Curtis Peebles With Contributions by Richard P. Hallion Library of Congress Cataloging-in-Publication Data Peebles, Curtis. Probing the sky : selected NACA/NASA research airplanes and their contributions to flight / by Curtis Peebles. pages cm Includes bibliographical references and index. 1. Research aircraft--United States--History. 2. United States.--National Aeronautics and Space Administration--Research. I. Title. TL567.R47P4349 2015 629.133’3--dc23 2014039331 Copyright © 2014 by the National Aeronautics and Space Administration. The opinions expressed in this volume are those of the authors and do not necessarily reflect the official positions of the United States Government or of the National Aeronautics and Space Administration. This publication is available as a free download at http://www.nasa.gov/ebooks. National Aeronautics and Space Administration Washington, DC Table of Contents Introduction: Toward—and Into—the Unknown v Chapter 1: Confronting the “Sound Barrier”: The Bell XS-1 ............................... 1 Chapter 2: Flying Test Tube: The Douglas D-558-1 Skystreak .......................... 45 Chapter 3: Proving the Swept Wing: The Douglas D-558-2 Skyrocket ............. 85 Chapter 4: Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto ................................................................ 135 Chapter 5: Versatile Minimalist: The Northrop X-4 Bantam ............................ 167 Chapter 6: Transformative Pioneer: The Bell X-5 .............................................. 207 Chapter 7: Progenitor of the Delta: The Convair XF-92A .................................. 243 Round One: A Reflection 269 Appendix: Technical Specifications for the Round One Aircraft 280 Acknowledgments 282 Selected Bibliography 283 Index 307 iii The scale of what’s involved in undertaking research flights is made clear in this iconic image of the D-558-2, the B-29 launch plane, and the ground support personnel and equipment. The photo was taken in front of the NACA hangar at South Base shortly before the move to the pres- ent facilities. (NASA Photo) Introduction Toward—and Into—the Unknown In the decades since the Wright brothers’ first flights, a body of knowledge and tools, created in an evolutionary process of small steps, had been built up to guide engineers and researchers in developing new aircraft. The early wood-and-fabric biplanes had given way to all-metal monoplanes. Aircraft size, range, and payload had also grown, until the oceans could be spanned in a fraction of the time a ship would take. Speed became the critical factor in both commercial and military operations. But by the early 1940s, speed itself had become the problem. With aircraft flying near the speed of sound, the old rules of subsonic aerodynamics no longer applied and the old tool of aeronautical research—the wind tunnel—no longer worked. Consequently, engineers lacked the means to determine if their designs would withstand actual flight conditions. The technology, tools, and procedures needed in this new realm of flight completely transformed the fields and practices of aerodynamics, propulsion, structures, design theory, materials, flight control systems, life-support systems, escape systems, safety procedures, wind tunnels, and data collection systems and methodologies. The body of knowledge for the supersonic era was effec- tively recast and made anew. Thomas Kuhn’s term “paradigm shift” is now much overused both as an expression and as an intellectual concept, but, in the case of the midcentury high-speed revolution, it is certainly appropriate. What aeronautical science had accrued for subsonic flight following the Wrights’ first flight in 1903 was rendered almost entirely irrelevant by the late 1940s and the onset of the supersonic era. The engine of transformation was the turbojet. It spelled the end of one era in aviation and the dawn of another, pushing people and machines across the sonic divide faster than they had gone before, forcing transonic and supersonic researchers to address the mysteries that were affecting airplanes entering this new and unknown realm. Ironically, while it was this new form of propulsion that drove the super- sonic breakthrough, the turbojet itself could not yet take the researchers and pilots far enough into the realm to conduct the vital research data they sought. v Probing the Sky The turbojet of the 1940s was a new and still immature technology. The temperatures in its combustion chambers (burner cans) and which the spin- ning turbine blades endured approached the limits of contemporary metal- lurgical science, making them unsuited then for supersonic flight. In order to explore the supersonic regime, researchers turned to rocket planes, relegating turbojets to slower speeds. Some of these turbojets became famous, such as the XS-1 (later designated simply the X-1, the first of the postwar “X-series” research airplanes). Others are little remembered today—but each played its own distinctive part in taking aviation from the subsonic to the supersonic era. Collectively, these first research aircraft are known as “Round One,” the tran- sonic and supersonic probers of the 1940s and 1950s that preceded “Round Two,” the hypersonic North American X-15 of the 1960s. This story is not simply one of strange-looking airplanes and their coura- geous test pilots. It offers, as well, insights into the history of aerospace tech- nology and science and of shifting technology and (yes) paradigms within that field. The turbojet and the rocket changed the rules. Together, they led to the X-1 and the other research airplanes. These airplanes were also why a small group of engineers came to a vast dry lakebed in the Mojave Desert, establish- ing there, arguably, the world’s premier center for aeronautical research and development, the Air Force Test Center–NASA Armstrong Flight Research Center complex (and the associated airspace) at Edwards Air Force Base, CA. Creating the new era, with its own rules and tools, took place against the dynamic background of the Cold War, forcing and feeding the need for ever- higher performance. But at the onset of that era, lacking reliable tools to deter- mine what would work and what would not, engineers had few means to decide what paths they should follow. In a real sense, they faced the same situation as the Wright brothers, when they were building their first kites and gliders in 1899–1902, with little reliable information, much of which was contradictory and misleading. They discovered that little of the available information was reli- able. They discovered that they had to reject treasured assumptions and awake to new realities. They had to rethink their concepts of stability and control. For the test pilots, there was yet something more: since the potential of failure and death was very real, when they went aloft, their skill had to be matched by even greater courage. Knowledge, dedication, expertise, and courage: of the mix of such was the crafting of the transonic and supersonic revolution, made manifest in the skies over the Mojave over a half century ago. vi A semispan airplane model on the wing of a P-51 in January 1946. In an adaptation of the wing-flow technique, a half-scale model of an airplane was attached to the upper surface of a wing. As the airplane made a dive, airflow over the wing accelerated to high transonic or supersonic velocity. (NASA) viii CHAPTER 1 Confronting the “Sound Barrier”: The Bell XS-1 The resistance of a wing shoots up like a barrier as we approach the speed of sound. —W.F. Hilton1 In 1935, when British aerodynamicist W.F. Hilton inadvertently coined the phrase “sound barrier,” the piston engine aircraft was nearing its performance peak, producing approximately one horsepower per pound of engine weight. But aeronautical engineers were already outrunning the knowledge they had acquired in the three decades since the Wright brothers’ first flight at Kitty Hawk, NC, in December 1903. Up to this time, designers had always treated the airflow over wings as if it were an “inviscid” or incompressible fluid. This simplified their calculations, and the errors induced by this convenient (if false) assumption were too small to be of significance. With aircraft now exceeding 400 miles per hour (mph), the compressibility of air could no longer be ignored. But engineers and scientists lacked the insight on how the required revolutionary changes could be accomplished. The wind tunnel, the aerodynamicist’s standard tool since before the Wright brothers, was of little help. As the airflow neared the speed of sound, shock waves formed on the models and supports and reflected onto the tunnel walls, rendering the data questionable from Mach 0.8, just below the speed of sound, to approximately Mach 1.2, just beyond the speed of sound. While tunnels could hint at some of the flow changes induced by high-speed flight—for example, a dramatic loss of lift and simultaneous sharp increase in drag—they could not furnish precise quantified data that would permit accurate analysis. Thus, very suddenly, a new realm of flight loomed—mysterious, unknown, dangerous, and destructive. Pilots making high-speed dives found that control surfaces would not move or did so to little effect. In some cases, their airplanes broke up or dove uncontrollably into the ground. Others, more fortunate, found that the controls would respond normally once the airplanes reached lower altitudes. The phrase “sound barrier” encapsulated the mystique of this realm. Very quickly, a mythology developed regarding supersonic flight. Myths unsupported by scientific fact gained traction, some of which bordered on the 1 Probing the Sky bizarre: Some held that a pilot’s voice would become caught in the throat as the plane “broke” the sound barrier, while others alleged that time would reverse and the pilot would become younger after going supersonic.2 The Onset of Transonic Research The first indications of these problems appeared in 1918. Aircraft of that time had top speeds of about 100 mph.