CASE
6
Even before the invention of the airplane, wind tunnels have been key in undertaking fundamental research in aerodynamics and evaluating design concepts and configurations. Wind tunnels are essential for aeronautical research, whether for subsonic, transonic, supersonic, or hypersonic flight. The swept wing, delta wing, blended wing body shapes, lifting bodies, hypersonic boost-gliders, and other flight concepts have been evaluated and refined in NACA and NASA tunnels.
NASA and the Evolution of the Wind Tunnel
Case-6 Cover Image: Blended Wing-Body free-flight model in the Langley Full-Scale Tunnel. NASA.
In November 2004, the small X-43A scramjet hypersonic research vehicle achieved Mach 9.8, roughly 6,600 mph, the fastest speed ever attained by an air-breathing engine. During the course of the vehicle’s 10-second engine burn over the Pacific Ocean, the National Aeronautics and Space Administration (NASA) offered the promise of a new revolution in aviation, that of high-speed global travel and cost-effective entry into space. Randy Voland, project engineer at Langley Research Center, exclaimed that the flight “looked really, really good” and that “in fact, it looked like one of our simulations.”[1] In the early 21st century, the public’s awareness of modern aeronautical research recognized advanced computer simulations and dramatic flight tests, such as the launching of the X-43A mounted to the front of a Pegasus rocket booster from NASA’s venerable B-52 platform. A key element in the success of the X-43A was a technology as old as the airplane itself: the wind tunnel, a fundamental research tool that also has evolved over the past century of flight.
NASA and its predecessor, the National Advisory Committee for Aeronautics (NACA), have been at the forefront of aerospace research since the early 20th century and on into the 21st. NASA made fundamental contributions to the development and refinement of aircraft and spacecraft—from commercial airliners to the Space Shuttle—for operation at various speeds. The core of this success has been NASA’s innovation, development, and use of wind tunnels. At crucial moments in the history of the United States, the NACA–NASA introduced state-of-the-art testing technologies as the aerospace community needed them, placing the organization onto the world stage.
The Anatomy of a Wind Tunnel
The design of an efficient aircraft or spacecraft involves the use of the wind tunnel. These tools simulate flight conditions, including Mach number and scale effects, in a controlled environment. Over the late 19th, 20th, and early 21st centuries, wind tunnels evolved greatly, but they all incorporate five basic features, often in radically different forms. The main components are a drive system, a controlled fluid flow, a test section, a model, and instrumentation. The drive system creates a fluid flow that replicates flight conditions in the test section. That flow can move at subsonic (up to Mach 1), transonic (Mach 0.75 to 1.25), supersonic (up to Mach 5), or hypersonic (above Mach 5) speeds. The placement of a scale model of an aircraft or spacecraft in the test section via balances allows the measurement of the physical forces acting upon that model with test instrumentation. The specific characteristics of each of these components vary from tunnel to tunnel and reflect the myriad of needs for this testing technology and the times in which experimenters designed them.[2]
Wind tunnels allow researchers to focus on isolating and gathering data about particular design challenges rooted in the four main systems of aircraft: aerodynamics, control, structures, and propulsion. Wind tunnels measure primarily forces such as lift, drag, and pitching moment, but they also gauge air pressure, flow, density, and temperature. Engineers convert those measurements into aerodynamic data to evaluate performance and design and to verify performance predictions. The data represent design factors such as structural loading and strength, stability and control, the design of wings and other elements, and, most importantly, overall vehicle performance.[3]
Most NACA and NASA wind tunnels are identified by their location, the size of their test section, the speed of the fluid flow, and the main design characteristic. For example, the Langley 0.3-Meter Transonic Cryogenic Tunnel evaluates scale models in its 0.3-meter test section between speeds of Mach 0.2 to 1.25 in a fluid flow of nitrogen gas. A specific application, 9- by 6-Foot Thermal Structures Tunnel, or the exact nature of the test medium, 8-Foot Transonic Pressure Tunnel, can be other characterizing factors for the name of a wind tunnel.
The Prehistory of the Wind Tunnel to 1958
The growing interest in and institutionalization of aeronautics in the late 19th century led to the creation of the wind tunnel.[4] English scientists and engineers formed the Royal Aeronautical Society in 1866. The group organized lectures, technical meetings, and public exhibitions, published the influential Annual Report of the Aeronautical Society, and funded research to spread the idea of powered flight. One of the more influential members was Francis Herbert Wenham. Wenham, a professional engineer with a variety of interests, found his experiments with a whirling arm to be unsatisfactory. Funded by a grant from the Royal Aeronautical Society, he created the world’s first operating wind tunnel in 1870–1872. Wenham and his colleagues conducted rudimentary lift and drag studies and investigated wing designs with their new research tool.[5]
Wenham’s wing models were not full-scale wings. In England, University of Manchester researcher Osborne Reynolds recognized in 1883 that the airflow pattern over a scale model would be the same for its full-scale version if a certain flow parameter were the same in both cases. This basic parameter, attributed to its discoverer as the Reynolds number, is a measure of the relative effects of the inertia and viscosity of air flowing over an aircraft. The Reynolds number is used to describe all types of fluid flow, including the shape of flow, heat transfer, and the start of turbulence.[6]
While Wenham invented the wind tunnel and Reynolds created the basic parameter for understanding its application to full-scale aircraft, Wilbur and Orville Wright were the first to use a wind tunnel in the systematic way that later aeronautical engineers would use it. The brothers, not aware of Wenham’s work, saw their “invention” of the wind tunnel become part of their revolutionary program to create a practical heavier-than-air flying machine from 1896 to 1903. Frustrated by the poor performance of their 1900 and 1901 gliders on the sandy dunes of the Outer Banks—they did not generate enough lift and were uncontrollable—the Wright brothers began to reevaluate their aerodynamic calculations. They discovered that Smeaton’s coefficient, one of the early contributions to aeronautics, and Otto Lilienthal’s groundbreaking airfoil data were wrong. They found the discrepancy through the use of their wind tunnel, a 6-foot-long box with a fan at one end to generate air that would flow over small metal models of airfoils mounted on balances, which they had created in their bicycle workshop. The lift and drag data they compiled in their notebooks would be the key to the design of wings and propellers during the rest of their experimental program, which culminated in the first controlled, heavier-than-air flight December 17, 1903.[7]
Over the early flight and World War I eras, aeronautical enthusiasts, universities, aircraft manufacturers, military services, and national governments in Europe and the United States built 20 wind tunnels. The United States built the most at 9, with 4 rapidly appearing during American involvement during the Great War. Of the European countries, Great Britain built 4, but the tunnels in France (2) and Germany (3) proved to be the most innovative. Gustav Eiffel’s 1912 tunnel at Auteiul, France, became a practical tool for the French aviation industry to develop high-performance aircraft for the Great War. At the University of Göttingen in Germany, aerodynamics pioneer Ludwig Prandtl designed what would become the model for all “modern” wind tunnels in 1916. The tunnel featured a closed circuit; a contraction cone, or nozzle, just before the test section that created uniform air velocity and reduced turbulence in the test section; and a chamber upstream of the test section that stilled any remaining turbulent air further.[8]
The NACA and the Wind Tunnel
For the United States, the Great War highlighted the need to achieve parity with Europe in aeronautical development. Part of that effort was the creation of the Government civilian research agency, the NACA, in March 1915. The committee established its first facility, Langley Memorial Aeronautical Laboratory—named in honor of aeronautical experimenter and Smithsonian Secretary Samuel P. Langley—2 years later near Hampton, VA, on the Chesapeake Bay. In June 1920, NACA Wind Tunnel No. 1 became operational. A close copy of a design built at the British National Physical Laboratory a decade earlier, the tunnel produced no data directly applicable to aircraft design.[9]
NACA Wind Tunnel No. 1 with a model of a Curtiss JN-4D Trainer in the test section. NASA.
One of the major obstacles facing the effective use of a wind tunnel was scale effects, meaning the Reynolds number of model did not match the full-scale airplane. Prandtl protege Max Munk proposed the construction of a high-pressure tunnel to solve the problem. His Variable Density Tunnel (VDT) could be used to test a 1/20th-scale model in an airflow pressurized to 20 atmospheres, which would generate identical Reynolds numbers to full-scale aircraft. Built in the Newport News shipyards, the VDT was radical in design with its boilerplate and rivets. More importantly, it proved to be a point of departure from previous tunnels with the data that it produced.[10]
The VDT became an indispensable tool to airfoil development that effectively reshaped the subsequent direction of American airfoil research and development after it became operational in 1923. Munk’s successor in the VDT, Eastman Jacobs, and his colleagues in the VDT pioneered airfoil design methods with the pivotal Technical Report 460, which influenced aircraft design for decades after its publication in 1933.[11] Of the 101 distinct airfoil sections employed on modern Army, Navy, and commercial airplanes by 1937, 66 were NACA designs. Those aircraft included the venerable Douglas DC-3 airliner, considered by many to be the first truly “modern” airplane, and the highly successful Boeing B-17 Flying Fortress of World War II.[12]
The NACA also addressed the fundamental problem of incorporating a radial engine into aircraft design in the pioneering Propeller Research Tunnel (PRT). Lightweight, powerful, and considered a revolutionary aeronautical innovation, a radial engine featured a flat frontal configuration that created a lot of drag. Engineer Fred E. Weick and his colleagues tested full-size aircraft structures in the tunnel’s 20-foot opening. Their solution, called the NACA cowling, arrived at the right moment to increase the performance of new aircraft. Spectacular demonstrations—such as Frank Hawks flying the Texaco Lockheed Air Express, with a NACA cowling installed, from Los Angeles to New York nonstop in a record time of 18 hours 13 minutes in February 1929—led to the organization’s first Collier Trophy, in 1929.
With the basic formula for the modern airplane in place, the aeronautical community began to push the limits of conventional aircraft design. The NACA built upon its success with the cowling research in the PRT and concentrated on the aerodynamic testing of full-scale aircraft in wind tunnels. The Full-Scale Tunnel (FST) featured a 30- by 60-foot test section and opened at Langley in 1931. The building was a massive structure at 434 feet long, over 200 feet wide, and 9 stories high. The first aircraft to be tested in the FST was a Navy Vought O3U-1 Corsair observation airplane. Testing in the late 1930s focused on removing as much drag from an airplane in flight as possible. NACA engineers—through an extensive program involving the Navy’s first monoplane fighter, Brewster XF2A-1 Buffalo—showed that attention to details such as air intakes, exhaust pipes, and gun ports effectively reduced drag.
In the mid- to late 1920s, the first generation of university-trained American aeronautical engineers began to enter work with industry, the Government, and academia. The philanthropic Daniel Guggenheim Fund for the Promotion of Aeronautics created aeronautical engineering schools, complete with wind tunnels, at the California Institute of Technology, Georgia Institute of Technology, Massachusetts Institute of Technology, University of Michigan, New York University, Stanford University, and University of Washington. The creation of these dedicated academic programs ensured that aeronautics would be an institutionalized profession. The university wind tunnels quickly made their mark. The prototype Douglas DC airliner, the DC-1, flew in July 1933. In every sense of the word, it was a streamline airplane because of the extensive amount of wind tunnel testing at Guggenheim Aeronautical Laboratory at the California Institute of Technology used in its design.
By the mid-1930s, it was obvious that the sophisticated wind tunnel research program undertaken by the NACA had contributed to a new level of American aeronautical capability. Each of the major American manufacturers built wind tunnels or relied upon a growing number of university facilities to keep up with the rapid pace of innovation. Despite those additions, it was clear in the minds of the editors at the influential trade journal Aviation that the NACA led the field with the grace, style, and coordinated virtuosity of a symphonic orchestra.[13]
World War II stimulated the need for sophisticated aerodynamic testing, and new wind tunnels met the need. Langley’s 20-Foot Vertical Spin Tunnel (VST) became operational in March 1941. The major difference between the VST and those that came before was its vertical closed-throat, annular return. A variable-speed three-blade, fixed-pitch fan provided vertical airflow at an approximate velocity of 85 feet per second at atmospheric conditions. Researchers threw dynamically scaled, free-flying aircraft models into the tunnel to evaluate their stability as they spun and tumbled out of control. The installation of remotely actuated control surfaces allowed the study of spin recovery characteristics. The NACA solution to spin problems for aircraft was to enlarge the vertical tail, raise the horizontal tail, and extend the length of the ventral fin.[14]
The NACA founded the Ames Aeronautical Laboratory on December 20, 1939, in anticipation of the need for expanded research and flight-test facilities for the West Coast aviation industry. The NACA leadership wanted to reach parity with European aeronautical research based on the belief that the United States would be entering World War II. The cornerstone facility at Ames was the 40 by 80 Tunnel capable of generating airflow of 265 mph for even larger full-scale aircraft when it opened in 1944. Building upon the revolutionary drag reduction studies pioneered in the FST, Ames researchers continued to modify existing aircraft with fillets and innovated dive recovery flaps to offset a new problem encountered when aircraft entered high-speed dives called compressibility.[15]
The NACA also desired a dedicated research facility that specialized in aircraft propulsion systems. Construction of the Aircraft Engine Research Laboratory (AERL) began at Cleveland, OH, in January 1941, with the facility becoming operational in May 1943.[16] The cornerstone facility was the Altitude Wind Tunnel (AWT), which became operational in 1944. The AWT was the only wind tunnel in the world capable of evaluating full-scale aircraft engines in realistic flight conditions that simulated altitudes up to 50,000 feet and speeds up to 500 mph. AERL researchers began first with large radial engines and propellers and continued with the new jet technology on through the postwar decades.[17]
The AERL soon became the center of the NACA’s work on alleviating aircraft icing. The Army Air Forces lost over 100 military transports along with their crews and cargoes over the “Hump,” or the Himalayas, as it tried to supply China by air. The problem was the buildup of ice on wings and control surfaces that degraded the aerodynamic integrity and overloaded the aircraft. The challenge was developing de-icing systems that removed or prevented the ice buildup. The Icing Research Tunnel (IRT) was the largest of its kind when it opened in 1944. It featured a 6- by 9-foot test section, a 160-horsepower electric motor capable of generating a 300 mph airstream, and a 2,100-ton refrigeration system that cooled the airflow down to -40 degrees Fahrenheit (ºF).[18] The tunnel worked well during the war and the following two decades, before NASA closed it. However, a new generation of icing problems for jet aircraft, rotary wing, and Vertical/Short Take-Off and Landing (V/STOL) aircraft resulted in the reopening of the IRT in 1978.[19]
During World War II, airplanes ventured into a new aerodynamic regime, the so-called “transonic barrier.” American propeller-driven aircraft suffered from aerodynamic problems caused by high-speed flight. Flight-testing of the P-38 Lightning revealed compressibility problems that resulted in the death of a test pilot in November 1941. As the Lightning dove from 30,000 feet, shock waves formed over the wings and hit the tail, causing violent vibration, which caused the airplane to plummet into a vertical, and unrecoverable, dive. At speeds approaching Mach 1, aircraft experienced sudden changes in stability and control, extreme buffeting, and, most importantly, a dramatic increase in drag, which created challenges for the aeronautical community involving propulsion, research facilities, and aerodynamics. Bridging the gap between subsonic and supersonic speeds was a major aerodynamic challenge.[20]
The transonic regime was unknown territory in the 1940s. Four approaches—putting full-size aircraft into terminal velocity dives, dropping models from aircraft, installing miniature wings mounted on flying aircraft, and launching models mounted on rockets—were used in lieu of an available wind tunnel in the 1940s for transonic research. Aeronautical engineers faced a daunting challenge rooted in developing tools and concepts because no known wind tunnel was able to operate and generate data at transonic speeds.
NACA Manager John Stack took the lead in American work in transonic development. As the central NACA researcher in the development of the first research airplane, the Bell X-1, he was well-qualified for high-speed research. His part in the first supersonic flight resulted in a joint award of the 1947 Collier Trophy. He ordered the conversion of the 8- and 16-Foot High-Speed Tunnels in spring 1948 to a slotted throat to enable research in the transonic regime. Slots in the tunnels’ test sections, or throats, enabled smooth operation at high subsonic speeds and low supersonic speeds. The initial conversion was not satisfactory. Physicist Ray Wright and engineers Virgil S. Ritchie and Richard T. Whitcomb hand-shaped the slots based on their visualization of smooth transonic flow. Working directly with Langley woodworkers, they designed and fabricated a channel at the downstream end of the test section that reintroduced air that traveled through the slots. Their painstaking work led to the inauguration of operations in the newly christened 8-Foot Transonic Tunnel (TT) 7 months later, on October 6, 1950.[21]
Rumors had been circulating throughout the aeronautical community about the NACA’s new transonic tunnels: the 8-Foot TT and the 16-Foot TT. The NACA wanted knowledge of their existence to remain confidential among the military and industry. Concerns over secrecy were deemed less important than the acknowledgement of the development of the slotted-throat tunnel, for which John Stack and 19 of his colleagues received a Collier Trophy in 1951. The award specifically recognized the importance of a research tool, which was a first in the 40-year history of the award. When used with already available wind tunnel components and techniques, the tunnel balance, pressure orifice, tuft surveys, and schlieren photographs, slotted-throat tunnels resulted in a new theoretical understanding of transonic drag. The NACA claimed that its slotted-throat transonic tunnels gave the United States a 2-year lead in the design of supersonic military aircraft.[22] John Stack’s leadership affected the NACA’s development of state-of-the-art wind tunnel technology. The researchers inspired by or working under him developed a generation of wind tunnels that, according to Joseph R. Chambers, became “national treasures.”[23]
The Transition to NASA
In the wake of the launch of Sputnik I in October 1957, the National Air and Space Act of 1958 combined the NACA’s research facilities at Langley, Ames, Lewis, Wallops Island, and Edwards with the Army and Navy rocket programs and the California Institute of Technology’s Jet Propulsion Laboratory to form NASA. Suddenly, the NACA’s scope of American civilian research in aeronautics expanded to include the challenges of space flight driven by the Cold War competition between the United States and the Soviet Union and the unprecedented growth of American commercial aviation on the world stage.
NASA inherited an impressive inventory of facilities from the NACA. The wind tunnels at Langley, Ames, and Lewis were the start of the art and reflected the rich four-decade legacy of the NACA and the ever- evolving need for specialized tunnels. Over the next five decades of NASA history, the work of the wind tunnels reflected equally in the first “A” and the “S” in the administration’s acronym.
The Unitary Plan Tunnels
In the aftermath of World War II and the early days of the Cold War, the Air Force, Army, Navy, and the NACA evaluated what the aeronautical industry needed to continue leadership and innovation in aircraft and missile development. Specifically, the United States needed more transonic and supersonic tunnels. The joint evaluation resulted in proposal called the Unitary Plan. President Harry S. Truman’s Air Policy Commission urged the passage of the Unitary Plan in January 1948. The draft plan, distributed to the press at the White House, proposed the installation of the 16 wind tunnels “as quickly as possible,” with the remainder to quickly follow.[24]
Congress passed the Unitary Wind Tunnel Plan Act, and President Truman signed it October 27, 1949. The act authorized the construction of a group of wind tunnels at U.S. Air Force and NACA installations for the testing of supersonic aircraft and missiles and for the high-speed and high-altitude evaluation of engines. The wind tunnel system was to benefit industry, the military, and other Government agencies.[25]
The portion of the Unitary Plan assigned to the U.S. Air Force led to the creation of the Arnold Engineering Development Center (AEDC) at Tullahoma, TN. Dedicated in June 1951, the AEDC took advantage of abundant hydroelectric power provided by the nearby Tennessee Valley Authority. The Air Force erected facilities, such as the Propulsion Wind Tunnel and two individual 16-Foot wind tunnels that covered the range of Mach 0.2 to Mach 4.75, for the evaluation of full-scale jet and rocket engines in simulated aircraft and missile applications. Starting with 2 wind tunnels and an engine test facility, the research equipment at the AEDC expanded to 58 aerodynamic and propulsion wind tunnels.[26] The Aeropropulsion Systems Test Facility, operational in 1985, was the finishing touch, which made the AEDC, in the words of one observer, “the world’s most complete aerospace ground test complex.”[27]
The sole focus of the AEDC on military aeronautics led the NACA to focus on commercial aeronautics. The Unitary Plan provided two benefits for the NACA. First, it upgraded and repowered the NACA’s existing wind tunnel facilities. Second, and more importantly, the Unitary Plan and provided for three new tunnels at each of the three NACA laboratories at the cost of $75 million. Overall, those three tunnels represented, to one observer, “a landmark in wind tunnel design by any criterion—size, cost, performance, or complexity.”[28]
The NACA provided a manual for users of the Unitary Plan Wind Tunnel system in 1956, after the facilities became operational. The document allowed aircraft manufacturers, the military, and other Government agencies to plan development testing. Two general classes of work could be conducted in the Unitary Plan wind tunnels: company or Government projects. Industrial clients were responsible for renting the facility, which amounted to between $25,000 and $35,000 per week (approximately $190,000 to $265,000 in modern currency), depending on the tunnel, the utility costs required to power the facility, and the labor, materials, and overhead related to the creation of the basic test report. The test report consisted of plotted curves, tabulated data, and a description of the methods and procedures that allowed the company to properly interpret the data. The NACA kept the original report in a secure file for 2 years to protect the interests of the company. There were no fees for work initiated by Government agencies.[29]
The Langley Unitary Plan Wind Tunnel began operations in 1955. NACA researcher Herbert Wilson led a design team that created a closed-circuit, continual flow, variable density supersonic tunnel with two test sections. The test sections, each measuring 4 by 4 feet and 7 feet long, covered the range between low Mach (1.5 to 2.9) and high Mach (2.3 to 4.6). Tests in the Langley Unitary Plan Tunnel included force and moment, surface pressure measurements and distribution, visualization of on- and off-surface airflow patterns, and heat transfer. The tunnel operated at 150 ºF, with the capability of generating 300–400 ºF in short bursts for heat transfer studies. Built at an initial cost of $15.4 million, the Langley facility was the cheapest of the three NACA Unitary Plan wind tunnels.