an aircraft would encounter at a given altitude. This means no shock waves, no turbulence, nothing but smooth flow until the air reaches the model. The task is delicate enough with a subsonic wind tunnel, but with supersonic flow the stakes are much higher. The upstream nozzle must be precisely curved; the slightest deviation from the design can cause a fan of shock waves to emanate, dirtying the pristine flow of the test section and rendering the tunnel unusable.

Zager also helped design the control room, where engineers could monitor both the model being tested as well as the terminal shock wave that is inevitable when running a supersonic tunnel. In the control room, camera monitors, pressure gauges, and a wall of alarms monitored the terminal shock—where supersonic flow terminates and subsonic flow begins—as it slowly progressed from the first nozzle down through the test section and past the model before settling near a second nozzle. If all went well, the shock wave would sit well behind the model, returning the wake of supersonic bomber models like the Convair B-58 and the Vought F8U to subsonic flow.

If things didn’t go well? The technical, if ungrammatical, term is “unstart.” When the tunnel unstarts, the shock wave shoots forward, striking the model from behind, and disappears through the first nozzle. According to Al Linne, an engineer at the tunnel today, “It’s the turbulence that really causes problems, because now you’ve got air that’s just going everywhere. Your model’s out there swinging around, doing things that visually, you wouldn’t believe.”

Because a supersonic wind tunnel consumes so much more power than a standard wind tunnel, the lab struck a deal with the Cleveland Electric Illuminating Company. If the tunnel engineers would request a weekly estimate of power, the power company would provide the electricity at a reduced rate, with two caveats: The tunnel would have to run only at night, and if the load on the grid was too high, the company could deny power.

“The toughest shift operation for me,” says retired test engineer Larry Smith, “was second shift, to make preparations for testing. That was back when my family was growing. You go to work at 10 at night and you come home at 9 the next morning. So you don’t see your family at all.”

But working at night had its perks. Says Bob Cubbison, one of the original research engineers at the 10 x 10, “The instrumentation guys used these kilns that would heat up and bond strain gauges to whatever it is they’d need to bond to. They found that a Magic Chef or a good Kenmore would work just as well as the industrial ovens. So they bought one of those, and you could have a hot cup of coffee at night, or the instrumentation guy would cook up a few hamburgs. The morning crew would come in and say: ‘What the heck went on here last night?’ ”

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