The Problem
The remarkably rapid evolution of aircraft design during the first four decades of this century had brought the state of the art to an apparent impasse by the late 1930s. Designers were beginning to conceive of aircraft capable of speeds in excess of 500 mph. Some truly daunting challenges faced them, however, as they considered the obstacles to be overcome. At 500 mph, they would be probing the lower limits of the transonic region, the little-understood area between Mach 0.7 and Mach 1.3 where an aircraft would encounter mixed subsonic and supersonic airflow. And, as it approached the speed of sound (Mach 1.0), a virtual "wall" of air would build up in front of it which one prominent aerodynamicist, speaking for many of his colleagues, likened to a "barrier against future progress."
Theoretical calculations seemed to indicate that, as an aircraft approached Mach 1.0, drag would reach infinity. Just how much power would be required to contend with "infinite" drag? And there were other, equally perplexing problems surrounding the phenomenon which aerodynamicists called "compressibility." At transonic speeds, an aircraft would encounter mixed subsonic and supersonic airflow conditions. Airflow accelerates as it passes over an airfoil and thus, while an airplane may only be flying at seven-tenths the speed of sound, the flow over its wings may well be moving at supersonic speeds. In this turbulent region of mixed flow conditions, aerodynamicists knew that shock waves would form on the aircraft and, moving back
and forth, violently disrupt the airflow and thereby dramatically change the aerodynamic trim of the vehicle and drastically alter its control response and controllability. Further, many aerodynamicists believed the turbulent flow could result in aircraft oscillations severe enough to cause structural damage.
Theoretical concerns became tragic realities by the early 40s. In the United States, the first air-craft to encounter serious compressibility problems was the Lockheed P-38 Lightning. Encountering violent buffeting during high-speed dives, its nose tended to "tuck under" while its tail surfaces shook wildly. The magnitude of the problem was brought home in terrifying fashion one morning in November of 1941, when veteran Lockheed test pilot Ralph Virden pushed over into a steep dive. As he accelerated downward, he lost elevator effectiveness and the P-38's dive angle grew increasingly steep. As the craft continued to pick up speed, ultimately accelerating to an estimated air-speed of 535 mph, the violently disturbed airflow coming off the wings overstressed the tail and literally tore it off. Caught in the grip of compressibility, a skillful test pilot had been reduced to the role of a helpless passenger on a journey toward destruction. A makeshift remedy--dive flaps attach-ed to the front wing spar on the lower surface of the wing--substantially alleviated the problem by, in effect, permitting the wing to retain enough lift at high speeds to provide pilots with sufficient control to pull out of dives at higher Mach numbers. This, however, was just a temporary remedy, not a solution.
Throughout the war, pilots of high-performance fighters continued to encounter the problem. During high-speed dives, they would suddenly discover that their control columns had frozen up or reversed control effectiveness altogether and, even if they were ultimately able to effect dive recoveries, the excessive aerodynamic loads imposed on the tails of their craft all too frequently resulted in catastrophic failures.
All of these very serious problems surfaced while prop-driven fighters still ruled supreme. The development of turbojet technology during the war years made the search for a real solution all the more compelling. By war's end, it was obvious that turbojet engines offered the potential to propel aircraft through the transonic and even, perhaps, into the supersonic region. And, indeed, in Where We Stand, the seminal assessment which Dr. Theodore von Karman submitted to General of the U.S. Army Air Forces (AAF) Henry H. "Hap" Arnold in August of 1945, he warned: "We cannot hope to secure air superiority in any future conflict without entering the supersonic speed range." This, in fact, was presented by Karman as the highest priority requirement confronting the postwar Air Force. The major question remained, however. Could a piloted airplane be designed and built to survive in that flight environment?
The answers, unfortunately, were not easily forthcoming. First and foremost, designers needed a much more complete knowledge of transonic aerodynamics--and their knowledge had to be based on concrete evidence, not theoretical calculations. Throughout the war years, however, wind tunnels remained practically useless in terms of transonic research. At Mach numbers below 0.8 and above 1.2, smooth airflow could be maintained and thus aerodynamicists were able to acquire accurate measurements. But, between those numbers, the tunnels "choked," as shock waves formed off test models and, in turn, reflected off tunnel walls, thereby inhibiting accurate measurement of flow characteristics around the model. The best solution to this problem, the slotted-throat transonic tunnel, would not arrive on the scene until the late 40s.
In the meantime, other methods of data acquisition--rocket-propelled models, free-falling instrumented missile shapes released from high altitudes, and wing-flow tests of airfoil shapes mounted to the upper surface of a P-51's wing--were employed as stopgap alternatives. While some useful data were acquired by these means, it was really of only limited value in terms of the magnitude of the problems to be overcome. These circumstances enhanced the appeal of a far more radical approach: to build and flight test a fully instrumented experimental aircraft.
Such an approach would, indeed, represent a radical departure from standard practice; in essence, a reversal of the time-honored process wherein researchers accumulated and analyzed their data before the designers built their aircraft. And, while there were legions of experts who scoffed at such an approach, there were a number of individuals who had labored long and hard to promote its merits. Among them, two men would play pivotal roles in the genesis of the experimental research airplane programs of the mid-to-late 40s: Major Ezra Kotcher, from the Engineering Division at Wright Field, and John Stack, director of the Compressibility Research Division at the National Advisory Committee for Aeronautics' (NACA) Langley Memorial Aeronautical Laboratory (LMAL). Both were confident that the sonic "wall" could be breached and both strove to convince their respective organizations that research airplanes offered the best means to demonstrate to the aeronautical community the validity of U.S. Navy Captain Walter S. Diehl’s contention that the so-called "sound barrier" was "just a steep hill."