Editors Note: The first half of this article gives us a feel for state of NACA's experimental process in the mid 1940's. (Before the National Aeronautics and Space Agency (NASA) was formed in 1958, our technical research organization was NACA, the National Advisory Committee for Aeronautics.)

Far out of sight of earthbound humans, and “seen” only by the magic eyes of two special radars, a B-29 Superfortress commences its bombing run some 40,000 feet over Virginia. From a strange-looking rack beneath the B-29’s port wing, drops the world’s fastest “bomb”.

The electronic gaze of the radars turns from bomber to bomb. Long and slender and beautifully streamlined, it resembles no bomb ever dropped. And it behaves unlike any missile ever borne aloft by a Superfort. By the time the silvery, eight-foot thing is halfway down in its plunge from the stratosphere, the highly polished missile is approaching earth at the speed of sound. In a matter of seconds it attains supersonic speed. With a dull crash, it smashes into the outer fringe of Langley Field at nearly 900 miles an hour and buries its broken length 60 feet under ground!

Terrific as the impact had been, there was no explosion. A “bomb” more in local parlance than actuality, the sleek form with the tiny tail fins was really a drop-test missile. Instead of explosives, it had been filled with precision instruments of versatile and compact design. Now these valuable electronic meters, recorders, and radio equipment lie buried and shattered in the wasteland back of Langley.

But their brief messages have been sent; their work is finished. During the few seconds of the descent, these instruments and others, sent to ground observers, certain data, never available before this Fall day of 1944. The data was about airflow, shock waves, and other information about supersonic speed. The transmitter relayed the data to the ground receiver and its complex instrument panel, through “radio telemetering,” all this high-speed intelligence from a sword-like antenna that protruded from the missile’s tail. And the speeds of the drop had been carefully measured and recorded by the two tracking radars. Here was vital information, impossible to attain by any prior means, that enabled the scientists of the NACA Compressibility Research Division to fathom may long-sought secrets of speedier flight.

It was this data, plus that gained from subsequent “bombings” with missiles of assorted shapes and sizes, that led to the starling predictions by these scientists that supersonic aircraft will be flying within a few months, and that within three years planes will be crossing the United States, in two and one-half hours, at speeds exceeding 1000 miles per hour.

Subsequent research has added even greater credence to these optimistic predictions. A valuable contribution was made to the NACA research fund by a project involving a number of terminal-velocity dives by test pilots of the Flight Research Division in specially equipped P-51H Mustang fighters whose top speed on the level approaches 480 miles per hour. Tiny scale models of planes, and parts of planes, were mounted on the topside of the Mustang wing at the exact point where the airflow speeds up to supersonic velocities during high-speed dives. Gauges and recorders installed inside the test ship’s wing, measured and evaluated the behavior of these models in the accelerated airflow and provided interesting data.

…………… Editor's note: Much text skipped .................

The second part of this article gives us a good knowledge of the problems with transonic flight. About one year after this article was written, NACA's Bell X-1 rocket plane flew faster than Mach 1.0, the speed of sound.

As most Air Trails Pictorial readers are aware, compressibility is that condition which presents itself when an airplane or boat begins to plow up waves in the air or water through which it is passing at a certain speed, instead of sliding smoothly through the fluid without kicking up a bow wave or leaving behind it a turbulent wake. Such disturbance causes drag, alters the flow of the fluid around the wing or hull, and induces severe buffeting. The amount of this disturbance varies according to the speed and, equally, the shape of the plane or ship. A blunt-nosed Liberty Ship, moving at only five knots, plows up a sizable bow wave; while a sharp-prowed destroyer creates little or no bow wave at ten knots. The streamlined P-80 slides through the air without encountering compressibility effects, where a larger, bulkier plane flying at half the speed would be practically bulling its way though the air.

Since speed is involved here to a large extent, we use a convenient decimal called a “Mach number” to express scientifically the relation of the plane’s airspeed to the speed of sound. A Mach number of 0.5 is half the speed of sound, and 1.0 equals the speed of sound. Now sound has nothing to do with how rapidly or slowly an airplane may fly. But it happens that all the sounds we hear travels to our ears in a series of pressure waves. All pressure waves-musical, spoken, or those created by a bomb blast, including those caused by the movement of an airplane’s wing though the air, travel at a definite speed. This wave-travel varies according to the temperature of the air and its density; which is why sea-level pressure waves move at 764 miles per hour (at 59 degrees F.) but travel only 664 miles per hour at 40,000 feet where the air is considerable colder and only on-fourth the density.

One scientist called the conquest of compressibility a “battle of patterns.” A wing is shaped so that the air flows around it in a reasonable even pattern that provides pressure against the underside and a suction area over the topside of the wing. The nature of this flow pattern, which maintains lift, is complicated by varying speeds of air flowing over the wing, and by the directions this air-flow takes. The layer of air (or “streamline”) that has to curve around the wing’s topside in the same length of time that a lower streamline passes across the relatively straight underside, must move at greater speed, since the former travels further. This is not all. The lower streamline is compressed slightly as it passes the high-pressure area under the wing, and the upper layer of air expands somewhat as it flows through the low-pressure region on top of the airfoil. We see here that, as the air gets out of the wing’s way to let it pass and then closes in behind it after the passage, several things happen to the air. Within this lift-providing pattern are different speeds, differing pressures, and varying volumes.

These cross-currents extend not only around the wing, but ahead of it, so that the layers of air, or streamlines, are parted to prepare for the wing’s entry. As the air begins to flow into the lift pattern before the wing arrives, the streamlines separate to let the wing pass freely. A plane flying at subsonic speeds below 0.5 Mach number has a flow pattern well in advance, but the faster it flies, the closer the plane comes to catching up with its pressure waves and the air ahead has less advance notice. But air in certain parts of the plane’s pressure pattern—around the windscreen and over the topside of the wing, for example—is moving at a much higher Mach number because it has to flow faster to get around these protrusions.

Here comes compressibility. Instead of the streamlines’ being separated, they are crowded together because they are unprepared for the plane’s entry. They pile up into a show wave not unlike the ship’s bow wave, and bubble over into a turbulent mass like a bow wave. A great drag is created which requires a power increase or additional fuel consumption, out of all proportion to the possible gain.

Shock waves start practically at right angles to the wing, but the faster the plane moves, the more the wave fronts slant rearward. The shock waves also slide further back on the wing. The angle at which a shock wave leans backward is called the “Mach cone.” Engineers are outsmarting nature successfully by shaping the wing to conform with this cone of hard, high-velocity air, which is the reason for providing the wing with such pronounced sweepback.