Category NASA’S CONTRIBUTIONS TO AERONAUTICS

The last of the CTAS tools, which can work independently but is more efficient when integrated into the full CTAS suite, is the Final Approach Spacing Tool. It assists the TRACON controllers to determine the most efficient sequence, schedule, and runway assignments for aircraft intend­ing to land. FAST takes advantage of information provided by the TMA and EDA tools in making its assessments and displaying advisories to

the controller, who then directs the aircraft as usual by radio communi­cation. FAST also makes its determinations by using live radar, weather and wind data, and a series of other static databases, such as aircraft performance models, each airline’s preferred operational procedures, and standard air traffic rules.[271]

Early tests of a prototype FAST system during the mid-1990s at the Dallas/Fort Worth International Airport TRACON showed imme­diate benefits of the technology. Using FAST’s runway assignment and sequence advisories during more than 25 peak traffic periods, control­lers measured a 10- to 20-percent increase in airport capacity, depend­ing on weather and airport conditions.[272]

This dramatic and elaborate crash test program of the early 1980s was one of the most ambitious and well-publicized experiments that NASA has conducted in its decades-long quest for increased aviation safety. In this 1980-1984 study, the NASA Dryden and Langley Research Centers joined with the FAA to quantitatively assess airline crashes. To do this, they set out to intentionally crash a remotely controlled Boeing 720 air­liner into the ground. The objective was not simply to crash the airliner, but rather to achieve an "impact-survivable” crash, in which many pas­sengers might be expected to survive.[369] This type of crash would allow a more meaningful evaluation of both the existing and experimental cabin safety features that were being observed. Much of the informa­tion used to determine just what was "impact-survivable” came from Boeing 707 fuselage drop tests conducted previously at Dryden’s Impact Dynamics Research Facility and a similar but complete aircraft drop conducted by the FAA.[370]

The FAA’s primary interest in the Controlled Impact Demonstration (CID, also sometimes jokingly referred to as "Crash in the Desert”) was to test an anti-misting kerosene (AMK) fuel additive called FM-9. This high – molecular-weight polymer, when combined with Jet-A fuel, had shown promise during simulated impact tests in inhibiting the spontaneous com­bustion of fuel spilling from ruptured fuel tanks. The possible benefits of this test were highly significant: if the fireball that usually follows an aircraft crash could be eliminated or diminished, countless lives might be saved. The FAA was also interested, secondarily, in testing new safety-related design features. NASA’s main interest in this study, on the other hand, was to measure airframe structural loads and collect crash dynamics data.[371]

With these objectives in mind, researchers from the two agencies filled the seats of the "doomed” passenger jet with anthropomorphic dummies instrumented to measure the transmission of impact loads. They also fit­ted the airliner with additional crash-survivability testing equipment, such as burn-resistant windows, fireproof cabin materials, experimental seat designs, flight data recorders, and galley and stowage-bin attachments.[372]

The series of tests included 15 remote-controlled flights, the first 14 of which included safety pilots onboard. The final flight took place on the morning of December 1, 1984. It started at Edwards AFB, NV, and ended with the intentional crash of the four-engine jet airliner onto the bed of Rogers Dry Lake. The designated target was a set of eight steel posts, or cut­ters, cemented into the lakebed to ensure that the jet’s fuel tanks ruptured. During this flight, NASA Dryden’s Remotely Controlled Vehicle Facility research pilot, Fitzhugh Fulton, controlled the aircraft from the ground.[373]

The crash was accomplished more or less as planned. As expected, the fuel tanks, containing 76,000 pounds of the anti-misting kerosene jet fuel, were successfully ruptured; unfortunately, the unexpectedly

spectacular fireball that ensued—and that took an hour to extinguish— was a major disappointment to the FAA. Because of the dramatic fail­ure of the anti-misting fuel, the FAA was forced to curtail its plan to require the use of this additive in airliners.[374]

In most other ways, however, the CID was a success. Of utmost importance were the lessons learned about crash survivability. New safety initiatives had been tested under realistic conditions, and the effects of a catastrophic crash on simulated humans were filmed inside the aircraft by multiple cameras and later visualized at the crash site. Analysis of these data showed, among many other things, that in a burn­ing airliner, seat cushions with fire-blocking layers were indeed supe­rior to conventional cushions. This finding resulted in FAA-mandated flammability standards requiring these safer seat cushions.[375] Another important safety finding that the crash-test data revealed was that the airliner’s adhesive-fastened tritium aisle lights, which would be of utmost importance during postcrash emergency egress, became dislodged and

nonfunctional during the crash. As a result, the FAA mandated that these lights be mechanically fastened, to maximize their time of usefulness after a crash.[376] These and other lessons from this unique research proj­ect have made commercial travel safer.

Charles Zimmerman energetically continued his interest in free-flight mod­els after the successful introduction of his 15-foot free-spinning tunnel. His next ambition was to provide a capability of investigating the dynamic stability and control of aircraft in conventional flight. His approach to this goal was to simulate the unpowered gliding flight of a model air­plane in still air but to accomplish this goal in a wind tunnel with the model within view of the tunnel operators. Without power, the model would be in equilibrium in descending flight, so the tunnel airstream had to be at an inclined angle relative to the horizon. Zimmerman designed a 5-foot-diameter wind tunnel that was mounted in a yoke-like support structure such that the tunnel could be pivoted and its airstream could

simulate various descent angles. Known as the Langley 5-Foot Free-Flight Tunnel, this exploratory apparatus was operated by two researchers—a tunnel operator, who controlled the airspeed and tilt angle of the tunnel, and a pilot, who controlled the model and assessed its behavior via a con­trol box with a fine wire connection to the model’s control actuators.[449]

Very positive results obtained in this proof-of-concept apparatus led to the design and construction of a larger 12-Foot Free-Flight Tunnel in 1939. Housed in a 60-foot-diameter sphere that permitted the tunnel to tilt upward and downward, the Langley 12-Foot Free-Flight Tunnel was designed for free-flight testing of powered as well as unpowered mod­els. A three-person crew was used in the testing, including a tunnel air­speed controller, a tunnel tilt-angle operator, and an evaluation pilot.

The tunnel operated as the premier NACA low-speed free-flight facil­ity for over 20 years, supporting advances in fundamental dynamic

Thrust Pilot

Safety Cable
Operator

» Pitch I Pilot ^ Test Conductor

Test setup for free-flight studies at Langley. The pitch pilot is in a balcony at the side of the test section. The pilot who controls the rolling and yawing motions is at the rear of the tunnel. NASA.

stability and control theory as well as specific airplane development programs. After the 1959 decision to transfer the free-flight activities to the Full-Scale Tunnel, the tunnel pivot was fixed in a horizontal position, and the facility has continued to operate as a NASA low-cost laboratory-type tunnel for exploratory testing of advanced concepts.

Relocation of the free-flight testing to the Full-Scale Tunnel made that tunnel the focal point of free-flight applications at Langley for the next 50 years.[450] The move required updates to the test technique and the free-flight models. The test crew increased to four or more individ­uals responsible for piloting duties, thrust control, tunnel operations, and model retrieval and was located at two sites within the wind tun­nel building. One group of researchers was in a balcony at one side of the open-throat test section, while a pilot who controlled the rolling and yawing motions of the model was in an enclosure at the rear of the test section within the structure of the tunnel exit-flow collector. Models of jet aircraft were typically powered by compressed air, and the level of

thrust was controlled by a thrust pilot in the balcony. Next to the thrust pilot was a pitch pilot who controlled the longitudinal motions of the model and conducted assessments of dynamic longitudinal stability and control during flight tests. Other key members of the test crew in the balcony included the test conductor and the tunnel airspeed operator.

A light, flexible cable attached to the model supplied the model with the compressed air, electric power for control actuators, and transmis­sion of signals for the controls and sensors carried within the model. A portion of the cable was made up of steel cable that passed through a pulley above the test section and was used to retrieve the model when the test was terminated or when an uncontrollable motion occurred. The flight cable was kept slack during the flight tests by a safety-cable opera­tor in the balcony who accomplished the job with a high-speed winch.[451]

Free-flight models in the Full-Scale Tunnel typically had model wing­spans of about 6 feet and weighed about 100 pounds. Propulsion was pro­vided by compressed air ejectors, miniature turbofans, and high thrust/ weight propeller motors. The materials used to fabricate models changed from the simple balsa free-flight construction used in the 12-Foot Free – Flight Tunnel to high-strength, lightweight composite materials. The control systems used by the free-flight models simulated the complex feedback and stabilization logic used in flight control systems for contem­porary aircraft. The control signals from the pilot stations were transmit­ted to a digital computer in the balcony, and a special software program computed the control surface deflections required in response to pilot inputs, sensor feedbacks, and other control system inputs. Typical sen­sor packages included control-position indicators, linear accelerometers, and angular-rate gyros. Many models used nose-boom-mounted vanes for feedback of angle of attack and angle of sideslip, similar to systems used on full-scale aircraft. Data obtained from the flights included opti­cal and digital recordings of model motions and pilot comments as well as analysis of the model’s response characteristics.

The NACA and NASA also developed wind tunnel free-flight testing techniques to determine high-speed aerodynamic characteristics, dynamic stability of aircraft, Earth atmosphere entry configurations, planetary probes, and aerobraking concepts. The NASA Ames Research Center led the development of such facilities starting in the 1940s with the Ames

Supersonic Free-Flight Tunnel (SFFT).[452] The SFFT, which was simi­lar in many respects to ballistic range facilities used for testing muni­tions, was designed for aerodynamic and dynamic stability research at high supersonic Mach numbers (Mach numbers in excess of 10). In the SFFT, the model was fired at high speeds upstream into a supersonic airstream (typically Mach 2.0). Windows for shadowgraph photography were along the top and sides of the test section.

Data obtained from motion time histories and measurements of the model’s attitudes during the brief flights were used to obtain aero­dynamic and dynamic stability characteristics. The small research mod­els had to be extremely strong to withstand high accelerations during the launch (up to 100,000 g’s), yet light enough to meet requirements for dynamic mass scaling (moments of inertia). Launching the models without angular disturbances or damage was challenging and required extensive development and experience. The SFFT was completed in late 1949 and became operational in the early 1950s.

Ames later brought online its most advanced aeroballistic testing capability, the Ames Hypervelocity Free-Flight Aerodynamic Facility (HFFAF), in 1964. This facility was initially developed in support of the Apollo program and utilized both light-gas gun and shock tube technol­ogy to produce lunar return and atmospheric entry. At one end of the test section, a family of light-gas gun was used to launch specimens into the test section, while at the opposite end, a large shock tube could be simultaneously used to produce a counterflowing airstream (the result being Mach numbers of about 30). This counterflow mode of operation proved to be very challenging and was used for only a brief time from 1968 to 1971. Throughout much of the 1970s and 1980s, this versatile facility was operated as a traditional aeroballistic range, using the guns to launch models into quiescent air (or some other test gas), or as a hypervelocity impact test facility. From 1989 through 1995, the facility was operated as a shock tube-driven wind tunnel for scramjet propul­sion testing. In 1997, the HFFAF underwent a major refurbishment and was returned to an aeroballistic mode of operation. It continues to oper­ate in this mode and is NASA’s only remaining aeroballistic test facility.[453]

In the mid-1950s, the NACA encountered an unexpected aerodynamic scale effect related to the long fuselage forebodies being introduced at the time. This experience led to one of the more important and last­ing lessons learned in the use of free-spinning models for spin predic­tions. One particular project stands out as a key experience regarding this topic. As part of the ongoing military requests for NACA support of new aircraft development programs, the Navy requested Langley to conduct spin tunnel tests of a model of its new Chance Vought XF8U-1 Crusader fighter in 1955. The results of spin tunnel tests of a 1/25-scale model indicated that the airplane would exhibit two spin modes.[509] The first mode would be a potentially dangerous fast, flat spin at an angle of attack of approximately 87 degrees, from which recoveries were unsat­isfactory or unobtainable. The second spin was much steeper, with a lower rate of rotation, and recoveries would probably be satisfactory.

As the spin tunnel results were analyzed, Chance Vought engineers directed their focus to identifying factors that were responsible for the flat spin exhibited by the model. The scope of activities stimulated by the XF8U-1 spin tunnel results included, in addition to extended spin tunnel tests, one-degree-of-freedom autorotation tests of a model of the

XF8U-1 configuration in the Chance Vought Low Speed Tunnel and a NACA wind tunnel research program that measured the aerodynamic sensitivity of a wide range of two-dimensional, noncircular cylinders to Reynolds number.[510] The wind tunnel tests were designed and con­ducted to include variations in Reynolds number from the low values associated with spin tunnel testing to much higher values more repre­sentative of flight.

With results from the static and autorotation wind tunnel studies in hand, researchers were able to identify an adverse effect of Reynolds number on the forward fuselage shape of the XF8U-1 such that, at the relatively low values of Reynolds number of the spin tunnel tests (about

90,0 based on fuselage-forebody depth), the spin model exhibited a powerful pro-spin aerodynamic yawing moment dominated by forces produced on the forebody. The pro-spin moment caused an autorota­tive spinning tendency, resulting in the fast flat spin observed in the spin tunnel tests. As the Reynolds number in the tunnel tests was increased to values approaching 300,000, however, the moments produced by the forward fuselage reversed direction and became antispin, remaining so for higher values of Reynolds number. Fundamentally, the researchers had clearly identified the importance of cross-sectional shapes of mod­ern aircraft—particularly those with long forebodies—on spin charac­teristics and the possibility of erroneous spin tunnel predictions because of the low test Reynolds number. When the full-scale spin tests were conducted, the XF8U-1 airplane exhibited only the steeper spin mode and the fast, flat spin predicted by the spin model that had caused such concern was never encountered.

During and after the XF8U-1 project, Langley’s spin tunnel per­sonnel developed expertise in the anticipation of potential Reynolds number effects on the forebody, and in the art of developing methods to geometrically modify models to minimize unrealistic spin predic­tions, caused by the phenomenon. In this approach, cross-sectional shapes of aircraft are examined before models are constructed, and if the forebody cross section is similar to those known to exhibit scale effects at low Reynolds number, static tests at other wind tunnels are

conducted for a range of Reynolds number to determine if artificial devices, such as nose-mounted strakes at specific locations, can be used to artificially alter the flow separation on the nose at low Reynolds number and cause it to more accurately simulate full-scale conditions.[511]

In addition to the XF8U-1, it was necessary to apply scale-correction fuselage strakes to the spin tunnel models of the Northrop F-5A and F-5E fighters, the Northrop YF-17 lightweight fighter prototype, and the Fairchild A-10 attack aircraft to avoid erroneous predictions because of fuselage forebody effects. In the case of the X-29, a specific study of the effects of forebody devices for correcting low Reynolds number effects was conducted in detail.[512]

The slotted-throat transonic tunnels pioneered by John Stack and his associates at Langley proved valuable, especially in the hands of one of the Center’s more creative minds, Richard. T. Whitcomb. In the 8-Foot TT, he investigated the transonic regime. Gaining a better understanding of aircraft speeds between Mach 0.75 and 1.25 was one of the major aero­dynamic challenges of the 1950s and a matter of national security during the Cold War. The Air Force’s Convair YF-102 Delta Dagger interceptor was unable to reach supersonic speeds during its first flights in 1953. Tests in the 8-Foot TT revealed that the increase in drag as an airplane approached supersonic speeds was not the result of shock waves form­ing at the nose but of those forming just behind the wings. Whitcomb created a rule of thumb that decreased transonic drag by narrowing, or pinching, the fuselage where it met the wings.[565] The improved YF-102A, with its new "area rule” fuselage, achieved supersonic flight in December 1954. The area rule fuselage increased the YF-102A’s top speed by 25 per­cent. Embraced by the aviation industry, Whitcomb’s revolutionary idea enabled a generation of military aircraft to achieve supersonic speeds.[566]

As he worked to validate the area rule concept, Whitcomb moved next door to the 8-Foot Transonic Pressure Tunnel (TPT) after it opened in 1953. His colleagues John Stack, Eugene C. Draley, Ray H. Wright, and Axel T. Mattson designed the facility from the outset as a slotted – wall transonic tunnel with a maximum speed of Mach 1.2.[567] In what quickly became known as "Dick Whitcomb’s tunnel,” he validated and made two additional aerodynamic contributions in the decades that followed—the supercritical wing and winglets.

Beginning in 1964, Whitcomb wanted to develop an airfoil for com­mercial aircraft that delayed the onset of high transonic drag near Mach 1 by reducing air friction and turbulence across an aircraft’s major aero­dynamic surface, the wing. Whitcomb went intuitively against conven­tional airfoil design by envisioning a smoother flow of air by turning a conventional airfoil upside down. Whitcomb’s airfoil was flat on top with a downward curved rear section. The blunt leading edge facilitated better takeoff, landing, and maneuvering performance as the airfoil slowed airflow, which lessened drag and buffeting and improved stabil­ity. Spending days at a time in the 8-Foot TPT, he validated his concept with a model he made with his own hands. He called his innovation a "supercritical wing,” combining "super” (meaning "beyond”) with "crit­ical” Mach number, which is the speed supersonic flow revealed itself above the wing.[568] After a successful flight program was conducted at NASA Dryden from 1971 to 1973, the aviation industry incorporated the supercritical wing into a new generation of aircraft, including sub­sonic transports, business jets, Short Take-Off and Landing (STOL) air­craft, and unmanned aerial vehicles (UAVs).[569]

Whitcomb’s continual quest to improve subsonic aircraft led him to investigate the wingtip vortex, the turbulent air found at the end of an airplane wing that created induced drag, as part of the Aircraft Energy Efficiency (ACEE) program. His solution was the winglet, a vertical wing­like surface that extended above and sometimes below the tip of each

wing. Whitcomb and his research team in the 8-Foot TPT investigated the drag-reducing properties of winglets for a first-generation, narrow – body subsonic jet transport from 1974 to 1976.[570] Whitcomb found that winglets reduced drag by approximately 20 percent and doubled the improvement in the lift-to-drag (L/D) ratio, to 9 percent, which boosted performance by enabling higher cruise speeds. The first jet-powered airplane to enter production with winglets was the Learjet Model 28 in 1977. The first large U. S. commercial transport to incorporate winglets, the Boeing 747-400, followed in 1985.[571]

By Weneth D. Painter

General Aviation has always been an essential element of American aeronautics. The NACA and NASA have contributed greatly to its efficiency, safety, and reliability via research across many technical disciplines. The mutually beneficial bonds linking research in civil and military aeronautics have resulted in such developments as the super­critical wing, electronic flight controls, turbofan propulsion, compos­ite structures, and advanced displays and instrumentation systems.

HOUGH COMMONLY ASSOCIATED IN THE PUBLIC MIND with small private aircraft seen buzzing around local airports and air parks, the term "General Aviation” (hereafter GA) is primarily a definition of aircraft utilization rather than a classification per se of aircraft phys­ical characteristics or performance. GA encompasses flying machines ranging from light personal aircraft to Mach 0.9+ business jets, com­prising those elements of U. S. civil aviation which are neither certified nor supplemental air carriers: kit planes and other home-built aircraft, personal pleasure aircraft, commuter airlines, corporate air transports, aircraft manufacturers, unscheduled air taxi operations, and fixed-base operators and operations.

Overall, NACA-NASA’s research has profoundly influenced all of this, contributing notably to the safety and efficiency of GA worldwide. Since the creation of the NACA in 1915, and continuing after establishment of NASA in 1958, Agency engineers have extensively investigated design concepts for GA, GA aircraft themselves, and the operating environment and related areas of inquiry affecting the GA community. In particu­lar, they have made great contributions by documenting the results of various wind tunnel and flight tests of GA aircraft. These results have strengthened both industrial practice within the GA industry itself and the educational training of America’s science, technology, engineering, and mathematics workforce, helping buttress and advance America’s stature as an aerospace nation. This study discusses the advancements
in GA through a review of selected applications of flight disciplines and aerospace technology.

NASA’s work with the RPV concept came of age when the agency applied RPV technology to support the Research, Development, Test, and Evaluation (RDT&E) of a new Air Force fighter, the McDonnell – Douglas (subsequently Boeing) F-15 Eagle. In 1969, the Air Force selected McDonnell-Douglas Aircraft Corporation to build the F-15, a Mach-2- capable air superiority fighter airplane designed using lessons learned during aerial combat over Vietnam. The prototype first flew in July 1972.

In the months leading up to that event, Maj. Gen. Benjamin Bellis, chief of the F-15 System Program Office at Wright-Patterson Air Force Base,

OH, requested NASA assistance in testing a three-eights-scale model F-15 RPRV to explore aerodynamic and control system characteristics of the F-15 configuration in spins and high-angle-of-attack flight. Such maneu­vers can be extremely hazardous. Rather than risk harm to a valuable test pilot and prototype, a ground pilot would develop stall/spin recov­ery techniques with the RPRV and pass lessons learned to test pilots fly­ing the actual airplanes.

In April 1972, NASA awarded McDonnell-Douglas a $762,000 con­tract to build three F-15 RPRV models. Other contractors provided electronic components and parachute-recovery equipment. NASA technicians installed avionics, hydraulics, and other subsystems. The F-15 RPRV was 23.5 feet long, was made primarily of fiberglass and wood, and weighed 2,500 pounds. It had no propulsion system and was designed for midair recovery using a helicopter. Each model cost a little over $250,000, compared with $6.8 million for a full-scale F-15 aircraft.[907] Every effort was made to use off-the-shelf components and equipment readily available at the Flight Research Center, including

hydraulic components, gyros, and telemetry systems from the lifting body research programs. A proportional uplink, then being used for instru­ment-landing system experiments, was acquired for the RPRV Ground Control Station (GCS). The ground cockpit itself was fashioned from a general-purpose simulator that had been used for stability-and – control studies. Data-processing computers were adapted for use in a programmable ground-based control system. A television cam­era provided forward visibility. The midair recovery system (MARS) parachute mechanism was taken from a Firebee drone.[908] The first F-15 RPRV arrived at the Flight Research Center in December 1972 but wasn’t flown until October 12, 1973. The model was carried to an altitude of about 45,000 feet beneath the wing of a modified B-52 Stratofortress known as the NB-52B. Following release from the launch pylon at a speed of 175 knots, ground pilot Einar Enevoldson guided the craft through a flawless 9-minute flight, during which he explored the vehi­cle’s basic handling qualities. At 15,000 feet altitude, a 12-foot spin – recovery parachute deployed to stabilize the descent. An 18-foot engage­ment chute and a 79-foot-diameter main chute then deployed so that the RPRV could be snagged in flight by a hook and cable beneath a helicopter, and set down gently on an inflated bag.[909] Enevoldson found the task of flying the RPRV very challenging, both physically and psychologically. The lack of physical cues left him feeling remote from the essential reassuring sensations of flight that provide a pilot with situational feed­back. Lacking sensory input, he found that his workload increased and that subjective time seemed to speed up. Afterward, he reenacted the mission in a simulator at 1.5 times actual time and found that the pace seemed the same as it had during the flight.

Researchers had monitored his heart rate during the flight to see if it would register the 70 to 80 beats per minute typical for a piloted test flight. They were surprised to see the readings indicate 130 to 140 beats per minute as the pilot’s stress level increased. Enevoldson found flying the F-15 RPRV less pleasant or satisfying than he normally did a difficult or demanding test mission.[910] "The results were gratifying,” he wrote in his postflight report, "and some satisfaction is gained from the

success of the technical and organizational achievement—but it wasn’t fun.”[911] In subsequent tests, Enevoldson and other research pilots explored the vehicle’s stability and control characteristics. Spin testing confirmed the RPRV’s capabilities for returning useful data, encouraging officials at the F-15 Joint Test Force to proceed with piloted spin trials in the preproduction prototypes at Edwards.[912] William H. "Bill” Dana piloted the fourth F-15 RPRV flight, on December 21, 1973. He collected about 100 seconds of data at angles of attack exceeding 30 degrees and 90 sec­onds of control-response data. Dana had a little more difficulty control­ling the RPRV in flight than he had in the simulator but otherwise felt everything went well. At Enevoldson’s suggestion, the simulator flights had been sped up to 1.4 times actual speed, and Dana later acknowl­edged that this had provided a more realistic experience.

During a postflight debriefing, Dana was asked how he liked flying the RPRV. He responded that it was quite different from sitting in the cockpit of an actual research vehicle, where he generally worried and
fretted until just before launch. Then he could settle down and just fly the airplane. With the RPRV, he said, he was calm and cool until launch and then felt keyed up through the recovery.[913] The first of several inci­dents involving the MARS parachute gear occurred during the ninth flight. The recovery helicopter failed to engage the chute, and the RPRV descended to the ground, where it was dragged upside down for about a quarter mile. Fortunately, damage was limited to the vertical tails, can­opy bulge, and nose boom. The RPRV was severely damaged at the end of the 14th flight, when the main parachute did not deploy because of failure of the MARS disconnect fitting.

Rather than repair the vehicle, it was replaced with the second F-15 RPRV. During the craft’s second flight, on January 16, 1975, research pilot Thomas C. McMurtry successfully completed a series of planned maneuvers and then deployed the recovery parachute. During MARS retrieval, with the RPRV about 3,000 feet above the ground, the towline separated. McMurtry quickly assumed control and executed an emer­gency landing on the Edwards Precision Impact Range Area (PIRA). As a result of this success and previous parachute-recovery difficulties, further use of MARS was discontinued. The RPRV was modified with landing skids, and all flights thereafter ended with horizontal touch­downs on the lakebed.[914] The F-15 RPRV project came to a halt December 17, 1975, following the 26th flight, but this did not spell the end of the vehicle’s career. In November 1977, flights resumed under the Spin Research Vehicle (SRV) project. Researchers were interested in evalu­ating the effect of nose shape on the spin susceptibility of modern high – performance fighters. Flight-testing with the F-15 model would augment previous wind tunnel experiments and analytical studies. Baseline work with the SRV consisted of an evaluation of the basic nose shape with and without two vortex strips installed. In November 1978, following nine baseline-data flights, the SRV was placed in inactive status pending the start of testing with various nose configurations for spin-mode determi­nation, forebody pressure-distribution studies, and nose-mounted spin – recovery parachute evaluation. Flights resumed in February 1981.[915]

When the SRV program ended in July 1981, the F-15 models had been carried aloft 72 times: 41 times for the RPRV flights and 31 times for the SRV. A total of 52 research missions were flown with the two aircraft: 26 free flights with each one. There had been only 2 ground aborts, 1 aborted planned-captive flight, and 15 air aborts prior to launch. Of 16 MARS recoveries, 13 were successful. Five landings occurred on the PIRA and 34 on the lakebed.[916] Flight data were correlated with wind tunnel and mathematical modeling results and presented in vari­ous technical papers. Tests of the subscale F-15 models clearly demon­strated the value of the RPRV concept for making bold, rapid advances in free-flight testing of experimental aircraft with minimal risk and max­imum return on investment. R. Dale Reed wrote that, "If information obtained from this program avoids the loss of just one full-scale F-15, then the program will have been a tremendous bargain.”[917]

Indeed it was: spin test results of the F-15 model identified a poten­tially dangerous "yaw-trip” problem with the full-scale F-15 if it had an offset airspeed boom. Such a configuration, the F-15 RPRV showed, might exhibit abrupt departure characteristics in turning flight as angle of attack increased. Subsequently, during early testing of F-15C aircraft equipped with fuselage-hugging conformal fuel tanks (like those subse­quently employed on the F-15E Strike Eagle) and an offset nose boom, Air Force test pilot John Hoffman experienced just such a departure. Review of the F-15 RPRV research results swiftly pinpointed the prob­lem and alleviated fears that the F-15 suffered from some inherent and major flaw that would force a costly and extensive redesign. This one "save” likely more than paid for the entire NASA F-15 RPRV effort.[918]

The NACA’s applied research was initially restricted to wind tunnel work. The wind tunnels had their own problems with supersonic flow, as shock waves formed and disturbed the flow, thus casting doubt on the model test results. This was especially true in the transonic regime, from Mach

0. 8 to 1.2, at which the shock waves were the strongest as the super­sonic flow slowed to subsonic in one single step; this was called a "nor­mal” shock, referring to the 90-degree angle of the shock wave to the vehicle motion. Free air experiments were necessary to validate and improve wind tunnel results. John Stack at NACA Langley developed a slotted wind tunnel that promised to reduce some of the flow irregu­larities. The Collier Trophy was awarded for this accomplishment, but validation of the supersonic tunnel results was still lacking. Pending the development of higher-powered engines for full-scale in-flight experi­ments, initial experimentation included attaching small wing shapes to NACA P-51 Mustangs, which then performed high-speed dives to and beyond their critical Mach numbers, allowing seconds of transonic
data collection. Heavy streamlined bomb shapes were released from NACA B-29s, the shapes going supersonic during their 30-45-second trajectories, sending pressure data to the ground via telemetry before impact.[1055] Supersonic rocket boosters were fired from the NACA facil­ity at Wallops Island, VA, carrying wind tunnel-sized models of wings and proposed aircraft configurations in order to gain research data, a test method that remained fruitful well into the 1960s. The NACA and the United States Air Force (USAF) formed a joint full-scale flight-test program of a supersonic rocket-powered airplane, the Bell XS-1 (subse­quently redesignated the X-1), which was patterned after a supersonic

0. 50-caliber machine gun projectile with thin wings and tail surfaces. The program culminated October 14, 1947, with the demonstration of a controllable aircraft that exceeded the speed of sound in level flight. The news media of the day hailed the breaking of the "sound barrier,” which would lead to ever-faster airplanes in the future. Speed records popularized in the press since the birth of aviation were "made to be broken”; now, the speed of sound was no longer the limit.

But the XS-1 flight in October was no more a practical solution to supersonic flight than the Wright brothers’ flights at Kitty Hawk in December 1903 were a director predecessor to transcontinental passen­ger flights. Rockets could produce the thrust necessary to overcome the drag of supersonic shock waves, but the thrust was of limited duration. Rocket motors of the era produced the greatest thrust per pound of engine, but they were dangerous and expensive, could not be throttled directly, and consumed a lot of fuel in a short time. Sustained supersonic flight would require a more fuel-efficient motor. The turbojet was an obvious choice, but in 1947, it was in its infancy and was relatively ineffi­cient, being heavy and producing only (at most) several thousand pounds of static thrust. Military-sponsored research continued on improving the efficiency and the thrust levels, leading to the introduction of after­burners, which would increase thrust from 10-30 percent, but at the expense of fuel flows, which doubled to quadrupled that of the more normal subsonic cruise settings. The NACA and manufacturers looked at another form of jet propulsion, the ramjet, which did away with the complex rotating compressors and turbines and relied on forward speed of the vehicle to compress the airflow into an inlet/diffuser, where fuel

would then be injected and combusted, with the exhaust nozzle further increasing the thrust.

NASA’s early research in SVS concepts almost immediately influenced broader perceptions of the field. Working with NASA researchers who reviewed and helped write the text, the Federal Aviation Administration crafted a definition of SVS published in Advisory Circular 120-29A, describing it as "a system used to create a synthetic image (e. g., typically a computer generated picture) representing the environment external to the airplane.” In 2000, NASA Langley researchers Russell V. Parrish, Daniel G. Baize, and Michael S. Lewis gave a more detailed definition as "a display system in which the view of the external environment is pro­vided by melding computer-generated external topography scenes from on-board databases with flight display symbologies and other informa­tion from on-board sensors, data links, and navigation systems. These systems are characterized by their ability to represent, in an intuitive manner, the visual information and cues that a flight crew would have in daylight Visual Meteorological Conditions (VMC).”[1136] This definition can

be expanded further to include sensor fusion. This provides the capability to blend in real time in varying percentages all of the synthetically derived information with video or infrared signals. The key requirements of SVS as stated above are to provide the pilot with an intuitive, equivalent-to – daylight VMC capability in all-weather conditions at any time on a tacti­cal level (with present and near future time and position portrayed on a head-up display [HUD] or primary flight display [PFD]) and far improved situation awareness on a strategic level (with future time and position portrayed on a navigation display [a NAV display, or ND]).

In the earliest days of proto-SVS development during the 1980s and early 1990s, the state of the art of graphics generators limited the terrain portrayal to stroke-generated line segments forming polygons to repre­sent terrain features. Superimposing HITS symbology on these displays was not difficult, but the level of situational awareness (SA) improve­ment was somewhat limited by the low-fidelity terrain rendering. In fact, the superposition of HITS projected flight paths to include a rectilinear runway presentation at the end of the approach segment on basic PFD displays inspired the development of improved terrain portrayal by sug­gesting the simple polygon presentation of terrain. The development of raster graphics generators and texturing capabilities allowed these sim­ple polygons to be filled, producing more realistic scenes. Aerial and sat­ellite photography providing "photo-realistic” quality images emerged in the mid-1990s, along with improved synthetic displays enhanced by constantly improving databases. With vastly improved graphics generators (reflecting increasing computational power), the early concept of co-displaying the desired vertical and lateral pathway guid­ance ahead of the airplane in a three-dimensional perspective has evolved from the crude representations of just two decades ago to the present examples of high-resolution, photo-realistic, and elevation-based three­dimensional displays, replete with overlaid pathway guidance, provid­ing the pilot with an unobstructed view of the world. Effectively, then, the goal of synthetically providing the pilot with an effective daylight, VMC view in all-weather has been achieved.[1137]

Though the expressions Synthetic Vision Systems, External Vision Systems (XVS), and Enhanced Vision Systems (EVS) have often been used interchangeably, each is distinct. Strictly speaking, SVS has come

to mean computer-generated imagery from onboard databases com­bined with precise Global Positioning System (GPS) navigation. SVS joins terrain, obstacle, and airport images with spatial and navigational inputs from a variety of sensor and reference systems to produce a real­istic depiction of the external world. EVS and XVS employ imaging sen­sor systems such as television, millimeter wave, and infrared, integrated with display symbologies (altitude/airspeed tapes on a PFD or HUD, for example) to permit all-weather, day-night operations.[1138]

Confusion in terminology, particularly in the early years, has char­acterized the field, including use of multiple terms. For example, in 1992, the FAA completed a flight test investigating millimeter wave and infrared sensors for all-weather operations under the name "Synthetic Vision Technology Demonstration.”[1139] SVS and EVS are often combined as one expression, SVS/EVS, and the FAA has coined another term as well, EFVS, for Enhanced Flight Vision System. Industry has undertaken its own developments, with its own corporate names and nuances. A number of avionics companies have implemented various forms of SVS technologies in their newer flight deck systems, and various airframe manufacturers have obtained certification of both an Enhanced Vision System and a Synthetic Vision System for their business and regional aircraft. But much still remains to be done, with NASA, the FAA, and industry having yet to fully integrate SVS and EVS/EFVS technology into a comprehensive architecture furnishing Equivalent Visual Operations (EVO), blending infrared-based EFVS with SVS and millimeter – wave sensors, thereby creating an enabling technology for the FAA’s planned Next Generation Air Transportation System.[1140]

The underlying foundation of SVS is a complete navigation and situational awareness system. This Synthetic Vision System consists mainly of integration of worldwide terrain, obstacle, and airport data­bases; real-time presentation of immediate tactical hazards (such as weather); an Inertial Navigation System (INS) and GPS navigation capability; advanced sensors for monitoring the integrity of the data­base and for object detection; presentation of traffic information; and a real-time synthetic vision display, with advanced pathway or equivalent guidance, effectively affording the aircrew a projected highway-in-the – sky ahead of them.[1141] Two enabling technologies were necessary for SVS to be developed: increased computer storage capacity and a global, real-

time, highly accurate navigation system. The former has been steadily developing over the past four decades, and the latter became available with the advent of GPS in the 1980s. These enabling technologies uti­lized or improved upon the Electronic Flight Information System (EFIS), or glass cockpit, architecture pioneered by NASA Langley in the 1970s and first flown on Langley’s Boeing 737 Advanced Transport Operating System (ATOPS) research airplane. It should be noted that the research accomplishments of this airplane—Boeing’s first production 737—in its two decades of NASA service are legendary. These included demonstra­tion of the first glass cockpit in a transport aircraft, evaluation of trans­port aircraft fly-by-wire technology, the first GPS-guided blind landing, the development of wind shear detection systems, and the first SVS – guided landings in a transport aircraft.[1142]

The development of GPS satellite navigation signals technology enabled the evolution of SVS. GPS began as an Air Force-Navy effort to build a satellite-based navigation system that could meet the needs of fast-moving aircraft and missile systems, something the older TRANSIT system, developed in the late 1950s, could not. After early studies by a variety of organizations—foremost of which was the Aerospace Corporation—the Air Force formally launched the GPS research and development program in October 1963, issuing hardware design con­tracts 3 years later. Known initially as the Navstar GPS system, the con­cept involved a constellation of 24 satellites orbiting 12,000 nautical miles above Earth, each transmitting a continual radio signal containing a

precise time stamp from an onboard atomic clock. By recording the time of each received satellite signal of a required 4 satellites and comparing the associated time stamp, a ground receiver could determine position and altitude to high accuracies. The first satellite was launched in 1978, and the constellation of 24 satellites was complete in 1995. Originally intended only for use by the Department of Defense, GPS was opened for civilian use (though to a lesser degree of precision) by President Ronald Reagan after a Korean Air Lines Boeing 747 commercial airliner was shot down by Soviet interceptors in 1983 after it strayed miles into Soviet territory. The utility of the GPS satellite network expanded dra­matically in 2000, when the United States cleared civilian GPS users to receive the same level of precision as military forces, thus increasing civilian GPS accuracy tenfold.[1143]

Database quality was essential for the development of SVS. The 1990s saw giant strides taken when dedicated NASA Space Shuttle mis­sions digitally mapped 80 percent of Earth’s land surface and almost 100 percent of the land between 60 degrees north and south latitude. At the same time, radar mapping from airplanes contributed to the dig­ital terrain database, providing sufficient resolution for SVS in route and specific terminal area requirements. The Shuttle Endeavour Radar Topography Mission in 2000 produced topographical maps far more precise than previously available. Digital terrain databases are being produced by commercial and government organizations worldwide.[1144]

With the maturation of the enabling technologies in the 1990s and its prior experience in developing glass cockpit systems, NASA Langley was poised to develop the concept of SVS as a highly effective tool for pilots to operate aircraft more effectively and safely. This did not hap­pen directly but was the culmination of experience gained by Langley research engineers and pilots on NASA’s Terminal Area Productivity (TAP)
and High-Speed Research (HSR) programs in the mid – to late 1990s. By 1999, when the SVS project of the AvSP was initiated and funded, Langley had an experienced core of engineers and research pilots eager to push the state of the art.

Before an aircraft can get into the winter sky and safely avoid the threat of icing, it first must take off from what the pilot hopes is a long, wide, dry runway at the beginning of the flight, as well as at the end of the flight. Likewise, NASA’s contributions to air safety in fighting the tyr­anny of temperature included research into ground operations. While NASA did not invent the plow to push snow off the runway, or flame­throwers to melt off any stubborn runway snow or ice, the Agency has been active in studying the benefits of runway grooves since the first civil runway was introduced in the United States at Washington National Airport in December 1965.[1260]

Runway grooves are intended to quickly channel water away from the landing strip without pooling on the surface so as to prevent

hydroplaning. The 3-mile-long runway at the Shuttle Landing Facility is probably the most famous runway in the Nation and known for being grooved. Of course, there is little chance of snow or ice accumulating on the Central Florida runway, so when NASA tests runway surfaces for cold weather conditions it turns to the Langley Aircraft Landing Dynamics Facility at NASA’s Langley Research Center (LaRC) in Hampton, VA. The facility uses pressurized water to drive a landing-gear-equipped platform down a simulated runway strip, while cameras and sensors keep an eye on tire pressure, tire temperature, and runway friction. Another runway at NASA’s Wallops Flight Facility also has been used to test various sur­face configurations. During the mid-1980s, tests were performed on 12 different concrete and asphalt runways, grooved and non-grooved, includ­ing dry; wet; and snow, slush, and ice-covered surface conditions. More than 200 test runs were made with two transport aircraft, and more than 1,100 runs were made with different ground test vehicles. Ground vehi­cle and B-737 aircraft friction tests were conducted on grooved and non­grooved surfaces under wet conditions. As expected, grooved runway surfaces had significantly greater friction properties than non-grooved surfaces, particularly at higher speeds.[1261]