Mach 1: Assaulting the Barrier

In 1947, no airplane had ever gone faster than the speed of sound.

The Douglas D-558-2 Skyrocket (shown here at Edwards Air Force Base circa May 1949) pushed past Mach 2 on November 20, 1953, beating an advanced X-1 to the record. (US Navy via National Air and Space Museum. Photo SI A-5168-C.)
Air & Space Magazine | Subscribe

(Continued from page 2)

Even more fascinated was NACA engineer Robert Gilruth, who realized that some of the local airflow over the wing of Cooper’s Mustang was going nicely, controllably, predictably supersonic. The NACA had been trying to do transonic research by dragging high-speed models to extreme altitudes aboard a B-29 and an F-82 Twin Mustang and then dropped them straight down onto a bombing range near Langley, Virginia—getting brief bits of data by tracking the plunging models with radar or by laboriously digging them out of the mud and reading the instrument recording that had survived.

Putting a tiny model atop a strut or “sting” on a P-51’s wing, right in the airflow that in places had accelerated to speeds of Mach 1.4, seemed much neater. From this experience, engineers realized they could reproduce the same effect right in their transonic wind tunnels by mounting models atop wing-shaped “bumps,” where they would also encounter supersonic air.

WHEN THE U.S. Army Air Forces imported Frank Whittle’s jet engine from England in 1942, the Jet Age did not arrive with it. The Bell P-59, the Air Force’s first operational jet-fighter, was still too slow to get itself in trouble. The next jet, the Lockheed P-80, was fast enough to suffer from aileron buzz caused by the capricious dance of shock waves on its control surfaces. But again the problem was encountered only in steep dives and again was counteracted by fortifying the control surfaces.

Not until after World War II did aircraft design take the radical turn toward supersonic flight that the jet invited, producing airplanes that looked strikingly different from their subsonic forebears. The changes derived mainly from two sources of information: NACA high-speed flight research and the scientific war spoils from Germany. The first easy answer to going faster, which turned out to be swept wings, came from both.

The NACA’s Robert Jones had been quietly studying the effect of sweep-back on the lift of large-span wings at the research lab in Langley, Virginia. He completed a formal report in April 1945, which the NACA issued on June 21 to military services and companies with security clearances. That May, Boeing engineer George Schairer accompanied von Kármán, then the Army Air Forces’ chief scientist, on an intelligence gathering mission to a once-secret aeronautics research installation in Braunschweig, Germany.

Poking into various offices in the laboratory, Schairer and von Kármán came upon a small model of an airplane with swept wings. Both had been closely following Jones’ studies and were anxious to get their hands on anything relating to the design of the model, but the sullen German aerodynamicists in Braunschweig shrugged off their questions. Von Kármán decided to play good cop/bad cop. Though the Soviets were nowhere nearby, he turned to his assistant and loudly said, “We’re through here. I think now it’s time to notify Russian intelligence to take over.” Terrified by the thought of a Soviet debriefing, the German director of engineering took von Kármán’s assistant to a nearby drywell and showed him where they had dumped all of their best research, including considerable wind tunnel data on the behavior of swept-back wings on the transonic regime.

Schairer immediately wrote Boeing headquarters, telling the company to stop work on a straight-wing dodo of a Mach 1 design that was already well under way. He returned to Seattle with microfilmed German data that resulted in the Boeing B-47, progenitor of the 707 and the B-52. The NACA’s equivalent data from Jones’ report convinced North American to put swept wings on a somewhat refined Air Force version of the FJ-1 Fury—a slow, tubby, straight-wing Navy jet that had already gone into limited production. The result was the F-86 Sabre, the first operational fighter in the world routinely capable of flying faster than sound (though it took a slight dive to do it) and one of the most aesthetically pleasing and operationally successful aircraft ever built.

Why the Bell X-1 challenged the “sound barrier” with straight wings when those of airplanes all around it, including its sibling rival, the Douglas D-558-2 Skyrocket, were swept has been a matter for interpretation since 1945. (The wings of the Douglas D-558-1 Skystreak were also straight.) When the Army Air technical Service Command awarded Bell the contract in March 1945, its engineers had already been briefed by wing sweep champion Robert Jones. But when word came back from Braunschweig, Army Air Forces General Alden R. Crawford accused the NACA of incompetence for not insisting on swept wings. The NACA responded by claiming prudence and pointed to the dearth of experimental evidence, especially at low speeds. A few who witnessed the petty spats between the two organizations later in the program have suggested that the NACA knew better but was determined to give the Army exactly what it asked for, which was a straight-wing rocket-propelled airplane, instead of the turbojet-powered design that the NACA favored. Whatever the reason for them, the straight, thin wings that carried the X-1 blithely past Mach 1 proved there was more than one way to skin a cat (see “Don’t Make Waves,” below)

One advantage straight wings had over swept-wing designs was rigidity: the more radically a wing was swept, the less torsionally rigid it became. This is why early swept-wing jets sometimes suffered “aileron reversal,” which was probably one source of the common misconception—helped along by Hollywood and the mid-1950s English classic film Breaking the Barrier—that “the controls reversed” as an airplane approached the speed of sound.

When the right aileron, say, on a weak-winged fighter such as the McDonnell F3H Demon was deflected upward at subsonic speeds, it would, as expected, command a roll to the right. But at near-supersonic speeds air pressure against the raised right aileron instead warped the trailing edge of that wing down, thus turning the entire right wing into an enormous aileron that commanded a left roll, to the pilot’s bafflement. Some airplanes could literally perform aileron rolls in the direction opposite full stick deflection, and F3Hs were known to return from combat practice maneuvers with permanently warped wings.

Comment on this Story

comments powered by Disqus