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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.)

Mach 1: Assaulting the Barrier

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

Sweeping the wing back has the same effect as reducing the ratio of thickness to chord and therefore also requires less work from the air. Since a swept wing meets the air at an angle, the distance the air must travel from the leading edge to the trailing edge of the wing is longer, and its acceleration is more gradual.

The supercritical airfoil is a much more recent invention, again by Richard Whitcomb, for delaying the formation of shock waves. The airfoil gets its name from the term “critical Mach number,” the speed, usually around Mach .7, at which the first shock waves appear on an airplane’s wing. Whitcomb found that by flattening the upper surface of the wing, he could both weaken the shock waves and push them aft to a point closer to the wing’s trailing edge. Reducing the camber, or curvature, of the wing also sacrifices lift, however. Whitcomb counteracted this effect by increasing the camber of the trailing edge. With a supercritical airfoil, the airplane can boost its critical Mach number; that is, it can fly faster without the loss of lift and increase in drag caused by the formation of shock waves close to its wing’s leading edge.

SIDEBAR: Not So Fast

Until the Navy develops supersonic submarines, Mach number will inevitably apply to objects zipping through the atmosphere. (A space vehicle might travel seventy-seven thousand miles an hour, but it’ll never break Mach 1 Out There: there is no sound, ergo, no speed of sound, in the vacuum of space.) But strictly speaking, Mach number is the ratio of the speed of an object to the speed of sound in whatever medium it’s traveling through. So Mach 1 for a bullet going through a bar of soap or a nail being hammered into a two-by-four by a particularly powerful carpenter will be quite different from Mach 1 for Tom Cruise buzzing the Miramar Tower.

Sound’s speed also changes with the temperature of the air, not simply its density, and though it’s typically about 742 mph at sea level, the strongest shout will generally travel only 661 mph anywhere between 26,000 and 60,000 feet, the band of pre-stratospheric tropopause at which air temperature normally remains constant.

To further complicate matters, it now turns out that H.C. Hardy, the physicist who established those specifics in 1942, based them on a dose of…well, maybe not bad air, but some pretty ordinary atmosphere literally picked up from a breeze through his lab window. Recent work by George S.K. Wong of the National Research Council of Canada has determined that the speed of sound at standard sea level conditions is not 741.5 mph, but 741.1. “You know physicists,” Wong said. ‘They always calculate things assuming this, assuming that.” Unfortunately, he did not know how reliable his corrections were.


Piling On

If you’ve ever seen the circles rippling outward from a pebble tossed into a pond you can imagine the invisible disturbance waves that radiate in three dimensions from an airplane—or any other disturbance—in the atmosphere. Disturbance or pressure waves are the very phenomena that transmit sound; the speed at which they pulse outward, therefore, is also the speed at which sound travels.

An airplane moving slower than the speed of sound stays comfortably behind the forward-moving wave fronts, which perform a certain service for the airplane by preparing the air for its arrival. At this speed the airplane politely bumps aside the air molecules in its path. Each molecule has time to move out of the way and to nudge its neighbor out of the way as well. Pick up the pace, however, and the wing (and/or the tail, cockpit canopy, radome, or any other airframe component) of an airplane flying close to or faster than the speed of sound arrives before the air knows it’s coming.

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