07 February 2010

When famed test pilot Alvin "Tex" Johnston came to Boeing just before the first flight of the XB-47 Stratojet prototype, it was on the recommendation of the USAF based on Johnston's prior experience with jet aircraft at Bell and that he had flown just about every aircraft in the Air Force's inventory. He had the nickname "Tex" as he always wore cowboy boots when he flew.

The flight test program of the Stratojet was quite literally exploring uncharted territory as it was the first large jet-powered swept-wing aircraft to fly. It fell to the Boeing test pilots like Tex Johnston to uncover all the aerodynamic pitfalls and report them to the engineers. At times, Johnston could be a constant irritant to the Stratojet engineering team but somehow his observations always seemed on the mark even if he wasn't seeing eye-to-eye with the engineers.

One famous example concerned the discovery that at high speed, the XB-47's nose would start pitching up as the Mach number increased. The oscillations had the potential to throw the Stratojet out of control and into a spin. Many of the test pilots had noted this high-Mach instability, but it took Tex Johnston's larger-than-life personality to get it taken seriously. After a test flight, he informed one of the Boeing engineers, T.A. Wilson (who would later head the B-52 program and eventually lead Boeing through the 70s and 80s), that the vibrations would get severe enough to cause a stall warning. Wilson wanted to know at what point the oscillations began as he wasn't seeing it on the flight data recording equipment printouts.

"I don't see anything that shows me it's a high-Mach stall".
"Goddammit, I felt it!"
"You felt it? Where?"
"Wilson, right in my ass!"

So T.A. Wilson decided to settle it once and for all. He removed the pilot's seat cushion and installed a data recorder right in the seat and had Tex Johnston take the XB-47 back up. Once he landed, Wilson took a look at the data and sure enough, the recorder picked up the signature of the onset of a high-Mach stall.

As the Stratojet approached higher Mach speeds, a forming shockwave from the wing's leading edge disturbed the airflow over the outer wings, causing them to lose lift. The solution was to put a series of vortex generators on the outer wing which energized the airflow over the outer wing and maintained lift at high speeds.

Source: Legend & Legacy: The Story of Boeing and Its People by Robert J. Serling. St. Martin's Press, 1992, p97-98.

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