the first hundred
are the hardest

In 1903, the Wrights launched their flyer. A national agency took off a short time later on a mission that has yet to land.

By Ahmed K. Noor, Samuel L. Venneri, and Jeremiah F. Creedon

After World War II, interest in wind tunnels for high-speed aerodynamic research increased. But, as airflow in the wind tunnels started approaching the speed of sound, the tunnels began choking on the shock waves that formed in the compressible fluid. As a result, the accuracy of measurements made in the tunnels diminished.

It is natural for technology to proceed in stages, so that new capabilities lead to new questions, which yield to new answers. It has been the role of NASA and its predecessor, the National Advisory Committee for Aeronautics, to face many of the questions raised by flight during the past century, and to contribute innovative answers.

For instance, NACA attacked the shock wave problem by positioning a series of slots at critical locations in the throat of a wind tunnel test section. This minimized measurement errors caused by shocks and extraneous compressibility, while it enabled aerodynamic characteristics to be evaluated up to and through the speed of sound.

Wake vortex study at Wallops Island, Va.

The slotted-throat concept went to work in the Langley 8-Foot High-Speed Tunnel in 1950. Later, slots were applied to the 16-Foot High-Speed Tunnel there and to several other NACA tunnels.

The first slotted-throat tunnel aided the discovery of the area rule as well as the principles behind supercritical wings and winglets. Each of these innovations was a significant leap ahead in the science of flight.

The Wright brothers, though not the first to build a wind tunnel, used one before solving the puzzle of powered flight and giving birth, many would say, to the field of aeronautical engineering.

NACA was formed in 1915, a little more than a decade after the Wrights' first flight. Orville himself served as a charter member on the committee beginning in 1920, only three years after the Langley Aeronautical Laboratory opened in Hampton, Va.

A mere two decades would pass before three more aeronautical research centers would open: Ames at Moffett, Calif., in 1940; Aircraft Engine Laboratory in Cleveland in 1941 (later to be named Lewis, then Glenn at Lewis Field); and, the Dryden test facility at Edwards, Calif., in 1946 (originally the Muroc flight test unit). NACA became the National Aeronautics and Space Administration in 1958.

Stepping Up the Flow

Like the Wrights, NACA and NASA built many of their facilities to solve specific problems. The lunar landing facility, for instance, replicated the one-sixth gravity of the moon so the challenge of landing on its surface could be worked through. Later, NASA would go on to investigate the controlled crashing of aircraft and study inexpensive ways of landing on Mars. As with the first slotted tunnel, NACA and, later, NASA often had to leap past the technology of the day just to design their facilities.

For example, a difficulty in aerodynamic prediction methodology was accounting for the differences in atmospheric conditions between wind tunnel models and full-scale aircraft. The Reynolds number, a key parameter in achieving similarity between sizes, measures the relative effects of the momentum (inertia) and viscosity of the air flowing over an aircraft.

Conventional wind tunnels cannot simulate full-scale conditions, so models are tested at less than full-scale values of the Reynolds number.

One approach to increasing the Reynolds number obtained in wind tunnels was pressurizing the air within a test section circuit. However, as aircraft grew larger and their operational envelopes expanded, generating sufficient levels of pressurization became impractical.

In the early 1970s, NASA Langley researchers demonstrated that full-scale Reynolds numbers for certain vehicles could be achieved by greatly lowering the air temperature. This breakthrough led to the development of the 0.3 Meter Cryogenic Tunnel and, later, the National Transonic Facility, both at Langley.

The agency's first wind tunnel, a 5-foot-diameter test section, accommodated models up to 3 1/2 feet wide. The largest tunnel now is the 80x120-foot installation at NASA Ames in California. It was built by adding a leg to the 40x80-foot tunnel built in 1944 for testing full-size aircraft. Wind tunnels have made it possible to test at speeds from 0 mph to nearly Mach 25—approximately 17,500 mph.

Other NASA test facilities support research into structures, materials, flight simulation, aeroacoustics, propulsion, and avionics. Many of the test facilities are unique, including the Impact Dynamics Research Facility, the National Transonic Facility, the Transonic Dynamics Tunnel, the 8-foot High-Temperature Tunnel, the Supersonic Low-Disturbance Tunnel, the 9x15-foot Anechoic Wind Tunnel for noise testing, the Aeroacoustic Propulsion Laboratory, and the Vertical Motion Simulator. Behind each of these facilities was a need to overcome barriers to improved performance.

Faster and Smoother

Studies at these facilities of airfoils, propellers, and other features of airplanes have greatly increased the efficiency of flight.

For instance, when in the late 1920s corrugated aluminum monoplanes began replacing biplanes constructed from wood and fabric, many aircraft used radial engines whose components sat right in the airstream, producing drag and hampering speed. NACA researchers conceived engine enclosures, or cowls, and tested various designs in the propeller research tunnel as well as in flight. The cowling reduced the aerodynamic resistance while maintaining adequate cooling flow and boosted speeds by 19 mph.

In the late 1930s, NACA researchers used a full-scale tunnel to evaluate and quantify drag-producing components, resulting in critical information for design modifications to reduce drag in flight. An increase in speed of up to 50 mph was achieved. The wind-tunnel tests were followed up with flight-research tests of the actual aircraft to verify the suggested modifications.

In the early 1920s, NACA started analytical studies of airfoils that could be integrated into wings to provide low drag for cruise conditions and ensure low-speed capability for takeoff and landing.
In the late 1930s, the Low-Turbulence Pressure Tunnel came on line and was used along with other tunnels to develop advanced airfoils that were applied to several aircraft in the early 1940s.

In the mid-1950s, NACA Langley researchers developed the propulsion-induced lift concept known as an externally blown flap. The idea was one of several concepts that were capable of reducing runway length. In this concept, lift is enhanced by directing jet engine exhaust onto the trailing edge flaps of a wing. This concept was used during the 1970s in the McDonnell Douglas YC-15 test-bed aircraft as well as in the C-17 Globemaster III military transport.

A variable-sweep wing concept for high- and low-speed flight dated back to World War II Germany. But a challenge lay in finding a mechanism that would permit an aircraft to sweep back its wings for reduced drag at high speeds, and extend them again to provide the lift necessary for low-speed takeoffs and landings. In 1959, NASA Langley researchers developed the outboard wing-pivot concept, in which the pivots of the moving wings were positioned at points within wing extensions outside the fuselage. The concept was used in several U.S., European, and Russian military aircraft, including the F-11, F-14, and B-1, the Tornado fighter-bomber, and the MiG-23, MiG-27, Tu-26, and Su-27.

In 1964, Richard Whitcomb and his team at NASA Langley made extensive wind tunnel studies of a new series of airfoils for high-speed flight. They wanted to retard drag by weakening the shock waves that appear on wings at high subsonic cruise conditions. They tested a slotted airfoil with a flattened forward camber, a blunt nose, and a slim cambered section behind the slot.

Because of concerns over manufacturing, the team decided to do away with the slot. This increased drag slightly. The joint Langley-Dryden program tested the design in flight during the early 1970s, producing a thickened airfoil by 1974.

This supercritical wing held promise for enhanced flight efficiency in subsonic jet airliner designs. It reduced drag and improved cruise efficiency and lift compared with conventional wings. Both the Boeing 777 long-range subsonic jet airliner and the C-17 Globemaster III military transport put the concept into service.

Slotted throat transonic wind tunnel (above) solved shock wave problems. NACA's first tunnel (middle) held models up to 3.5 feet wide. Wind tunnel results led to inflight tests, such as that on the X-29 research aircraft (top).

Once airplanes exceeded the speed of sound, a new set of conditions arose. As an aircraft passes through the speed of sound at transonic speeds, shock waves form and drag increases. In 1953, Whitcomb of NACA conceived a radical solution for decreasing drag by reducing a portion of the area where the wing met the fuselage, producing a fuselage that had a distinct wasp-waist, or Coke-bottle, shape. The "area rule"—as this solution became known—revolutionized conventional aircraft design and was later applied to virtually all high-speed fighters and bombers. It was applied, as well, to civil transport aircraft in situations involving mitigation of strong shock waves emanating from components such as engine nacelles, wings, and pylons.

In 1956, the Royal Aircraft Establishment in the United Kingdom discovered vortex lift. Four years later, the phenomenon enhanced the low-speed lift capability of the Concorde SST. NASA researchers found that canards placed upstream of the main wing improved lift on fighter aircraft at subsonic speeds because the vortex they generated interacted favorably with the wings.

High-swept, wing-root extensions that were integrated into the configurations yielded similar benefits. Wing-root extensions also radically enhanced the maneuverability of fighters.

By sharpening the leading edges of the wing-root extensions, NASA engineers intensified the vortices that were generated, increasing lift. Sharpened leading edges, called wing-body strakes, also alleviated buffeting on aircraft that were maneuvering at transonic speeds.

NASA researchers also have added to the understanding of the wake vortex, which affects air traffic.

A flying aircraft generates a horizontal swirling vortex behind it. The strength and duration of this wake vortex are directly dependent on the size of the aircraft. They can last for several minutes and stretch for many kilometers behind an aircraft. When a following aircraft encounters such disturbed airflow, the plane can accelerate uncontrollably. Because of the severity of these wake disturbances, the FAA restricts the distance between aircraft during takeoffs and landings, which can congest runways and the skies overhead.

NASA has suggested changes in airplane configurations that minimize the dangers of trailing vortices. Recently, it integrated a wake vortex monitoring and advisory system, combining atmospheric properties, aircraft properties, and vortex avoidance strategies, into a program called the Aircraft Vortex Spacing System.

Living with the Wind

Aircraft landing through wind shear or "down bursts" were a leading cause of fatalities in air travel from the mid-1960s to the mid-1980s. NASA reduced this threat by learning about the dynamics of wind shear, then developing an "F-factor," a metric that characterizes the severity of the threat posed by a wind shear.

NASA also developed a sensor that detects severe microbursts with enough time for a pilot to evade them. The wind shear detection and avoidance capabilities are required on all large commercial transports.

Research on composite materials intensified in the 1970s as engineers strove for lighter, better-performing designs. NASA published design guides and developed new materials. It also introduced modeling and simulation capabilities for predicting the response, failure, and performance of composite materials and structures.

In 1972, NASA initiated the Composites Flight Service program, which led to the equipping of airliners and helicopters with more than 300 new composite parts. Several other composite programs have since been initiated or managed by NASA, among them the Aircraft Energy Efficiency program in 1976 and the Advanced Composites Technology program in 1988.

Turbofan is tested at the 9x15 low-speed Anechoic Wind Tunnel at NASA Glenn Research Center (above). The spin parachute and truss assembly on the X-29 undergoes high-angle-of-attack tests at Dryden (top).

Noise reduction has been a concern for the agency for more than 50 years. NACA began investigating the sound emissions of propellers with studies that included the physics of noise generation and transmission. NACA and NASA, with the FAA and industry, have since conducted and sponsored extensive studies on jet noise reduction and the impact of noise on the community.

The research activities helped lead to an overall 75 percent reduction of noise in today's jets compared with those of the 1950s.

By the 1960s, new low-bypass-ratio turbofan engines used relatively low engine core exhaust flows mixed with bypass flows, which lowered exhaust velocities and quieted jet exhaust. NASA researchers added acoustic liners to engine nacelle inlets to absorb noise from the combustor and turbine engine stages. They designed exhaust nozzle mixers as well, and evaluated steeper approach patterns to minimize noise on the ground.

In recent years, NASA researchers used advanced CFD codes and sophisticated instrumentation to develop fundamental understanding of noise generation from propulsion systems as well as other aircraft components.

At high angles of attack, aircraft can lose control and aerodynamic stability, and enter spins from which they cannot recover. Highly maneuverable military aircraft, such as the F-15, F-16, and F-18, call for designers to ensure that the planes can operate at high angles of attack.

In the late 1980s, NASA researchers made ground-based and piloted simulator studies that focused on critical technologies for high-angle-of-attack operations, including aerodynamics, flight controls, and test techniques. These studies contributed to the F-22 and the Joint Strike Fighter programs, and provided invaluable tools and design guidelines for next-generation, high-performance aircraft.

In 1993, as part of the High-Speed Research program, NASA and industry investigated artificial vision for supersonic transport pilots to replace the Concorde's drooped nose as a way to see during takeoff and landing. A goal was significant weight savings. Researchers conducted extensive simulator and flight tests to define the advanced displays that were to be used in the system.

In the early 1980s, Heinz Erzberger of NASA Ames began developing guidance algorithms for aircraft. He adapted these algorithms to the operation of aircraft in the national airspace system. His work promised improved throughput and reduced delays at major airports.

Working with the FAA, Erzberger developed decision support tools for air traffic controllers, evaluating the tools under actual air traffic conditions. Today, decision support tools populate control towers at many U.S. airports and improve the operation of the complex national airspace.

In 2002, NASA published an Aeronautics Blueprint, laying out its vision for the future of air and space flight. It will include investigations into advanced propulsion technologies, lightweight, high-strength adaptable structures, adaptive controls, and advanced vehicle designs—even collaborative design tools that may be used to develop a Mars airplane.

The vision also outlines a new model for aviation safety and security in which humans and automation will form a synergistic partnership.

The vision describes revolutionary aircraft of practically every kind—subsonic, supersonic, general aviation, extremely short takeoff and landing, and unmanned. The blueprint calls for these vehicles to fly in a complex airspace, a highly integrated system of systems.

If the next 100 years are anything like the last, it should be quite a flight.

Ahmed K. Noor is Eminent Scholar and William E. Lobeck Professor of Aerospace Engineering and the director of the Center for Advanced Engineering Environments at Old Dominion University in Norfolk, Va. He is also adjunct professor of mechanical and aerospace engineering at the University of Florida in Gainesville. Samuel L. Venneri is the former chief technologist of NASA. Jeremiah F. Creedon is director of transportation research at Old Dominion University and a former associate administrator of aerospace technology of NASA.

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